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Machine Translation Using Semantic Web Technologies: A Survey <s> Translating KBs <s> This publication has emanated from research supported in part by a research grant from Science ::: Foundation Ireland (SFI) under Grant Number SFI/12/RC/2289 and by the European Union under Grant No. 248458 for the Monnet project. <s> BIB001 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Translating KBs <s> We propose two novel model architectures for computing continuous vector representations of words from very large data sets. The quality of these representations is measured in a word similarity task, and the results are compared to the previously best performing techniques based on different types of neural networks. We observe large improvements in accuracy at much lower computational cost, i.e. it takes less than a day to learn high quality word vectors from a 1.6 billion words data set. Furthermore, we show that these vectors provide state-of-the-art performance on our test set for measuring syntactic and semantic word similarities. <s> BIB002 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Translating KBs <s> Ontology localization is the task of adapting an ontology to a different cultural context, and has been identified as an important task in the context of the Multilingual Semantic Web vision. The key task in ontology localization is translating the lexical layer of an ontology, i.e., its labels, into some foreign language. For this task, we hypothesise that the translation quality can be im-proved by adapting a machine translation system to the domain of the ontology. To this end, we build on the success of existing statistical machine translation (SMT) approaches, and investigate the impact of different domain adaptation techniques on the task. In particular, we investigate three techniques: i) en-riching a phrase table by domain-specific translation candidates acquired from existing Web resources, ii) relying on Explicit Semantic Analysis as an additional technique for scoring a certain translation of a given source phrase, as well as iii) adaptation of the language model by means of weighting n-grams with scores obtained from topic modelling. We present in detail the impact of each of these three techniques on the task of translating ontology labels. We show that these techniques have a generally positive effect on the quality of translation of the ontology and that, in combination, they provide a significant improvement in quality. <s> BIB003 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Translating KBs <s> Linked Open Data has been recognized as a valuable source for background information in data mining. However, most data mining tools require features in propositional form, i.e., a vector of nominal or numerical features associated with an instance, while Linked Open Data sources are graphs by nature. In this paper, we present RDF2Vec, an approach that uses language modeling approaches for unsupervised feature extraction from sequences of words, and adapts them to RDF graphs. We generate sequences by leveraging local information from graph sub-structures, harvested by Weisfeiler-Lehman Subtree RDF Graph Kernels and graph walks, and learn latent numerical representations of entities in RDF graphs. Our evaluation shows that such vector representations outperform existing techniques for the propositionalization of RDF graphs on a variety of different predictive machine learning tasks, and that feature vector representations of general knowledge graphs such as DBpedia and Wikidata can be easily reused for different tasks. <s> BIB004 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Translating KBs <s> Many recent works have demonstrated the benefits of knowledge graph embeddings in completing monolingual knowledge graphs. Inasmuch as related knowledge bases are built in several different languages, achieving cross-lingual knowledge alignment will help people in constructing a coherent knowledge base, and assist machines in dealing with different expressions of entity relationships across diverse human languages. Unfortunately, achieving this highly desirable crosslingual alignment by human labor is very costly and errorprone. Thus, we propose MTransE, a translation-based model for multilingual knowledge graph embeddings, to provide a simple and automated solution. By encoding entities and relations of each language in a separated embedding space, MTransE provides transitions for each embedding vector to its cross-lingual counterparts in other spaces, while preserving the functionalities of monolingual embeddings. We deploy three different techniques to represent cross-lingual transitions, namely axis calibration, translation vectors, and linear transformations, and derive five variants for MTransE using different loss functions. Our models can be trained on partially aligned graphs, where just a small portion of triples are aligned with their cross-lingual counterparts. The experiments on cross-lingual entity matching and triple-wise alignment verification show promising results, with some variants consistently outperforming others on different tasks. We also explore how MTransE preserves the key properties of its monolingual counterpart TransE. <s> BIB005 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Translating KBs <s> Our work presented in this paper focuses on the translation of domain-specific expressions represented in semantically structured resources, like ontologies or knowledge graphs. To make knowledge accessible beyond language borders, these resources need to be translated into different languages. The challenge of translating labels or terminological expressions represented in ontologies lies in the highly specific vocabulary and the lack of contextual information, which can guide a machine translation system to translate ambiguous words into the targeted domain. Due to the challenges, we train and translate the terminological expressions in the medial and financial domain with statistical as well as with neural machine translation methods. We evaluate the translation quality of domain-specific expressions with translation systems trained on a generic dataset and experiment domain adaptation with terminological expressions. Furthermore we perform experiments on the injection of external knowledge into the translation systems. Through these experiments, we observed a clear advantage in domain adaptation and terminology injection of NMT methods over SMT. Nevertheless, through the specific and unique terminological expressions, subword segmentation within NMT does not outperform a word based neural translation model. <s> BIB006

According to the surveyed articles, it is clear that SWT may be used for translating KBs in order to be applied in MT systems. For instance, some contents provided by the German Wikipedia version are not contained in the Portuguese one. Therefore, the semantic structure (i.e., triples) provided by DBpedia versions of these respective Wikipedia versions would be able to help translate from German to Portuguese. For example, the terms contained in triples would be translated to a given target language using a dictionary containing domain words. This dictionary may be acquired in two different ways. First, by performing localisation, as in the works by J. P. McCrae BIB003 and Arcan BIB001 which translates the terms contained in a monolingual ontology, thus generating a bilingual ontology. Second, by creating embeddings of both DBpedia versions in order to determine the similarity between entities through their vectors. This insight is supported by some recent works, such as Ristoski et al. BIB004 , which creates bilingual embeddings using RDF based on Word2vec BIB002 algorithms, and Muhao Chen et al. BIB005 , which generates multilingual knowledge graph embeddings for aligning entities across languages. Therefore, we suggest investigating a MT approach mainly based on SWT using NN for translating KBs. Once the KB are translated, we suggest including them into the language models for improving the translation of entities. Besides C. Shi et al , one of the recent attempts to this direction was carried out by Arčan and Buitelaar BIB006 by which the authors aimed at translating domain-specific expressions represented by English KBs in order to make the knowledge accessible for other languages. They claimed that KBs are mostly in English therefore they can not contribute to the problem of MT to other languages. Thus, they translated two KBs which belong to medical and financial domains along with the English Wikipedia to German. Once translated, the KBs were used as external resources in the translation of German-English. For the sake of brevity, they evaluated their approach in Phrase-based and NMT systems. The results were quite appealing then the investigation of this type of research requires to be fully exploited.

Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> Over the last decades, several billion Web pages have been made available on the Web. The ongoing transition from the current Web of unstructured data to the Web of Data yet requires scalable and accurate approaches for the extraction of structured data in RDF (Resource Description Framework) from these websites. One of the key steps towards extracting RDF from text is the disambiguation of named entities. While several approaches aim to tackle this problem, they still achieve poor accuracy. We address this drawback by presenting AGDISTIS, a novel knowledge-base-agnostic approach for named entity disambiguation. Our approach combines the Hypertext-Induced Topic Search (HITS) algorithm with label expansion strategies and string similarity measures. Based on this combination, AGDISTIS can efficiently detect the correct URIs for a given set of named entities within an input text. We evaluate our approach on eight different datasets against state-of-the-art named entity disambiguation frameworks. Our results indicate that we outperform the state-of-the-art approach by up to 29% F-measure. <s> BIB001 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> In this paper we present a Web interface and a RESTful API for our state-of-the-art multilingual word sense disambiguation and entity linking system. The Web interface has been developed, on the one hand, to be user-friendly for non-specialized users, who can thus easily obtain a first grasp on complex linguistic problems such as the ambiguity of words and entity mentions and, on the other hand, to provide a showcase for researchers from other fields interested in the multilingual disambiguation task. Moreover, our RESTful API enables an easy integration, within a Java framework, of state-of-the-art language technologies. Both the Web interface and the RESTful API are available at http://babelfy.org <s> BIB002 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> This paper proposes an extension of METEOR, a well-known MT evaluation metric, for multiple target languages using an in-house lexical resource called DBnary (an extraction from Wiktionary provided to the community as a Multilingual Lexical Linked Open Data). Today, the use of the synonymy module of METEOR is only exploited when English is the target language (use of WordNet). A synonymy module using DBnary would allow its use for the 21 languages (covered up to now) as target languages. The code of this new instance of METEOR, adapted to several target languages, is provided to the community via a github repository. We also show that our DBnary augmented METEOR increases the correlation with human judgements on the WMT 2013 and 2014 metrics dataset for English-to-(French, Russian, German, Spanish) language pairs. <s> BIB003 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> Contributive resources, such as Wikipedia, have proved to be valuable to Natural Language Processing or multilingual Information Retrieval applications. This work focusses on Wiktionary, the dictionary part of the resources sponsored by the Wikimedia foundation. In this article, we present our extraction of multilingual lexical data from Wiktionary data and to provide it to the community as a Multilingual Lexical Linked Open Data (MLLOD). This lexical resource is structured using the LEMON Model. This data, called DBnary, is registered at http://thedatahub.org/dataset/dbnary. <s> BIB004 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> This paper presents an approach combining lexico-semantic resources and distributed representations of words applied to the evaluation in machine translation (MT). This study is made through the enrichment of a well-known MT evaluation metric: METEOR. This metric enables an approximate match (synonymy or morphological similarity) between an automatic and a reference translation. Our experiments are made in the framework of the Metrics task of WMT 2014. We show that distributed representations are a good alternative to lexico-semantic resources for MT evaluation and they can even bring interesting additional information. The augmented versions of METEOR, using vector representations, are made available on our Github page. <s> BIB005 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> Abstract With the ever increasing availability of linked multilingual lexical resources, there is a renewed interest in extending Natural Language Processing (NLP) applications so that they can make use of the vast set of lexical knowledge bases available in the Semantic Web. In the case of Machine Translation, MT systems can potentially benefit from such a resource. Unknown words and ambiguous translations are among the most common sources of error. In this paper, we attempt to minimise these types of errors by interfacing Statistical Machine Translation (SMT) models with Linked Open Data (LOD) resources such as DBpedia and BabelNet. We perform several experiments based on the SMT system Moses and evaluate multiple strategies for exploiting knowledge from multilingual linked data in automatically translating named entities. We conclude with an analysis of best practices for multilingual linked data sets in order to optimise their benefit to multilingual and cross-lingual applications. <s> BIB006 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> Entity linking has recently been the subject of a significant body of research. Currently, the best performing approaches rely on trained mono-lingual models. Porting these approaches to other languages is consequently a difficult endeavor as it requires corresponding training data and retraining of the models. We address this drawback by presenting a novel multilingual, knowledge-base agnostic and deterministic approach to entity linking, dubbed MAG. MAG is based on a combination of context-based retrieval on structured knowledge bases and graph algorithms. We evaluate MAG on 23 data sets and in 7 languages. Our results show that the best approach trained on English datasets (PBOH) achieves a micro F-measure that is up to 4 times worse on datasets in other languages. MAG on the other hand achieves state-of-the-art performance on English datasets and reaches a micro F-measure that is up to 0.6 higher than that of PBOH on non-English languages. <s> BIB007 </s> Machine Translation Using Semantic Web Technologies: A Survey <s> Conclusions and Future Work <s> We present Grid Beam Search (GBS), an algorithm which extends beam search to allow the inclusion of pre-specified lexical constraints. The algorithm can be used with any model that generates a sequence $ \mathbf{\hat{y}} = \{y_{0}\ldots y_{T}\} $, by maximizing $ p(\mathbf{y} | \mathbf{x}) = \prod\limits_{t}p(y_{t} | \mathbf{x}; \{y_{0} \ldots y_{t-1}\}) $. Lexical constraints take the form of phrases or words that must be present in the output sequence. This is a very general way to incorporate additional knowledge into a model's output without requiring any modification of the model parameters or training data. We demonstrate the feasibility and flexibility of Lexically Constrained Decoding by conducting experiments on Neural Interactive-Predictive Translation, as well as Domain Adaptation for Neural Machine Translation. Experiments show that GBS can provide large improvements in translation quality in interactive scenarios, and that, even without any user input, GBS can be used to achieve significant gains in performance in domain adaptation scenarios. <s> BIB008

In this paper, we detailed a systematic literature review of MT using SWT for translating natural language sentences. The review aimed to answer the four research questions defined in Section 2 by a thorough analysis of the 21 most relevant papers. Our goal was to provide a clear understanding of how SWT have helped the translation process within MT systems. Few studies have been found, suggesting that this method is still in its infancy. The surveyed articles demonstrate that SWT have been mainly used for the disambiguation task in MT systems and their capabilities have steadily increased. Considering the decision power of SWT, they can not be ignored by future MT systems. Nevertheless, there are still significant drawbacks. Although the research works provide strong evidence of general SW advantages in the translation process measured by automatic evaluation metrics. The real semantic contributions were assessed manually and the evaluators could measure improvement according to the respective domains through which the work was approached. BLEU was the automatic evaluation metric basically used by all research works and it lacks semantic measurement. Thus, decreasing the real contribution of SW in MT. Recent works on MT evaluation have shown promising advances for dealing with semantics in related metrics. These recent works BIB003 BIB005 attached SWT, in this case DBnary BIB004 , to METEOR to handle meanings for non-English languages. In addition, MEANT has become the state-of-the-art in terms of evaluating semantics correlations in translated texts. Therefore, we expect that upcoming research works attempting to combine SWT with MT will be better evaluated by using these referred semantic metrics. How can SWT enhance MT quality? Regarding this main research question, we identified that both semantic and syntactic disambiguation including entities, structural divergence and OOV words problems can all be tackled with SWT. Additionally, SWT and MT approaches face the problem of languages to be inherently ambiguous, in terms of lexicon, syntax, semantics and pragmatics. Identifying concepts in a KB or finding the right translation for a word are instances of the same WSD problem. Therefore, a deeper understanding of how pieces of information obtained from ontologies and KB on the one hand, and parallel and monolingual corpora on the other hand may contribute to solve these problems of ambiguity in MT. The two most recently surveyed articles, BIB006 , which have implemented a translation model relying on structured knowledge, support our conclusion. They showed promising results even using an automatic evaluation metric, which does not consider semantics. Therefore, this work increases the evidence that SWT can significantly enhance the quality of MT systems. Furthermore, we answered the first sub-question RQ1 by listing and discussing all surveyed articles. RQ2 is summarized in Table 3 where we list all applied SW methods and respective SW resources. For RQ3, in Section 4 we also discussed the impact of applying an ontology to MT. We conclude that ontological knowledge is generally beneficial for certain translation quality issues, mostly related to disambiguation in morphologically rich languages and sign languages. The ontological knowledge was a crucial resource for improving the translation between sign and spoken natural languages. We sought to answer RQ4, about the availability of SW-tools, and we could perceive that most of them were discontinued. Only AGDIS-TIS BIB007 BIB001 , DBpedia Spotlight and Babelfy BIB002 , which had supported some of the surveyed articles, are currently available. Moreover, we noticed that these SW tools performed only a specific task -disambiguation, for supporting the translations. Apart from the tools, BabelNet and DBpedia were used as KBs by almost of all surveyed works. As a next step, we intend to elaborate a novel MT approach based on our suggestions made in Section 4, which means an approach capable of gathering knowledge from different SW resources simultaneously for addressing the ambiguity of named entities which can also alleviate the problem of OOV words. This insight relies on recent works BIB008 which have guided the usage of external knowledge in NMT systems for overcoming the vocabulary limitation of NN models. Also, we aim at creating a method to structure natural language sentences into triples for supporting the generation task. Future works that can be expected from fellow researchers, based on statements in their own papers, include the creation of linguistic ontologies describing the syntax of rich morphologically languages for supporting MT approaches. In addition, alignment between ontologies is expected to try to bridge gaps which are not addressed by the current SMT models. Wellknown bilingual dictionaries have been mapped to RDF, then the creation of multilingual dictionaries have become easier for content translation. These RDF dictionaries can help to improve MT steps, such as alignment, or even translate, based entirely on such semantic resources.

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> Over the past five years, there has been considerable growth and established value in the practice of modeling automotive software requirements. Much of this growth has been centered on requirements of software associated with the established functional areas of an automobile, such as those associated with powertrain, chassis, body, safety and infotainment. This paper makes a case for modeling four additional attributes that are increasingly important as vehicles become information conduits: security, privacy, usability, and reliability. These four attributes are important in creating specifications for embedded in-vehicle automotive software. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> This report is one of a series resulting from a project entitled "Competitiveness by Leveraging Emerging Technologies Economically" (COMPLETE), carried out by JRC-IPTS. Each of the COMPLETE studies illustrates in its own right that European companies are active on many fronts of emerging and disruptive ICT technologies and are supplying the market with relevant products and services. Nevertheless, the studies also show that the creation and growth of high tech companies is still very complex and difficult in Europe, and too many economic opportunities seem to escape European initiatives and ownership. COMPLETE helps to illustrate some of the difficulties experienced in different segments of the ICT industry and by growing potential global players. This report reflects the findings of a study conducted by Egil Juliussen and Richard Robinson, two senior experts from iSuppli Corporation on the Competitiveness of the European Automotive Embedded Software industry. The report starts by introducing the market, its trends, the technologies, their characteristics and their potential economic impact, before moving to an analysis of the competitiveness of the corresponding European industry. It concludes by suggesting policy options. The research, initially based on internal expertise and literature reviews, was complemented with further desk research, expert interviews, expert workshops and company visits. The results were ultimately reviewed by experts and also in a dedicated workshop. The report concludes that currently ICT innovation in the automotive industry is a key competence in Europe, with very little ICT innovation from outside the EU finding its way into EU automotive companies. A major benefit of a strong automotive ICT industry is the resulting large and valuable employment base. But future maintenance of automotive ICT jobs within the EU will only be possible if the EU continues to have high levels of product innovation. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> Reliability and dependability of software in modern cars is of utmost importance. Predicting these properties for software under development is therefore important for modern car OEMs, and using reliability growth models (e.g. Rayleigh, Goel-Okumoto) is one approach. In this paper we evaluate a number of standard reliability growth models on a real software system from automotive industry. The results of the evaluation show that models can be fitted well with defect inflow data, but certain parameters need to be adjusted manually in order to predict reliability more precisely in the late test phases. In this paper we provide recommendations for how to adjust the models and how the adjustments should be used in the development process of software in the automotive domain by investigating data from an industrial project. <s> BIB003 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> Providing various wireless connectivities for vehicles enables the communication between vehicles and their internal and external environments. Such a connected vehicle solution is expected to be the next frontier for automotive revolution and the key to the evolution to next generation intelligent transportation systems (ITSs). Moreover, connected vehicles are also the building blocks of emerging Internet of Vehicles (IoV). Extensive research activities and numerous industrial initiatives have paved the way for the coming era of connected vehicles. In this paper, we focus on wireless technologies and potential challenges to provide vehicle-to-x connectivity. In particular, we discuss the challenges and review the state-of-the-art wireless solutions for vehicle-to-sensor, vehicle-to-vehicle, vehicle-to-Internet, and vehicle-to-road infrastructure connectivities. We also identify future research issues for building connected vehicles. <s> BIB004 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> Automotive infotainment systems are driving changes to automobiles, and to driver behavior. <s> BIB005 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> Autonomous and semiautonomous vehicles are currently being developed by over14 companies. These vehicles may improve driving safety and convenience, or they may create new challenges for drivers, particularly with regard to situation awareness (SA) and autonomy interaction. I conducted a naturalistic driving study on the autonomy features in the Tesla Model S, recording my experiences over a 6-month period, including assessments of SA and problems with the autonomy. This preliminary analysis provides insights into the challenges that drivers may face in dealing with new autonomous automobiles in realistic driving conditions, and it extends previous research on human-autonomy interaction to the driving domain. Issues were found with driver training, mental model development, mode confusion, unexpected mode interactions, SA, and susceptibility to distraction. New insights into challenges with semiautonomous driving systems include increased variability in SA, the replacement of continuous control with seria... <s> BIB006 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> I. INTRODUCTION <s> This paper presents a scientometric and bibliometric review of the research on autonomous vehicles (AVs) to identify its main characteristics, evolution, and potential trends for future studies. Relevant articles were searched on WoS, yielding a research corpus of 10,580 papers, and the software CiteSpace was subsequently used for analysis. The results showed that AV research is heterogeneous and registered a growing demand over time. Multidisciplinarity is present, with 96 science fields being identified. As in any other sector, it is necessary to understand broader aspects of this industry such as the market factors surrounding it, as well as other economic and managerial issues. In this sense, we observed a migration of the research field from multidisciplinarity to pluridisciplinarity with a greater number of studies focusing on the latter. We understand that terminology standardisation contributes to achieving pluridisciplinarity. As such, it is important to highlight that sustainability, public policies, liability, and safety, as well as business issues such as performance and business models are some of the tendencies in the field of AVs. For future studies, we suggest a more in-depth analysis of publications in terms of individual search terms, as well as the sub-areas identified as trends in this paper. <s> BIB007

In the last several decades, software has become an essential part of everyday life. Working, shopping, traveling, communicating and playing are all infused with and mediated by software . Just as steam was the driver of the industrial age in the 19th century, software is the driver of today's information age. There is not a single industry today that is not at least partially governed by software, and automotive industry is no different. In 2008, a conservative assessment of the average software value per car was $425 and it is estimated to increase to $575 by 2020 BIB002 , indicating a rise in value of over 35%. Furthermore, the worldwide market value of automotive embedded software is estimated to grow from $30 billion in 2008 to over $52 billion in 2020 BIB002 . All of this, including the growing complexity of software in automotive systems, indicates that software engineering is an increasingly important discipline within automotive systems development and it will become even more crucial for vehicle manufacturers to equip their vehicles with software of the highest quality to stay relevant in today's market. Software is an indispensable part of today's vehicles and its importance is perhaps the most significant in electric vehicles (EVs), as they are actively pushing for a bigger share in the automotive market. The number of electric cars The associate editor coordinating the review of this manuscript and approving it for publication was Razi Iqbal . surpassed 2 million vehicles worldwide in 2016 after crossing the 1 million threshold in 2015 [9] . Country targets, EV equipment manufacturer announcements and continuous technology improvements indicate that the electric car stock will range between 9 million and 20 million by 2020 and between 40 million and 70 million by 2025 [9] . With the advent of electric vehicles comes the increase of the number of interconnected electronic control units (ECUs) and, consequently, the complexity of software needed to manage them. Today, some vehicles contain upward of 100 ECUs, running millions of lines of software code BIB005 . It is estimated that by 2020 75% of cars will be built with the necessary hardware to connect to the Internet , enabling automated links to all other connected objects such as smart-phones, tracking devices, traffic lights and other motor vehicles . The interaction with other devices and information systems creates an information-rich travel environment for the passengers and enhances the situational awareness of the vehicle BIB004 . Software plays a prominent part in the development of connected cars, because the information collected from other vehicles and the grid has to be processed and displayed to the driver as quickly as possible. Connected vehicles provide a basis for the emergence of autonomous driving. Autonomous vehicles are vehicles with action and motion capabilities which do not require any sort of driver or teleoperation control BIB007 . They are set to be the next step in the automotive evolution, bringing change not only to automotive industry, but to the way transportation is perceived. Today, autonomous cars are being developed by several manufacturers BIB006 and software is taking some of the critical functions (e.g. controlling the vehicle and analyzing the vehicle's surroundings), with a goal to eventually replace human drivers. Connected and autonomous vehicles (CAVs) are currently among the most researched automotive technologies. With the growing environmental concerns and greenhouse gas reduction goals, the future CAVs are expected to be electric, indicating that a connected and autonomous electric vehicle (CAEV) should become a common feature on the road within the next decade . To be competitive, CAEVs need to have a well-designed information and communication technology (ICT) architecture in the form of on-board electronics, together with associated automotive software, as this could improve a vehicles energy efficiency, driving comfort and passenger safety, all of which are significant aspects to consider when buying a new vehicle. In addition to the ICT architecture design, CAEV software development offers challenges in the form of many quality requirements that need to be satisfied. Software reliability is critical since CAEVs are software-driven (e.g. drive-by-wire systems) and any kind of software failure can put the lives of the passengers to risk BIB003 . Next, the software has to be secure to prevent unauthorized access to the in-vehicle network and with modern vehicles ''knowing'' more about their passengers than ever before, it needs to ensure privacy protection . Furthermore, usability is particularly important because of its impact on safety, as the vehicle interface must provide access to information and controls without distracting the driver BIB001 . Modern vehicles also need to be adaptable to the needs of the driver, enabling higher personalization, and software and hardware updates. Bearing in mind the ever-increasing relevance of CAEVs and the importance of automotive software in them, a literature survey of the field was made ( Table 1 ). The four search terms related to the focus of the paper were chosen for the survey: automotive software as the core search term, and electric vehicle (or vehicles), connected and autonomous as optional terms. The search terms were applied to the title, the abstract and keywords of the papers in Scopus, Elsevier's database of peer-reviewed literature, and to the metadata of conference, journal and magazine papers, books and early access articles in IEEE Xplore, the IEEE research database. Although there were 694 and 331 papers found in Scopus and IEEE Xplore databases respectively for the term automotive software, there was only four of them found for the combination of automotive software and electric vehicles, and none were found for the combination of all four search terms, suggesting that an overview of the research area is needed. Therefore, this paper aims to provide a systematic summary of automotive software found in today's CAEVs. Such a summary, in addition to providing an introduction to the field, could also be useful for the identification of trends and challenges which could potentially be tackled in future work. The remainder of the paper has the following structure. Section II describes the automotive software and ICT architecture found in CAEVs, with Section III mapping that architecture on today's most popular EVs. In Section IV, the future trends and challenges of automotive software development are described, while Section V concludes the paper.

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> A. ELECTRIC ARCHITECTURE <s> The battery is a fundamental component of electric vehicles, which represent a step forward towards sustainable mobility. Lithium chemistry is now acknowledged as the technology of choice for energy storage in electric vehicles. However, several research points are still open. They include the best choice of the cell materials and the development of electronic circuits and algorithms for a more effective battery utilization. This paper initially reviews the most interesting modeling approaches for predicting the battery performance and discusses the demanding requirements and standards that apply to ICs and systems for battery management. Then, a general and flexible architecture for battery management implementation and the main techniques for state-of-charge estimation and charge balancing are reported. Finally, we describe the design and implementation of an innovative BMS, which incorporates an almost fully-integrated active charge equalizer. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> A. ELECTRIC ARCHITECTURE <s> This paper gives an overview of the system architecture and software design challenges for Electric Vehicles (EVs). First, we introduce the EV-specific components and their control, considering the battery, electric motor, and electric powertrain. Moreover, technologies that will help to advance safety and energy efficiency of EVs such as drive-by-wire and information systems are discussed. Regarding the system architecture, we present challenges in the domain of communication and computation platforms. A paradigm shift towards time-triggered in-vehicle communication systems becomes inevitable for the sake of determinism, making the introduction of new bus systems and protocols necessary. At the same time, novel computational devices promise high processing power at low cost which will make a reduction in the number of Electronic Control Units (ECUs) possible. As a result, the software design has to be performed in a holistic manner, considering the controlled component while transparently abstracting the underlying hardware architecture. For this purpose, we show how middleware and verification techniques can help to reduce the design and test complexity. At the same time, with the growing connectivity of EVs, security has to become a major design objective, considering possible threats and a security-aware design as discussed in this paper. <s> BIB002

Although there are different types of EVs (e.g. hybrid EVs, plug-in hybrid EVs and solar-powered EVs), today's highway-capable EVs are mostly battery electric vehicles (BEVs). Their architecture is fairly simple; it consists of a battery pack, battery management system, electric motor and a motor controller. Battery pack is used for storing electric energy and it is the most expensive component in EVs BIB002 . Battery pack consists of series-connected battery cells, and its capacity determines the driving range of the EV. To keep the battery cells within their safe operating range a Battery Management System (BMS) is required BIB001 . BMS monitors the state of charge of battery cells, their voltage levels, the current drawn from the cells and cell temperature. The key component of an EV is the electric motor, as it is responsible for performing the vehicle's main functiondriving. The electric motor can also be used as a generator of electricity through regenerative or dynamic braking. Between the electric motor and the battery is an inverter, which converts the DC source from the battery pack to AC current driving the motor. To ensure efficient and reliable operation of the electric motor, a motor controller is needed. The controller normally controls the power supplied to the motor, and with it, the vehicle speed. Finally, to create an energy efficient EV, it is not enough to simply replace the ICE and fuel tank in traditional ICE cars with an electric motor and battery; the entire powertrain has to be modelled for the motor and battery.

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. SOFTWARIZED ARCHITECTURE <s> This work presents a study of current and future bus systems with respect to their security against various malicious attacks. After a brief description of the most well-known and established vehicular communication systems, we present feasible attacks and potential exposures for these automotive networks. We also provide an approach for secured automotive communication based on modern cryptographic mechanisms that provide secrecy, manipulation prevention and authentication to solve most of the vehicular bus security issues. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. SOFTWARIZED ARCHITECTURE <s> The deployment of onboard electronic systems and software in the motor vehicle has increased steadily since the early 1970s. The software-based implementation of vehicle functions must take into consideration higher demands on safety and reliability in relation to lower cost ceilings, longer product life cycles and shorter development times. The driver-vehicle-environment system, essential system basics, development processes for electronic systems / software, and the methods and tools used in the development, manufacture, and service of electronic systems are described in this book, which was translated from German into English by Roger Carey. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. SOFTWARIZED ARCHITECTURE <s> The design of electric vehicles require a complete paradigm shift in terms of embedded systems architectures and software design techniques that are followed within the conventional automotive systems domain. It is increasingly being realized that the evolutionary approach of replacing the engine of a car by an electric engine will not be able to address issues like acceptable vehicle range, battery lifetime performance, battery management techniques, costs and weight, which are the core issues for the success of electric vehicles. While battery technology has crucial importance in the domain of electric vehicles, how these batteries are used and managed pose new problems in the area of embedded systems architecture and software for electric vehicles. At the same time, the communication and computation design challenges in electric vehicles also have to be addressed appropriately. This paper discusses some of these research challenges. <s> BIB003 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. SOFTWARIZED ARCHITECTURE <s> With the rapidly evolving technology of the smart grid and electric vehicles (EVs), the battery has emerged as the most prominent energy storage device, attracting a significant amount of attention. The very recent discussions about the performance of lithium-ion (Li-ion) batteries in the Boeing 787 have confirmed so far that, while battery technology is growing very quickly, developing cells with higher power and energy densities, it is equally important to improve the performance of the battery management system (BMS) to make the battery a safe, reliable, and cost-efficient solution. The specific characteristics and needs of the smart grid and EVs, such as deep charge/discharge protection and accurate state-of-charge (SOC) and state-of-health (SOH) estimation, intensify the need for a more efficient BMS. The BMS should contain accurate algorithms to measure and estimate the functional status of the battery and, at the same time, be equipped with state-of-the-art mechanisms to protect the battery from hazardous and inefficient operating conditions. <s> BIB004 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. SOFTWARIZED ARCHITECTURE <s> The connected car—a vehicle capable of accessing to the Internet, of communicating with smart devices as well as other cars and road infrastructures, and of collecting real-time data from multiple sources—is likely to play a fundamental role in the foreseeable Internet Of Things. In a context ruled by very strong competitive forces, a significant amount of car manufacturers and software and hardware developers have already embraced the challenge of providing innovative solutions for new-generation vehicles. Today’s cars are asked to relieve drivers from the most stressful operations needed for driving, providing them with interesting and updated entertainment functions. In the meantime, they have to comply with the increasingly stringent standards about safety and reliability. The aim of this article is to provide an overview of the possibilities offered by connected functionalities on cars and the associated technological issues and problems, as well as to enumerate the currently available hardware and software solutions and their main features. <s> BIB005 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. SOFTWARIZED ARCHITECTURE <s> Hybrid Electric Vehicles with different voltage levels, powertrain structures, and vehicle variants are driving OEMs to develop and maintain multiple software systems including functions like torque assist, regenerative braking, generation and start-stop. Development of such complex software systems includes challenges like reusability, maintenance, variant management, complexity, testability and redundancy of these functions. This situation provides the opportunity for OEMs to explore common software system with acceptable effort, calculable risk and even time-to-market. Furthermore, the OEM has the possibility of being more flexible when selecting suppliers for base software and hardware and reducing dependency of a single system supplier. One of the main goals for the OEM is to improve containment of product/ process complexity, risk in addressing the challenges of complex software system and ensure it is hardware independent for seamless porting into various supplier hardware. Presented in this paper is a creative way of utilizing AUTOSAR with Advanced Software Developing Methods to scale this complex software system and optimize the business trend. <s> BIB006

In EVs, automotive software is the foundation of all vehicle functions. It would be impossible to count all the components running automotive software in EVs, so a high-level approach will be taken, providing an overview of automotive electronics, bus systems and software architecture. Well-defined software architecture is especially important in automotive software design, as it helps to understand and predict the performance of the system by examining all of its subsystems, components and modules. Software and electronics in automotive systems go hand in hand, which is why software architecture cannot be viewed as stand-alone, without being connected to vehicle electronic systems. Electronic Control Units (ECUs) are embedded systems on which software that controls one or more electronic systems or subsystems in a vehicle is executed. Each ECU is responsible for performing a certain vehicle function. Assembly language or the C programming language can be used to program the ECUs , which, along with the software running on them, form the electrical and software system of the car. When electronics was first started being deployed in vehicles, the operation of ECUs was largely autonomous with no interaction among different ECUs. The networking of different ECUs was made possible with the introduction of automotive bus systems. Automotive bus systems interconnect components inside a vehicle, allowing multiple access to various sensors and enabling higher-level software functions that run on several subsystems. They can be divided into four different communication groups based on their application area and technical properties: • event-triggered bus systems (e.g. CAN); • time-triggered bus systems (e.g. FlexRay); • subnetworks (e.g. LIN); • multimedia bus systems (e.g. MOST). The comparison of the mentioned representatives of each automotive bus system communication group can be seen in Table 2 . Control Area Network (CAN) is an event-triggered bus system mainly used for soft real-time in-car serial communication between ECUs, networking for example the antilock braking system (ABS) BIB001 or the battery management system BIB004 . It enables data rates up to 1 Mbit/s and broadcasts messages to all connected nodes which independently decide whether to process the message or not. CAN ensures the quickest transmission of top priority messages using CSMA/CD (Carrier Sense Multiple Access / Collision Detection) access control method and it detects transfer errors using cyclic redundancy check (CRC) and parity bits, and initiates retransmission of affected messages BIB001 . FlexRay is a time-triggered bus system developed in 1999 for future safety-relevant high-speed automotive networks, targeting highly safety relevant applications such as drive-by-wire and powertrain BIB001 . It supports data rates up to 10 Mbit/s and up to 64 connected nodes. It uses the cyclic TDMA (Time Division Multiple Access) that provides fixed time slots during which devices can access the bus and mini-slots which can enlarge if devices have additional data to be sent that doesn't fit in their designated time slot . The error tolerance is achieved by redundant channels, CRC and a bus guardian that detects and handles logical errors BIB001 . Local Interconnect Network (LIN) is a local sub network usually connected as a sub-bus to a more powerful bus, such as CAN. It has been developed for less complex networks where the bandwidth and versatility of CAN bus is not required, such as, automatic door locking mechanisms, power-windows and temperature and rain sensors BIB001 . LIN supports data transmission of up to 20 kbit/s and error protection in the form of checksums and parity bits. Multimedia bus systems such as Media Oriented Systems Transport (MOST) are responsible for the transmission of high-quality audio, voice and video data streams used for in-vehicle entertainment. MOST features a data rate of up to 24.8 Mbit/s in synchronous and 14.4 Mbit/s in asynchronous transmission mode with a 700 kbit/s control channel . Access control in synchronous mode is realized by TDM (Time Division Multiplex) and by CSMA/CA in asynchronous mode. An internal MOST system service is in charge of error management, detecting errors over parity bits, CRC and checksums. Vehicle electronics and software architecture can be viewed as multiple subsystems comprised of different ECUs that communicate over bus systems. Although each subsystem is responsible for a certain part of the vehicle, they communicate with one another, distributing some of the more complex functions over several subsystems and ECUs. The main subsystems found in vehicles are: • powertrain subsystem; • chassis subsystem; • body subsystem; • multimedia subsystem. The powertrain subsystem is in charge of the components that generate power and deliver it to the road surface; e.g. electric motor, motor controller, battery pack and battery management system. It has a relatively small number of user interfaces; the start/stop switch for starting and shutting off the vehicle and the accelerator pedal for increasing the speed of the vehicle. Along with controlling the powertrain according to user inputs, software functions of the powertrain subsystem include collecting information about the working parameters of the electric motor and battery pack, and notifying the driver via information systems if anything is out of order. The throttle-by-wire system, needed for the control of autonomous vehicles without human input, is also implemented in the powertrain subsystem of the electric vehicle. The chassis subsystem encompasses vehicle's axles and wheels, brakes, steering system, suspension and shock absorbers. It includes systems such as the antilock braking system (ABS), electronic stability program (ESP), parking brake, tire pressure monitoring system, suspension, power steering, brake-by-wire and steer-by-wire systems BIB002 . Functional failure of any of the aforementioned systems can put the passengers' lives at risk, which is why the safety requirements for the chassis subsystem are very stringent, with sensitive monitoring mechanisms in place for each of the safety critical systems. Similar to the powertrain system, the chassis subsystem also has a small number of user interfaces; the brake pedal, the steering wheel and the parking brake. The vehicle software and electronics responsible for passive safety and comfort and convenience are included in the body subsystem BIB002 . The passive safety systems are all onboard systems in charge of protecting the safety of vehicle's passengers (e.g. airbags, seat belt sensors and tighteners). The comfort and convenience systems include the central locking system, powered windows, wipers, heating and air conditioning, mirror adjusters, headlamp controls, parking aid features and other similar auxiliary systems which improve the passengers' travel comfort. Practically all of the comfort and convenience systems provide some kind of user interface, while passive safety systems usually provide none. The body subsystem contains the largest number of independent software functions due to the amount of different functionalities implemented in the subsystem. The multimedia subsystem includes the instrument cluster and the infotainment system which provides added comfort value to the passengers. The instrument cluster usually displays vehicle information such as current speed, battery level and indicators about vehicle status, and it must comply with regulations and laws to ensure that the information is properly displayed to the driver. To be able to display this information and to provide controls to elements from other subsystems, the ECUs of the multimedia subsystem are networked with the ECUs from the powertrain, chassis and body subsystems BIB002 . Infotainment system providing audio and video entertainment, voice control and smart phone integration is also a part of the multimedia subsystem. Examples of modern vehicle infotainment platforms are the QNX Car platform by Blackberry, Windows Embedded Automotive by Microsoft, GENIVI by GENIVI Alliance, Tizen IVI and Automotive Grade Linux by the Linux Foundation, and Android by Google BIB005 . Currently, most suppliers provide new functionalities to vehicles as hardware devices. This is one of the main reasons for the growing number of ECUs and the increase in complexity of the aforementioned subsystems. This approach is not scalable, so a transition from distributed hardware to distributed software and services will be necessary. For this purpose, flexible architectures and standardized software component approaches for vehicle software like AUTOSAR (Automotive Open System Architecture) will become essential for CAEVs BIB003 . AUTOSAR is a global standardization consortium of automotive manufacturers and suppliers developing an open industry standard for automotive electronic architectures. It addresses the challenge of rising code complexity by providing an open automotive software architecture which supports the development of standardized electronic systems that improve quality, performance, safety and environmental friendliness BIB006 . AUTOSAR enables modeling ECUs and entire electronic systems at a higher level using a top-down approach with the architecture of the ECU software as the starting point and simplifies the process of updating software over the vehicle's lifetime. AUTOSAR offers two solutions for the development of automotive software. AUTOSAR Classic Platform is the standard software platform for deeply embedded ECUs, and in CAEVs it remains the first choice for the development of software for the powertrain, chassis and body subsystems as they have high demands on safety, real-time and determinism while running on low cost hardware . New use cases, such as connected and autonomous vehicles, required the development of the AUTOSAR Adaptive Platform. Adaptive platform is meant to be used in applications that demand high-end computing power, such as multimedia, automated driving and communication with traffic infrastructure and cloud servers.

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. CONNECTED ARCHITECTURE <s> Vehicular environments impose a set of new requirements on today's wireless communication systems. Vehicular safety communications applications cannot tolerate long connection establishment delays before being enabled to communicate with other vehicles encountered on the road. Similarly, non-safety applications also demand efficient connection setup with roadside stations providing services (e.g. digital map update) because of the limited time it takes for a car to drive through the coverage area. Additionally, the rapidly moving vehicles and complex roadway environment present challenges at the PHY level. The IEEE 802.11 standard body is currently working on a new amendment, IEEE 802.1 lp, to address these concerns. This document is named wireless access in vehicular environment, also known as WAVE. As of writing, the draft document for IEEE 802.11p is making progress and moving closer towards acceptance by the general IEEE 802.11 working group. It is projected to pass letter ballot in the first half of 2008. This paper provides an overview of the latest draft proposed for IEEE 802.11p. It is intended to provide an insight into the reasoning and approaches behind the document. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. CONNECTED ARCHITECTURE <s> Vehicular ad hoc networks (VANETs) are classified as an application of mobile ad hoc network (MANET) that has the potential in improving road safety and in providing travellers comfort. Recently VANETs have emerged to turn the attention of researchers in the field of wireless and mobile communications, they differ from MANET by their architecture, challenges, characteristics and applications. In this paper we present aspects related to this field to help researchers and developers to understand and distinguish the main features surrounding VANET in one solid document, without the need to go through other relevant papers and articles starting from VANET architecture and ending up with the most appropriate simulation tools to simulate VANET protocols and applications. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. CONNECTED ARCHITECTURE <s> The connected car—a vehicle capable of accessing to the Internet, of communicating with smart devices as well as other cars and road infrastructures, and of collecting real-time data from multiple sources—is likely to play a fundamental role in the foreseeable Internet Of Things. In a context ruled by very strong competitive forces, a significant amount of car manufacturers and software and hardware developers have already embraced the challenge of providing innovative solutions for new-generation vehicles. Today’s cars are asked to relieve drivers from the most stressful operations needed for driving, providing them with interesting and updated entertainment functions. In the meantime, they have to comply with the increasingly stringent standards about safety and reliability. The aim of this article is to provide an overview of the possibilities offered by connected functionalities on cars and the associated technological issues and problems, as well as to enumerate the currently available hardware and software solutions and their main features. <s> BIB003

Connected vehicle is a vehicle capable of communicating with other vehicles, the infrastructure, smart devices and, most importantly, the Internet. A connected vehicle can exchange information by several different types of communication -vehicle to vehicle, vehicle to road infrastructure, vehicle to internet and vehicle to smart device. Vehicle To Vehicle (V2V) communication is the direct transmission of data between two or more vehicles. V2V communication can be used for accident avoidance, route optimization, multimedia information sharing and social interaction BIB003 . Communicating vehicles form a vehicular ad hoc network (VANET) which changes constantly and quickly. VANET enables communication between moving vehicles using dedicated short range communication (DSRC) BIB002 , a short-range wireless communication channel designed for automotive use, standardized by the IEEE 802.11p standard, also known as WAVE (Wireless Access in Vehicular Environments) BIB001 . The WAVE system is also used in the Vehicle To Road Infrastructure (V2I) communication. The main system components enabling V2I communication are onboard units (OBUs) and roadside units (RSUs). An OBU is a device mounted on a vehicle used for exchanging information with RSUs (or with other OBUs in case of V2V communication) BIB002 . The RSUs are usually fixed along the side of the road or on light poles, traffic lights and road signs. RSU's main functions are extending the range of VANET by redistributing information from OBUs to other OBUs, running safety applications such as accident or work zone warnings, and providing Internet connectivity to OBUs BIB002 . Internet communication is the fundamental requirement for a connected vehicle. There are three different solutions by which a vehicle can achieve an Internet connection: an embedded solution, a tethered solution and an integrated solution BIB003 . In the embedded solution, a SIM card is permanently installed in the vehicle during the manufacturing process. The module carrying the SIM card in the embedded solution is called the Telematics Control Unit (TCU). The tethered solution offers a modem that allows the insertion of a personal SIM decoupled from the vehicle, while keeping the application intelligence inside the vehicle. In the integrated approach, both intelligence and Internet connectivity are provided by a user's smart device, with the applications running on the smartphone displayed on the vehicles interface. Today, vehicle-to-Internet connection is most commonly achieved through 4G mobile communication. Connected vehicles generate high amounts of data, and with more and more vehicles becoming connected every year, the amount of generated data is set to increase. Vehicle manufacturers and telecommunications companies such as Audi, BWM, Deutsche Telekom, Ericsson and Vodafone have already started to prepare for the increase in vehicle generated data by forming the Fifth-Generation Automotive Association (5GAA) . 5G mobile networks will have reduced latency, massive device connectivity and will be able to handle much more data, and the 5GAA aims to use this technology to address society's connected mobility needs. Finally, connected vehicles need to offer a way to communicate with the user's personal smart devices. The smart device integration is usually achieved by a Bluetooth or Wi-Fi connection to the infotainment system. The infotainment system provides a user interface (UI) and the radio/entertainment/media functionality . Current trends in infotainment systems steer away from embedded applications capable of running on only one system, and towards multiplatform applications capable of running on systems adopted by more than just one car manufacturer. The most widespread solutions for smart device integration are Mir-rorLink by Car Connectivity Consortium, AppLink by Ford, Apple CarPlay by Apple and Android Auto by the Open Automotive Alliance BIB003 .

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> D. AUTONOMOUS ARCHITECTURE <s> X-by-Wire is a generic term referring to the replacement of mechanical or hydraulic systems, such as braking or steering, by electronic ones. In this chapter, we analyze the real-time and dependability constraints of X-by-Wire systems, review the fault-tolerant services that are needed and the communication protocols (TTP/C, FlexRay and TTCAN) considered for use in such systems. Using a Steer-by-Wire case-study, we detail the design principles and verification methods that can be used to ensure the stringent constraints of X-by-Wire systems. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> D. AUTONOMOUS ARCHITECTURE <s> This paper gives an overview of the system architecture and software design challenges for Electric Vehicles (EVs). First, we introduce the EV-specific components and their control, considering the battery, electric motor, and electric powertrain. Moreover, technologies that will help to advance safety and energy efficiency of EVs such as drive-by-wire and information systems are discussed. Regarding the system architecture, we present challenges in the domain of communication and computation platforms. A paradigm shift towards time-triggered in-vehicle communication systems becomes inevitable for the sake of determinism, making the introduction of new bus systems and protocols necessary. At the same time, novel computational devices promise high processing power at low cost which will make a reduction in the number of Electronic Control Units (ECUs) possible. As a result, the software design has to be performed in a holistic manner, considering the controlled component while transparently abstracting the underlying hardware architecture. For this purpose, we show how middleware and verification techniques can help to reduce the design and test complexity. At the same time, with the growing connectivity of EVs, security has to become a major design objective, considering possible threats and a security-aware design as discussed in this paper. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> D. AUTONOMOUS ARCHITECTURE <s> An autonomous car is a self-driving vehicle that has the capability to perceive the surrounding environment and navigate itself without human intervention. For autonomous driving, complex autonomous driving algorithms, including perception, localization, planning, and control, are required with many heterogeneous sensors, actuators, and computers. To manage the complexity of the driving algorithms and the heterogeneity of the system components, this paper applies distributed system architecture to the autonomous driving system, and proposes a development process and a system platform for the distributed system of an autonomous car. The development process provides the guidelines to design and develop the distributed system of an autonomous vehicle. For the heterogeneous computing system of the distributed system, a system platform is presented, which provides a common development environment by minimizing the dependence between the software and the computing hardware. A time-triggered network protocol, FlexRay, is applied as the main network of the software platform to improve the network bandwidth, fault tolerance, and system performance. Part II of this paper will provide the evaluation of the development process and system platform by using an autonomous car, which has the ability to drive in an urban area. <s> BIB003 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> D. AUTONOMOUS ARCHITECTURE <s> Part I of this paper proposed a development process and a system platform for the development of autonomous cars based on a distributed system architecture. The proposed development methodology enabled the design and development of an autonomous car with benefits such as a reduction in computational complexity, fault-tolerant characteristics, and system modularity. In this paper (Part II), a case study of the proposed development methodology is addressed by showing the implementation process of an autonomous driving system. In order to describe the implementation process intuitively, core autonomous driving algorithms (localization, perception, planning, vehicle control, and system management) are briefly introduced and applied to the implementation of an autonomous driving system. We are able to examine the advantages of a distributed system architecture and the proposed development process by conducting a case study on the autonomous system implementation. The validity of the proposed methodology is proved through the autonomous car A1 that won the 2012 Autonomous Vehicle Competition in Korea with all missions completed. <s> BIB004

An autonomous or a self-driving vehicle is a vehicle capable of perceiving its surroundings and navigating itself without human intervention. For autonomous driving to be made possible, a vehicle must be equipped with electronic control VOLUME 7, 2019 systems and surrounding detection sensors, along with complex autonomous driving algorithms, including: • perception -sensing the surrounding environment using various types of sensor techniques such as RADAR, LIDAR and computer vision; • localization -finding the car's position using the Global Positioning System (GPS); • planning -determining the behaviour and motion of the vehicle based on information from perception and localization; • control -executing the planned function by steering, accelerating and braking; • system management -supervising the driving system with functions such as fault management and logging BIB003 . The information on surrounding environment is collected using several types of sensor components. RADAR system uses radio waves to determine the range, angle and velocity of objects. The LIDAR system creates a digital 3D representation of the surrounding objects by illuminating them with pulsed laser light and measuring the reflected pulses with a sensor. Cameras and computer vision are then used to recognize and distinguish various visual objects BIB004 . Localization is achieved by a GPS system using additional information such as vehicle motion sensors, environment perception data and digital maps to account for the errors that can occur in bad GPS satellite signal conditions. Planning algorithms are usually embedded in the vehicle's autopilot functionality and achieved entirely by software functions. Autopilot algorithms are divided in three stages: global routing (i.e. finding the fastest and safest way from the initial position to the goal position), behaviour reasoning (i.e. assessing the driving situation and determining the behaviour of the autonomous vehicle based on the global route and perception information), and local motion planning (i.e. avoiding static and dynamic obstacle collision) BIB004 . Autopilot software in autonomous vehicles also has some interesting ethical requirements as it needs to replicate the human decision making process . The key technology enabling the control of an autonomous vehicle is drive-by-wire. Drive-by-wire is referring to the replacement of mechanical or hydraulic systems, such as acceleration (throttle-by-wire), braking (brake-bywire) or steering (steer-by-wire), by electronic ones BIB001 . Additional ECUs are added so that the software in autonomous vehicles can perform all driving functions. Although drive-by-wire is emerging for ICE vehicles, it is especially important for EVs. In EVs, brake-by-wire technology enables energy saving in the form of energy recuperation during braking by decoupling the braking pedal from the brakes BIB002 . Naturally, drive-by-wire is a highly safetycritical system which is why it must be designed in a faulttolerant fashion, with control system redundancies. For a critical drive-by-wire system, a management system must ensure that a system failure does not lead to endangerment of human lives or the vehicle environment, and that a failure of a single component does not lead to the failure of the whole drive-by-wire system BIB001 .

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> A. TRENDS IN AUTOMOTIVE SOFTWARE AND SYSTEM DESIGN <s> Disruptive technologies have the potential to change markets dramatically. The switch from internal combustion engines to electrical engines is such a change. But electric engines for vehicles are only the catalyst for the real change. Most significantly, the architecture and role of information and communication technology (ICT) will change for the vehicle of the future. This paper discusses the results of a study conducted in Germany on the role of ICT architectures. Furthermore, it will present an experimental platform that implements the vision of this study. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> A. TRENDS IN AUTOMOTIVE SOFTWARE AND SYSTEM DESIGN <s> This paper gives an overview of the system architecture and software design challenges for Electric Vehicles (EVs). First, we introduce the EV-specific components and their control, considering the battery, electric motor, and electric powertrain. Moreover, technologies that will help to advance safety and energy efficiency of EVs such as drive-by-wire and information systems are discussed. Regarding the system architecture, we present challenges in the domain of communication and computation platforms. A paradigm shift towards time-triggered in-vehicle communication systems becomes inevitable for the sake of determinism, making the introduction of new bus systems and protocols necessary. At the same time, novel computational devices promise high processing power at low cost which will make a reduction in the number of Electronic Control Units (ECUs) possible. As a result, the software design has to be performed in a holistic manner, considering the controlled component while transparently abstracting the underlying hardware architecture. For this purpose, we show how middleware and verification techniques can help to reduce the design and test complexity. At the same time, with the growing connectivity of EVs, security has to become a major design objective, considering possible threats and a security-aware design as discussed in this paper. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> A. TRENDS IN AUTOMOTIVE SOFTWARE AND SYSTEM DESIGN <s> In this article, the authors discuss the evolution of self-driving cars and examine current technology developments as well as potential safety, mobility, and productivity benefits of these vehicles. The basic aspects of autonomous vehicle technology, as put forth by the National Highway Transportation Safety Administration (NHTSA), are outlined. The authors also describe some of the challenges that accommodating self-driving vehicles may pose for those who plan and design transportation facilities, and they suggest actions the industry can take to address the anticipated challenges. <s> BIB003 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> A. TRENDS IN AUTOMOTIVE SOFTWARE AND SYSTEM DESIGN <s> Safety remains a problem on U.S. roadways, with more than 32,000 fatalities, 2.2 million injuries and 6 million crashes each year. Travelers, shippers and the economy are exposed to increasing amounts of congestion, unreliability, delay, emissions and excess energy consumption, which impede the efficient movement of people and goods. The U.S. Department of Transportation (DOT) had embarked upon a major research program toward implementing connected vehicle safety technologies, applications and systems using dedicated, short-range wireless communications (DSRC). Previous research by the National Highway Traffic Safety Administration (NHTSA) demonstrated that 80% of unimpaired driver crash types could be addressed by the connected vehicle technology. Through the year-long Safety Pilot that took place in Michigan from 2012 to 2013, U.S. DOT tested the effectiveness of wireless connected vehicle technology in real-world, multimodal driving conditions; collecting data about how ordinary drivers adapt to the use of connected vehicle technology; and identifying the potential safety benefits of connected vehicle technology. This work was performed in recognition of a February 3, 2014 NHTSA agency decision for light vehicles and a similar decision expected soon for commercial vehicles that will likely launch regulatory processes to require or incentivize all new vehicles to be equipped with DSRC devices. Communication among and between vehicles and the infrastructure (including traffic signals, work zone equipment, or pavement sensors, and other infrastructure elements) would also have data and mobility benefits (including data-driven applications such as traveler information for freight and passengers, transit operations, network flow optimization, traffic signal systems and incident response, emergency staging, and evacuation as well as sustainability-related applications). This paper describes an ongoing effort to explore opportunities for the state of Oregon, USA, to participate in future funded pilot deployments of mobility and environmental related applications in the coming years—possibly including a set of regional pilots as well as smaller, more self-contained projects focused on priority applications. As connected vehicle research moves into deployment, state, local and transit agencies, Metropolitan Planning Organizations and the private sector will start experiencing the effects of vehicles, after-market devices, mobile devices, and infrastructure with DSRC and other wireless connectivity at their cores. Along with other states and regions, the Oregon Department of Transportation (ODOT) can benefit from preliminary scoping, evaluation, and assessment of the impact of connected vehicles and infrastructure and a wide range of potential cooperative system applications. With this in mind, ODOT can determine whether or not to pursue the next phases of federal connected vehicle application funding. It can also make an informed choice about taking a potential national leadership role in the connected vehicle arena, and assess opportunities to join projects with other partners. This paper provides a summary of an internal survey conducted within ODOT along with insights gained from the analysis of the survey results. Next steps in the process are also described. <s> BIB004

The trends in automotive software and ICT architecture design are driven by two streams: societal and technological. The societal trends approach the future of mobility from an environmental point of view and the point of view of vehicle passengers. On the other hand, the technological trends back the societal requirements, while steering the automotive systems development in a way that incorporates emerging technologies and future sustainability. According to Buckl et al. BIB001 , the main societal trends shaping the automotive software development today are: 1) Energy and cost efficiency: The required energy per kilometer needs to be reduced. This can be achieved by replacing existing hardware functions with software, which decreases the vehicle's weight, or by adding intelligent predictive management functions. 2) Zero accidents: To reduce the number of accidents on the road, a number of pro-active safety functions need to be implemented. Fully autonomous vehicles are expected to be the ultimate solution for reducing the number of accidents. 3) Seamless connectivity: Modern vehicles will need to enable connections to other smart devices and the cloud, and provide frequent software updates to keep track with advances in multimedia and infotainment technology. 4) Personalization: Closely connected to the demand for seamless connectivity is the demand for personalization. Certain secondary vehicle functions will have to be transferred to a personal mobile device to provide users with information about their vehicle wherever they go. Vendor independent infotainment systems could allow users to create a consistently similar driving experience in different cars by a simple transfer of predefined settings from a mobile device. With the constantly changing landscape of the automotive industry, predicting and leveraging rising technologies is of paramount importance to the automotive software developers. Based on the CAEV ICT architecture and the ICT architectures of today's most popular EVs described in this paper, the main technological trends in today's automotive software industry can be identified: Ethernet is expected to be the replacement for current communication buses because of its high-speed, cost efficient and light-weight implementation BIB002 . 3) Connectivity and cooperation: Cars will have to be able to communicate with each other and the infrastructure through mobile and ad-hoc networks BIB004 . All modern cars are expected to provide smart device integration through Bluetooth or Wi-Fi connectivity. 4) Autonomous functions: Creating self-driving vehicles is the next step in automotive evolution and it entails a large amount of complexity in various vehicle systems BIB003 . Safety-critical systems, as well as verification and validation methods for software in cars will have to become even more stringent and advanced .

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. CHALLENGES <s> Disruptive technologies have the potential to change markets dramatically. The switch from internal combustion engines to electrical engines is such a change. But electric engines for vehicles are only the catalyst for the real change. Most significantly, the architecture and role of information and communication technology (ICT) will change for the vehicle of the future. This paper discusses the results of a study conducted in Germany on the role of ICT architectures. Furthermore, it will present an experimental platform that implements the vision of this study. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> B. CHALLENGES <s> The effort for the integration of new functionalities in today's vehicles is increasing as the interconnection and verification of the growing amount of heterogeneous and distributed electric control units (ECU's) becomes more difficult. The demand for a new architectural approach that can cope with the increasing complexity and offers possibilities for a smooth integration of future technologies is urgent. Such technologies are drive-by-wire systems or advanced driver assistance systems. This paper extends the previously introduced ICT architecture for future vehicles by the analysis of a possible system, hardware and software architecture and their properties. In addition, a migration path from the current vehicle architecture is suggested and economic impacts of suggested improvements are shown. A short description of differences to AUTOSAR is given. Demonstrators for proof-of-concept and evaluation are also discussed. With this work we have brought the previously described ICT architecture one step closer to the large-scale implementation in the automotive domain. <s> BIB002

Automotive software, as a dominant factor in the automotive industry, brings various challenges whose solutions can prove decisive for vehicle manufacturers to stay ahead of the competition. Future trends can be used to identify the main challenges in the automotive software and ICT architecture design that the manufacturers have to solve. These challenges are: 1) Reducing ICT architecture complexity: The evolution of vehicle ICT architectures shows a trend of architectures becoming more complex than required for the corresponding gain in functionalities. This trend is identified for some time, and vehicle manufacturers have already moved to answer it. The number of ECUs in an average vehicle in 2010 was around 70 BIB001 , while in today's electric vehicles it goes as low as three (as seen on Table 3 ). The centralization of ECUs impacts a vehicle's weight and cost, because of the decreased number of needed components, wiring optimizations, and reduced integration and testing effort. The ECU consolidation is almost an imperative in autonomous vehicles, as centralization decreases the complexity of development of advanced assisted driving functions, reduces the number of operations that can go wrong and increases reliability.The hardware architecture of such a centralized system should consist of a small number of central processing units parallelized to ensure redundancy, interconnected with smart sensors and actuators via a homogeneous and optionally redundant network BIB002 . Perceived events, like video camera signals can be evaluated much faster if the processing of signal input, decision and output is located in a single system . The relocation of software responsibility on a single, central ECU indicates that the workload on the ECU will increase, so the developed automotive software needs to be extremely resource efficient to decrease the hardware cost. Another way to reduce vehicle ICT architecture complexity is to use a domain controlled architecture, which reorders the corresponding vehicle functionality in a domain specific manner (typical domains are described in Section II-B powertrain, chassis, body and multimedia) . In this architecture model, every domain has an additional abstraction layer in the form of server ECUs called domain controllers. Every domain controller can control several bus systems, deactivating them individually to save energy when their operation is not needed. Furthermore, applications which must always be available during car usage can be integrated on domain controller ECUs and can be shared with other ECUs from the same domain. The challenge in using this approach is that achieving parallelization of ECUs from the same domain cannot be done using typical software parallelization scenarios as used for personal computer software, because of the implied high functional safety demands.

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> Over the past five years, there has been considerable growth and established value in the practice of modeling automotive software requirements. Much of this growth has been centered on requirements of software associated with the established functional areas of an automobile, such as those associated with powertrain, chassis, body, safety and infotainment. This paper makes a case for modeling four additional attributes that are increasingly important as vehicles become information conduits: security, privacy, usability, and reliability. These four attributes are important in creating specifications for embedded in-vehicle automotive software. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> Road vehicles have an increasing reliance on electronic systems to control their functionality and to deliver the feature and attribute demands made by manufacturers, legislators and consumers. This trend is particularly evident in the new generation of more energy-efficient vehicles that includes hybrid vehicles and full electric vehicles. The architectures of these vehicles are characterized by a greater degree of integration and interaction between the systems, as well as the introduction of new types of system with unique potential failure modes. As a result, system safety is a central part of the design and implementation process for these vehicles. ::: ::: In this respect a new standard, ISO 26262 "Road vehicles --- Functional safety" is in preparation. It sets out requirements for managing functional safety, hazard analysis and risk assessment, and the development and verification of systems, hardware and software. Nevertheless, in hybrid and electric vehicles functional safety is only one part of the overall process of system safety, which encompasses other domains such as electrical safety and crashworthiness. ::: ::: This paper will give a brief introduction to the concepts and challenges of system safety when applied to such vehicles, including a discussion of the role of ISO 26262 and some of the key principles of that standard, including the concepts of automotive safety integrity level (ASIL), safety goals and safety concepts. The implications of the standard on emerging vehicle technology will also be examined. Finally, the need for an holistic approach to system safety in such vehicles will be presented. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> Modern automobiles are pervasively computerized, and hence potentially vulnerable to attack. However, while previous research has shown that the internal networks within some modern cars are insecure, the associated threat model--requiring prior physical access--has justifiably been viewed as unrealistic. Thus, it remains an open question if automobiles can also be susceptible to remote compromise. Our work seeks to put this question to rest by systematically analyzing the external attack surface of a modern automobile. We discover that remote exploitation is feasible via a broad range of attack vectors (including mechanics tools, CD players, Bluetooth and cellular radio), and further, that wireless communications channels allow long distance vehicle control, location tracking, in-cabin audio exfiltration and theft. Finally, we discuss the structural characteristics of the automotive ecosystem that give rise to such problems and highlight the practical challenges in mitigating them. <s> BIB003 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> Model-Based Design with production code generation has been extensively utilized throughout the automotive software engineering community because of its ability to address complexity, productivity, and quality challenges. With new applications such as lane departure warning or electromechanical steering, engineers have begun to consider Model-Based Design to develop embedded software for applications that need to comply with functional safety standards such as ISO 26262. For the development of high-integrity in-vehicle software, ISO 26262 is considered state-of-the-art or generally accepted rules of technology (GART) [Fal02, Lov06]. Developers of in-vehicle software need to understand and implement the standard's requirements pertaining to software development. Due to the widespread utilization of Model-Based Design to develop automotive E/E systems, it is of particular importance to setup ISO 26262 compliant Model-Based Design processes and tool chains. This paper discusses a verification and validation workflow for developing in-vehicle software components which need to comply with ISO 26262-6 using Model-Based Design. It discusses tool support by using a Simulink® family tool chain for Model-Based Design as an example. <s> BIB004 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> This paper gives an overview of the system architecture and software design challenges for Electric Vehicles (EVs). First, we introduce the EV-specific components and their control, considering the battery, electric motor, and electric powertrain. Moreover, technologies that will help to advance safety and energy efficiency of EVs such as drive-by-wire and information systems are discussed. Regarding the system architecture, we present challenges in the domain of communication and computation platforms. A paradigm shift towards time-triggered in-vehicle communication systems becomes inevitable for the sake of determinism, making the introduction of new bus systems and protocols necessary. At the same time, novel computational devices promise high processing power at low cost which will make a reduction in the number of Electronic Control Units (ECUs) possible. As a result, the software design has to be performed in a holistic manner, considering the controlled component while transparently abstracting the underlying hardware architecture. For this purpose, we show how middleware and verification techniques can help to reduce the design and test complexity. At the same time, with the growing connectivity of EVs, security has to become a major design objective, considering possible threats and a security-aware design as discussed in this paper. <s> BIB005 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> Embedded electronic components, so-called ECU (Electronic Controls Units), are nowadays a prominent part of a car's architecture. These ECUs, monitoring and controlling the different subsystems of a car, are interconnected through several gateways and compose the global internal network of the car. Moreover, modern cars are now able to communicate with other devices through wired or wireless interfaces such as USB, Bluetooth, WiFi or even 3G. Such interfaces may expose the internal network to the outside world and can be seen as entry points for cyber attacks. In this paper, we present a survey on security threats and protection mechanisms in embedded automotive networks. After introducing the different protocols being used in the embedded networks of current vehicles, we then analyze the potential threats targeting these networks and describe how the attackers' opportunities can be enhanced by the new communication abilities of modern cars. Finally, we present the security solutions currently being devised to address these problems. <s> BIB006 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> This paper is an introduction to security challenges for the design of automotive hardware/software architectures. State-of-the-art automotive architectures are highly heterogeneous and complex systems that rely on distributed functions based on electronics and software. As cars are getting more connected with their environment, the vulnerability to attacks is rapidly growing. Examples for such wireless communication are keyless entry systems, WiFi, or Bluetooth. Despite this increasing vulnerability, the design of automotive architectures is still mainly driven by safety and cost issues rather than security. In this paper, we present potential threats and vulnerabilities, and outline upcoming security challenges in automotive architectures. In particular, we discuss the challenges arising in electric vehicles, like the vulnerability to attacks involving tampering with the battery safety. Finally, we discuss future automotive architectures based on Ethernet/IP and how formal verification methods might be used to increase their security. <s> BIB007 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> 2) Increasing in-vehicle network communication speed: <s> Machine learning (ML) plays an ever-increasing role in advanced automotive functionality for driver assistance and autonomous operation; however, its adequacy from the perspective of safety certification remains controversial. In this paper, we analyze the impacts that the use of ML as an implementation approach has on ISO 26262 safety lifecycle and ask what could be done to address them. We then provide a set of recommendations on how to adapt the standard to accommodate ML. <s> BIB008

As vehicle manufacturers are making an effort to reduce the number of ECUs in vehicles, another challenge arises. In a centralized architecture, the central processing unit can be further away from the data sources it needs to operate correctly. Autonomous cars need to collect and process the surroundings data as fast as possible for their decision making system to react on time. The communication bus system used for gathering data from those sources will have to provide much higher speeds than available in CAN and FlexRay buses of today. Ethernet is emerging as a communication system to potentially answer those needs with its Gigabit speeds BIB005 , but it is still not widely accepted in today's EVs (as seen on Table 3 ). 3) Ensuring automotive software reliability: In CAEVs, software is responsible not only for controlling the vehicle, but for providing new functionalities and driving innovation. As such, the demand for automotive software quality and reliability is very high to guarantee that the vehicles are safe for drivers, passengers and other road users. To ensure reliability, several functional safety standards such as ISO 26262 are in place. ISO 26262 is a standard that regulates functional safety of road vehicles and it is the main functional safety standard for the development of software for highintegrity in-vehicle applications BIB004 . It recommends the use of a Hazard Analysis and Risk Assessment (HARA) method which helps to identify hazardous events in the system and to specify safety goals that mitigate the hazards BIB008 . Another key requirement of ISO 26262 is the use of Automotive Safety Integrity Level (ASIL) which is a risk classification scheme for an item (e.g. component, software unit) in an automotive system. The ASILs are allocated through a process which covers hazard identification and classification, and risk assessment BIB002 . Each ASIL depicts the degree of rigor required (e.g. testing techniques, types of documentation required) to reduce the risk of an item BIB008 . ISO 26262 also provides a specification of safety requirements which need to be allocated to each ASIL to indicate how to prevent or mitigate the hazards. ISO 26262 was defined in 2011 and it applies to all systems developed after its publication, meaning that it plays an important role in the development of CAEV automotive software . As CAEVs rely heavily on software for the control and coordination of various components and subsystems, automotive software developers need to understand and implement the standard's requirements, constraints and verification and validation processes to create software which can be used in vehicles. 4) Improving software security and ensuring privacy protection: With the implementation of new communication systems arrive new security threats. A connected vehicle with a centralized architecture, can be attacked through the mobile network or Bluetooth BIB003 . If the vehicle is not prepared to prevent such an attack, the attacker could be able to control the vehicle, potentially putting the lives of the passengers at risk. An attacker could also use the vehicle to retrieve personal information such as the driver's phone directory, call history or GPS coordinates BIB006 . To prevent the attacker from accessing various nodes of the invehicle network, security measures like authentication are required, but a lack of those measures is not uncommon . In electric vehicles, battery management system hacking and intrusion via the charging plug BIB007 are a reality and security-aware design has to become a major design objective for automotive architectures to prevent these kinds of attacks from happening. 5) Improving adaptability and usability: With new security threats potentially being discovered after the deployment of a vehicle on the market, the subsequent addition of new software (OTA updates and patches), and hardware (additional sensors) features will become necessary for fully autonomous vehicles, as their safety will depend on it. Future vehicles will also have to be adaptable to the needs of their passengers. The infotainment systems have to be customizable and provide smartphone integration support for mobile operation systems that come after the vehicle is put on market, not allowing the vehicle OS to become outdated too soon. The integration of mobile phone services such as voicemail, messaging and email into the vehicle's interface makes its usability especially important. Infotainment system needs to allow the driver to focus on driving, while providing seamless access to information and comfort and convenience features BIB001 .

Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> Traditionally, economists and market researchers have been interested in identifying the factors that affect consumers' car buying behaviors to estimate market share, and to that end they have developed various models of vehicle type choice. However, they do not usually consider consumers' travel attitudes, personality, lifestyle, and mobility as factors that may affect the vehicle type choice. The purpose of this study is to explore the relationship of such factors to individuals' vehicle type choices, and to develop a disaggregate choice model of vehicle type based on these factors as well as typical demographic variables. The data for this study comes from a 1998 mail-out/mail-back survey of 1904 residents in the San Francisco Bay Area. The dependent variable (the vehicle type the respondent drives most often) is classified into nine categories: small, compact, mid-sized, large, luxury, sports, minivan/van, pickup, and sport utility vehicle. Based on these categories, we first related vehicle type to travel attitude, personality, lifestyle, mobility, and demographic variables individually, using one-way analysis of variance and chi-squared tests. Then, a multinomial logit model for vehicle type choice was estimated. The final model (which possessed the IIA property) confirmed that the variables analyzed significantly affect an individual's vehicle type choice. These results provide useful background not only to vehicle manufacturers, but also to decision makers and planners of transportation policy related to vehicle ownership, traffic congestion, and energy consumption. <s> BIB001 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> The issues of global warming and depletion of fossil fuels have paved opportunities to electric vehicle (EV). Moreover, the rapid development of power electronics technologies has even realized high energy-efficient vehicles. EV could be the alternative to decrease the global green house gases emission as the energy consumption in the world transportation is high. However, EV faces huge challenges in battery cost since one-third of the EV cost lies on battery. This paper reviews state-of-the-art of the energy sources, storage devices, power converters, low-level control energy management strategies and high supervisor control algorithms used in EV. The comparison on advantages and disadvantages of vehicle technology is highlighted. In addition, the standards and patterns of drive cycles for EV are also outlined. The advancement of power electronics and power processors has enabled sophisticated controls (low-level and high supervisory algorithms) to be implemented in EV to achieve optimum performance as well as the realization of fast-charging stations. The rapid growth of EV has led to the integration of alternative resources to the utility grid and hence smart grid control plays an important role in managing the demand. The awareness of environmental issue and fuel crisis has brought up the sales of EV worldwide. <s> BIB002 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> Current trends suggest that there is a substantial increase in the overall usage of electric vehicles (EVs). This, in turn, is causing drastic changes in the transportation industry and, more broadly, in business, policy making, and society. One concrete challenge brought by the increase in the number of EVs is a higher demand for charging stations. This paper presents a methodology to address the challenge of EV charging station deployment. The proposed methodology combines multiple sources of heterogeneous real-world data for the sake of deriving insights that can be of a great value to decision makers in the field, such as EV charging infrastructure providers and/or local governments. Our starting point is the business data, \textit{; ; i.e.}; ; , data describing charging infrastructure, historical data about charging transactions, and information about competitors in the market. Another type of data used are geographical data, such as places of interest located around chargers (\textit{; ; e.g.}; ; , hospitals, restaurants, and shops) and driving distances between available chargers. The merged data from different sources are used to predict charging station utilization when EV charging infrastructure and/or contextual data change, \textit{; ; e.g.}; ; , when another charging station or a place of interest is created. Based on such predictions, we suggest where to deploy new charging stations. We foresee that the proposed methodology can be used by EV charging infrastructure providers and/or local governments as a decision support tool that prescribes an optimal area to place a new charging station while keeping a desired level of utilization of the charging stations. We showcase the proposed methodology with an illustrative example involving the Dutch EV charging infrastructure through the period from 2013 to 2016. Specifically, we prescribe the optimal location for new ELaadNL charging stations based on different objectives such as maximizing the overall charging network utilization and/or increasing the number of chargers in scarcely populated areas. <s> BIB003 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> The recent advent of electric vehicles (EVs) marks the beginning of a new positive era in the transportation sector. Although the environmental benefits of EVs are well-known today, planning and managing EV charging infrastructure are activities that are still not well-understood. In this paper, we are investigating how the so-called EV-enabled parking lot, a parking lot that is equipped with a certain number of chargers, can define an appropriate parking policy in such a way that satisfies two challenges: EV owners’ needs for recharging as well as the parking lot operator’s goal of profit maximization. Concretely, we present three parking policies that are able to simultaneously deal with both EVs and internal combustion engine vehicles. Detailed sensitivity analysis, based on real-world data and simulations, evaluates the proposed parking policies in a case study concerning parking lots in Melbourne, Australia. This paper produces results that are highly prescriptive in nature because they inform a decision maker under which circumstances a certain parking policy operates optimally. Most notably, we find that the dynamic parking policy, which takes the advantage of advanced information technology (IT) and charging infrastructure by dynamically changing the role of parking spots with chargers, often outperforms the other two parking policies, because it maximizes the profit and minimizes the chance of cars being rejected by the parking lot. We also discuss how making a few parking spots EV-exclusive might be a good policy when the number of available chargers is small and/or the required IT infrastructure is not in place for using the dynamic policy. We conclude this paper proposing a technology roadmap for transforming parking lots into smart EV-enabled parking lots based on the three studied parking policies. <s> BIB004 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> In coming years, attraction to alternative urban mobility paradigms such as Connected and Autonomous Electric Vehicles (CAEVs) will increase since CAEVs can significantly contribute to not only optimize traffic flow and improve road safety but also minimize dependence on fossil fuel and reduce carbon emission in urban areas. Nonetheless, there are several barriers towards widespread adoption of CAEVs. In order to have significant growth of CAEVs in urban areas, adequate number of charging facilities in urban areas is needed as well as an efficient smart CAEV charging management is required for managing and allocating charging station resources. In this paper, we have designed and implemented a system utilizing automated reservation based charging strategies that include effective reservation management and efficient allocation of time slots of wireless charging stations. <s> BIB005 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> The impending environmental issues and growing concerns for global energy crises are driving the need for new opportunities and technologies that can meet significantly higher demand for cleaner and sustainable energy systems. This necessitates the development of transportation and power generation systems. The electrification of the transportation system is a promising approach to green the transportation systems and to reduce the issues of climate change. This paper inspects the present status, latest deployment, and challenging issues in the implementation of Electric vehicles (EVs) infrastructural and charging systems in conjunction with several international standards and charging codes. It further analyzes EVs impacts and prospects in society. A complete assessment of charging systems for EVs with battery charging techniques is explained. Moreover, the beneficial and harmful impacts of EVs are categorized and thoroughly reviewed. Remedial measures for harmful impacts are presented and benefits obtained therefrom are highlighted. Bidirectional charging offers the fundamental feature of vehicle to grid technology. In this paper, the current challenging issues due to the massive deployment of EVs, and upcoming research trends are also presented. It is envisioned that the researchers interested in such areas can find this paper valuable and an informative one-stop source. <s> BIB006 </s> Automotive Software in Connected and Autonomous Electric Vehicles: A Review <s> C. IMPACT OF AUTOMOTIVE SOFTWARE <s> Abstract Electric vehicles (EVs) are currently the most promising technology with the potential to transform transportation and energy landscapes to make industry and society more sustainable. Although their adoption has been much faster than anticipated a few years ago, resulting in a significant increase from 400,000 EVs sold in 2013 to more than 3 million in 2017, the coming years will be crucial for determining whether EVs are to become a new transportation industry “standard”, the role of which has been held by internal combustion engine (ICE) vehicles over the last hundred years. Not only will innovation in EV batteries determine a “win” or “fail” for EVs, but a significant role in its acceptance will be successful development of a comprehensive EV eco-system, establishing an efficient network of EV chargers as well as attractive and affordable charging tariffs. Furthermore, having insight as to how much money EV users are willing to pay for the charging service (i.e., willingness to pay) in various circumstances is one of the essential factors in building a successful EV eco-system. That being said, this paper utilizes a gamified survey for exploring a person's willingness to pay for EV charging. The experimental setup includes both the classic (text-only) questionnaire, as well as the game-based questionnaire. Therefore, not only have interesting insights on the willingness to pay for EV charging been identified, but analysis of user experience analysis from the questionnaires suggests that use of gamification is a good approach to performing a survey as it exhibits superior hedonic quality in comparison to classic methods (i.e., text-only questionnaire). <s> BIB007

Since its introduction in the 1970s, automotive software's importance in vehicle development has been growing exponentially. Today's CAEVs provide a wide range of infotainment, telematics and driving features, all made possible by automotive software. The next stage of connectedness VOLUME 7, 2019 will bring Software Defined Cars (SDCs) with abilities of over-the-air (OTA) updates capable of changing powertrain capabilities, vehicle dynamics and onboard services, enabling the vehicles to continually adapt to the needs of their passengers . Such changes to the automotive industry, where software is taking over the key role in bringing virtually all new features to life, will impact all stakeholders involved in mobility. Infrastructure owners are greatly influenced by the developments in the automotive industry. With the number of EVs on the road continually growing, the energy infrastructure owners have to deploy more and more EV charging stations and various research on the methodology of the deployment BIB003 , BIB004 , charging station reservation BIB005 and willingness-to-pay for EV charging BIB007 is already being made. Power grid will also have to be adapted to endure the additional charging demand generated by the rising number of EVs BIB006 . Municipalities and governments responsible for the design, construction, management, safety and maintenance of the road infrastructure were adapting to the mobility needs of the population since the inception of the automobile. When the fully autonomous vehicles become a reality, the road infrastructure will have to be entirely revised. Autonomous vehicle safety demands are stringent, so it will be of the utmost importance to prepare and equip the roads with the technology needed to comply with those demands. If it becomes widely accepted and standardized, the V2I technology will have to be deployed along the existing infrastructure by the municipalities in collaboration with telecommunication companies and vehicle manufacturers. All of this will considerably reduce traffic congestions, making it easier to maintain the roads and infrastructure, saving the municipal money in the process. Telecommunication companies will also have to prepare their infrastructure for the surge in data traffic which will gradually happen when the majority of vehicles on the road become connected to the Internet. Quick and efficient implementation of those changes could bring financial gain to all the aforementioned infrastructure owners. Vehicle manufacturers are the ones developing new features and deciding the course of automotive software development and the automotive industry as a whole. Selling vehicles is one of the main goals of vehicle manufacturers, and software is one of today's main selling points. This is why it is important for the automotive companies to identify rising trends in the ICT industry as early as possible to be on the forefront of automotive software innovation. Research and development teams play a key part in this process. Vehicle owners are the ones who are paying for the vehicle to acquire ownership. Personality, lifestyle, travel attitude and mobility factors all affect the individuals' vehicle type choices BIB001 , and software can tailor the vehicle to the needs of the buyer. Smart phones are deeply integrated into today's lifestyle, and an average person today is more tech savvy than ever. Just as the people with a pro-environmental attitude are more likely to buy an electric vehicle, average people are more likely to buy vehicles providing the option of integration of their favourite smart phone brand on the vehicle's infotainment system. Vehicle owners are also responsible for vehicle's maintenance after the purchase. All but essential mechanical components are slowly being replaced by automotive software, making it easier and cheaper for owners to maintain their vehicle. Recent trends in car sharing and subscription services make it necessary to differentiate vehicle owners and vehicle users. Vehicle users are people using the vehicle for transport. This can be a family using a car, passengers in a taxi or truckdrivers driving company provided trucks to transport goods. For a vehicle user, vehicle efficiency and comfort are the key concerns. Energy management systems BIB002 and intuitive infotainment services are entirely made possible by automotive software, and a major factor in improving those vehicle traits. A summary of roles and interests of different stakeholders, along with the impact automotive software has on them, can be seen in Table 4 .

Social networking and academic performance: A review <s> Introduction <s> This study examines the relationship between use of Facebook, a popular online social network site, and the formation and maintenance of social capital. In addition to assessing bonding and bridging social capital, we explore a dimension of social capital that assesses one’s ability to stay connected with members of a previously inhabited community, which we call maintained social capital. Regression analyses conducted on results from a survey of undergraduate students (N = 286) suggest a strong association between use of Facebook and the three types of social capital, with the strongest relationship being to bridging social capital. In addition, Facebook usage was found to interact with measures of psychological well-being, suggesting that it might provide greater benefits for users experiencing low self-esteem and low life satisfaction. <s> BIB001 </s> Social networking and academic performance: A review <s> Introduction <s> This study advances our knowledge of students’ online self-disclosures by examining a random sample of students’ Facebook profiles at a large, public research University in the Northeast. A quantitative content analysis revealed that the vast majority of students had a Facebook account and very small proportions restricted access of their profile to University staff. Substantial proportions of students’ profiles contained contact information, course schedules, positive references to the University, and pictures of students’ consuming alcohol. Detailed implications for practice are discussed. <s> BIB002 </s> Social networking and academic performance: A review <s> Introduction <s> Social network sites (SNSs) are increasingly attracting the attention of academic and industry researchers intrigued by their affordances and reach. This special theme section of the Journal of Computer-Mediated Communication brings together scholarship on these emergent phenomena. In this introductory article, we describe features of SNSs and propose a comprehensive definition. We then present one perspective on the history of such sites, discussing key changes and developments. After briefly summarizing existing scholarship concerning SNSs, we discuss the articles in this special section and conclude with considerations for future research. <s> BIB003 </s> Social networking and academic performance: A review <s> Introduction <s> There is much talk of a change in modern youth - often referred to as digital natives or Homo Zappiens - with respect to their ability to simultaneously process multiple channels of information. In other words, kids today can multitask. Unfortunately for proponents of this position, there is much empirical documentation concerning the negative effects of attempting to simultaneously process different streams of information showing that such behavior leads to both increased study time to achieve learning parity and an increase in mistakes while processing information than those who are sequentially or serially processing that same information. This article presents the preliminary results of a descriptive and exploratory survey study involving Facebook use, often carried out simultaneously with other study activities, and its relation to academic performance as measured by self-reported Grade Point Average (GPA) and hours spent studying per week. Results show that Facebook(R) users reported having lower GPAs and spend fewer hours per week studying than nonusers. <s> BIB004 </s> Social networking and academic performance: A review <s> Introduction <s> Abstract We investigated how Facebook use and attitudes relate to self-esteem and college adjustment, and expected to find a positive relationship between Facebook and social adjustment, and a negative relationship between Facebook, self-esteem, and emotional adjustment. We examined these relationships in first-year and upper-class students and expected to find differences between the groups. Seventy undergraduate students completed Facebook measures (time, number of friends, emotional and social connection to Facebook), the Rosenberg Self-Esteem Scale, and the Student Adaptation to College Scale. First-year students had a stronger emotional connection to and spent more time on Facebook while they reported fewer friends than upper-class students did. The groups did not differ in the adjustment scores. The number of Facebook friends potentially hinders academic adjustment, and spending a lot of time on Facebook is related to low self-esteem. The number of Facebook friends was negatively associated with emo... <s> BIB005 </s> Social networking and academic performance: A review <s> Introduction <s> Educators and others are interested in the effects of social media on college students, with a specific focus on the most popular social media website-Facebook. Two previous studies have examined the relationship between Facebook use and student engagement, a construct related to positive college outcomes. However, these studies were limited by their evaluation of Facebook usage and how they measured engagement. This paper fills a gap in the literature by using a large sample (N = 2368) of college students to examine the relationship between frequency of Facebook use, participation in Facebook activities, and student engagement. Student engagement was measured in three ways: a 19-item scale based on the National Survey of Student Engagement, time spent preparing for class, and time spent in co-curricular activities. Results indicate that Facebook use was significantly negatively predictive of engagement scale score and positively predictive of time spent in co-curricular activities. Additionally, some Facebook activities were positively predictive of the dependent variables, while others were negatively predictive. <s> BIB006 </s> Social networking and academic performance: A review <s> Introduction <s> Because of the social media platform's widespread adoption by college students, there is a great deal of interest in how Facebook use is related to academic performance. A small number of prior studies have examined the relationship between Facebook use and college grade point average (GPA); however, these studies have been limited by their measures, sampling designs and failure to include prior academic ability as a control variable. For instance, previous studies used non-continuous measures of time spent on Facebook and self-reported GPA. This paper fills a gap in the literature by using a large sample (N=1839) of college students to examine the relationship among multiple measures of frequency of Facebook use, participation in Facebook activities, and time spent preparing for class and actual overall GPA. Hierarchical (blocked) linear regression analyses revealed that time spent on Facebook was strongly and significantly negatively related to overall GPA, while only weakly related to time spent preparing for class. Furthermore, using Facebook for collecting and sharing information was positively predictive of the outcome variables while using Facebook for socializing was negatively predictive. <s> BIB007 </s> Social networking and academic performance: A review <s> Introduction <s> Over 500 million people interact daily with Facebook. Yet, whether Facebook use influences subjective well-being over time is unknown. We addressed this issue using experience-sampling, the most reliable method for measuring in-vivo behavior and psychological experience. We text-messaged people five times per day for two-weeks to examine how Facebook use influences the two components of subjective well-being: how people feel moment-to-moment and how satisfied they are with their lives. Our results indicate that Facebook use predicts negative shifts on both of these variables over time. The more people used Facebook at one time point, the worse they felt the next time we text-messaged them; the more they used Facebook over two-weeks, the more their life satisfaction levels declined over time. Interacting with other people “directly” did not predict these negative outcomes. They were also not moderated by the size of people's Facebook networks, their perceived supportiveness, motivation for using Facebook, gender, loneliness, self-esteem, or depression. On the surface, Facebook provides an invaluable resource for fulfilling the basic human need for social connection. Rather than enhancing well-being, however, these findings suggest that Facebook may undermine it. <s> BIB008 </s> Social networking and academic performance: A review <s> Introduction <s> Abstract Studies on the mental health implications of social media have generated mixed results. Drawing on a survey of college students (N=513), this research uses structural equation modeling to assess the relationship between Facebook interaction and psychological distress and two underlying mechanisms: communication overload and self-esteem. It is the first study, to our knowledge, that examines how communication overload mediates the mental health implications of social media. Frequent Facebook interaction is associated with greater distress directly and indirectly via a two-step pathway that increases communication overload and reduces self-esteem. The research sheds light on new directions for understanding psychological well-being in an increasingly mediated social world as users share, like, and comment more and more. <s> BIB009 </s> Social networking and academic performance: A review <s> Introduction <s> Studies have shown that multitasking with technology, specifically using Social Networking Sites (SNSs), decreases both efficiency and productivity in an academic setting. This study investigates multitasking's impact on the relationship between SNS use and Grade Point Average (GPA) in United States (US; n=451) and European (n=406) university students using quantitative and qualitative data analysis. Moderated Multiple Regression analysis results showed that the negative relationship between SNS use and GPA was moderated by multitasking only in the US sample. This may be due to European students being less prone to ''disruptive'' multitasking. The results provide valuable cautionary information about the impact of multitasking and using SNSs in a learning environment on university students' GPAs. <s> BIB010 </s> Social networking and academic performance: A review <s> Introduction <s> Online surveys were used to examine adults' Facebook disclosures.Greater general social media use is related to decreased well-being.Facebook use is unrelated to stress or quality of life.Sharing important, bad health news is related to decreased well-being.Not sharing to protect oneself is linked to decreased well-being. The current study examined the relationship between general perceived levels of stress, quality of life, social networking usage, and disclosing important life events on Facebook in order to better understand the complex relationship between online disclosure and individual well-being. An online survey was completed by adult Facebook users aged 18-70. Results indicate that the more time spent on and the more social network memberships, the higher stress and lower quality of life; Facebook-specific usage was unrelated to either well-being variable. Together, these findings suggest that the current increase in social media variety and usage may be detrimental to user well-being. Users who shared important, bad health news on Facebook had higher stress and lower quality of life than those who did not, with no significant differences for sharing good health news. The more that users did not share important news on Facebook for self-protection and friend unresponsiveness reasons, the greater their stress. The self-protection reason was also negatively related to quality of life. These inconsistent findings can likely be partially explained by the nature of the information that is shared. These findings are discussed in light of disclosure and relationship patterns on social networks. <s> BIB011 </s> Social networking and academic performance: A review <s> Introduction <s> This paper presents, for further discussion, a review of the scientific literature produced internationally on the use of Social Network Sites (SNS) in different levels of education and settings. A total of 62 articles published in international scientific journals with peer review have been analysed. The main objective of this paper is to discuss the most recurrent lines of research since the emergence of these tools in education. The level of education in which research is carried out is also analysed, as well as the paradigms and methods used for data collection. The analysis leads to the conclusion that most of the studies analysed have been carried out in higher education, from a quantitative paradigm, focusing on the use of SNS as educational tools. This article shows that research into SNS in education is at an early stage of development, and it demonstrates the need to widen lines of research on SNS in media education to include as yet unexplored dimensions. <s> BIB012 </s> Social networking and academic performance: A review <s> Introduction <s> This paper examines the impact of Facebook usage on students' academic performance. In addition, it also analyse whether Socialization influences Facebook usage. Empirical data was collected from 1165 Malaysian university students using a questionnaire survey. It was found that the construct Socially Accepted influences Facebook usage while Acculturation does not have any significant relationship with usage. The results also illustrated that there is a positive relationship between students' Academic Performance and Facebook usage i.e. the higher the usage the better they perceived they perform. <s> BIB013 </s> Social networking and academic performance: A review <s> Introduction <s> The quantitative study to explore the most popular amongst Saudi students.The study establishes relation among the social media usage and academic grades.Poor time management and football also have negative impact on academic grades.Normality tests are to find relationship among social media usage and GPA scores.The hypothesis about existence of relationship among GPA and SN is not satisfied. Social media is a popular method for communication amongst university students in Saudi Arabia. However excessive social media use can raise questions about whether academic performance is affected. This research explores this question by conducting a survey on university students in Saudi Arabia in regards to social media usage and their academic performance. The survey also explored which social network is the most popular amongst Saudi students, what students thought about their social media usage and factors besides social media usage which negatively affect academic performance.The survey received 108 responses and descriptive statistics including normality tests i.e. scatter plots were used to examine the relationship between the average number of hours students spent of social media a week and GPA scores of the students. The results demonstrated that there was no linear relationship between social media usage in a week and GPA score. Students highlighted that besides social media use, time management is a factor which affects students 'studies negatively.The findings of the paper can be used to propose the effective plans for improving the academic performance of the students in such a way that a balance in the leisure, information exchange and academic performance can be maintained. <s> BIB014

can consume, produce, and/or interact with stream of user-generated content provide by their connections on the site^(p. 158). This updated definition was proffered in light of the changes that have occurred since the initial definition was first purveyed in BIB003 , which defined SNS as allowing users to: B(1) construct a public or semi-public profile within a bounded system, (2) articulate a list of other users with whom they share a connection, and (3) view and traverse their list of connections and those made by others within the system. The nature and nomenclature of these connections may vary from site to site^(p. 211). The genesis of SNS can be found in the website, Six Degrees, launched in 1997-a couple of years before the bursting of the dot-com bubble BIB003 . The advent of SNS provided users with a new set of powerful affordances to draw connections and interact with others easily, and in more ways than ever before. Almost twenty years after their conception, today, one can hardly escape the reach of social networks. A plethora of SNS were born in the initial wave of the social media era. MySpace founded in 2003 (MySpace n.d.), was the most recognizable of the lot as well as the dominant social network in the early part of the era. Following on the heels of MySpace's founding, another social network, Facebook, was unveiled a year later , which now reigns supreme as it houses more than a billion users (Newsroom, 2016) . Undeniably, SNS such as Facebook have drastically transformed the way in which we connect, interact, engage, share, and create social ties. The proliferation of social networks arguably represents one of the most significant phenomena in the history of computer-mediated communications. The evidence of the pervasiveness of SNS in our everyday lives is hard to miss. The number of SNS users has risen to about 2.3 billion globally, which is in parallel with the number of Internet users counting almost one-half of the global population . Likewise, nearly two-thirds of American adults now use SNS, with the figure particularly higher for young adults (90%) . In addition to the number of users indicating the popularity of social networking, the amount of time users spend on social networking underscores the preponderance and importance of SNS to people's daily lives. According to a recent survey by Global Web Index , the average user spends about 1.72 h per day on social networking; this figure amounts to about 28% of all online activity. Although SNS receive patronage from almost every demographic group, one such group is particularly well represented: students . SNS have undeniably become ubiquitous and integral in the lives of students . In fact, heavy SNS use by students is regularly reported -students spend anywhere from 30 min to over 2 h on Facebook per day BIB005 . The uptake of SNS by this population has naturally attracted academic interest. Given the widespread use and importance of SNS BIB012 , the issue of how they affect students is of clear relevance to educators as well as researchers, and warrants further investigation. This paper proceeds from and highlights the importance of SNS and the influences resulting from use. Both anecdotal and empirical evidence report that users are affected by SNS. Recent scholarship has shed light on the outcomes of SNS use-with the psychosocial effects of SNS use representing the dominant concepts addressed apropos outcomes in the continually accumulating body of work. In this substantial stream of work, for example many studies examine SNS outcomes with respect to: subjective well-being and social capital BIB001 ); life satisfaction, social trust, civic engagement, and political participation ); and, student engagement BIB006 ) to name but a few. Furthermore, many works have associated SNS use with both positive and negative socio-psychological phenomena BIB008 ). While a majority of research in this area has developed around the positive affordances of SNS, evidence has started to surface that highlights negative influences as well. Despite the many affordances of SNS, research has associated high usage of SNS with: psychological distress BIB009 , lower quality of life BIB011 , and reduced subjective wellbeing BIB008 ), among others. But whereas the body of work examining the effects of SNS use on psychosocial outcomes is growing and generally tending towards consensus, the link between SNS use and academic outcomes remains both inadequately covered and understood. The phenomenal popularity and pervasiveness of SNS, such as Facebook, has not escaped the attention of educational scholars, and as such, recent years have seen a concomitant rise in investigations on SNS use BIB005 ; given this backdrop, it is easy to understand the motivations-particularly in the scholarshipto endorse this push. Although the body of research on SNS use is quickly expanding BIB012 , one direction of effort-connecting SNS use and various academic phenomenaremains sparsely addressed and lacking consensus (e.g., Ahn 2011; BIB013 BIB014 BIB010 . Hence, an important question in this stream of research concerns the effects of SNS use on academic outcomes. Of course, much of the prior work on academic-SNS research has indeed proceeded from and focused on the link between SNS use and academic performance. However, within this stream of work, the SNS use and academic performance link is unstable and widely contested because of the mixed findings within published results (e.g., BIB013 BIB014 BIB007 BIB002 BIB004 . Nevertheless, despite the emergent nature of this research, we believe that this review, given the rapid and continued growth of SNS use, is timely because it helps curate the published findings.

Social networking and academic performance: A review <s> Rationale for the review and research question <s> Abstract We investigated how Facebook use and attitudes relate to self-esteem and college adjustment, and expected to find a positive relationship between Facebook and social adjustment, and a negative relationship between Facebook, self-esteem, and emotional adjustment. We examined these relationships in first-year and upper-class students and expected to find differences between the groups. Seventy undergraduate students completed Facebook measures (time, number of friends, emotional and social connection to Facebook), the Rosenberg Self-Esteem Scale, and the Student Adaptation to College Scale. First-year students had a stronger emotional connection to and spent more time on Facebook while they reported fewer friends than upper-class students did. The groups did not differ in the adjustment scores. The number of Facebook friends potentially hinders academic adjustment, and spending a lot of time on Facebook is related to low self-esteem. The number of Facebook friends was negatively associated with emo... <s> BIB001 </s> Social networking and academic performance: A review <s> Rationale for the review and research question <s> The purpose of this article is to review current published research studies focusing on the use of Facebook by students and teachers. The aim of the review is not to solely discuss Facebook in relation to teaching or learning purposes, or about its educational value per se, but also to present a detailed account of the participants' Facebook usage profile or the extent to which users are engaged in Facebook activities. The emphasis of this review will be upon empirical findings rather than opinion- or theoretical explanations. Following the review guidelines set by Creswell (Research Design Qualitative and Quantitative Approaches, 1994), I summarize the hitherto accumulated state of knowledge concerning Facebook and highlight questions or issues that research has left unresolved. This review is organized into three sections that cover the major topics of current research: (a) students' Facebook usage profile or extent of Facebook use (e.g., time students spend on Facebook each day, students' motives for using Facebook, as well as various factors that may affect these usage profiles), (b) the effects of Facebook use (e.g., effects of Facebook self-disclosure on teacher credibility, effects of Facebook use on student social presence and discussion, and effects of Facebook on students' academic performance), and (c) students' attitudes toward Facebook. The conclusions overall suggest that Facebook thus far has very little educational use, that students use Facebook mainly to keep in touch with known individuals, and that students tend to disclose more personal information about themselves on Facebook; hence attracting potential privacy risks upon themselves. <s> BIB002 </s> Social networking and academic performance: A review <s> Rationale for the review and research question <s> Abstract The popularity of social network sites (SNSs) among college students has stimulated scholarship examining the relationship between SNS use and college adjustment. The present research furthers our understanding of SNS use by studying the relationship between loneliness, varied dimensions of Facebook use, and college adjustment among first-year students. We looked at three facets of college adjustment: social adjustment, academic motivation, and perceived academic performance. Compulsive use of Facebook had a stronger association with academic motivation than habitual use of Facebook, but neither were directly correlated with academic performance. Too much time spent on Facebook was weakly but directly associated with poorer perceived academic performance. Loneliness was a stronger indicator of college adjustment than any dimension of Facebook usage. <s> BIB003 </s> Social networking and academic performance: A review <s> Rationale for the review and research question <s> This paper presents, for further discussion, a review of the scientific literature produced internationally on the use of Social Network Sites (SNS) in different levels of education and settings. A total of 62 articles published in international scientific journals with peer review have been analysed. The main objective of this paper is to discuss the most recurrent lines of research since the emergence of these tools in education. The level of education in which research is carried out is also analysed, as well as the paradigms and methods used for data collection. The analysis leads to the conclusion that most of the studies analysed have been carried out in higher education, from a quantitative paradigm, focusing on the use of SNS as educational tools. This article shows that research into SNS in education is at an early stage of development, and it demonstrates the need to widen lines of research on SNS in media education to include as yet unexplored dimensions. <s> BIB004 </s> Social networking and academic performance: A review <s> Rationale for the review and research question <s> A mixed-methods study of academic performance and Facebook among U.S.-based college students.A qualitative account of how a U.S.-sample of college students conferred meanings to their academic lives on Facebook.Academic performance may determine college students' Facebook use, rather than the reverse.Those with lower GPAs may disclose negative affective states in their academic disclosure on Facebook. This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N=261; male=66; female=195; M age ź 22years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N=714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students' Facebook use, rather than the reverse. Implications for student support services are discussed. <s> BIB005

As mentioned above, it bears emphasis that while there has been increasing interest examining the use and effects of SNS BIB001 BIB004 , research has thus far largely understudied the effects of SNS use on various dimensions of academic phenomena . Identifying the academic impacts of the use of SNS is essential-one that has potentially broad interest-to comprehending whether such influences are of any consequence. Given this context, we concentrate our attention on outcomes related to academic phenomena. Although most extant studies on SNS use outcomes has focused on outcomes pertaining to the psychosocial domain ), a separate stream of research has started to investigate the academic outcomes of SNS use-that is, the effect of SNS use on academic performance. The SNS-academic performance nexus is an important yet insufficiently investigated topic. While previous attempts to examine the link between SNS use and academic performance exist, notably, they have mostly yielded mixed results, and still remain contested. This apparent disagreement provides both an opportunity and motivation to further study and unlock this topic-it seems, therefore, worthwhile to commission a review on this topic for disambiguation. Moreover, BIB005 suggest that the topic deserves attention, as it is crucial to understand whether the use of SNS impacts academic performance or if changes in academic performance propel students to gravitate toward SNS. Addressing such a question is an important endeavor. A cursory look at the recent literature examining the link between SNS and academic performance immediately highlights the existence of mixed findings. BIB003 note, Bit is difficult to draw any definitive conclusions from the literature on SNS usage and academic performance^(p.159). Because of the lack of agreement, to better understand and clarify this link, a good starting point, then, is to take stock of the related work. Moreover, given the disparate findings across studies, examining these studies as a whole affords benefits such as discerning patterns, themes, recurrent ideas, and/or trends. Given the absence of a systematic literature review covering this topic, the present review aims to fill this gap in the literature by unearthing, compiling, and showcasing the extant empirical evidence apropos this topic. In accordance with the aim of the paper-to clarify the association between SNS use and academic performance-the central question addressed by this review is: what is the link between SNS use and academic performance? Thus, the goal of this paper is to explore and better understand not just how the constructs are linked, but also to find explanatory mechanisms for the association, if any. In light of the many dimensions of SNS use, while we acknowledge that some students do use SNS for academic undertakings BIB002 , in this paper, we specifically narrow our focus on general SNS use-we reasoned that lumping together studies that consider the educational impact of SNS use and general SNS could lead to conflating the findings for this review, in addition to hindering the comparability of the included studies. The work on students' academic use of SNS merits its own focus and review, and thus, we exclude such works from the present review. The remainder of the article proceeds as follows. In Section 2, we will present and expound on the review methodology. Section 3 will present the record and summary of articles included in the review along with the results of the analysis. Following this, Section 4 will discuss the findings, highlight some important limitations of the present review, and offer some suggestions for future research. Section 5 concludes the review.

Social networking and academic performance: A review <s> Distribution by platforms <s> The proliferation and ease of access to information and communication technologies (ICTs) such as Facebook, text messaging, and instant messaging has resulted in ICT users being presented with more real-time streaming data than ever before. Unfortunately, this has also resulted in individuals increasingly engaging in multitasking as an information management strategy. The purpose of this study was to examine how college students multitask with ICTs and to determine the impacts of this multitasking on their college grade point average (GPA). Using web survey data from a large sample of college students at one university (N=1839), we found that students reported spending a large amount of time using ICTs on a daily basis. Students reported frequently searching for content not related to courses, using Facebook, emailing, talking on their cell phones, and texting while doing schoolwork. Hierarchical (blocked) linear regression analyses revealed that using Facebook and texting while doing schoolwork were negatively associated with overall college GPA. Engaging in Facebook use or texting while trying to complete schoolwork may tax students' capacity for cognitive processing and preclude deeper learning. Our research indicates that the type and purpose of ICT use matters in terms of the educational impacts of multitasking. <s> BIB001 </s> Social networking and academic performance: A review <s> Distribution by platforms <s> This paper is an exploratory study of the relationship between personality and problematic social network use among Chinese university students and the impact of problematic social network use on academic performance. Students who scored higher on the personality traits of agreeableness and conscientiousness were less likely to be associated with emotional/physical problems related to social network use. In addition, students who report spending excessive time on social networking sites and those who believe that their academic performance suffers because of social network use are more likely to have lower academic performance. Male students in China who score low on agreeableness are more likely to engage in problematic social network use. The results suggest that academic advisors should consider counseling students on the negative impact of problematic social network use and should plan interventions for students whose academic performance is negatively affected by problematic social network use. <s> BIB002 </s> Social networking and academic performance: A review <s> Distribution by platforms <s> This paper examines the impact of Facebook usage on students' academic performance. In addition, it also analyse whether Socialization influences Facebook usage. Empirical data was collected from 1165 Malaysian university students using a questionnaire survey. It was found that the construct Socially Accepted influences Facebook usage while Acculturation does not have any significant relationship with usage. The results also illustrated that there is a positive relationship between students' Academic Performance and Facebook usage i.e. the higher the usage the better they perceived they perform. <s> BIB003 </s> Social networking and academic performance: A review <s> Distribution by platforms <s> Young adults, especially college students, are consistently engaging in multiple tasks simultaneously. They are texting, reading, and using social media while studying and attending class. While there are a variety of contexts and relationships likely influenced by this, the present research project examines the influence of media multitasking in the context of students in technology-saturated classrooms and how this is impacting learning and academic performance. A survey of college students examined the impact of technology-based multitasking behaviors both within and outside classrooms. Data demonstrate that those who multitask frequently in-class have lower current college GPAs. This relationship remained significant even after controlling for perceived multitasking efficacy and time spent studying outside of class. Texting emerged as a dominant activity both while attending class and while doing homework. Females seem to use technologies more for maintaining mediated interpersonal interactions and social connections. Males seem to use technology more for online information seeking and for consuming online videos. Those who reported multitasking while doing homework spent more time spent studying outside of class, thereby contributing to inefficient study habits. Implications for technology use, best practices and policies in academic settings are discussed. <s> BIB004 </s> Social networking and academic performance: A review <s> Distribution by platforms <s> The quantitative study to explore the most popular amongst Saudi students.The study establishes relation among the social media usage and academic grades.Poor time management and football also have negative impact on academic grades.Normality tests are to find relationship among social media usage and GPA scores.The hypothesis about existence of relationship among GPA and SN is not satisfied. Social media is a popular method for communication amongst university students in Saudi Arabia. However excessive social media use can raise questions about whether academic performance is affected. This research explores this question by conducting a survey on university students in Saudi Arabia in regards to social media usage and their academic performance. The survey also explored which social network is the most popular amongst Saudi students, what students thought about their social media usage and factors besides social media usage which negatively affect academic performance.The survey received 108 responses and descriptive statistics including normality tests i.e. scatter plots were used to examine the relationship between the average number of hours students spent of social media a week and GPA scores of the students. The results demonstrated that there was no linear relationship between social media usage in a week and GPA score. Students highlighted that besides social media use, time management is a factor which affects students 'studies negatively.The findings of the paper can be used to propose the effective plans for improving the academic performance of the students in such a way that a balance in the leisure, information exchange and academic performance can be maintained. <s> BIB005 </s> Social networking and academic performance: A review <s> Distribution by platforms <s> This study examined the simultaneous impact of Facebook (FB) and smart phone usage (SP) on the leisure activities (LA) and college adjustment (SACQ) of students in Serbia. The moderating effects of gender on the observed relationships were also examined. An exploratory study of students in Serbia (N?=?485) revealed that: 1. Students in Serbia spend a daily average of 2.76?h on Facebook, while the average total daily smart phone use is 8:34?h 2. Facebook and smart phone use has no decisive influence on the allocation of time for leisure activities and college adjustment. 3. Facebook and smart phone use still has a certain positive effect on leisure activities, but this influence can become slightly negative if Facebook and smart phones are used too much. 4. In cases of a lack of time, students are more likely to sacrifice academic work, rather than time for Facebook, smart phones, or leisure activities. 5. The moderating effects of students' gender on the observed relationships are weakly expressed. Finally, Facebook and smart phone use has become a common and integral part of life for the majority of Serbian students, and the time for these activities is integrated into their overall time. Presented are the results of the influence of Facebook and smart phone usage.The questionnaires were completed by 485 students.Facebook and smart phone use has no decisive influence o leisure activities.Slightly negative influence if Facebook and smart phones are used too much.Sacrificing academic work, rather than time for Facebook. <s> BIB006

A large proportion of the studies focused on the use of specific platforms (see Fig. 4 ). Facebook was the most prominently used platform BIB003 BIB004 BIB006 BIB001 BIB002 ) and multiple SNS BIB005 . Additionally, Facebook and smart phone usage was studied simultaneously in the study by BIB006 .

Social networking and academic performance: A review <s> SNS-academic performance relationship formulation <s> A mixed-methods study of academic performance and Facebook among U.S.-based college students.A qualitative account of how a U.S.-sample of college students conferred meanings to their academic lives on Facebook.Academic performance may determine college students' Facebook use, rather than the reverse.Those with lower GPAs may disclose negative affective states in their academic disclosure on Facebook. This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N=261; male=66; female=195; M age ź 22years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N=714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students' Facebook use, rather than the reverse. Implications for student support services are discussed. <s> BIB001 </s> Social networking and academic performance: A review <s> SNS-academic performance relationship formulation <s> This paper examines the impact of Facebook usage on students' academic performance. In addition, it also analyse whether Socialization influences Facebook usage. Empirical data was collected from 1165 Malaysian university students using a questionnaire survey. It was found that the construct Socially Accepted influences Facebook usage while Acculturation does not have any significant relationship with usage. The results also illustrated that there is a positive relationship between students' Academic Performance and Facebook usage i.e. the higher the usage the better they perceived they perform. <s> BIB002

The extant literature mostly approaches the SNS-academic performance nexus, either explicitly or implicitly, as being linear. Additionally, the relationship between SNS use and academic performance was decidedly formulated as unidirectional from SNS use to academic performance. This suggests that the researchers have worked with this underlying presumption of the directionality of this relationship. By contrast, the yield on the other end was miniscule. Alternatively, only one study considered the relationship from academic performance to SNS use BIB001 ; this study calls into question the framing of the relation, and in turn contrarily suggests for a reconsideration of the directionality of the relationship. While this is reasonable to assume and opens up a new lens to view the SNS-academic relationship, however, it should be noted that this study, like most studies reviewed, is also cross-sectional, so any claims on directionality should be assessed with caution. Regardless, a critical question that arises, then, is whether the formulation of the relationship proceeds from SNS use to academic performance, or vice versa. Toward this end, we suggest going beyond cross-section research, that is, that further research employing longitudinal design is required to better explain the formulation of this relationship and the temporal arrangement of the two constructs. Another possibility that proceeds from this suggestion relates to testing the reciprocal relationship between the two constructs-one that can also be tested with longitudinal design efforts. A more recent line of thinking positions SNS use as a mediator in the relationship between social acceptance and academic performance BIB002 . We see such formulations deserving of extended efforts for adding to the evidence base for reconceptualization of the outcomes of SNS use. Looping back to the point about the linearity approach assumed by the reviewed studies, we contend that it may be beneficial, when relating SNS use to academic performance, to consider the potential for warped relationships as opposed to the tested relationship that has been the norm in the literaturelinear. It is worth noting that variations in findings may result from a variety of study characteristics, such as research design. Thus, the variations in the design across the studies impose some limitations in gelling the disparate evidence pieces.

Social networking and academic performance: A review <s> Variable construction/measurement <s> This study advances our knowledge of students’ online self-disclosures by examining a random sample of students’ Facebook profiles at a large, public research University in the Northeast. A quantitative content analysis revealed that the vast majority of students had a Facebook account and very small proportions restricted access of their profile to University staff. Substantial proportions of students’ profiles contained contact information, course schedules, positive references to the University, and pictures of students’ consuming alcohol. Detailed implications for practice are discussed. <s> BIB001 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> There is much talk of a change in modern youth - often referred to as digital natives or Homo Zappiens - with respect to their ability to simultaneously process multiple channels of information. In other words, kids today can multitask. Unfortunately for proponents of this position, there is much empirical documentation concerning the negative effects of attempting to simultaneously process different streams of information showing that such behavior leads to both increased study time to achieve learning parity and an increase in mistakes while processing information than those who are sequentially or serially processing that same information. This article presents the preliminary results of a descriptive and exploratory survey study involving Facebook use, often carried out simultaneously with other study activities, and its relation to academic performance as measured by self-reported Grade Point Average (GPA) and hours spent studying per week. Results show that Facebook(R) users reported having lower GPAs and spend fewer hours per week studying than nonusers. <s> BIB002 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> Abstract Little is known about the influence of electronic media use on the academic and social lives of university students. Using time-diary and survey data, we explore the use of various types of electronic media among first-year students. Time-diary results suggest that the majority of students use electronic media to multitask. Robust regression results indicate a negative relationship between the use of various types of electronic media and first-semester grades. In addition, we find a positive association between social-networking-site use, cellular-phone communication, and face-to-face social interaction. <s> BIB003 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> Studies have shown that multitasking with technology, specifically using Social Networking Sites (SNSs), decreases both efficiency and productivity in an academic setting. This study investigates multitasking's impact on the relationship between SNS use and Grade Point Average (GPA) in United States (US; n=451) and European (n=406) university students using quantitative and qualitative data analysis. Moderated Multiple Regression analysis results showed that the negative relationship between SNS use and GPA was moderated by multitasking only in the US sample. This may be due to European students being less prone to ''disruptive'' multitasking. The results provide valuable cautionary information about the impact of multitasking and using SNSs in a learning environment on university students' GPAs. <s> BIB004 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> Electronic communication is emotionally gratifying, but how do such technological distractions impact academic learning? The current study observed 263 middle school, high school and university students studying for 15min in their homes. Observers noted technologies present and computer windows open in the learning environment prior to studying plus a minute-by-minute assessment of on-task behavior, off-task technology use and open computer windows during studying. A questionnaire assessed study strategies, task-switching preference, technology attitudes, media usage, monthly texting and phone calling, social networking use and grade point average (GPA). Participants averaged less than six minutes on task prior to switching most often due to technological distractions including social media, texting and preference for task-switching. Having a positive attitude toward technology did not affect being on-task during studying. However, those who preferred to task-switch had more distracting technologies available and were more likely to be off-task than others. Also, those who accessed Facebook had lower GPAs than those who avoided it. Finally, students with relatively high use of study strategies were more likely to stay on-task than other students. The educational implications include allowing students short ''technology breaks'' to reduce distractions and teaching students metacognitive strategies regarding when interruptions negatively impact learning. <s> BIB005 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> Tweeting is the preferred method of active SNS usage for college age narcissists.No relationship between active Facebook usage and narcissism in college sample.Platform differences explain the differences in active usage on Facebook versus Twitter.Narcissistic motives mediate the relationship between narcissism and active usage. The amount of research on social networking sites (SNS) and narcissism is accumulating quickly requiring greater levels of variable specification and more fine-tuned hypothesis testing to clearly determine the relationships among key variables. The current investigation examines two of the most popular SNS, Facebook and Twitter, formulating hypotheses around the specific features of each site within college and adult samples. Unlike previous research that has focused almost exclusively on SNS usage, we focused on active usage (i.e., SNS content generation) as opposed to passive usage (i.e., SNS consumption) and included reasons for usage as a potential black box in the narcissism to SNS usage relationship. Results suggest that the features of Twitter make tweeting the preferred means of active usage among narcissists in the college sample, but not the adult sample, who prefer Facebook. In fact, we found no significant direct or indirect relationship with active usage on Facebook for the college sample, calling into question popular press articles linking Millennial narcissism with Facebook use. Additionally platform differences (i.e., microblogging versus profile-based) may explain the importance of active usage on Twitter relative to Facebook. That is, with Twitter, narcissistic motives for usage all manifest through tweeting while Facebook provides other mechanisms to achieve narcissistic motives. <s> BIB006 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> Abstract The popularity of social network sites (SNSs) among college students has stimulated scholarship examining the relationship between SNS use and college adjustment. The present research furthers our understanding of SNS use by studying the relationship between loneliness, varied dimensions of Facebook use, and college adjustment among first-year students. We looked at three facets of college adjustment: social adjustment, academic motivation, and perceived academic performance. Compulsive use of Facebook had a stronger association with academic motivation than habitual use of Facebook, but neither were directly correlated with academic performance. Too much time spent on Facebook was weakly but directly associated with poorer perceived academic performance. Loneliness was a stronger indicator of college adjustment than any dimension of Facebook usage. <s> BIB007 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> A mixed-methods study of academic performance and Facebook among U.S.-based college students.A qualitative account of how a U.S.-sample of college students conferred meanings to their academic lives on Facebook.Academic performance may determine college students' Facebook use, rather than the reverse.Those with lower GPAs may disclose negative affective states in their academic disclosure on Facebook. This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N=261; male=66; female=195; M age ź 22years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N=714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students' Facebook use, rather than the reverse. Implications for student support services are discussed. <s> BIB008 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> This paper examines the impact of Facebook usage on students' academic performance. In addition, it also analyse whether Socialization influences Facebook usage. Empirical data was collected from 1165 Malaysian university students using a questionnaire survey. It was found that the construct Socially Accepted influences Facebook usage while Acculturation does not have any significant relationship with usage. The results also illustrated that there is a positive relationship between students' Academic Performance and Facebook usage i.e. the higher the usage the better they perceived they perform. <s> BIB009 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> The quantitative study to explore the most popular amongst Saudi students.The study establishes relation among the social media usage and academic grades.Poor time management and football also have negative impact on academic grades.Normality tests are to find relationship among social media usage and GPA scores.The hypothesis about existence of relationship among GPA and SN is not satisfied. Social media is a popular method for communication amongst university students in Saudi Arabia. However excessive social media use can raise questions about whether academic performance is affected. This research explores this question by conducting a survey on university students in Saudi Arabia in regards to social media usage and their academic performance. The survey also explored which social network is the most popular amongst Saudi students, what students thought about their social media usage and factors besides social media usage which negatively affect academic performance.The survey received 108 responses and descriptive statistics including normality tests i.e. scatter plots were used to examine the relationship between the average number of hours students spent of social media a week and GPA scores of the students. The results demonstrated that there was no linear relationship between social media usage in a week and GPA score. Students highlighted that besides social media use, time management is a factor which affects students 'studies negatively.The findings of the paper can be used to propose the effective plans for improving the academic performance of the students in such a way that a balance in the leisure, information exchange and academic performance can be maintained. <s> BIB010 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> Young adults, especially college students, are consistently engaging in multiple tasks simultaneously. They are texting, reading, and using social media while studying and attending class. While there are a variety of contexts and relationships likely influenced by this, the present research project examines the influence of media multitasking in the context of students in technology-saturated classrooms and how this is impacting learning and academic performance. A survey of college students examined the impact of technology-based multitasking behaviors both within and outside classrooms. Data demonstrate that those who multitask frequently in-class have lower current college GPAs. This relationship remained significant even after controlling for perceived multitasking efficacy and time spent studying outside of class. Texting emerged as a dominant activity both while attending class and while doing homework. Females seem to use technologies more for maintaining mediated interpersonal interactions and social connections. Males seem to use technology more for online information seeking and for consuming online videos. Those who reported multitasking while doing homework spent more time spent studying outside of class, thereby contributing to inefficient study habits. Implications for technology use, best practices and policies in academic settings are discussed. <s> BIB011 </s> Social networking and academic performance: A review <s> Variable construction/measurement <s> This study examined the simultaneous impact of Facebook (FB) and smart phone usage (SP) on the leisure activities (LA) and college adjustment (SACQ) of students in Serbia. The moderating effects of gender on the observed relationships were also examined. An exploratory study of students in Serbia (N?=?485) revealed that: 1. Students in Serbia spend a daily average of 2.76?h on Facebook, while the average total daily smart phone use is 8:34?h 2. Facebook and smart phone use has no decisive influence on the allocation of time for leisure activities and college adjustment. 3. Facebook and smart phone use still has a certain positive effect on leisure activities, but this influence can become slightly negative if Facebook and smart phones are used too much. 4. In cases of a lack of time, students are more likely to sacrifice academic work, rather than time for Facebook, smart phones, or leisure activities. 5. The moderating effects of students' gender on the observed relationships are weakly expressed. Finally, Facebook and smart phone use has become a common and integral part of life for the majority of Serbian students, and the time for these activities is integrated into their overall time. Presented are the results of the influence of Facebook and smart phone usage.The questionnaires were completed by 485 students.Facebook and smart phone use has no decisive influence o leisure activities.Slightly negative influence if Facebook and smart phones are used too much.Sacrificing academic work, rather than time for Facebook. <s> BIB012

There was quite a bit of variability in how researchers measured SNS use, such as: log of recorded activities BIB003 ; minutes of use (e.g., BIB004 ; frequency and hours of use (e.g., Rouis 2012); amount of time spent on Facebook and Fig. 9 Item density-Total Link Strength Fig. 10 Item density-Occurrences frequency of checking Facebook (e.g., Lee 2014); average daily use of Facebook (e.g., BIB008 ; Facebook Intensity Scale (e.g., BIB009 ; frequency of use and average length of Facebook use (e.g., Lambić 2016); and, number of hours per week spent on SNS (e.g., BIB010 . The most popular measurement technique used for SNS use was self-reports. The use of self-reported SNS use seems logical and convenient given the difficulty in attaining actual usage data from users. However, measuring SNS use with the SNS scale or time on SNS is inherently deficient in that such measures fail to take into account how users' use SNS (i.e., the activities that they engage in). It is also worth noting that the way SNS use is operationalized in the literature in some ways reflects the impressive developments that have occurred with the SNS use landscape. Early works on the topic considered the use and non-use of SNS (e.g., BIB002 BIB001 , while over time there has been a shift away from such an operationalization of use, as SNS has now become, in many ways, ubiquitous and entrenched in our lives-thus, mirroring the situation in practice. There were a few studies that employed measurement techniques different from self-reports. BIB005 integrated participant observations into their study, and observed the participants' use of various technologies as well as their behaviors using a studying observation form; this was complemented with a post-studying questionnaire. Another study used a log of recorded activities for measuring the time spent on SNS (Jacobsen and Forste 2011). Junco (2013) notes that the literature on SNS use and academic performance more often than not uses GPA as the measure of academic performance. Academic performance was measured primarily via: self-reported GPA (e.g., BIB011 , GPA obtained from registrar (e.g., Junco and Cotten 2012), and perceived performance (e.g., BIB009 BIB012 . Overwhelmingly, studies resorted to the use of self-reported GPA and/or perceived performance compared to actual GPA obtained from the registrar/ student records; this probably speaks to the difficulty in obtaining actual GPAs, along with the convenience that measures such as self-reported GPA and perceived performance afford in gathering data. Thus, the studies that employ such proxies suffer from limitations inherent to such use. The conflicting and inconsistent findings in the published studies may in some fashion reflect, or even stem from, the differences in how the variables are constructed and/or measured. BIB006 point out that an issue with the extant research on SNS is Ba lack of specification regarding the type of sites included under the umbrella of Bsocial networking^^(p.2); furthermore, they argue that we need to reconsider the reliance on standard SNS usage variables as such efforts can be guilty of oversimplification. Additionally, BIB007 point out that the attempts to extract any decisive conclusions from the published work are complicated by the fact that Bthe concept and measurement of SNS usage varied across studies^(p.2). Thus, in some ways the evidence is likely muddied by the inconsistent and imprecise operationalization and methods of measurements of the constructs. Given the sensitivity of the findings to the operationalization of the variables and the variability in the designs, as pointed out earlier, one has to exercise some caution in drawing generalizations from such synthesis.

Social networking and academic performance: A review <s> Analysis <s> A recent draft manuscript suggested that Facebook use might be related to lower academic achievement in college and graduate school (Karpinski, 2009). The report quickly became a media sensation and was picked up by hundreds of news outlets in a matter of days. However, the results were based on correlational data in a draft manuscript that had not been published, or even considered for publication. This paper attempts to replicate the results reported in the press release using three data sets: one with a large sample of undergraduate students from the University of Illinois at Chicago, another with a nationally representative cross sectional sample of American 14- to 22-year-olds, as well as a longitudinal panel of American youth aged 14-23. In none of the samples do we find a robust negative relationship between Facebook use and grades. Indeed, if anything, Facebook use is more common among individuals with higher grades. We also examined how changes in academic performance in the nationally representative sample related to Facebook use and found that Facebook users were no different from non-users. <s> BIB001 </s> Social networking and academic performance: A review <s> Analysis <s> There is much talk of a change in modern youth - often referred to as digital natives or Homo Zappiens - with respect to their ability to simultaneously process multiple channels of information. In other words, kids today can multitask. Unfortunately for proponents of this position, there is much empirical documentation concerning the negative effects of attempting to simultaneously process different streams of information showing that such behavior leads to both increased study time to achieve learning parity and an increase in mistakes while processing information than those who are sequentially or serially processing that same information. This article presents the preliminary results of a descriptive and exploratory survey study involving Facebook use, often carried out simultaneously with other study activities, and its relation to academic performance as measured by self-reported Grade Point Average (GPA) and hours spent studying per week. Results show that Facebook(R) users reported having lower GPAs and spend fewer hours per week studying than nonusers. <s> BIB002 </s> Social networking and academic performance: A review <s> Analysis <s> The proliferation and ease of access to information and communication technologies (ICTs) such as Facebook, text messaging, and instant messaging has resulted in ICT users being presented with more real-time streaming data than ever before. Unfortunately, this has also resulted in individuals increasingly engaging in multitasking as an information management strategy. The purpose of this study was to examine how college students multitask with ICTs and to determine the impacts of this multitasking on their college grade point average (GPA). Using web survey data from a large sample of college students at one university (N=1839), we found that students reported spending a large amount of time using ICTs on a daily basis. Students reported frequently searching for content not related to courses, using Facebook, emailing, talking on their cell phones, and texting while doing schoolwork. Hierarchical (blocked) linear regression analyses revealed that using Facebook and texting while doing schoolwork were negatively associated with overall college GPA. Engaging in Facebook use or texting while trying to complete schoolwork may tax students' capacity for cognitive processing and preclude deeper learning. Our research indicates that the type and purpose of ICT use matters in terms of the educational impacts of multitasking. <s> BIB003 </s> Social networking and academic performance: A review <s> Analysis <s> Electronic communication is emotionally gratifying, but how do such technological distractions impact academic learning? The current study observed 263 middle school, high school and university students studying for 15min in their homes. Observers noted technologies present and computer windows open in the learning environment prior to studying plus a minute-by-minute assessment of on-task behavior, off-task technology use and open computer windows during studying. A questionnaire assessed study strategies, task-switching preference, technology attitudes, media usage, monthly texting and phone calling, social networking use and grade point average (GPA). Participants averaged less than six minutes on task prior to switching most often due to technological distractions including social media, texting and preference for task-switching. Having a positive attitude toward technology did not affect being on-task during studying. However, those who preferred to task-switch had more distracting technologies available and were more likely to be off-task than others. Also, those who accessed Facebook had lower GPAs than those who avoided it. Finally, students with relatively high use of study strategies were more likely to stay on-task than other students. The educational implications include allowing students short ''technology breaks'' to reduce distractions and teaching students metacognitive strategies regarding when interruptions negatively impact learning. <s> BIB004 </s> Social networking and academic performance: A review <s> Analysis <s> Abstract The popularity of social network sites (SNSs) among college students has stimulated scholarship examining the relationship between SNS use and college adjustment. The present research furthers our understanding of SNS use by studying the relationship between loneliness, varied dimensions of Facebook use, and college adjustment among first-year students. We looked at three facets of college adjustment: social adjustment, academic motivation, and perceived academic performance. Compulsive use of Facebook had a stronger association with academic motivation than habitual use of Facebook, but neither were directly correlated with academic performance. Too much time spent on Facebook was weakly but directly associated with poorer perceived academic performance. Loneliness was a stronger indicator of college adjustment than any dimension of Facebook usage. <s> BIB005 </s> Social networking and academic performance: A review <s> Analysis <s> A mixed-methods study of academic performance and Facebook among U.S.-based college students.A qualitative account of how a U.S.-sample of college students conferred meanings to their academic lives on Facebook.Academic performance may determine college students' Facebook use, rather than the reverse.Those with lower GPAs may disclose negative affective states in their academic disclosure on Facebook. This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N=261; male=66; female=195; M age ź 22years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N=714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students' Facebook use, rather than the reverse. Implications for student support services are discussed. <s> BIB006

Most studies presented descriptive analyses for the data. The early studies on this topic predominantly treated the analysis through correlations (e.g., BIB001 ). Likewise, it should be noted that a majority of the studies in the early part of the topic development used correlation analysis. This could be a function of the follower mechanism where subsequent studies simply try to follow the line of analysis conducted by the early works. Regression analysis (e.g., BIB003 BIB004 , MANOVA (e.g., BIB002 ) and structural equation modeling (e.g., BIB006 BIB005 were other methods of analysis employed in the studies. Given the nature of the analysis conducted in the bulk of the studies, it is difficult to speak to the causality of the tested relationships, and as such should temper the findings of such an evidence base. It behooves us to note that given that a user can engage in various activities on SNS, analysis techniques frequented in the studies may be deficient in fully explicating such complex relationships in the absence of both context and temporality.

Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Studies suggest a bias against the publication of null (p >.05) results. Instead of significance, we advocate reporting effect sizes and confidence intervals, and using replication studies. If statistical tests are used, power tests should accompany them. <s> BIB001 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> This study advances our knowledge of students’ online self-disclosures by examining a random sample of students’ Facebook profiles at a large, public research University in the Northeast. A quantitative content analysis revealed that the vast majority of students had a Facebook account and very small proportions restricted access of their profile to University staff. Substantial proportions of students’ profiles contained contact information, course schedules, positive references to the University, and pictures of students’ consuming alcohol. Detailed implications for practice are discussed. <s> BIB002 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> A recent draft manuscript suggested that Facebook use might be related to lower academic achievement in college and graduate school (Karpinski, 2009). The report quickly became a media sensation and was picked up by hundreds of news outlets in a matter of days. However, the results were based on correlational data in a draft manuscript that had not been published, or even considered for publication. This paper attempts to replicate the results reported in the press release using three data sets: one with a large sample of undergraduate students from the University of Illinois at Chicago, another with a nationally representative cross sectional sample of American 14- to 22-year-olds, as well as a longitudinal panel of American youth aged 14-23. In none of the samples do we find a robust negative relationship between Facebook use and grades. Indeed, if anything, Facebook use is more common among individuals with higher grades. We also examined how changes in academic performance in the nationally representative sample related to Facebook use and found that Facebook users were no different from non-users. <s> BIB003 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> There is much talk of a change in modern youth - often referred to as digital natives or Homo Zappiens - with respect to their ability to simultaneously process multiple channels of information. In other words, kids today can multitask. Unfortunately for proponents of this position, there is much empirical documentation concerning the negative effects of attempting to simultaneously process different streams of information showing that such behavior leads to both increased study time to achieve learning parity and an increase in mistakes while processing information than those who are sequentially or serially processing that same information. This article presents the preliminary results of a descriptive and exploratory survey study involving Facebook use, often carried out simultaneously with other study activities, and its relation to academic performance as measured by self-reported Grade Point Average (GPA) and hours spent studying per week. Results show that Facebook(R) users reported having lower GPAs and spend fewer hours per week studying than nonusers. <s> BIB004 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Abstract Little is known about the influence of electronic media use on the academic and social lives of university students. Using time-diary and survey data, we explore the use of various types of electronic media among first-year students. Time-diary results suggest that the majority of students use electronic media to multitask. Robust regression results indicate a negative relationship between the use of various types of electronic media and first-semester grades. In addition, we find a positive association between social-networking-site use, cellular-phone communication, and face-to-face social interaction. <s> BIB005 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> This research study examines the impact of facebook usage on the academic performance and the quality of life of college students. With the advent of the internet, more denizens are spending time on social networks as a way to expand both their personal and business relationships. We used structural equation modeling to evaluate the hypotheses. Findings indicate that the dispositional factors may influence the endogenous variables in our model. We discuss the limitations of the study, the implications, and future research directions. <s> BIB006 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> The proliferation and ease of access to information and communication technologies (ICTs) such as Facebook, text messaging, and instant messaging has resulted in ICT users being presented with more real-time streaming data than ever before. Unfortunately, this has also resulted in individuals increasingly engaging in multitasking as an information management strategy. The purpose of this study was to examine how college students multitask with ICTs and to determine the impacts of this multitasking on their college grade point average (GPA). Using web survey data from a large sample of college students at one university (N=1839), we found that students reported spending a large amount of time using ICTs on a daily basis. Students reported frequently searching for content not related to courses, using Facebook, emailing, talking on their cell phones, and texting while doing schoolwork. Hierarchical (blocked) linear regression analyses revealed that using Facebook and texting while doing schoolwork were negatively associated with overall college GPA. Engaging in Facebook use or texting while trying to complete schoolwork may tax students' capacity for cognitive processing and preclude deeper learning. Our research indicates that the type and purpose of ICT use matters in terms of the educational impacts of multitasking. <s> BIB007 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Online social networks (OSNs) have permeated all generations of Internet users, becoming a prominent communications tool, particularly in the student community. Thus, academic institutions and faculty are increasingly using social networking sites, such as Facebook and LinkedIn, to connect with current and potential students and to deliver instructional content. This has led to a rise in questions about the impact of OSN on academic performance and the possibility of using it as an effective teaching tool. To learn more about the impact on academic performance, we conducted a survey of business students at a large state university. Survey results were analyzed using structural equation modeling (SEM). The results revealed a statistically significant negative relationship between time spent by students on OSN and their academic performance. The time spent on OSN was found to be heavily influenced by the attention span of the students. Specifically, we determined that the higher the attention span, the lower is the time spent on OSN. Further, attention span was found to be highly correlated with characteristics that predict or influence student behavior, such as their perceptions about society's view of social networking, their likes and dislikes of OSN, ease of use of OSN, etc. <s> BIB008 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Abstract In the great expansion of the social networking activity, young people are the main users whose choices have vast influence. This study uses the flow theory to gauge the impact of Facebook usage on Tunisian students' achievements, with the presumption that the high usage level might reduce students' scholar achievements. The research design suggests that this impact would vary among students with different interests for the university and multitasking capabilities. Facebook usage would develop students' satisfaction with friends and family, which could enhance their academic performance. Analyses from 161 Tunisian students show that Facebook usage does not affect significantly students' academic performance and their satisfaction with the family, whereas it decreases their actual satisfaction with friends. Yet, a high level of satisfaction of the student with his family continues to enhance his academic performance. Overall, though, Facebook usage appears to do not have a significant effect on un... <s> BIB009 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Studies have shown that multitasking with technology, specifically using Social Networking Sites (SNSs), decreases both efficiency and productivity in an academic setting. This study investigates multitasking's impact on the relationship between SNS use and Grade Point Average (GPA) in United States (US; n=451) and European (n=406) university students using quantitative and qualitative data analysis. Moderated Multiple Regression analysis results showed that the negative relationship between SNS use and GPA was moderated by multitasking only in the US sample. This may be due to European students being less prone to ''disruptive'' multitasking. The results provide valuable cautionary information about the impact of multitasking and using SNSs in a learning environment on university students' GPAs. <s> BIB010 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Electronic communication is emotionally gratifying, but how do such technological distractions impact academic learning? The current study observed 263 middle school, high school and university students studying for 15min in their homes. Observers noted technologies present and computer windows open in the learning environment prior to studying plus a minute-by-minute assessment of on-task behavior, off-task technology use and open computer windows during studying. A questionnaire assessed study strategies, task-switching preference, technology attitudes, media usage, monthly texting and phone calling, social networking use and grade point average (GPA). Participants averaged less than six minutes on task prior to switching most often due to technological distractions including social media, texting and preference for task-switching. Having a positive attitude toward technology did not affect being on-task during studying. However, those who preferred to task-switch had more distracting technologies available and were more likely to be off-task than others. Also, those who accessed Facebook had lower GPAs than those who avoided it. Finally, students with relatively high use of study strategies were more likely to stay on-task than other students. The educational implications include allowing students short ''technology breaks'' to reduce distractions and teaching students metacognitive strategies regarding when interruptions negatively impact learning. <s> BIB011 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> This paper is an exploratory study of the relationship between personality and problematic social network use among Chinese university students and the impact of problematic social network use on academic performance. Students who scored higher on the personality traits of agreeableness and conscientiousness were less likely to be associated with emotional/physical problems related to social network use. In addition, students who report spending excessive time on social networking sites and those who believe that their academic performance suffers because of social network use are more likely to have lower academic performance. Male students in China who score low on agreeableness are more likely to engage in problematic social network use. The results suggest that academic advisors should consider counseling students on the negative impact of problematic social network use and should plan interventions for students whose academic performance is negatively affected by problematic social network use. <s> BIB012 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> The present study explores the usage and attitude toward Facebook and text messaging among African American and Hispanic teenagers. It also examines the influence of personality and gender on the n... <s> BIB013 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Abstract The popularity of social network sites (SNSs) among college students has stimulated scholarship examining the relationship between SNS use and college adjustment. The present research furthers our understanding of SNS use by studying the relationship between loneliness, varied dimensions of Facebook use, and college adjustment among first-year students. We looked at three facets of college adjustment: social adjustment, academic motivation, and perceived academic performance. Compulsive use of Facebook had a stronger association with academic motivation than habitual use of Facebook, but neither were directly correlated with academic performance. Too much time spent on Facebook was weakly but directly associated with poorer perceived academic performance. Loneliness was a stronger indicator of college adjustment than any dimension of Facebook usage. <s> BIB014 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> As technology use continues its steady growth among college students, both within and outside of the classroom, its effect on academic performance becomes an increasingly important question to address. Cognitive theory and multitasking research strongly support a negative effect while other studies have found little to no effect. Using a large sample of students, this study attempted to address these opposing results and help find clarity. We explored the relationship of the frequency of students' use of technologies and their academic performance as measured by GPA, SAT scores, study hours, and predicted course grade. In order to help understand our findings, we also examined the role of gender and employment status in this relationship. Our hypotheses were partially supported, with frequency of technology use negatively related to academic performance. We discuss our findings and limitations of the research. <s> BIB015 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> This paper examines the impact of Facebook usage on students' academic performance. In addition, it also analyse whether Socialization influences Facebook usage. Empirical data was collected from 1165 Malaysian university students using a questionnaire survey. It was found that the construct Socially Accepted influences Facebook usage while Acculturation does not have any significant relationship with usage. The results also illustrated that there is a positive relationship between students' Academic Performance and Facebook usage i.e. the higher the usage the better they perceived they perform. <s> BIB016 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Abstract Although some research has shown a negative relation between Facebook use and academic performance, more recent research suggests that this relation is likely mitigated by multitasking. This study examined the time students at different class ranks spent on Facebook, the time they spent multitasking with Facebook, as well as the activities they engaged in on the site ( N = 1649). The results showed that seniors spent significantly less time on Facebook and spent significantly less time multitasking with Facebook than students at other class ranks. Time spent on Facebook was significantly negatively predictive of GPA for freshmen but not for other students. Multitasking with Facebook was significantly negatively predictive of GPA for freshmen, sophomores, and juniors but not for seniors. The results are discussed in relation to freshmen transition tasks and ideas for future research are provided. <s> BIB017 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> A mixed-methods study of academic performance and Facebook among U.S.-based college students.A qualitative account of how a U.S.-sample of college students conferred meanings to their academic lives on Facebook.Academic performance may determine college students' Facebook use, rather than the reverse.Those with lower GPAs may disclose negative affective states in their academic disclosure on Facebook. This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N=261; male=66; female=195; M age ź 22years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N=714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students' Facebook use, rather than the reverse. Implications for student support services are discussed. <s> BIB018 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> The quantitative study to explore the most popular amongst Saudi students.The study establishes relation among the social media usage and academic grades.Poor time management and football also have negative impact on academic grades.Normality tests are to find relationship among social media usage and GPA scores.The hypothesis about existence of relationship among GPA and SN is not satisfied. Social media is a popular method for communication amongst university students in Saudi Arabia. However excessive social media use can raise questions about whether academic performance is affected. This research explores this question by conducting a survey on university students in Saudi Arabia in regards to social media usage and their academic performance. The survey also explored which social network is the most popular amongst Saudi students, what students thought about their social media usage and factors besides social media usage which negatively affect academic performance.The survey received 108 responses and descriptive statistics including normality tests i.e. scatter plots were used to examine the relationship between the average number of hours students spent of social media a week and GPA scores of the students. The results demonstrated that there was no linear relationship between social media usage in a week and GPA score. Students highlighted that besides social media use, time management is a factor which affects students 'studies negatively.The findings of the paper can be used to propose the effective plans for improving the academic performance of the students in such a way that a balance in the leisure, information exchange and academic performance can be maintained. <s> BIB019 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> Young adults, especially college students, are consistently engaging in multiple tasks simultaneously. They are texting, reading, and using social media while studying and attending class. While there are a variety of contexts and relationships likely influenced by this, the present research project examines the influence of media multitasking in the context of students in technology-saturated classrooms and how this is impacting learning and academic performance. A survey of college students examined the impact of technology-based multitasking behaviors both within and outside classrooms. Data demonstrate that those who multitask frequently in-class have lower current college GPAs. This relationship remained significant even after controlling for perceived multitasking efficacy and time spent studying outside of class. Texting emerged as a dominant activity both while attending class and while doing homework. Females seem to use technologies more for maintaining mediated interpersonal interactions and social connections. Males seem to use technology more for online information seeking and for consuming online videos. Those who reported multitasking while doing homework spent more time spent studying outside of class, thereby contributing to inefficient study habits. Implications for technology use, best practices and policies in academic settings are discussed. <s> BIB020 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> This study examined the simultaneous impact of Facebook (FB) and smart phone usage (SP) on the leisure activities (LA) and college adjustment (SACQ) of students in Serbia. The moderating effects of gender on the observed relationships were also examined. An exploratory study of students in Serbia (N?=?485) revealed that: 1. Students in Serbia spend a daily average of 2.76?h on Facebook, while the average total daily smart phone use is 8:34?h 2. Facebook and smart phone use has no decisive influence on the allocation of time for leisure activities and college adjustment. 3. Facebook and smart phone use still has a certain positive effect on leisure activities, but this influence can become slightly negative if Facebook and smart phones are used too much. 4. In cases of a lack of time, students are more likely to sacrifice academic work, rather than time for Facebook, smart phones, or leisure activities. 5. The moderating effects of students' gender on the observed relationships are weakly expressed. Finally, Facebook and smart phone use has become a common and integral part of life for the majority of Serbian students, and the time for these activities is integrated into their overall time. Presented are the results of the influence of Facebook and smart phone usage.The questionnaires were completed by 485 students.Facebook and smart phone use has no decisive influence o leisure activities.Slightly negative influence if Facebook and smart phones are used too much.Sacrificing academic work, rather than time for Facebook. <s> BIB021 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> In this paper a research on the application of Facebook in education is presented. A total of 139 students of the Faculty of Education in Sombor (Serbia) participated in the research. The results establish a positive correlation between the academic performance of students and the frequency of use of Facebook for educational purposes. The factor of perceived usefulness greatly affected the decision of students to use Facebook as a learning aid. No significant difference in the frequency of use of Facebook for general purposes has been reported between the groups of students divided by the academic performance. Frequency of educational use of Facebook positively influenced the academic performance.Perceived usefulness affected students use of Facebook as a learning aid.Facebook use for general purposes does not affect the academic performance. <s> BIB022 </s> Social networking and academic performance: A review <s> Positive, negative, and/or insignificant findings <s> The future generation’s competency in the English language is highly dependent on the performance of our very own TESL students as they are the future English language teachers who can make all the differences. Hence, any issues that might jeopardise their performance can never be taken lightly. This research was carried out to investigate whether TESL students’ addiction to social networking sites (SNSs) influences their academic performance. A survey questionnaire method was used to gather data from 93 Semester 8 TESL students from the Faculty of Education, UiTM Shah Alam. The SPSS version 16.0 was used to analyse data. Despite initial prediction that student addiction towards SNSs is related to their academic performance, this study found no significant relationship between the two aforementioned variables. The finding contradicts several other studies on the relationship between SNSs addiction and academic achievement. <s> BIB023

Contradictory data points on the association between SNS use and academic performance emerged from the evidence base (see Fig. 11 ). On one hand, one study detected a positive relationship BIB016 . Likewise, BIB007 , who despite finding a negative relationship between the time spent on Facebook and GPA, note that behaviors such as sharing links and checking on friends were positively linked to GPA. On the other hand, some studies revealed a negative relationship BIB012 BIB005 BIB021 BIB007 BIB007 BIB017 BIB010 BIB004 BIB013 BIB018 BIB008 BIB011 BIB014 . Still others found no significant relationship BIB019 BIB020 BIB006 BIB002 BIB022 BIB003 BIB023 BIB009 . From this, while it may be easy to make a negative appraisal, caution is warranted given some of the issues pointed in the earlier subsections of the findings. Of course, there were some qualifiers in the findings that need mentioning. For instance, Junco (2013) found the negative link between time on Facebook and GPA to be significant only for freshman students. Moreover, multitasking with Facebook was negatively linked to GPA for freshmen, sophomores, and juniors, but not for seniors. While, the results still seem spotty and generally channel toward two main results (negative and no relationship), a simple frequency weighing of the studies suggests a negative bent in the association. This negative echo is in line with another strand of the literature that has examined the association between general technology use and academic performance (e.g., BIB015 . While many possible explanations have been offered, we find the one offered by Junco & Cotten (2011) to be reasonable: engaging in SNS use Bwhile trying to complete schoolwork may tax students' capacity for cognitive processing and preclude deeper learning^(p.1). However, such an explanation needs to be supported by additional work. Despite the strong appeal of placing a negative lens to view the relationship, still more evidence is required to assert such an outcome. Further, one cannot simply discount the counterbalancing evidence detected in the studies that provides some resistance to the negative bent. Moreover, we should also view the findings with the effects of publication bias in mind given the preponderance to publish statistically significant scientific results coupled with the publication bias against null findings BIB001 .

Social networking and academic performance: A review <s> Additional variables <s> A recent draft manuscript suggested that Facebook use might be related to lower academic achievement in college and graduate school (Karpinski, 2009). The report quickly became a media sensation and was picked up by hundreds of news outlets in a matter of days. However, the results were based on correlational data in a draft manuscript that had not been published, or even considered for publication. This paper attempts to replicate the results reported in the press release using three data sets: one with a large sample of undergraduate students from the University of Illinois at Chicago, another with a nationally representative cross sectional sample of American 14- to 22-year-olds, as well as a longitudinal panel of American youth aged 14-23. In none of the samples do we find a robust negative relationship between Facebook use and grades. Indeed, if anything, Facebook use is more common among individuals with higher grades. We also examined how changes in academic performance in the nationally representative sample related to Facebook use and found that Facebook users were no different from non-users. <s> BIB001 </s> Social networking and academic performance: A review <s> Additional variables <s> Online social networking has deeply penetrated university campuses, influencing multiple aspects of student life. We investigate the impacts of individual online social networking engagement (e.g., on Facebook) from a pedagogical standpoint. Based on social learning theory, we argue that two socialization processes, social acceptance and acculturation, bridge individual online social networking engagement with three domains of social learning outcomes. Results from a survey accompanied by focus group discussions demonstrate the substantial impacts of university student online social networking engagement on social learning processes and outcomes. Online social networking not only directly influences university students' learning outcomes, but also helps the students attain social acceptance from others and adapt to university culture, both of which play prominent roles in improving their learning outcomes. <s> BIB002 </s> Social networking and academic performance: A review <s> Additional variables <s> There is much talk of a change in modern youth - often referred to as digital natives or Homo Zappiens - with respect to their ability to simultaneously process multiple channels of information. In other words, kids today can multitask. Unfortunately for proponents of this position, there is much empirical documentation concerning the negative effects of attempting to simultaneously process different streams of information showing that such behavior leads to both increased study time to achieve learning parity and an increase in mistakes while processing information than those who are sequentially or serially processing that same information. This article presents the preliminary results of a descriptive and exploratory survey study involving Facebook use, often carried out simultaneously with other study activities, and its relation to academic performance as measured by self-reported Grade Point Average (GPA) and hours spent studying per week. Results show that Facebook(R) users reported having lower GPAs and spend fewer hours per week studying than nonusers. <s> BIB003 </s> Social networking and academic performance: A review <s> Additional variables <s> Abstract In the great expansion of the social networking activity, young people are the main users whose choices have vast influence. This study uses the flow theory to gauge the impact of Facebook usage on Tunisian students' achievements, with the presumption that the high usage level might reduce students' scholar achievements. The research design suggests that this impact would vary among students with different interests for the university and multitasking capabilities. Facebook usage would develop students' satisfaction with friends and family, which could enhance their academic performance. Analyses from 161 Tunisian students show that Facebook usage does not affect significantly students' academic performance and their satisfaction with the family, whereas it decreases their actual satisfaction with friends. Yet, a high level of satisfaction of the student with his family continues to enhance his academic performance. Overall, though, Facebook usage appears to do not have a significant effect on un... <s> BIB004 </s> Social networking and academic performance: A review <s> Additional variables <s> Studies have shown that multitasking with technology, specifically using Social Networking Sites (SNSs), decreases both efficiency and productivity in an academic setting. This study investigates multitasking's impact on the relationship between SNS use and Grade Point Average (GPA) in United States (US; n=451) and European (n=406) university students using quantitative and qualitative data analysis. Moderated Multiple Regression analysis results showed that the negative relationship between SNS use and GPA was moderated by multitasking only in the US sample. This may be due to European students being less prone to ''disruptive'' multitasking. The results provide valuable cautionary information about the impact of multitasking and using SNSs in a learning environment on university students' GPAs. <s> BIB005 </s> Social networking and academic performance: A review <s> Additional variables <s> This paper is an exploratory study of the relationship between personality and problematic social network use among Chinese university students and the impact of problematic social network use on academic performance. Students who scored higher on the personality traits of agreeableness and conscientiousness were less likely to be associated with emotional/physical problems related to social network use. In addition, students who report spending excessive time on social networking sites and those who believe that their academic performance suffers because of social network use are more likely to have lower academic performance. Male students in China who score low on agreeableness are more likely to engage in problematic social network use. The results suggest that academic advisors should consider counseling students on the negative impact of problematic social network use and should plan interventions for students whose academic performance is negatively affected by problematic social network use. <s> BIB006 </s> Social networking and academic performance: A review <s> Additional variables <s> Tweeting is the preferred method of active SNS usage for college age narcissists.No relationship between active Facebook usage and narcissism in college sample.Platform differences explain the differences in active usage on Facebook versus Twitter.Narcissistic motives mediate the relationship between narcissism and active usage. The amount of research on social networking sites (SNS) and narcissism is accumulating quickly requiring greater levels of variable specification and more fine-tuned hypothesis testing to clearly determine the relationships among key variables. The current investigation examines two of the most popular SNS, Facebook and Twitter, formulating hypotheses around the specific features of each site within college and adult samples. Unlike previous research that has focused almost exclusively on SNS usage, we focused on active usage (i.e., SNS content generation) as opposed to passive usage (i.e., SNS consumption) and included reasons for usage as a potential black box in the narcissism to SNS usage relationship. Results suggest that the features of Twitter make tweeting the preferred means of active usage among narcissists in the college sample, but not the adult sample, who prefer Facebook. In fact, we found no significant direct or indirect relationship with active usage on Facebook for the college sample, calling into question popular press articles linking Millennial narcissism with Facebook use. Additionally platform differences (i.e., microblogging versus profile-based) may explain the importance of active usage on Twitter relative to Facebook. That is, with Twitter, narcissistic motives for usage all manifest through tweeting while Facebook provides other mechanisms to achieve narcissistic motives. <s> BIB007 </s> Social networking and academic performance: A review <s> Additional variables <s> This paper examines the impact of Facebook usage on students' academic performance. In addition, it also analyse whether Socialization influences Facebook usage. Empirical data was collected from 1165 Malaysian university students using a questionnaire survey. It was found that the construct Socially Accepted influences Facebook usage while Acculturation does not have any significant relationship with usage. The results also illustrated that there is a positive relationship between students' Academic Performance and Facebook usage i.e. the higher the usage the better they perceived they perform. <s> BIB008 </s> Social networking and academic performance: A review <s> Additional variables <s> Young adults, especially college students, are consistently engaging in multiple tasks simultaneously. They are texting, reading, and using social media while studying and attending class. While there are a variety of contexts and relationships likely influenced by this, the present research project examines the influence of media multitasking in the context of students in technology-saturated classrooms and how this is impacting learning and academic performance. A survey of college students examined the impact of technology-based multitasking behaviors both within and outside classrooms. Data demonstrate that those who multitask frequently in-class have lower current college GPAs. This relationship remained significant even after controlling for perceived multitasking efficacy and time spent studying outside of class. Texting emerged as a dominant activity both while attending class and while doing homework. Females seem to use technologies more for maintaining mediated interpersonal interactions and social connections. Males seem to use technology more for online information seeking and for consuming online videos. Those who reported multitasking while doing homework spent more time spent studying outside of class, thereby contributing to inefficient study habits. Implications for technology use, best practices and policies in academic settings are discussed. <s> BIB009 </s> Social networking and academic performance: A review <s> Additional variables <s> This study examined the simultaneous impact of Facebook (FB) and smart phone usage (SP) on the leisure activities (LA) and college adjustment (SACQ) of students in Serbia. The moderating effects of gender on the observed relationships were also examined. An exploratory study of students in Serbia (N?=?485) revealed that: 1. Students in Serbia spend a daily average of 2.76?h on Facebook, while the average total daily smart phone use is 8:34?h 2. Facebook and smart phone use has no decisive influence on the allocation of time for leisure activities and college adjustment. 3. Facebook and smart phone use still has a certain positive effect on leisure activities, but this influence can become slightly negative if Facebook and smart phones are used too much. 4. In cases of a lack of time, students are more likely to sacrifice academic work, rather than time for Facebook, smart phones, or leisure activities. 5. The moderating effects of students' gender on the observed relationships are weakly expressed. Finally, Facebook and smart phone use has become a common and integral part of life for the majority of Serbian students, and the time for these activities is integrated into their overall time. Presented are the results of the influence of Facebook and smart phone usage.The questionnaires were completed by 485 students.Facebook and smart phone use has no decisive influence o leisure activities.Slightly negative influence if Facebook and smart phones are used too much.Sacrificing academic work, rather than time for Facebook. <s> BIB010

While there is apparent usefulness and reason in examining the explicit link between SNS use and academic performance, being bounded by such constraints leaves much to be elucidated. BIB002 note that Bthe hidden complexity and dynamism of social networking behavior necessitates exploration of the underlying linkage mechanism between online social networking and desirable learning outcomes^(p. 3). Surprisingly, the literature has paid little attention to spillover effects of SNS use. This is, we contend, in part attributable to the narrow focus on the SNS-academic performance link. Some of the articles in the review illustrate SNS to have additional spillover influences, such as: Facebook users spend fewer hours studying per week compared to non-Facebook users BIB003 . Likewise, students are more likely, when faced with a lack of time, to give up academic work for Facebook use BIB010 . In light of such disclosures, additional attempts should consider the influences of SNS use on additional variables such as study habits and study strategies to provide a more wholesome picture of the academic influences of SNS use. Across the reviewed articles, an aspect that the studies have scantily considered is the influence of either mediators and/or moderators on the SNS-academic performance link. We suggest that additional research may examine whether a consideration of such influences can help better explicate the mechanisms underlying this link. The idea that SNS use effects academic performance was initially the go-to way of approaching this topic. However, in recent examinations, adjustments to this line of thinking have been made, as illustrated in the recent consideration of the possible mediating role of SNS use BIB008 . Some studies have suggested for an examination of potential influences of other variables and contextual factors on this link. In this context, there are certain salient variables that deserve consideration. Multitasking has been offered as a moderating variable in the link between SNS use and academic performance (e.g., Junco, 2013; BIB005 ); however, it should be noted that both studies included some qualifiers in the findings. In the Junco (2013) study, multitasking with Facebook was negatively linked to GPA for freshmen, sophomores, and juniors, but not for seniors (suggesting the variations in findings based on the class standing of students). Likewise, the moderating role of multitasking was only significant in the US sample and not in the European sample BIB005 . Another study included considerations for moderating roles of polychronicity and students' interest for the university BIB004 . Thus, we view such efforts as a promising line of enquiry for the explication of this link. Additionally, some studies, such as BIB009 controlled for multitasking efficacy and class preparation time in their investigations. While, studies such as the one by BIB001 incorporate control variables, including, age, gender, race/ethnicity, and socio-economic status (SES), however, rather limited research has considered the influences of such demographic factors. Even though considerations for the influence of personality traits on SNS use was offered by a few studies BIB006 BIB007 , note that SNS research has tended to ignore the influences of variables such as personality. Thus, future efforts could focus on the influences of demographic factors and personality characteristics.

Social networking and academic performance: A review <s> Discussion <s> The purpose of this article is to review current published research studies focusing on the use of Facebook by students and teachers. The aim of the review is not to solely discuss Facebook in relation to teaching or learning purposes, or about its educational value per se, but also to present a detailed account of the participants' Facebook usage profile or the extent to which users are engaged in Facebook activities. The emphasis of this review will be upon empirical findings rather than opinion- or theoretical explanations. Following the review guidelines set by Creswell (Research Design Qualitative and Quantitative Approaches, 1994), I summarize the hitherto accumulated state of knowledge concerning Facebook and highlight questions or issues that research has left unresolved. This review is organized into three sections that cover the major topics of current research: (a) students' Facebook usage profile or extent of Facebook use (e.g., time students spend on Facebook each day, students' motives for using Facebook, as well as various factors that may affect these usage profiles), (b) the effects of Facebook use (e.g., effects of Facebook self-disclosure on teacher credibility, effects of Facebook use on student social presence and discussion, and effects of Facebook on students' academic performance), and (c) students' attitudes toward Facebook. The conclusions overall suggest that Facebook thus far has very little educational use, that students use Facebook mainly to keep in touch with known individuals, and that students tend to disclose more personal information about themselves on Facebook; hence attracting potential privacy risks upon themselves. <s> BIB001 </s> Social networking and academic performance: A review <s> Discussion <s> Abstract Little is known about the influence of electronic media use on the academic and social lives of university students. Using time-diary and survey data, we explore the use of various types of electronic media among first-year students. Time-diary results suggest that the majority of students use electronic media to multitask. Robust regression results indicate a negative relationship between the use of various types of electronic media and first-semester grades. In addition, we find a positive association between social-networking-site use, cellular-phone communication, and face-to-face social interaction. <s> BIB002 </s> Social networking and academic performance: A review <s> Discussion <s> Recent controversies in psychology have spurred conversations about the nature and quality of psychological research. One topic receiving substantial attention is the role of replication in psychological science. Using the complete publication history of the 100 psychology journals with the highest 5-year impact factors, the current article provides an overview of replications in psychological research since 1900. This investigation revealed that roughly 1.6% of all psychology publications used the term replication in text. A more thorough analysis of 500 randomly selected articles revealed that only 68% of articles using the term replication were actual replications, resulting in an overall replication rate of 1.07%. Contrary to previous findings in other fields, this study found that the majority of replications in psychology journals reported similar findings to their original studies (i.e., they were successful replications). However, replications were significantly less likely to be successful when t... <s> BIB003 </s> Social networking and academic performance: A review <s> Discussion <s> Studies have shown that multitasking with technology, specifically using Social Networking Sites (SNSs), decreases both efficiency and productivity in an academic setting. This study investigates multitasking's impact on the relationship between SNS use and Grade Point Average (GPA) in United States (US; n=451) and European (n=406) university students using quantitative and qualitative data analysis. Moderated Multiple Regression analysis results showed that the negative relationship between SNS use and GPA was moderated by multitasking only in the US sample. This may be due to European students being less prone to ''disruptive'' multitasking. The results provide valuable cautionary information about the impact of multitasking and using SNSs in a learning environment on university students' GPAs. <s> BIB004 </s> Social networking and academic performance: A review <s> Discussion <s> Abstract This study investigated how social networking sites (SNSs) use by Chinese international students in Japan influenced their perceived social capital and psychological well-being. In addition, it examined how, as sojourners, Chinese international students' perceived acculturative stress varied. Data were collected from 142 Chinese international students. The results indicated that the intensity of SNS use was unable to predict individuals' perceived social capital and psychological well-being. The effect of SNS use varied according to the functions it serves. Specifically, SNS use for social and informational functions (SIF) increased individuals' levels of perceived bridging social capital and perceived life satisfaction, while SNS use for entertaining recreational functions (ERF) was unable to predict perceived social capital but increased individuals' levels of loneliness. It was also found that, in the intercultural environment, Chinese international students' levels of perceived acculturative ... <s> BIB005 </s> Social networking and academic performance: A review <s> Discussion <s> As technology use continues its steady growth among college students, both within and outside of the classroom, its effect on academic performance becomes an increasingly important question to address. Cognitive theory and multitasking research strongly support a negative effect while other studies have found little to no effect. Using a large sample of students, this study attempted to address these opposing results and help find clarity. We explored the relationship of the frequency of students' use of technologies and their academic performance as measured by GPA, SAT scores, study hours, and predicted course grade. In order to help understand our findings, we also examined the role of gender and employment status in this relationship. Our hypotheses were partially supported, with frequency of technology use negatively related to academic performance. We discuss our findings and limitations of the research. <s> BIB006 </s> Social networking and academic performance: A review <s> Discussion <s> A mixed-methods study of academic performance and Facebook among U.S.-based college students.A qualitative account of how a U.S.-sample of college students conferred meanings to their academic lives on Facebook.Academic performance may determine college students' Facebook use, rather than the reverse.Those with lower GPAs may disclose negative affective states in their academic disclosure on Facebook. This paper uses a mixed-methods approach to examine the relation between online academic disclosure and academic performance. A multi-ethnic sample of college students (N=261; male=66; female=195; M age ź 22years) responded to open-ended questions about their Facebook use. Thematic analysis revealed that over 14% of the Facebook wall posts/status updates (N=714) contained academic themes; positive states were more frequent than negative and neutral states and students with lower GPAs expressed negative states more often. A path analysis suggested that academic performance may determine college students' Facebook use, rather than the reverse. Implications for student support services are discussed. <s> BIB007 </s> Social networking and academic performance: A review <s> Discussion <s> This paper presents, for further discussion, a review of the scientific literature produced internationally on the use of Social Network Sites (SNS) in different levels of education and settings. A total of 62 articles published in international scientific journals with peer review have been analysed. The main objective of this paper is to discuss the most recurrent lines of research since the emergence of these tools in education. The level of education in which research is carried out is also analysed, as well as the paradigms and methods used for data collection. The analysis leads to the conclusion that most of the studies analysed have been carried out in higher education, from a quantitative paradigm, focusing on the use of SNS as educational tools. This article shows that research into SNS in education is at an early stage of development, and it demonstrates the need to widen lines of research on SNS in media education to include as yet unexplored dimensions. <s> BIB008

We stated early in the paper that a cursory search of the literature revealed strikingly divergent findings. Having arrived at the finish line, the initial assessment has not changed. Despite the steadily growing scholarship examining the topic under consideration, the review, consisting of 23 papers, leaves us at a juncture where the guiding question posed remains unresolved and is, instead, open for further exploration and elaboration. Despite this, the review does help bring the disparate pieces and discussion together to at least provide a better picture of what researchers have been working with-offering a foundation for future efforts. While the mixed evidence base might be disheartening, the progress made so far in the explication of the topic is commendable. In view of the above, and in keeping with the short history of the evidence base, we begin the discussion by calling for the need to contextualize the influence of SNS use on academic performance, and to avoid making any sweeping generalizations from the still early literature on this topic. Rooting the argument in the bad or good influences of SNS as it relates to academic performance must be done with caution because of the ramifications such arguments may lead to-such as imposing various adherences to undercooked evidence, or worse, contributing to a narrative that overtakes the evidence. Even if one proceeds from the notion that SNS have a negative impact on academic performance, the ramifications ought to, in some ways, be rationalized by considering the various positive benefits accorded to users. Further, simply trying to draw linear and/or direct connections between the two constructs could impinge on our understanding and assessment of the influence of SNS. BIB005 put it aptly, Bthere is no absolute standard to decide whether using SNSs is a good thing because it is not one dimensional^(p.2). Then, a consideration of Bnot [just] the intensity but the functions of SNS use^ (Guo et al. 2014, p.4 ) might provide a broader perspective and, may even help tease apart the mechanics of the relationship. So while the efforts to show how the constructs are related is commendable-which is what the extant literature has and continues to do-it is also important to be able to explain the why of this particular phenomenon, and that is where the challenge lies. But whereas the review provides some developing understanding of the topic, some open questions surface. Given the renewed recognition for the importance of non-significant findings, of note in this review is the appearance, albeit rather modest, of such findings; given that it is less frequent to find non-significant findings reported in the literature, the mere fact that such findings found home in such a small body of work is notable. While the negative stance in the evidence base of this review connects in some ways to the larger body of work examining the association between general technology use and academic performance (e.g., BIB006 , in light of the mixed evidence, further robust findings are warranted. A notable detection was the heterogeneity in the pool of studies; this we contend could be a function of the early nature of the topic and the lack of theoretical frameworks guiding the research. In the review, we see some openings for future research, which we now discuss. Prior work has generally relied on cross-sectional design, which while helpful, only permits static explications of relationships. Future explorations should consider longitudinal designs-which has been mostly overlooked in the studies-to better capture the temporal arrangement of the two constructs and the causal effects, if any, of SNS use over time. Such extensions take on special importance as it helps alleviate any sweeping generalizations about the causality of the constructs made with crosssectional attempts. Further, the approach for examination of the relationship has generally been from SNS use to academic performance; BIB007 flipped the script and suggested a reexamination of the relationship in the other direction. Such an alternative view, while seemingly anomalous, is interesting in that it provides a new perspective on the formulation of the relationship, and thus, deserves careful consideration to gain deeper insights. BIB008 note that the literature on SNS use by students has highlighted that students generally tend to use SNS for social and/or recreational purposes; by contrast, others have noted that students do engage in activities that can be Bacademic^ BIB001 . Thus, future efforts could examine the effects of students' use of SNS as platforms for educational endeavors and assess if there is any value add in such activities, and consequently what the influence of such use is on academic performance. We acknowledge that different SNS have different features and are used differently; thus, providing a direction for future research that could examine the SNS-academic performance nexus for individual platforms. Most studies did not report the length of experience and the number of SNS memberships that the participants had; this review reveals a need for additional research that considers the influence of such contextual factors. The existing work has generally only presented the influence of SNS on academic performance in terms of how one construct is related to another; thus, additional work is warranted to better explicate the mechanisms through which the SNS-academic performance nexus function. Another challenge in this stream of work as identified by BIB002 relates to the difficulty in distinguishing Bbetween various types of media use in order to estimate associations with academic performance^(p.275); thus, highlighting the complication in boxing the influence of SNS on academic outcomes. As pointed out in the literature, inconsistencies in findings begin to surface when a phenomenon is tested in cross-cultural contexts, which may be attributable to cultural, technological, and economic differences . Given the preponderance of investigations in the US, it is crucial to address this question in more diverse settings and contexts. As an illustration, BIB004 tested the role of multitasking as a moderating variable in the link between SNS use and academic performance-the evidence suggested that the moderating role of multitasking was only significant in the US sample and not in the European sample. The review also found that a notable missing angle concerned the lack of approaches that leverage theoretical frameworks to investigate the link. We may be well served by future efforts that are guided by relevant theoretical groundings. Finally, given the recent calls encouraging scholars to consider replicating published studies (e.g., BIB003 Maxwell, Lau, & Howard, 2015) , we agree that the topic would benefit from replication studies as, Bresearch findings require replication because of their influence, not despite it^ (Makel et al. 2012, p.5) . Maxwell, Lau, and Howard (2015) go one step further and exhort that Bthat multiple replication studies will often be needed to resolve apparent inconsistencies in the literature^(p.495). Given the emergent nature of this stream of work, it is difficult to pin a particular explanation for the elusiveness of an accordant answer to the question of the association. Likewise, drawing any strong conclusions may be hasty in light of both the short innings and little generalizable evidence that the topic enjoys. In line with this, we contend that time and accumulating investigations in this stream of work ought to help reconcile the apparent tension in the findings that this review has detected. With that, it also behooves such a review to highlight the caveats attached to this review-we hope that offering these limitations can also provide some leverage and openings for future research. Of course, the data collection method employed in this review may not have fully captured all the relevant articles; such a possibility is a natural concern. On a related point, we focused on the general use of SNS and the link to academic performance; such an emphasis may have led to exclusion of some articles of peripheral relevance. Furthermore, our strict inclusion criterion of only using peer-reviewed journal articles constrains the representation of findings; other publication types could be worth considering for expanding the review tent to provide a broader perspective on the topics. Likewise, we should also note that since we only reviewed published work, the potential for publication bias must be considered. As such, before distilling any hard insights from the review, we caution readers to be cognizant of an important caveat attached to this review which concerns the representative, not exhaustive, nature of the endeavor. The present review focused strictly on empirical works, and thus, suffers from the lack of complementary insights on both the qualifications and contexts afforded by qualitative studies. Furthermore, a single author conducted the literature search and coding of the articles; future review efforts could benefit from having multiple eyes and perspectives to strengthen the rigor of this research.

Gross substitutability: An algorithmic survey <s> <s> Competitive adjustment processes in labor markets with perfect information but heterogeneous firms and workers are studied. Generalizing results of Shapley and Shubik [7], and of Crawford and Knoer [1], we show that equilibrium in such markets exists and is stable, in spite of workers' discrete choices among jobs, provided that all workers are gross substitutes from each firm's standpoint. We also generalize Gale and Shapley's [3] result that the equilibrium to which the adjustment process converges is biased in favor of agents on the side of the market that makes offers, beyond the class of economies to which it was extended by Crawford and Knoer [1]. Finally, we use our techniques to establish the existence of equilibrium in a wider class of markets, and some sensible comparative statics results about the effects of adding agents to the market are obtained. THE ARROW-DEBREU THEORY of general economic equilibrium has long been recognized as a powerful and elegant tool for the analysis of resource allocation in market economies. Not all markets fit equally well into the Arrow-Debreu framework, however. Consider, for example, the labor market-or the housing market, which provides an equally good example for most of our purposes. Essential features of the labor market are pervasive uncertainty about market opportunities on the part of participants, extensive heterogeneity, in the sense that job satisfaction and productivity generally differ (and are expected to differ) interactively and significantly across workers and jobs, and large set-up costs and returns to specialization that typically limit workers to one job. All of these features can be fitted formally into the Arrow-Debreu framework. State-contingent general equilibrium theory, for example, provides a starting point for studying the effects of uncertainty. But this analysis has been made richer and its explanatory power broadened by the examination of equilibrium with incomplete markets, search theory, and market signaling theory. The purpose of this paper is to attempt some improvements in another dimension: we study the outcome of competitive sorting processes in markets where complete heterogeneity prevails (or may prevail). To do this, we take as given the implications of set-up costs and returns to specialization by assuming that, while firms can hire any number of workers, workers can take at most one job. We also return to the simplification of perfect information. In the customary view of competitive markets, agents take market prices as given and respond noncooperatively to them. In this framework equilibrium cannot exist in general unless the goods traded in each market are truly homogeneous; heterogeneity therefore generally requires a very large number of markets. And since these markets are necessarily extremely thin-in many cases containing only a single agent on each side-the traditional stories supporting the plausibility of price-taking behavior are quite strained. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> <s> In this paper we study the problem of efficient production and allocation of indivisible objects among a set of consumers. We assume that the agents’ preferences depend on the bundle of objects and the quantity of money they consume. Furthermore, we assume that preferences are quasilinear in money, and that agents have a large endowment of money. With indivisibilities, it is well-known that many familiar properties of the utility functions fail to ensure existence. In their striking analysis of the matching problem, Kelso and Crawford [5] introduce the gross substitutes (GS) condition which ensures the non-emptiness of the core. We propose two new conditions, and show that with quasilinearity they are equivalent Article ID jeth.1999.2531, available online at http:www.idealibrary.com on <s> BIB002

1. Gross substitutes and Walrasian tatônnement. The notion of gross substitutes was introduced by Kelso and Crawford BIB001 in order to analyze two sided matching markets of workers and firms. Originally it was defined as a condition on the gross product generated by a set of workers for a given firm, hence the name gross substitutes. Such condition allowed a natural salary adjustment process to converge to a point where each worker is hired by some firm and no worker is over-demanded. Gul and Stacchetti BIB002 later use the same notion to analyze the existence of price equilibria in markets with indivisible goods. For this survey, we adopt the Gul and Stacchetti terminology and talk about buyers/items/prices instead of firms/workers/salaries as in Kelso and Crawford. Before we proceed, we fix some notation: we denote by [n] = {1, . . . , n} a set of items (goods). A valuation over such items is a function v : 2 [n] → R such that v(∅) = 0 1 . Given a price vector p ∈ R n and a set S ⊆ [n], we denote p(S) = j∈S p j . We will define v p (S) = v(S) − p(S) as the value of a subset S under the price vector p. This corresponds to the utility of an agent with this valuation for acquiring such set under those prices. Given disjoint sets S, T we define the marginal value of T with respect to S as v(T |S) = v(T ∪ S) − v(S). We sometimes omit braces in the representation of sets when this is clear from the context, for example, by v(i, j|S) we denote v({i, j}|S). An economy with indivisible goods is composed by a set [n] of items (goods) and [m] of buyers (agents) where each agent i ∈ [m] has a valuation v i : 2 [n] → R. We use the notion of the demand correspondence to define an equilibrium of this economy: Definition 1.1 (demand correspondence). Given a valuation function v : 2 [n] → R and a vector of prices p ∈ R n , we define the demand correspondence as the family of sets that maximize the utility of an agent under a price vector p :

Gross substitutability: An algorithmic survey <s> Theorem 1.4 (Kelso and Crawford <s> We propose a massively parallelizable algorithm for the classical assignment problem. The algorithm operates like an auction whereby unassigned persons bid simultaneously for objects thereby raising their prices. Once all bids are in, objects are awarded to the highest bidder. The algorithm can also be interpreted as a Jacobi — like relaxation method for solving a dual problem. Its (sequential) worst — case complexity, for a particular implementation that uses scaling, is O(NAlog(NC)), where N is the number of persons, A is the number of pairs of persons and objects that can be assigned to each other, and C is the maximum absolute object value. Computational results show that, for large problems, the algorithm is competitive with existing methods even without the benefit of parallelism. When executed on a parallel machine, the algorithm exhibits substantial speedup. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> Theorem 1.4 (Kelso and Crawford <s> In this paper we study the problem of efficient production and allocation of indivisible objects among a set of consumers. We assume that the agents’ preferences depend on the bundle of objects and the quantity of money they consume. Furthermore, we assume that preferences are quasilinear in money, and that agents have a large endowment of money. With indivisibilities, it is well-known that many familiar properties of the utility functions fail to ensure existence. In their striking analysis of the matching problem, Kelso and Crawford [5] introduce the gross substitutes (GS) condition which ensures the non-emptiness of the core. We propose two new conditions, and show that with quasilinearity they are equivalent Article ID jeth.1999.2531, available online at http:www.idealibrary.com on <s> BIB002 </s> Gross substitutability: An algorithmic survey <s> Theorem 1.4 (Kelso and Crawford <s> In most of microeconomic theory, consumers are assumed to exhibit decreasing marginal utilities. This paper considers combinatorial auctions among such submodular buyers. The valuations of such buyers are placed within a hierarchy of valuations that exhibit no complementarities, a hierarchy that includes also OR and XOR combinations of singleton valuations, and valuations satisfying the gross substitutes property. Those last valuations are shown to form a zero-measure subset of the submodular valuations that have positive measure. While we show that the allocation problem among submodular valuations is NP-hard, we present an efficient greedy 2-approximation algorithm for this case and generalize it to the case of limited complementarities. No such approximation algorithm exists in a setting allowing for arbitrary complementarities. Some results about strategic aspects of combinatorial auctions among players with decreasing marginal utilities are also presented. <s> BIB003 </s> Gross substitutability: An algorithmic survey <s> Theorem 1.4 (Kelso and Crawford <s> March 2002 A family of ascending package auction models is introduced in which bidders may determine their own packages on which to bid. In the proxy auction (revelation game) versions, the outcome is a point in the core of the exchange economy for the reported preferences. When payoffs are linear in money and goods are substitutes, sincere reporting constitutes a Nash equilibrium and the outcome coincides with the Vickrey auction outcome. Even when goods are not substitutes, ascending proxy auction equilibria lie in the core with respect to the true preferences. Compared to the Vickrey auction, the proxy auctions generate higher equilibrium revenues, are less vulnerable to collusion, can handle budget constraints much more robustly, and may provide better ex ante investment incentives. Working Papers Index <s> BIB004 </s> Gross substitutability: An algorithmic survey <s> Theorem 1.4 (Kelso and Crawford <s> There has been much interest in the machine learning and algorithmic game theory communities on understanding and using submodular functions. Despite this substantial interest, little is known about their learnability from data. Motivated by applications, such as pricing goods in economics, this paper considers PAC-style learning of submodular functions in a distributional setting. A problem instance consists of a distribution on {0,1}n and a real-valued function on {0,1}n that is non-negative, monotone, and submodular. We are given poly(n) samples from this distribution, along with the values of the function at those sample points. The task is to approximate the value of the function to within a multiplicative factor at subsequent sample points drawn from the same distribution, with sufficiently high probability. We develop the first theoretical analysis of this problem, proving a number of important and nearly tight results. For instance, if the underlying distribution is a product distribution then we give a learning algorithm that achieves a constant-factor approximation (under some assumptions). However, for general distributions we provide a surprising Omega(n1/3) lower bound based on a new interesting class of matroids and we also show a O(n1/2) upper bound. Our work combines central issues in optimization (submodular functions and matroids) with central topics in learning (distributional learning and PAC-style analyses) and with central concepts in pseudo-randomness (lossless expander graphs). Our analysis involves a twist on the usual learning theory models and uncovers some interesting structural and extremal properties of submodular functions, which we suspect are likely to be useful in other contexts. In particular, to prove our general lower bound, we use lossless expanders to construct a new family of matroids which can take wildly varying rank values on superpolynomially many sets; no such construction was previously known. This construction shows unexpected extremal properties of submodular functions. <s> BIB005

In some sense, gross substitutability is also necessary for the existence of Walrasian equilibria. Gul and Stacchetti BIB002 show the following: let C be a class of valuation functions that contains all unit demand valuations, i.e., all valuations of the type v(S) = max j∈S v({j}). Then if C is such that for all v 1 , . . . , v m ∈ C there is a Walrasian equilibrium, then all valuations in C are gross substitutes. 2. Examples, Non-Examples and Submodularity. It is instructive to have in mind a couple of examples and non-examples of gross substitute functions, to guide our intuition in the following sections. Three simple classes of valuations that can be readily recognized as gross substitutes from Definition 1.3 are: 1. additive valuations, i.e., valuations for which v(S) = i∈S v({i}) 2. unit-demand valuations, i.e., valuations for which v(S) = max i∈S v({i}) 3. symmetric concave valuations, i.e., valuations of the form v(S) = f (|S|) for some monotone concave function f : R + → R + . A less obvious example is the class of valuations known as assignment valuations introduced by Shapley , also called OXS valuations by Lehmann, Lehmann and Nisan BIB003 . An assignment valuation with n items can be represented as an n × k matrix V with non-negative entries, where each row corresponds to an item and each column to a position. Entry V i,j corresponds to the value of item i in position j. Each item can be assigned to a single position and each position can be assigned only one item. The value of a subset S of items is the value of the optimal assignment of items in S to positions. Items and positions are allowed to be left unassigned. Mathematically: k x kj ≤ 1, k x ik ≤ 1 and x ij ∈ {0, 1}, ∀i, j} We postpone until Section 8 a proof that all assignment valuations are gross substitutes, but we remark that unit-demand functions are the special case where there is only one position, additive functions correspond to the case where the matrix is an n × n diagonal matrix and the symmetric concave valuations to the case in which all the rows of the matrix are equal. Hatfield and Milgrom define a generalization of assignment valuations, which they call endowed assignment valuations. The show that this class also satisfied the gross substitutes property and that it captures most practical applications of gross substitutes valuations in matching markets. A valuation v on n items is said to be an endowed assignment valuation if there is an assignment valuation w on a larger set of items such that v(S) = w(S|T ). Another important example of gross substitutes is the class of matroid rank functions. This survey won't formally require any knowledge of matroid theory, basic familiarity with matroids will be useful to guide the reader's intuition later on. The topic is too extensive to survey here, but we point to Lawler , Oxley or Schrijver for a comprehensive discussion. We remark that even though matroid rank functions are gross substitute valuations, sums of matroid rank functions might not be. For example, given three items {a, b, c} define for each i ∈ {a, b, c}, the function r i : 2 {a,b,c} → R such that: , and r i (S) = 2 for all remaining subsets S. Notice that for each i, r i is a matroid rank function and hence satisfy the gross substitutability. However, the valuation v = r a + 2 · r b + 3 · r c does not satisfy gross substitutability. In order to see that, observe that for the price vector p = BIB005 BIB001 BIB005 , D(v; p) = {{a}, {c}, {a, c}, {b, c}}. If the price of item c increases to ∞, i.e., p = [4, 5, ∞], then demand set becomes D(v; p ) = {{a}}. So the increase in price of item c makes item b no longer belong to any demanded set, violating Definition 1.3. Gul and Stachetti BIB002 observe that gross substitutes are a subclass of submodular functions: Definition 2.1 (submodularity). A valuation function is said to be submodular if for all subsets S, Theorem 2.2 (Gul and Stacchetti BIB002 ). Every gross substitute valuation function is submodular. Proof. Let v be a gross substitute valuation functions. Given S ⊆ [n] and i, j / ∈ S, consider the price vector 3 such that p t = ∞ for t / ∈ S ∪ {i, j}, p t = −∞ for t ∈ S ∪ {j} and p i = v(i|S ∪ {j}). Clearly S ∪ {i, j} ∈ D(v, p). Now, if one defines p such that p j = ∞ and p t = p t for all other t, then by gross substitutability, S ∪ {i} must be a demanded set. Therefore: v(i|S) ≥ v(i|S ∪ {j}). Since the example v = r a +2·r b +3·r c earlier in this section is the sum of matroid rank functions, and hence submodular, it is clear that gross substitutes is a strict subclass of submodular functions. Another good source of examples of submodular but not gross substitute functions comes from the class of budget additive functions. We say that a valuation function is budget additive if it is of the form: v(S) = min{B, i∈S w i } for non-negative real numbers B, w 1 , . . . , w n . The following example due to Lehmann, Lehmann and Nisan BIB003 shows function that is budget additive but not gross substitutes: consider three items {a, b, c} with weights w a = w b = 1 and w c = 2 and budget B = 2. In order to see that the associated budget additive function is not gross substitutes, notice that for the prices p = [ BIB004 , 1], then the demand correspondence becomes D(v, p ) = {{c}}, i.e., the increase in the price of a makes item b be no longer demanded. 3. Gross substitutes, greedy demand oracles and local search. An important property of gross substitute valuations is that it is the exact class of functions for which demand oracles can be computed via a greedy or local search algorithms. In this section we make this statement precise by defining matroidal maps and discrete concave valuations. The first part of the survey will be devoted to show that those three concepts are equivalent. An algorithmic primitive needed to implement the Walrasian tatônnement procedure is the computation of a set in the demand correspondence X ∈ D(v, p). This is usually referred as the demand oracle problem. A simple heuristic to compute demand oracles is the greedy algorithm: start with the empty set X and keep adding the element j / ∈ X that gives the maximum improvement to v p (X). In other words: Definition 3.1 (matroidal map). A valuation function is a matroidal map if for all price vectors p ∈ R n , the greedy algorithm implements a demand oracle, i.e., is the output of Algorithm 2. Before we proceed with the task of proving the previous theorem, we also mention 3 allowing prices to take values ∞ and −∞ simplifies the arguments. To be more precise, one can view such prices as M or −M for M = 1 + max S v(S).

Gross substitutability: An algorithmic survey <s> 4. <s> In labor market models as well as in exchange economies with indivisible goods gross substitutability is used as a property to guarantee the existence of competitive equilibria. This paper develops an easy way to check gross substitutability for utility functions concerning a finite set of indivisible goods (or employees) and money. Concavity is one of the conditions that has to be satisfied. Only one other, but similar, type of relation must be checked to guarantee gross substitutability. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> 4. <s> In most of microeconomic theory, consumers are assumed to exhibit decreasing marginal utilities. This paper considers combinatorial auctions among such submodular buyers. The valuations of such buyers are placed within a hierarchy of valuations that exhibit no complementarities, a hierarchy that includes also OR and XOR combinations of singleton valuations, and valuations satisfying the gross substitutes property. Those last valuations are shown to form a zero-measure subset of the submodular valuations that have positive measure. While we show that the allocation problem among submodular valuations is NP-hard, we present an efficient greedy 2-approximation algorithm for this case and generalize it to the case of limited complementarities. No such approximation algorithm exists in a setting allowing for arbitrary complementarities. Some results about strategic aspects of combinatorial auctions among players with decreasing marginal utilities are also presented. <s> BIB002 </s> Gross substitutability: An algorithmic survey <s> 4. <s> In their 1982 article, Kelso and Crawford proposed a gross substitutes condition for the existence of core (and equilibrium) in a two-sided matching model. Since then, this condition has often been used in the literature on matching models and equilibrium models in the presence of indivisibilities. In this paper we prove that a reservation value (or utility) function satisfies the gross substitutes condition if and only if it is anM?-concave function defined on the unit-hypercube, which is a discrete concave function recently introduced by Murota and Shioura (1999). <s> BIB003 </s> Gross substitutability: An algorithmic survey <s> 4. <s> We propose two different methods for presenting substitutes (a.k.a. gross-substitutes) valuations. Each provides short descriptions for a family of substitutes valuations. We also show that substitutes valuation are closed under k-satiation. <s> BIB004

A price independent local characterization. We make a brief detour and look at a different, yet very related question about gross substitutes. In the previous section we gave two alternative algorithmic characterizations of gross substitutes. All characterizations given so far involve prices, i.e., they are of the form: a valuation v satisfied the gross substitutes property if for all price vectors p, the pair (v, p) has some given property. The question of giving an explicit characterization of gross substitutes was resolved simultaneously by Fujishige and Yang BIB003 and Reijnierse, Gellekom and Potters BIB001 . The first paper provides a powerful connection to the theory of Discrete Convex Analysis, which we discuss in more detail in Section 7. We focus first on the definition given by Reijnierse et al BIB001 . Theorem 4.1 (Reijnierse, Gellekom and Potters BIB001 ). A valuation function has the gross substitutes property iff it is submodular and for all sets S ⊆ [n] and all distinct i, j, k / ∈ S, the following holds: Proof sketch. The high level picture of their proof is quite simple and illuminating. Here we provide a brief sketch of it. First, they show that a function doesn't have the gross substitutes property iff it is possible to find the following certificate: a price vector p ∈ R n such that Notice that the existence of such certificate clearly shows the violation of gross substitutability: the increase in the price of j would make the demand set become {S} in case (i) and {S ∪ {k}} in case (ii). Therefore, item i would no longer be in the demand set. Proving the other direction requires more work, but its essence is to search for a minimal violation of gross substitutability by starting with an arbitrary one and changing the price vector so to shrink the demand set until it is minimal. The second step is to transform the existence of certificates as above in simple conditions on v. This is based on two observations: There exists a certificate of type (i) iff the v is not submodular. There exists a certificate of type (ii) iff there is a violation of condition (RGP). First, assume that we have a certificate of type (i). So, there are prices such that which is a violation of submodularity. Conversely, if you have a violation of submodularity for i, j, S, take p t = −∞ for t ∈ S, p t = ∞ for t / ∈ S ∪ {i, j} and p i = v(i|S) + and p j = v(j|S) + for some tiny and this gives us a certificate of type (i). Assume now that we have a certificate of type (ii). So, there are prices such Summing the inequalities such that the prices cancel, we get: Conversely, if you have a violation of the condition (RGP) for i, j, k, S, let φ > 0 be the value of the violation, i.e., φ = v(i, j|S) + v(k|S) − max{v(i|S) + v(j, k|S), v(j|S) + v(i, k|S)}. Now, consider prices p t = −∞ for t ∈ S, p t = ∞ for t / ∈ S ∪ {i, j, k} and It is straightforward to check that such prices give us a certificate of type (ii). Now we explain what we mean by a local characterization. Given a valuation function v and two sets S, R ⊆ [n] we can define a restriction v R|S : 2 R → R by v R|S (T ) = v(T |S). We say that this is a k-restriction if |R| = k. Observe that usual properties as monotonicity and submodularity are properties of the restrictions, i.e., a function is monotone iff each 1-restriction is monotone. A function is submodular iff each 2-restriction is submodular. A corollary of Theorem 4.1 is that: Corollary 4.2. A valuation function satisfies the gross substitutes property iff every 3-restriction satisfied the gross substitutes property. An equivalent characterization of the one in Theorem 4.1 was also observed in Lehmann, Lehmann and Nisan BIB002 and Bing, Lehmann and Milgrom BIB004 . The latter characterization defines for each valuation v and subset S ⊆ [n] a measure of how two goods i and j are substitutes to each other. Given i, j / ∈ S, let: and observe that (RGP) is equivalent to is a metric satisfying the following stronger version of the triangle inequality: . Such metrics are called ultra-metrics and have the interesting properties that all triangles are isosceles. This translates back to α S as saying that given {i, j, k}, then up to renaming we have: We observe one interesting non-trivial and useful consequence of the isosceles triangle property, which will be useful later: Proof. Let P = {i 1 , i 2 , j 1 , j 2 } and M = min {t1,t2}∈P α S (t 1 , t 2 ). Define the length of the edge between (t 1 , t 2 ) as α S (t 1 , t 2 ). By property (Iso), every triangle is isosceles with the smaller edge appearing at least twice. So, if we look the graph of the edges of minimal length between P , then either one of two things happen: (i) there is a cycle, i.e., there are α S ( In order to see that, let x 1 , x 2 be nodes in P such that α S (x 1 , x 2 ) = M . Let x 3 be some other node, so the triangle x 1 , x 2 , x 3 must have two sides of length then by property (Iso) applied on the triangles x 1 , x 2 , x 4 and x 2 , x 3 , x 4 , we must have α S (x 1 , x 4 ) = α S (x 3 , x 4 ) = M , in which case we are in case (i). Now, proving the statement of the lemma in each of the two cases is simple: if we are in case (i), then we can assume (swapping the names of , which is equivalent to the statement in the lemma. In case (ii), say i 1 is the center of the star, i.e., α . Swapping the names of j 1 and j 2 if necessary, we can assume that: which is equivalent to the statement in the lemma.

Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> Given a finite set E of cardinality, say, N and a map ƒ:P(E) →R ≔ R∪{−∞}, we define a sequence e1, …, eN of elements of E to be ƒ-greedy if ei ∉ {e1, …, ei−1} and ƒ({e1, …, ei}) ≥ ƒ({e1, …, ei−1} ∪ {e}) for all i ∈ {1, …, N} and all e ∈ E⧸{e1, …, ei−1}. In addition, for any map η : E →R we put ƒη(x) ≔ ƒ(x) + Σe∈xη(e) for all x ⊆ E, and we define ƒ to be a well-layered map if for every η : E →R, every ƒη-greedy sequence e1, …, eN ∈ E and every layer Pi(E) ≔ {y ⊆ E | #y = i} (i = 0, …, N) we have ƒη({e1, …, ei}) = max{ƒη(y) | y ∈ Pi(E)}. In this note, we characterize well-layered maps by some appropriate quantified version of the greedoid exchange condition or—equivalently—by some appropriate greedoidal version of the quantitative relations defining valuated matroids and valuated Δ-matroids, respectively. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> Given a set function, that is, a map @?: P(E) -> R @? R @? {-~} from the set P(E) of subsets of a finite set E into the reals including -~, the standard greedy algorithm (GA) for optimizing @? starts with the empty set and then proceeds by enlarging this set greedily, element by element. A set function @? is said to be tractable if in this way a sequence x"0 @? @A, x"1, . . ., x"N @? E (N @? #E) of subsets with max(@?) @? {@?(x"0), @?(x"1), . . ., @?(x"N)} will always be found. In this note, we will reinterpret and transcend the traditions of classical GA-theory (cf., e.g., [KLS]) by establishing necessary and sufficient conditions for a set function @? not just to be tractable as it stands, but to give rise to a whole family of tractable set functions @?(@h) : P(E) -> R : x @? @?(x) + @S"e" "@?" "x@h(e), where @h runs through all real valued weighting schemes @h : E -> R, in which case @? will be called rewarding. In addition, we will characterize two important subclasses of rewarding maps, viz. truncatably rewarding (or well-layered) maps, that is, set functions @? such that [formula] is rewarding for every i = 1, . . ., N, and matroidal maps, that is, set functions @? such that for every @h : E -> R and every @?"e"t"a-greedy sequence x"0, x"1, . . ., x"N as above, one has max(@?"@h) = @?"@h(x"i) for the unique i @? {0, . . ., N} with @?"@h(x"0) = @?"@h(x"i" "+" "1). <s> BIB002 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> Abstract “Convex analysis” is developed for functions defined on integer lattice points. We investigate the class of functions which enjoy a variant of Steinitz's exchange property. This includes linear functions on matroids, valuations on matroids (in the sense of Dress and Wenzel), and separable concave functions on the integral base polytope of submodular systems. It is shown that a function ω has the Steinitz exchange property if and only if it can be extended to a concave function ω such that the maximizers of ( ω +any linear function) form an integral base polytope. A Frenchel-type min–max theorem and discrete separation theorems are established which imply, as immediate consequences, Frank's discrete separation theorem for submodular functions, Edmonds' intersection theorem, Fujishige's Fenchel-type min–max theorem for submodular functions, and also Frank's weight splitting theorem for weighted matroid intersection. <s> BIB003 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> Abstract We study economies (with indivisibilities) that satisfy the gross substitutes (GS) condition. We define an excess demand set with the property that increasing the prices of all goods in excess demand eventually leads to the smallest Walrasian prices. This procedure is a generalization of the auction studied by G. Demange, D. Gale and M. J. Sotomayor, Polit. Econ. 94 (1986), 863–872. In our auction, truthful revelation of demand is a perfect Bayesian equilibrium if the smallest Walrasian prices correspond to the Vickrey–Clarke–Groves payments. However, no dynamic auction can reveal sufficient information to implement the Vickrey mechanism if all GS preferences are allowed. Journal of Economic Literature Classification Numbers: D4, D44, D5, D51. <s> BIB004 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> In labor market models as well as in exchange economies with indivisible goods gross substitutability is used as a property to guarantee the existence of competitive equilibria. This paper develops an easy way to check gross substitutability for utility functions concerning a finite set of indivisible goods (or employees) and money. Concavity is one of the conditions that has to be satisfied. Only one other, but similar, type of relation must be checked to guarantee gross substitutability. <s> BIB005 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> March 2002 A family of ascending package auction models is introduced in which bidders may determine their own packages on which to bid. In the proxy auction (revelation game) versions, the outcome is a point in the core of the exchange economy for the reported preferences. When payoffs are linear in money and goods are substitutes, sincere reporting constitutes a Nash equilibrium and the outcome coincides with the Vickrey auction outcome. Even when goods are not substitutes, ascending proxy auction equilibria lie in the core with respect to the true preferences. Compared to the Vickrey auction, the proxy auctions generate higher equilibrium revenues, are less vulnerable to collusion, can handle budget constraints much more robustly, and may provide better ex ante investment incentives. Working Papers Index <s> BIB006 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> In their 1982 article, Kelso and Crawford proposed a gross substitutes condition for the existence of core (and equilibrium) in a two-sided matching model. Since then, this condition has often been used in the literature on matching models and equilibrium models in the presence of indivisibilities. In this paper we prove that a reservation value (or utility) function satisfies the gross substitutes condition if and only if it is anM?-concave function defined on the unit-hypercube, which is a discrete concave function recently introduced by Murota and Shioura (1999). <s> BIB007 </s> Gross substitutability: An algorithmic survey <s> Definition 5.1 (well-layered map). A function v : 2 <s> There has been much interest in the machine learning and algorithmic game theory communities on understanding and using submodular functions. Despite this substantial interest, little is known about their learnability from data. Motivated by applications, such as pricing goods in economics, this paper considers PAC-style learning of submodular functions in a distributional setting. A problem instance consists of a distribution on {0,1}n and a real-valued function on {0,1}n that is non-negative, monotone, and submodular. We are given poly(n) samples from this distribution, along with the values of the function at those sample points. The task is to approximate the value of the function to within a multiplicative factor at subsequent sample points drawn from the same distribution, with sufficiently high probability. We develop the first theoretical analysis of this problem, proving a number of important and nearly tight results. For instance, if the underlying distribution is a product distribution then we give a learning algorithm that achieves a constant-factor approximation (under some assumptions). However, for general distributions we provide a surprising Omega(n1/3) lower bound based on a new interesting class of matroids and we also show a O(n1/2) upper bound. Our work combines central issues in optimization (submodular functions and matroids) with central topics in learning (distributional learning and PAC-style analyses) and with central concepts in pseudo-randomness (lossless expander graphs). Our analysis involves a twist on the usual learning theory models and uncovers some interesting structural and extremal properties of submodular functions, which we suspect are likely to be useful in other contexts. In particular, to prove our general lower bound, we use lossless expanders to construct a new family of matroids which can take wildly varying rank values on superpolynomially many sets; no such construction was previously known. This construction shows unexpected extremal properties of submodular functions. <s> BIB008

[n] → R is called welllayered iff for each p ∈ R n the sets S 0 , S 1 , S 2 , . . . obtained by the greedy algorithm (i.e., S 0 = ∅ and Theorem 5.2 (Dress and Terhalle BIB001 ). A map v : 2 [n] → R is well-layered iff for any triple of disjoint sets S, {i}, T with |T | ≥ 2, Before we proceed to the proof of Theorem 5.2 it is useful to notice its relation with the condition of Reijnierse et al: Conditions (RGP) and (WL) are equivalent. Proof. One readily recognizes condition (RGP) to be a special case of (WL) with |T | = 2. For the other direction, we show by induction on |T | that (RGP) implies (WL). For |T | = 2, this is trivial. Now, suppose we proved it for |T | = t − 1. Given S, {i}, T with |T | = t, choose k ∈ T minimizing α S (i, k). Then by the induction hypothesis applied to S ∪ k, {i}, T \ k, we have that there is j ∈ T \ k such that: which can be re-written as: now notice that by the choice of k, it must be the case that for all j, α S (i, k) ≤ α S (k, j), since the smaller α S -value in the triangle i, j, k appears twice and . Summing with the previous inequality gives us condition (WL) for |T | = t. Proof of Theorem 5.2. First, assume that the condition (WL) holds and let's prove that S t ∈ argmax S;|S|=t v p (S) by induction on t. The case t = 1 is trivial. Assume we proved for t − 1 and assume there is S with |S | = t and v p (S ) > v p (S t ). Choose such set maximizing k such that {x 1 , . . . , x k } ⊆ S . Since |S | = |S t | = t, clearly k < t. Now, applying (WL) for {x 1 , . . . , x k }, x k+1 , T = S \ {x 1 , . . . , x k } we get that there is j ∈ T such that: For the other direction, assume that (WL) is violated. Since (WL) is equivalent to (RGP), (WL) must be violated by some |T | = {j, k}. So assume S, i, {j, k} for which (WL) is not valid. Now, define prices such that The greedy algorithm will first pick all the elements in S, then i. Now, observe that for t = |S| + 2 the optimal set is either S ∪{i, j}, or S ∪{j, k} or S ∪{i, k}. The fact that (WL) is violated implies that v(i|S) + v(j, k|S) > v(j|S) + v(i, k|S). Substituting v(i|S) and v(j|S) by the prices, we get that (for sufficiently small ) v(j, k|S) e., S ∪ {j, k} is strictly preferable then S ∪ {i, k}. The exact same argument works swapping j and k, so the only set of size |S| + 2 maximizing v p is S ∪ {j, k}. Therefore v can't be a well-layered map, since the greedy algorithm picked i in step |S| + 1. This finishes the proof of Theorem 5.2. The concept of well layered maps guarantees that the greedy algorithm will find the optimal set of each cardinality. In order to guarantee that the greedy algorithm as described in Section 3 will find the optimal, we need to guarantee that once a layer doesn't improve over the previous, we can stop. Dress and Terhalle BIB002 observe that in order for this to happen, it is necessary and sufficient that the valuation is both well-layered and submodular. They call such functions matroidal maps. Proof. Submodularity guarantees that the sets S t will be such that . This guarantees that t → v p (S t ) is concave. For the converse, if a function is not submodular, then there exist i, j, S such that v(i, j|S) > v(i|S) + v(j|S). So one can set prices p t = −∞ for t ∈ S, p t = ∞ for t / ∈ S ∪ {i, j}, p i = v(i|S) + and p j = v(j|S) + for some small . For such prices the greedy algorithm will terminate on S, while the optimum is S ∪ {i, j}. Theorem 5.4 establishes our main claim that Definitions 3.1 and 1.3 are equivalent. In particular, for any gross substitute valuation function, a set S ∈ D(v; p) can be found using the greedy algorithm (Algorithm 2). The following remark states that the greedy algorithms finds not only one, but all demanded sets. This fact is somehow implicit in the proof of Theorem 5.4 but we state and prove it again for completeness: Remark 5.5. Given a gross substitute valuation function v, a price vector p and a demanded set S ∈ D(v; p), then there is a tie breaking rule BIB008 for the greedy algorithm that produces the set S. Formally, if x 1 , x 2 , . . . , x k is an ordering of elements in S such that Proof. This fact follows directly from the (WL) condition. Assume that there is i such that for some v p (y|x 1 , . . . , x i−1 ) > v p (x i |x 1 , . . . , x i−1 ). Since x 1 , . . . , x k are in greedy order, it must be that y / ∈ S. By applying (WL) with sets X i−1 = {x 1 , . . . , x i−1 }, {y}, {x i , . . . , x k }, we get that there is x t for t ≥ i such that: We would like to finish by pointing out that many combinatorial structures such as matroids, polymatroids, valuated matroids , among others, can be defined as the class of objects for which a certain problem admits a greedy solution. For a more extensive exposition on such combinatorial structures we refer to the classical text of Korte, Lovász and Schrader on greedoids [26] . Similar arguments can be used to argue about local search: Theorem 5.6. A valuation function satisfied Definition 3.3 iff it is well-layered and submodular. Proof. First we argue that if a valuation v is well-layered and submodular, then local search can't get stuck in a local maximum for any price p. Then we want to argue that there is S ∈ N where N is the neighborhood of S as in the local search procedure in Section 3. First observe that if v is well-layered and submodular, then v p has also those two properties. We consider three cases: , where the last inequality follows from submodularity. Therefore, there is some i for which v p (i|S) > 0, then we can take S = S ∪ {i}. . Now, we consider two possibilities: (iii-a) If i ∈ S * , we use condition (WL) with S ∩ S * , i, S \ S * . This gives us j ∈ S \ S * such that: By the choice of i, we know that then we can swap i and j and consider the sub-case where i ∈ S. (iii-b) If i ∈ S, we use condition (WL) with S ∩ S * , i, S * \ S. Doing as above, we find j ∈ S * \ S such that , which holds with equality since S * is optimal. This way we obtain an optimal set closer to S. Then we can repeat the above procedure with S * ∪ i \ j instead of S * a finite number of times until we reach some set S ∈ N or we reach one of the previous cases. For the converse direction, we want to show that if a valuation is not well-layered or not submodular, local search can get stuck in suboptimal local minima. For this, we can use the same examples used to show that in such case the greedy algorithm can be suboptimal. We end this section with Figure 5 .1 summarizing the equivalences proved so far. The dashed arrow corresponds to a claim proved in BIB005 but not fully proved in this survey. To make the survey self-contained, we give a direct proof that the concept of matroidal maps (Definition 3.1) implies the original definition of gross substitutes due to Kelso and Crawford (Definition 1.3): Theorem 5.7. Every matroidal map satisfied the gross substitutes condition. Proof. Consider a matroidal map v, a price vector p and a set S ∈ D(v; p). In order to show that the conditions in Definition 1.3 hold, we only need to show for a price vector p such that p i > p i for some i and p j = p j for all j = i, since we can increase prices one at a time. Let S be a set in argmax , then S ∈ D(v; p ) and we are done. Now, consider the case where p i > p i +v p (S)−v p (S ). For this, first definep such thatp i = p i +v p (S)−v p (S ) andp j = p j for all j = i. Clearly S ∈ D(v;p). By Remark 5.5, there is a tie breaking rule for which the greedy algorithm picks S. Since vp(i|S \ i) = 0, we can assume that i is the last element picked under this tie breaking rule. One can use the same tie breaking rule to pick under price vector p . It is straightforward to see that the greedy algorithm behaves the same way as inp until i. Therefore it will choose a set containing S \ i. 6. Duality theorem for gross substitutes. The duality between gross substitutes and submodular functions was observed in many places, as in Fujishige and Yang BIB007 and Murota BIB003 , Gul and Stacchetti BIB004 and Ausubel and Milgrom BIB006 . Given a valuation function v, they consider the utility function u : R n → R which maps a set of prices p ∈ R n to the optimal utility that can be obtained under such prices u(p) = max S v p (S). They relate it to the concept of R n -valued submodular function, which are functions f : , where ∨ and ∧ are the component-wise maximum and minimum respectively. Analogously to submodular set functions, this is equiva- for any δ i , δ j > 0 and i = j, where e i is the i-th coordinate vector. Theorem 6.1 (duality, Ausubel and Milgrom BIB006 ). A valuation function v : 2 [n] → R has the gross substitutes property iff its associate utility u(p) = max S v p (S) is an R n -valued submodular function. Proof. Since u is continuous, it is enough to prove for almost all p, δ i , δ j and then we can extend to all by continuity. Define Γ = ∪ S,T ⊆[n] {p ∈ R n ; v p (S) = v p (T )}. Then Γ is a measure zero subset of R n , since it is a finite collection of hyperplanes. For p / ∈ Γ, |D(v, p)| = 1. For p / ∈ Γ, denote by D(v, p) the unique set demanded at those prices. Given such p, there is ball around p such that for all price vectors, the demand set is the same, so since the increase in p i can't remove j from the demand set by gross substitutability. The converse can be proved by the same argument backwards. A different view of the same duality can be obtained by the characterization of demand sets as basis of a matroid: Theorem 6.2 (duality, Gul and Stacchetti BIB004 ). A valuation function v : 2 [n] → R has the gross substitutes property then for any price p ∈ R n , the set of the demanded sets of minimum size, form the set of basis of a matroid. One characterization of basis-set B ⊂ 2 [n] of matroids is via the exchange property: for any S, T ∈ B and s ∈ S \ T , there is t ∈ T \ S such that S ∪ t \ s ∈ B. We notice that we proved exactly this fact in case (iii) of the proof of Theorem 5.6.

Gross substitutability: An algorithmic survey <s> Connection to Discrete Convex Analysis and Valuated Matroids. <s> Abstract “Convex analysis” is developed for functions defined on integer lattice points. We investigate the class of functions which enjoy a variant of Steinitz's exchange property. This includes linear functions on matroids, valuations on matroids (in the sense of Dress and Wenzel), and separable concave functions on the integral base polytope of submodular systems. It is shown that a function ω has the Steinitz exchange property if and only if it can be extended to a concave function ω such that the maximizers of ( ω +any linear function) form an integral base polytope. A Frenchel-type min–max theorem and discrete separation theorems are established which imply, as immediate consequences, Frank's discrete separation theorem for submodular functions, Edmonds' intersection theorem, Fujishige's Fenchel-type min–max theorem for submodular functions, and also Frank's weight splitting theorem for weighted matroid intersection. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> Connection to Discrete Convex Analysis and Valuated Matroids. <s> The independent assignment problem (or the weighted matroid intersection problem) is extended using Dress and Wenzel's matroid valuations, which are attached to the vertex set of the underlying bipartite graph as an additional weighting. Specifically, the problem considered is as follows: given a bipartite graph $G=(V\sp{+}, V\sp{-}; A)$ with arc weight $w: A \to \hbox{{\bf R}}$ and matroid valuations $\omega\sp{+}$ and $\omega\sp{-}$ on $V\sp{+}$ and $V\sp{-}$, respectively, find a matching $M (\subseteq A)$ that maximizes $ \sum \{w(a) \mid a \in M \} + \omega\sp{+}(\partial\sp{+} M) + \omega\sp{-}(\partial\sp{-} M) $, where $\partial\sp{+} M$ and $\partial\sp{-} M$ denote the sets of vertices in $V\sp{+}$ and $V\sp{-}$ incident to $M$. As natural extensions of the previous results for the independent assignment problem, two optimality criteria are established: one in terms of potentials and the other in terms of negative cycles in an auxiliary graph. <s> BIB002 </s> Gross substitutability: An algorithmic survey <s> Connection to Discrete Convex Analysis and Valuated Matroids. <s> The concept of M-convex function, introduced by Murota 1996, is a quantitative generalization of the set of integral points in an integral base polyhedron as well as an extension of valuated matroid of Dress and Wenzel 1990. In this paper, we extend this concept to functions on generalized polymatroids with a view to providing a unified framework for efficiently solvable nonlinear discrete optimization problems. <s> BIB003 </s> Gross substitutability: An algorithmic survey <s> Connection to Discrete Convex Analysis and Valuated Matroids. <s> In this paper we study the problem of efficient production and allocation of indivisible objects among a set of consumers. We assume that the agents’ preferences depend on the bundle of objects and the quantity of money they consume. Furthermore, we assume that preferences are quasilinear in money, and that agents have a large endowment of money. With indivisibilities, it is well-known that many familiar properties of the utility functions fail to ensure existence. In their striking analysis of the matching problem, Kelso and Crawford [5] introduce the gross substitutes (GS) condition which ensures the non-emptiness of the core. We propose two new conditions, and show that with quasilinearity they are equivalent Article ID jeth.1999.2531, available online at http:www.idealibrary.com on <s> BIB004 </s> Gross substitutability: An algorithmic survey <s> Connection to Discrete Convex Analysis and Valuated Matroids. <s> In their 1982 article, Kelso and Crawford proposed a gross substitutes condition for the existence of core (and equilibrium) in a two-sided matching model. Since then, this condition has often been used in the literature on matching models and equilibrium models in the presence of indivisibilities. In this paper we prove that a reservation value (or utility) function satisfies the gross substitutes condition if and only if it is anM?-concave function defined on the unit-hypercube, which is a discrete concave function recently introduced by Murota and Shioura (1999). <s> BIB005 </s> Gross substitutability: An algorithmic survey <s> Connection to Discrete Convex Analysis and Valuated Matroids. <s> We show that any communication ...nding a Pareto e¢cient allocation in a private-information economy must also discover supporting Lindahl prices. In particular, e¢cient allocation of L indivisible objects requires naming a price for each of the 2Li1 bundles. Furthermore, exponential communication in L is needed just to ensure a higher share of surplus than that realized by auctioning all items as a bundle, or even a higher expected surplus (for some probability distribution over valuations). When the valuations are submodular, e¢ciency still requires exponential communication (and fully polynomial approximation is impossible). When the objects are homogeneous, arbitrarily good approximation is obtained using exponentially less communication than that needed for exact e¢ciency. ¤This paper supersedes our working paper “The Communication Complexity of E¢cient Allocation Problems.” We are grateful to Paul Milgrom, Luis Rayo, Yoav Shoham, Steve Tadelis, Moshe Tenneholtz, and participants of seminars at Harvard-MIT, Princeton, Stanford, Toronto, and the DIMACSWorkshop on Computational Issues in Game Theory and Mechanism Design for useful comments. The ...rst author was supported by a grant from the Israeli Academy of Sciences. The second author was supported by the Guggenheim Foundation and the National Science Foundation, and was hosted by the Institute for Advanced Study in Princeton during his work on the project. yInstitute of Computer Science, Hebrew University, Jerusalem, Israel, email: [email protected] zDepartment of Economics, Stanford University, Stanford, CA 94305; email: [email protected] <s> BIB006

Fujishige and Yang BIB005 showed a powerful connection between gross substitute valuations and the concept of M -concave functions in Discrete Convex Analysis. Discrete Convex Analysis is a theory developed by Murota BIB001 that defines a very general class of functions f : Z n → R on the integral lattice for which it is possible to prove strong duality theorems. Such theorems enable the design of efficient greedy and flow-like algorithmic solutions for various discrete optimization problems involving such functions. Murota and Shioura BIB003 define M -concave functions based on the concept of M -concavity of Murota BIB001 . They originally define M -concave functions on the integral lattice Z n , but for the purposes of this survey, we will consider their restriction to {0, 1} n : Definition 7.1 (M -concave functions, Murota and Shioura BIB003 ). A function v : 2 [n] → R is M -concave if for all S, T ⊆ [n] and s ∈ S \ T , Proof. The fact that M -concavity implies gross substitutability is easy to see, since taking T ⊆ S \ s we recover submodularity: , which is exactly (WL). For the other direction, assume that v is gross substitutes and we want to show is satisfied (M ). First, consider the following transformation: given v : 2 [n] → R, define another valuation function on 2n items by adding n dummy items: ω : . It is straightforward to check that if v is gross substitutes then so does ω. Now we define the condition (M) on ω as follows: for all sets S, T ⊆ [2n] with |S| = |T | and s ∈ S \ T : So, we only need to prove that if ω is a gross substitutes valuation, then it also satisfied (M). For |S \ T | = |T \ S| = 1, the property in trivial. For |S \ T | = |T \ S| = 2, this follows directly from Lemma 4.3. Now we prove by induction on k = |S \ T | = |T \ S|. Fix some arbitrarys ∈ S \ (T ∪ s) and findt ∈ T \ S maximizing ω(T ∪s \t) − ω(S ∪t \ s). Now, apply induction on the sets S and T ∪s \t. We get that there is t ∈ T \ (S ∪t) such that: By the case with k = 2 with sets T and T ∪ {s,s} \ {t,t}, we know that: If the maximum corresponds to the first expression, this together with the previous inequality, gives us exactly what we want to prove, i.e., ω(S) + ω(T ) ≤ ω(S ∪ t \ s) + ω(T ∪s\t), which corresponds to condition (M). If the maximum is the second expression we use the choice oft to see that: ω(T ∪s\t)−ω(S ∪t\s) ≥ ω(T ∪s\t)−ω(S ∪t\s). This together with the previous inequalities also leads to condition (M). Valuated Matroids. The characterization of gross substitutability by the (M ) also connects it to the concept of valuated matroids, due to Dress and Wenzel : ; |S| = k}. We say that a map ω : k → R is a valuated matroid if is satisfied the following version of the exchange property: given S, T ∈ [n] k and s ∈ S \ T , there exists t ∈ T \ S such that: In particular, Theorem 7.2 together with the discussion in its proof imply that: [n] → R satisfies the gross substitutes property iff the map ω : Proof. We will give an algorithmic proof of the previous theorem based on a couple of observations about the Walrasian tatônnement procedure. First note that we can find S ∈ D(v 1 * v 2 , p) by finding a Walrasian equilibrium in an economy with items [n] and three players with valuations v 1 , v 2 , u where u(S) = j∈S p j . Let S 1 , S 2 , U be the partition of the items induced by such equilibrium. Then, this is the partition maximizing In particular, S 1 , S 2 must be the optimal partition of S = S 1 ∪ S 2 among v 1 , v 2 , therefore, S maximizes (v 1 * v 2 )(S) − p(S) and therefore S ∈ D(v 1 * v 2 , p). Second observe that for gross substitute valuations the Walrasian tatônnement procedure (Algorithm 1) always outputs a partition of the goods if we are careful to always select X i ∈ D(v i , p i ) such that S i ⊆ X i . This can be easily implemented by computing X i via the greedy algorithm (Algorithm 2) initialized with X i = S i . Moreover, the partition is such that For rational valuations, we can rescale them such that v i (S i ) are integers. In such case, taking δ < 1 n guarantees that Walrasian tatônnement outputs the optimal allocation. Finally, in the description of Algorithm 1 we initialized the prices as zero and the allocations such that agent 1 initially has all the goods. Notice that it enough to initialize with a price p ∈ R n + and a partition S 1 , . . . , S n such that there is Those observations together can be used to give an elementary proof of Theorem 8.1. We show that v 1 * v 2 satisfy the Definition 1. . Consider a Walrasian equilibrium in the economy formed by v 1 , v 2 , u. Let q be the price vector in such equilibrium. By the Second Welfare Theorem, we take the allocation in equilibrium as S 1 , S 2 , U = [n] \ (S 1 ∪ S 2 ). Let p be a price vector with p ≤ p . We want to show that there is a set X ∈ D(v 1 * v 2 , p ) such that S ∩ {j; p j = p j } ⊆ X. For that, define u (S) = j∈S p j and consider the Walrasian tatônnement procedure for the economy defined by v 1 , v 2 , u . Initialize such procedure with allocation S 1 , S 2 , U and price vector q. This is a valid initialization, since S i ∈ D(v i , q) and also, U ⊆ {j; q j ≤ p j } ∈ D(u , q). Now, let S 1 , S 2 , U be the final outcome of the Walrasian tatônnement procedure. Observe that if j ∈ S 1 ∪ S 2 , then q j ≥ p j , otherwise q wouldn't be Walrasian for v 1 , v 2 , u. Now, if p j = p j , then such item couldn't have been acquired by u , since it would never be in his demand for such price. Therefore A corollary of Theorem 8.1 is that assignment valuations are gross substitutes: it is straightforward from the definition that an assignment valuation function can be written as a convolution of unit-demand functions, one for each right-side node in the bipartite graph. 9. Computing Walrasian Prices for gross substitutes. The problem of computing a Walrasian equilibrium of an economy consisting of n items and m agents with gross substitutes valuations v 1 , . . . , v m has two components: the first is called the welfare problem, which consists in finding a partition S 1 , . . . , S m maximizing The second is the computation of Walrasian prices. There are various approaches for those problems: the perhaps more classical line of approach is to use variations of the tatônnement procedure. Nisan and Segal BIB006 propose a solution that explores properties of gross substitutes to build a suitable linear program. Finally, Murota BIB002 gives a strongly polynomial time algorithm for this problem based on a cycle-canceling approach. We start by discussing how to obtain Walrasian prices from a solution to the welfare problem. The first method is based on an idea by Gul and Stachetti BIB004 : Lemma 9.1 (Gul and Stachetti BIB004 ). Let W be the optimal welfare of an economy with a set [n] of items and agents with gross substitute valuations v 1 , . . . , v m . Also, let W −j be the welfare with the economy with the same agents and items [n] \ j. Then the price vector p with p j = W − W −j is a vector of Walrasian prices for the original economy. The method proposed by Gul and Stachetti to compute Walrasian prices needs access to the optimal allocation for n+1 economies: the original one and the economy after each good is removed. An alternative approach is given by Murota BIB002 . First, consider the following definition: Lemma 9.3 (Murota BIB002 ). If the allocation is optimal, the exchange graph has no negative cycles and therefore, the shortest-path distance is well defined. For each i ∈ [n + m], let φ i be the distance from dummy node d 1 to i. Then φ i ≤ 0 and the vector p i = −φ i is a vector of Walrasian prices. Proof. First assume that the graph has no negative cycles. In such case, the concept of distance is well defined. Given a pair of dummy nodes d i , d j , the weight of the arcs w di,dj = w dj ,di = 0, then φ di = 0 for all dummy nodes. Also, for all items k ∈ S i , the weight of the arc from a dummy node d i to k is w di,k = −v i (S i ∪k)+v i (S i ) ≤ 0, so φ k ≤ 0 for all nodes k. This allows us to define prices as p k = −φ k . Since φ is the shortest path distance, for all j ∈ S i , k / ∈ S i : φ k ≤ φ j + w jk , which is equivalent to: Since k and j are possibly dummy items, this also implies that: The last three inequalities show that S i is a local optimal of the local search procedure (Algorithm 3), hence S i ∈ D(v i , p) by Theorem 5.6. Therefore, p is a vector of Walrasian prices. Now we argue that if the allocation is optimal, then there are no negative cycles. Given an optimal allocation, let p be a vector of Walrasian prices supporting this allocation. Now define φ j = −p j for all j ∈ [n] and φ d = 0 for all dummy items d. By the same argument as above, the fact that p is a vector of Walrasian prices implies that: w ij ≥ φ i − φ j , therefore for every cycle i 1 , i 2 , . . . , i k , i 1 we have:

Gross substitutability: An algorithmic survey <s> The exchange graph has O(n + m) nodes and O((n + m) <s> A collection of items is to be distributed among several bidders, and each bidder is to receive at most one item. Assuming that the bidders place some monetary value on each of the items, it has been shown that there is a unique vector of equilibrium prices that is optimal, in a suitable sense, for the bidders. In this paper we describe two dynamic auction mechanisms: one achieves this equilibrium and the other approximates it to any desired degree of accuracy. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> The exchange graph has O(n + m) nodes and O((n + m) <s> We propose a massively parallelizable algorithm for the classical assignment problem. The algorithm operates like an auction whereby unassigned persons bid simultaneously for objects thereby raising their prices. Once all bids are in, objects are awarded to the highest bidder. The algorithm can also be interpreted as a Jacobi — like relaxation method for solving a dual problem. Its (sequential) worst — case complexity, for a particular implementation that uses scaling, is O(NAlog(NC)), where N is the number of persons, A is the number of pairs of persons and objects that can be assigned to each other, and C is the maximum absolute object value. Computational results show that, for large problems, the algorithm is competitive with existing methods even without the benefit of parallelism. When executed on a parallel machine, the algorithm exhibits substantial speedup. <s> BIB002

2 ) edges. Checking if a graph has negative cycles can be done in time O(N · E) using the Bellman-Ford algorithm, where N is the number of nodes and E is the number of edges. This gives an O((n + m) 3 ) algorithm to compute Walrasian prices for gross substitute valuations given the optimal allocation. 10. Welfare Problem for gross substitutes. Finally, we discuss algorithmic solutions to the welfare problem for gross substitute valuations. Before we start, we mention a couple of important special cases of this problem. If v i (S) = max j∈S w ij for all i ∈ [m], then this is the traditional maximum weighted matching problem. If v i is the rank function of a matroid, then this corresponds to a special case of the matroid intersection problem. For example, the problem of deciding if a graph has k disjoint spanning trees naturally maps to the welfare problem with k agents where the items correspond to edges of the graph and valuation functions correspond to the rank function of the graphical matroid. We discuss three approaches for this problem: tatônnement, linear-programming and cycle canceling. The first approach has a natural economic intuition but yields only an approximation scheme. The second approach produces an exact solution and runs in polynomial time. The third approach is purely combinatorial and yields a strongly polynomial time algorithm. 10.1. Algorithms via the tatônnement procedure. In Section 1, the Walrasian tatônnement procedure (Algorithm 1) was used as a proof device to show the existence of Walrasian equilibria for gross substitute valuations. In this section we discuss how to use it as an actual algorithm. We start by analyzing the running time of Algorithm 1 using the greedy algorithm (initialized with X i = S i ) to compute the demand oracle. Then we discuss variants of the implementation. We assume that v i (S) is an integer (rescaling the input, if necessary) and define . We argued in Section 1 that each price can increase at most M/δ times. This gives a bound of nM/δ on the number of total price increases. In what follows we argue that there are at most m + nM/δ executions of the while loop in Algorithm 1. Consider the following implementation of the while loop: start with a queue containing all the agents 1, . . . , m. At each time, pop agent i from the the queue and compute X i ∈ D(v i , p i ) with S i ⊆ X i . If X i = S i , execute the while loop and for each k = i such that S k changes during the while loop, add k to the queue if he is not already there. Noticed at this point we removed i from the queue. After the execution of the while loop, we don't need to look at i again, unless S i changes, i.e., unless some item j is taken away from i, since by the fact that valuations are gross substitutes and the prices only increase during the process, The previous version runs in pseudo-polynomial time due to the linear dependency on M . This can be easily improved to a dependency on log(M ) by updating the prices in a multiplicative fashion. Initialize all prices to zero and define the following price update rule: U (0) = δ and U (p j ) = p j (1 + δ). So each price is in the set {0, δ, δ(1 + δ), δ(1 + δ) 2 , . . .}. Now, we change Algorithm 1 in two ways: first we calculate p i as p i j = p j if j ∈ S i and p i j = U (p j ) otherwise. Also, when we update prices inside the while loop, we update p j to U (p j ). By the same argument as before, prices never rise past M , so there are at most n · log 1+δ (M ) price updates. This produces a running time of O(n 2 m + 1 δ n 3 log M ). The solution produced is such that . Taking T = S * i (the optimal partition) and summing for all i, we get: Maximum matching. It is illuminating to look at the case of weighted maximum matching, i.e. v i (S) = max j∈S w ij . For this particular case, the Walrasian tatônnement procedure takes the form of the auction method from Bertsekas BIB002 and the ascending auction of Demange, Gale and Sotomayor BIB001 . It also closely resembles Kuhn's Hungarian Method . For this particular case, the demand oracle can be computed in time O(n), which gives us complexity O(

Gross substitutability: An algorithmic survey <s> Linear Programming algorithms. <s> A simple procedure is given for solving minimal cost flow problems in which feasible flows are maintained throughout. It specializes to give primal algorithms for the assignment and transportation problems. Convex cost problems can also be handled. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> Linear Programming algorithms. <s> A classical algorithm for finding a minimum-cost circulation consists of repeatedly finding a residual cycle of negative cost and canceling it by pushing enough flow around the cycle to saturate an arc. We show that a judicious choice of cycles for canceling leads to a polynomial bound on the number of iterations in this algorithm. This gives a very simple strongly polynomial algorithm that uses no scaling. A variant of the algorithm that uses dynamic trees runs in O ( nm (log n ) min{log( nC ), m log n }) time on a network of n vertices, m arcs, and arc costs of maximum absolute value C . This bound is comparable to those of the fastest previously known algorithms. <s> BIB002 </s> Gross substitutability: An algorithmic survey <s> Linear Programming algorithms. <s> Abstract For solving minimum cost flow problems Goldberg and Tarjan [7] prove strongly polynomial bounds on the negative circuit method of Klein [9] which previously was not even known to be finite. Following the proposal of Goldberg and Tarjan, Cui and Fujishige [1] discuss the use of minimum mean circuits for solving the much more general minimum cost submodular flow problem and prove finiteness where the minimum mean circuit is chosen using a secondary criterium. We introduce certain additional positive weights on negative circuits and propose selecting a negative circuit with minimum ration of cost and weight. The resulting method for solving minimum cost submodular flow problems is pseudopolynomial. In fact, it terminates after at most m·U minimum ratio computations where m denotes the number of arcs and U the maximum capacity of an arc. <s> BIB003 </s> Gross substitutability: An algorithmic survey <s> Linear Programming algorithms. <s> The independent assignment problem (or the weighted matroid intersection problem) is extended using Dress and Wenzel's matroid valuations, which are attached to the vertex set of the underlying bipartite graph as an additional weighting. Specifically, the problem considered is as follows: given a bipartite graph $G=(V\sp{+}, V\sp{-}; A)$ with arc weight $w: A \to \hbox{{\bf R}}$ and matroid valuations $\omega\sp{+}$ and $\omega\sp{-}$ on $V\sp{+}$ and $V\sp{-}$, respectively, find a matching $M (\subseteq A)$ that maximizes $ \sum \{w(a) \mid a \in M \} + \omega\sp{+}(\partial\sp{+} M) + \omega\sp{-}(\partial\sp{-} M) $, where $\partial\sp{+} M$ and $\partial\sp{-} M$ denote the sets of vertices in $V\sp{+}$ and $V\sp{-}$ incident to $M$. As natural extensions of the previous results for the independent assignment problem, two optimality criteria are established: one in terms of potentials and the other in terms of negative cycles in an auxiliary graph. <s> BIB004 </s> Gross substitutability: An algorithmic survey <s> Linear Programming algorithms. <s> We show that any communication ...nding a Pareto e¢cient allocation in a private-information economy must also discover supporting Lindahl prices. In particular, e¢cient allocation of L indivisible objects requires naming a price for each of the 2Li1 bundles. Furthermore, exponential communication in L is needed just to ensure a higher share of surplus than that realized by auctioning all items as a bundle, or even a higher expected surplus (for some probability distribution over valuations). When the valuations are submodular, e¢ciency still requires exponential communication (and fully polynomial approximation is impossible). When the objects are homogeneous, arbitrarily good approximation is obtained using exponentially less communication than that needed for exact e¢ciency. ¤This paper supersedes our working paper “The Communication Complexity of E¢cient Allocation Problems.” We are grateful to Paul Milgrom, Luis Rayo, Yoav Shoham, Steve Tadelis, Moshe Tenneholtz, and participants of seminars at Harvard-MIT, Princeton, Stanford, Toronto, and the DIMACSWorkshop on Computational Issues in Game Theory and Mechanism Design for useful comments. The ...rst author was supported by a grant from the Israeli Academy of Sciences. The second author was supported by the Guggenheim Foundation and the National Science Foundation, and was hosted by the Institute for Advanced Study in Princeton during his work on the project. yInstitute of Computer Science, Hebrew University, Jerusalem, Israel, email: [email protected] zDepartment of Economics, Stanford University, Stanford, CA 94305; email: [email protected] <s> BIB005

The second approach, proposed by Nisan and Segal BIB005 , is based on linear programming. They observe that the welfare problem can be cast as the following integer program: Let W LP correspond to the linear programming relaxation of the previous problem, i.e., to the program obtained by relaxing the last constraint to 0 ≤ x iS ≤ 1. Since it is a relaxation, W IP ≤ W LP . Bikhchandani and Mamer observe that when v i are gross substitute valuations, this holds with equality, for the following reason: by the duality theorem in Linear Programming, W LP corresponds to the solution of the following dual program: An optimal solution to the integer program corresponds to the welfare of a Walrasian equilibrium W IP = i v i (S i ). If p are the corresponding Walrasian prices and corresponds to a feasible solution to the dual. Therefore Given this observation, Nisan and Segal propose solving the welfare problem by solving the dual linear program above using a separation based linear programming algorithm, such as the ellipsoid method. The program has n + m variables but an exponential number of constraints. In order to solve it, we need to provide a separation oracle, i.e., an polynomial-time algorithm to decide, for each (u, p) if it is feasible and if not, produce a violated constraint. The problem that the separation oracle needs to solve is to decide for each agent i if u i ≥ max S v i (S) − j∈S p j . For gross substitute valuations, this can be easily solved by the greedy algorithm (Algorithm 2). The algorithm described above computes the value W * = i v i (S i ) of the optimal allocation. In order to compute the optimal allocation itself, one can proceed as follows: fix an arbitrary item j and for all agents i compute the value of the optimal allocation, conditioned on agent i receiving j. In order words, compute the value of the optimal allocation of items [n] \ j to agents with valuationṽ i = v i for i = i and v i (S) = v i (S|j) and let W * j→i be the solution. Then there must be some agent i for which W * = v i (j) + W * j→i . Allocate j to this agent and recursively solve the allocation problem with on [n] \ j for valuationsṽ. 10.3. Cycle-canceling algorithms. Finally we describe a purely combinatorial approach proposed by Murota BIB004 based on the Klein's cycle-canceling technique BIB001 for the minimum-cost flow problem. This approach has the advantage that it leads to a strongly polynomial time algorithm. The algorithm is presented in three parts: first we provide a generic cycle canceling algorithm that finishes in finite time but without additional running time guaranteed. Then, we show that a careful choice of cycles can lead to a (weakly) polynomial time algorithm, following an analysis by Goldberg and Tarjan BIB002 . Finally, an additional idea due to Zimmermann BIB003 leads to a strongly polynomial time algorithm. 10.3.1. Improving the allocation along negative cycles. Murota's optimality criteria (Lemma 9.3) states that an allocation S 1 , . . . , S m is suboptimal iff the exchange graph (Definition 9.2) contains a negative-weight cycle. Let C be this negative weight cycle. First we note that an edge going out of S i (S i augmented with a dummy item) corresponds to the exchange of a (possibly dummy) element a i ∈ S i by an element b i / ∈ S i . The value of the edge corresponds to the change in value for i by replacing a i by b . This observation suggests the following approach to improve the allocation: perform the exchanges prescribed by such cycle, i.e., if

Gross substitutability: An algorithmic survey <s> 10.3.2. <s> Abstract A simple proof is given that limn−t8(log2 log2gn)/n = 1, where gn denotes the number of distinct combinatorial geometries on n point <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> Competitive adjustment processes in labor markets with perfect information but heterogeneous firms and workers are studied. Generalizing results of Shapley and Shubik [7], and of Crawford and Knoer [1], we show that equilibrium in such markets exists and is stable, in spite of workers' discrete choices among jobs, provided that all workers are gross substitutes from each firm's standpoint. We also generalize Gale and Shapley's [3] result that the equilibrium to which the adjustment process converges is biased in favor of agents on the side of the market that makes offers, beyond the class of economies to which it was extended by Crawford and Knoer [1]. Finally, we use our techniques to establish the existence of equilibrium in a wider class of markets, and some sensible comparative statics results about the effects of adding agents to the market are obtained. THE ARROW-DEBREU THEORY of general economic equilibrium has long been recognized as a powerful and elegant tool for the analysis of resource allocation in market economies. Not all markets fit equally well into the Arrow-Debreu framework, however. Consider, for example, the labor market-or the housing market, which provides an equally good example for most of our purposes. Essential features of the labor market are pervasive uncertainty about market opportunities on the part of participants, extensive heterogeneity, in the sense that job satisfaction and productivity generally differ (and are expected to differ) interactively and significantly across workers and jobs, and large set-up costs and returns to specialization that typically limit workers to one job. All of these features can be fitted formally into the Arrow-Debreu framework. State-contingent general equilibrium theory, for example, provides a starting point for studying the effects of uncertainty. But this analysis has been made richer and its explanatory power broadened by the examination of equilibrium with incomplete markets, search theory, and market signaling theory. The purpose of this paper is to attempt some improvements in another dimension: we study the outcome of competitive sorting processes in markets where complete heterogeneity prevails (or may prevail). To do this, we take as given the implications of set-up costs and returns to specialization by assuming that, while firms can hire any number of workers, workers can take at most one job. We also return to the simplification of perfect information. In the customary view of competitive markets, agents take market prices as given and respond noncooperatively to them. In this framework equilibrium cannot exist in general unless the goods traded in each market are truly homogeneous; heterogeneity therefore generally requires a very large number of markets. And since these markets are necessarily extremely thin-in many cases containing only a single agent on each side-the traditional stories supporting the plausibility of price-taking behavior are quite strained. <s> BIB002 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> A classical algorithm for finding a minimum-cost circulation consists of repeatedly finding a residual cycle of negative cost and canceling it by pushing enough flow around the cycle to saturate an arc. We show that a judicious choice of cycles for canceling leads to a polynomial bound on the number of iterations in this algorithm. This gives a very simple strongly polynomial algorithm that uses no scaling. A variant of the algorithm that uses dynamic trees runs in O ( nm (log n ) min{log( nC ), m log n }) time on a network of n vertices, m arcs, and arc costs of maximum absolute value C . This bound is comparable to those of the fastest previously known algorithms. <s> BIB003 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> In this paper we study the problem of efficient production and allocation of indivisible objects among a set of consumers. We assume that the agents’ preferences depend on the bundle of objects and the quantity of money they consume. Furthermore, we assume that preferences are quasilinear in money, and that agents have a large endowment of money. With indivisibilities, it is well-known that many familiar properties of the utility functions fail to ensure existence. In their striking analysis of the matching problem, Kelso and Crawford [5] introduce the gross substitutes (GS) condition which ensures the non-emptiness of the core. We propose two new conditions, and show that with quasilinearity they are equivalent Article ID jeth.1999.2531, available online at http:www.idealibrary.com on <s> BIB004 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> Abstract We study economies (with indivisibilities) that satisfy the gross substitutes (GS) condition. We define an excess demand set with the property that increasing the prices of all goods in excess demand eventually leads to the smallest Walrasian prices. This procedure is a generalization of the auction studied by G. Demange, D. Gale and M. J. Sotomayor, Polit. Econ. 94 (1986), 863–872. In our auction, truthful revelation of demand is a perfect Bayesian equilibrium if the smallest Walrasian prices correspond to the Vickrey–Clarke–Groves payments. However, no dynamic auction can reveal sufficient information to implement the Vickrey mechanism if all GS preferences are allowed. Journal of Economic Literature Classification Numbers: D4, D44, D5, D51. <s> BIB005 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> Preface I. Introduction to Structural Approach --- Overview of the Book 1 Structural Approach to Index of DAE 1.1 Index of differential-algebraic equations 1.2 Graph-theoretic structural approach 1.3 An embarrassing phenomenon 2 What Is Combinatorial Structure? 2.1 Two kinds of numbers 2.2 Descriptor form rather than standard form 2.3 Dimensional analysis 3 Mathematics on Mixed Polynomial Matrices 3.1 Formal definitions 3.2 Resolution of the index problem 3.3 Block-triangular decomposition II. Matrix, Graph and Matroid 4 Matrix 4.1 Polynomial and algebraic independence 4.2 Determinant 4.3 Rank, term-rank and generic-rank 4.4 Block-triangular forms 5 Graph 5.1 Directed graph and bipartite graph 5.2 Jordan-Holder-type theorem for submodular functions 5.3 Dulmage-Mendelsohn decomposition 5.4 Maximum flow and Menger-type linking 5.5 Minimum cost flow and weighted matching 6 Matroid 6.1 From matrix to matroid 6.2 Basic concepts 6.3 Examples 6.4 Basis exchange properties 6.5 Independent matching problem 6.6 Union 6.7 Bimatroid (linking system) III. Physical Observations for Mixed Matrix Formulation 7 Mixed Matrix for Modeling Two Kinds of Numbers 7.1 Two kinds of numbers 7.2 Mixed matrix and mixed polynomial matrix 8 Algebraic Implications of Dimensional Consistency 8.1 Introductory comments 8.2 Dimensioned matrix 8.3 Total unimodularity of dimensioned matrices 9 Physical Matrix 9.1 Physical matrix 9.2 Physical matrices in a dynamical system IV. Theory and Application of Mixed Matrices 10 Mixed Matrix and Layered Mixed Matrix 11 Rank of Mixed Matrices 11.1 Rank identities for LM-matrices 11.2 Rank identities for mixed matrices 11.3 Reduction to independent matching problems 11.4 Algorithms for the rank 11.4.1 Algorithm for LM-matrices 11.4.2 Algorithm for mixed matrices 12 Structural Solvability of Systems of Equations 12.1 Formulation of structural solvability 12.2 Graphical conditions for structural solvability 12.3 Matroidal conditions for structural solvability 13. Combinatorial Canonical Form of LM-matrices 13.1 LM-equivalence 13.2 Theorem of CCF 13.3 Construction of CCF 13.4 Algorithm for CCF 13.5 Decomposition of systems of equations by CCF 13.6 Application of CCF 13.7 CCF over rings 14 Irreducibility of LM-matrices 14.1 Theorems on LM-irreducibility 14.2 Proof of the irreducibility of determinant 15 Decomposition of Mixed Matrices 15.1 LU-decomposition of invertible mixed matrices 15.2 Block-triangularization of general mixed matrices 16 Related Decompositions 16.1 Partition as a matroid union 16.2 Multilayered matrix 16.3 Electrical network with admittance expression 17 Partitioned Matrix 17.1 Definitions 17.2 Existence of proper block-triangularization 17.3 Partial order among blocks 17.4 Generic partitioned matrix 18 Principal Structures of LM-matrices 18.1 Motivations 18.2 Principal structure of submodular systems 18.3 Principal structure of generic matrices 18.4 Vertical principal structure of LM-matrices 18.5 Horizontal principal structure of LM-matrices V. Polynomial Matrix and Valuated Matroid 19 Polynomial/Rational Matrix 19.1 Polynomial matrix and Smith form 19.2 Rational matrix and Smith-McMillan form at infinity 19.3 Matrix pencil and Kronecker form 20 Valuated Matroid 20.1 Introduction 20.2 Examples 20.3 Basic operations 20.4 Greedy algorithms 20.5 Valuated bimatroid 20.6 Induction through bipartite graphs 20.7 Characterizations 20.8 Further exchange properties 20.9 Valuated independent assignment problem 20.10 Optimality criteria 20.10.1 Potential criterion 20.10.2 Negative-cycle criterion 20.10.3 Proof of the optimality criteria 20.10.4 Extension to VIAP(k) 20.11 Application to triple matrix product 20.12 Cycle-canceling algorithms 20.12.1 Algorithms 20.12.2 Validity of the minimum-ratio cycle algorithm 20.13 Augmenting algorithms 20.13.1 Algorithms 20.13.2 Validity of the augmenting algorithm VI. Theory and Application of Mixed Polynomial Matrices 21 Descriptions of Dynamical Systems 21.1 Mixed polynomial mat <s> BIB006 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> In most of microeconomic theory, consumers are assumed to exhibit decreasing marginal utilities. This paper considers combinatorial auctions among such submodular buyers. The valuations of such buyers are placed within a hierarchy of valuations that exhibit no complementarities, a hierarchy that includes also OR and XOR combinations of singleton valuations, and valuations satisfying the gross substitutes property. Those last valuations are shown to form a zero-measure subset of the submodular valuations that have positive measure. While we show that the allocation problem among submodular valuations is NP-hard, we present an efficient greedy 2-approximation algorithm for this case and generalize it to the case of limited complementarities. No such approximation algorithm exists in a setting allowing for arbitrary complementarities. Some results about strategic aspects of combinatorial auctions among players with decreasing marginal utilities are also presented. <s> BIB007 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> This paper discusses two advancements in the theory of designing truthful randomized mechanisms. ::: ::: Our first contribution is a new framework for developing truthful randomized mechanisms. The framework enables the construction of mechanisms with polynomially small failure probability. This is in contrast to previous mechanisms that fail with constant probability. Another appealing feature of the new framework is that bidding truthfully is a stronglydominant strategy. The power of the framework is demonstrated by an $O(\sqrt m)$-mechanism for combinatorial auctions that succeeds with probability $1-O(\frac {\log m} {\sqrt m})$. ::: ::: The other major result of this paper is an O(logmloglogm) randomized truthful mechanism for combinatorial auction with subadditivebidders. The best previously-known truthful mechanism for this setting guaranteed an approximation ratio of $O(\sqrt m)$. En route, the new mechanism also provides the best approximation ratio for combinatorial auctions with submodularbidders currently achieved by truthful mechanisms. <s> BIB008 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> We propose a new Walrasian tatonnement process called a double-track procedure for efficiently allocating multiple heterogeneous indivisible items in two distinct sets to many buyers who view items in the same set as substitutes but items across the two sets as complements. In each round of the process, a Walrasian auctioneer first announces the current prices for all items, buyers respond by reporting their demands at these prices, and then the auctioneer adjusts simultaneously the prices of items in one set upward but those of items in the other set downward. It is shown that this procedure converges globally to a Walrasian equilibrium in finitely many rounds. Copyright 2009 The Econometric Society. <s> BIB009 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> Submodular functions are a key concept in combinatorial optimization. Algorithms that involve submodular functions usually assume that they are given by a (value) oracle. Many interesting problems involving submodular functions can be solved using only polynomially many queries to the oracle, e.g., exact minimization or approximate maximization. ::: ::: In this paper, we consider the problem of approximating a non-negative, monotone, submodular function f on a ground set of size n everywhere, after only poly(n) oracle queries. Our main result is a deterministic algorithm that makes poly(n) oracle queries and derives a function f such that, for every set S, f(S) approximates f(S) within a factor α(n), where α(n) = √n+1 for rank functions of matroids and α(n), = O(√n log n) for general monotone submodular functions. Our result is based on approximately finding a maximum volume inscribed ellipsoid in a symmetrized polymatroid, and the analysis involves various properties of submodular functions and polymatroids. ::: ::: Our algorithm is tight up to logarithmic factors. Indeed, we show that no algorithm can achieve a factor better than Ω(√n/log n), even for rank functions of a matroid. <s> BIB010 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> We consider the problem of maximizing welfare when allocating $m$ items to $n$ players with subadditive utility functions. Our main result is a way of rounding any fractional solution to a linear programming relaxation to this problem so as to give a feasible solution of welfare at least $1/2$ that of the value of the fractional solution. This approximation ratio of $1/2$ is an improvement over an $\Omega(1/\log m)$ ratio of Dobzinski, Nisan, and Schapira [Proceedings of the 37th Annual ACM Symposium on Theory of Computing (Baltimore, MD), ACM, New York, 2005, pp. 610-618]. We also show an approximation ratio of $1-1/e$ when utility functions are fractionally subadditive. A result similar to this last result was previously obtained by Dobzinski and Schapira [Proceedings of the 17th Annual ACM-SIAM Symposium on Discrete Algorithms (Miami, FL), SIAM, Philadelphia, 2006, pp. 1064-1073], but via a different rounding technique that requires the use of a so-called “XOS oracle.” The randomized rounding techniques that we use are oblivious in the sense that they only use the primal solution to the linear program relaxation, but have no access to the actual utility functions of the players. <s> BIB011 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> There has been much interest in the machine learning and algorithmic game theory communities on understanding and using submodular functions. Despite this substantial interest, little is known about their learnability from data. Motivated by applications, such as pricing goods in economics, this paper considers PAC-style learning of submodular functions in a distributional setting. A problem instance consists of a distribution on {0,1}n and a real-valued function on {0,1}n that is non-negative, monotone, and submodular. We are given poly(n) samples from this distribution, along with the values of the function at those sample points. The task is to approximate the value of the function to within a multiplicative factor at subsequent sample points drawn from the same distribution, with sufficiently high probability. We develop the first theoretical analysis of this problem, proving a number of important and nearly tight results. For instance, if the underlying distribution is a product distribution then we give a learning algorithm that achieves a constant-factor approximation (under some assumptions). However, for general distributions we provide a surprising Omega(n1/3) lower bound based on a new interesting class of matroids and we also show a O(n1/2) upper bound. Our work combines central issues in optimization (submodular functions and matroids) with central topics in learning (distributional learning and PAC-style analyses) and with central concepts in pseudo-randomness (lossless expander graphs). Our analysis involves a twist on the usual learning theory models and uncovers some interesting structural and extremal properties of submodular functions, which we suspect are likely to be useful in other contexts. In particular, to prove our general lower bound, we use lossless expanders to construct a new family of matroids which can take wildly varying rank values on superpolynomially many sets; no such construction was previously known. This construction shows unexpected extremal properties of submodular functions. <s> BIB012 </s> Gross substitutability: An algorithmic survey <s> 10.3.2. <s> Motivated by the problem of querying and communicating bidders' valuations in combinatorial auctions, we study how well different classes of set functions can be sketched. More formally, let f be a function mapping subsets of some ground set [n] to the non-negative real numbers. We say that f' is an α-sketch of f if for every set S, the value f'(S) lies between f(S)/α and f(S), and f' can be specified by poly(n) bits. ::: ::: We show that for every subadditive function f there exists an α-sketch where α = n1/2. O(polylog(n)). Furthermore, we provide an algorithm that finds these sketches with a polynomial number of demand queries. This is essentially the best we can hope for since: ::: ::: 1. We show that there exist subadditive functions (in fact, XOS functions) that do not admit an o(n1/2) sketch. (Balcan and Harvey [3] previously showed that there exist functions belonging to the class of substitutes valuations that do not admit an O(n1/3) sketch.) ::: ::: 2. We prove that every deterministic algorithm that accesses the function via value queries only cannot guarantee a sketching ratio better than n1−e. ::: ::: We also show that coverage functions, an interesting subclass of submodular functions, admit arbitrarily good sketches. ::: ::: Finally, we show an interesting connection between sketching and learning. We show that for every class of valuations, if the class admits an α-sketch, then it can be α-approximately learned in the PMAC model of Balcan and Harvey. The bounds we prove are only information-theoretic and do not imply the existence of computationally efficient learning algorithms in general. <s> BIB013

Polynomial running time via canceling minimum mean weight cycles. The cycles we will consider are the cycles with minimum mean weight, i.e., cycles minimizing: w(C)/|C|, where w(C) = e∈C w e . A minimum mean cycle in a directed graph can be found in time O(N ·E) where N is the number of nodes and E is the number of edges using an algorithm 5 due to Karp . We note in particular that the algorithm in Lemma 10.2 can be used to find a cycle minimum mean weight cycle C t satisfying the Unique Minimum Matching Condition, since the cycles C t produced by the algorithm suggested by the lemma are such that: This completes the description of the Minimum mean weight cycle canceling algorithm. In order to show that it runs in weakly polynomial time, we bound the number of iterations using a potential function. Given an allocation S 1 , . . . , S m we define the slackness (S 1 , . . . , S m ) as the minimum ≥ 0 such that there exist a price p, such that for all agents i, a ∈ S i and b / ∈ S i (possibly dummy items), A solution is optimal iff (S 1 , . . . , S m ) = 0. In some sense, this function measures how sub-optimal an allocation is. The presentation is an adaptation of the proof of Goldberg and Tarjan BIB003 for the minimum cost circulation algorithm in strongly polynomial time via cycle canceling. The first lemma relates to the value of the minimum mean-weight cycle: Lemma 10.3 (Goldberg, Tarjan BIB003 ). Given a certain allocation S 1 , . . . , S m , then (S 1 , . . . , S m ) = −µ where µ is the value of the minimum mean weight cycle in the exchange graph corresponding to this allocation. Proof. This can be obtained by writing the natural Linear Program that computes and taking the dual. Given a directed graph (N, E) with costs c ij on edges, consider the program: By the duality theorem, this corresponds to the solution of the following Linear Program: which corresponds to the circulation of minimum mean-weight. Since each circulation x ∈ R E + can be decomposed in weighted sum of cycles x = t α t · z t where α t ∈ R + and z t ∈ R E + is the indicator vector of a cycle. Therefore, there must be a mean-weight circulation that is a cycle and this must be the mean-weight cycle. The following corollary follows from the previous proof and complementarity slackness: Corollary 10.4. Given a certain allocation S 1 , . . . , S m and a price vector with respect to which the allocation is (S 1 , . . . , S m )-optimal, then if C is the mean weight minimum cycle, then along the edges (a, b) of the cycle, w ab + p a − p b = − . In particular, if the cycle satisfies the Unique Minimum Weight Matching Condition, performing the changes prescribed by the cycle produces an improvement of · |C| to the value of the allocation. The next lemma uses this fact to shows that canceling a mean weight negative cycle in Algorithm 4 doesn't increase the slackness (S 1 , . . . , S m ). Lemma 10.5. Given an allocation S 1 , . . . , S m , then if C is a minimum mean weight cycle satisfying the Unique Minimum Weight Condition, then performing the exchanges prescribed by the cycle can't reduce (S 1 , . . . , S m ). Corollary 10.8. In the setting of the previous lemma, after the exchanges are performed, either the new allocation is optimal, or in all negative cycles of the new exchange graph there is at least one edge (a, b) with a ∈ S i and β ib = 1. Proof. If C is a negative cycle in the resulting graph then summing the inequalities w ab + p b − p a ≥ − β i(a)b , then we get 0 > w(C) ≥ − β (C), so β (C) > 0. The previous lemmas suggest the following algorithm: initialize β with β ib = 1 for all i ∈ [m] and b ∈ [n + m]. Then run the cycle canceling algorithm picking the cycle minimizing w(C)/β(C). After canceling the cycle, update β by setting to zero the components β ib according to Lemma 10.7. Corollary 10.8 guarantees that while the allocation is sub-optimal there will be a negative cycle with positive β-value, so the algorithm finishes on an optimal allocation. Given that the chosen cycle in each iteration has at least one edge corresponding to a 1-entry and this is set to zero in the end of each iteration, there are at most (n + m) 2 iterations. Computing a minimum ratio cycle can be done in time where N is the number of edges and E is the number of edges. If T is the cost of evaluating a valuation function, the overall complexity of the algorithm is: O((T · (n + m) 4 + (n + m) 6 log(n + m))). Theorem 10.9 (Murota ). The algorithm that finds a minimum weight mean cycle with minimum number of edges among such cycles (Algorithm 4) is a strongly polynomial time algorithm for the gross substitute welfare problem. We remark that Murota gives a strongly polynomial time algorithm for a more general problem called the Valuated Matroid Intersection problem. In this problem there is a bipartite graph defined by (U, V, E) a weight function w : E → R and gross substitute functions v U : 2 U → R and v V : 2 V → R, find the matching M maximizing where M U and M V are the subsets of U and V covered by the matching. The techniques for solving this problem are very similar to the ones presented in this survey: they involve building an exchange graph and improving the allocation along suitable cycles. Besides the cycle canceling approach, Murota also proposes an additional strongly-polynomial time algorithm based on a flow-like primal-dual approach. 11. Further remarks: Applications, Representation, Approximability and Relations to other classes. Gross substitutes have a wide range of applications in many different fields and therefore it is not surprising that this concept has been rediscovered many times in different contexts. In economics it was originally conceived as a natural condition to express substitutability among workers in two-sided labor markets. To the present date, the original paper by Kelso and Crawford BIB002 has more then 800 citations and the term "gross substitutes" appears in more then 3500 publications according in Google Scholar. A comprehensive exposition on all the work is out of the scope of this survey, but we point the reader to some key generalizations and applications of gross substitutes in economics that will serve as starting point to the reader: Roth , Hatfield and Milgrom , Sun and Yang BIB009 , Gul and Stachetti BIB004 BIB005 and Bikhchandani and Mamer . In Discrete Mathematics, the concept of introduced by Dress and Wenzel to generalize the Grassmann-Plücker relations in p-adic analysis. Their prototypical example of a valuated matroid (which roughly corresponds to a gross substitute valuation defined only on sets of same cardinality, see Section 7 for a complete discussion) is the following function parametrized by a prime number p: ω p : (Q n ) n → Z that associates every n-tuple of n-dimensional rational vectors x 1 , . . . , x n ∈ Q n to: for integers a and b not divisible by p. The greedy algorithm for valuated matroids is presented in as a generalization of the greedy algorithm that finds among a finite set of Q n -vectors a basis minimizing the p-adic valuation of the determinant. We refer to Murota's book BIB006 for a comprehensive survey of applications of valuated matroids and gross substitute valuations to problems in engineering and operations research. We finalize this survey discussing issues around the representation of gross substitute valuations and relations to other classes of valuation functions. We decided to keep the discussion of those points in the further remarks section since those are less well-understood properties, unlike the other topics presented in this survey. Also, unlike the rest of the survey, we assume for this section familiarity of the reader with Matroid Theory . 11.1. Representation and Approximation. One of the central issues in auction design is how to design a language in which agents can represent their valuation in a compact and expressible way. The fact that the demand oracle problem for gross substitute valuations allows for simple and efficient greedy algorithms might raise suspicion that valuations in this class may admit simple and compact representation. Also contributing to this suspicion is the observation by Lehmann, Lehmann and Nisan BIB007 that the set of gross substitute valuations has measure zero with respect to the set of all valuation functions. To make this statement precise, consider the representation of a valuation function v : 2 [n] → R as a vector in R BIB007 follows as a consequence of the isosceles triangle property of the ultra-metric induced by gross substitute valuations (Iso) discussed in Section 4. Equation (Iso) implies that certain linear relations among the components of vector representation of each valuation function must be satisfied. Therefore G(GrossSubstitutes) is contained in the union of finitely many hyperplanes and hence has measure zero. We also know from Section 2 that G(GrossSubstitutes) is not convex. This is in sharp contrast to G(Submodular) which is a full dimensional convex polyhedral set. The observations in the previous paragraph motivate the question of whether gross substitute valuations admit a compact representation. To make this question precise, we need to define what we mean by representation. Intuitively, we will measure the size of a representation as the amount of memory a computer would require to store this valuation function. Also, to avoid discussing how to represent real numbers, we restrict our attention to integer-valued valuations, i.e., valuations of the type v : 2 [n] → Z. Each integer z ∈ Z requires log 2 (z + 1) of memory space in order to store its binary representation. The naive representation of any integer-valued valuation function such that v(S) ≤ M for each S requires at most (2 n − 1) · log 2 (M + 1) space. Certain classes of valuations can be represented with a lot less space, for example, unit demand valuations can be represented by n integers corresponding to the singleton values v({i}) for all i ∈ [n], therefore requiring n · log 2 (M + 1) space. For any class of valuation we can ask the same question: what is the minimum amount of space necessary to represent an integer-valued valuations of this class over n items with values at most M ? The main reason to study this question is because often the representation size is a proxy for complexity and expressivity of a valuation function. An equally compelling reason is that compact representations allow for concise bidding that facilitate auction design. This question is also closely related to counting the number of distinct valuation of this class over n items and value at most M . If V (n, M ) is this number, then it is possible to assign an unique index to each of those valuations and represent each such valuation by its index. Since each valuation is now represented by an index between 1 and V (n, M ), the size of representation is log 2 V (n, M ) . Although not a very useful representation, this gives a lower bound on the size of any given representation: any representation must have size at least log 2 V (n, M ) . The exact space required to represent gross substitute functions is unknown. It is strictly smaller then (2 n − 1) · log 2 (M + 1) since by the discussion in Section 4, the values v(S) satisfy various linear relations. Here we ask how smaller it is ? More precisely, can the exponential dependency on n be improved upon ? A negative answer comes from the fact that matroid rank functions are a subclass of gross substitutes. A matroid rank function takes values in {0, 1, . . . , n} so admits a naive representation of size O(2 n log n). Knuth BIB001 proved that log 2 log 2 m n ≥ n − O(log n) where m n is the total number of matroid rank functions with base set [n] . Since a representation of size s can encode at most 2 s different valuation functions, in order to represent all matroid rank functions we require at least 2 n /poly(n) size. This in particular implies that the exponential dependency in n can't be improved. Balcan and Harvey BIB012 show that even if we settle for an approximation of the valuation function v, we still can't improve the exponential dependency on n. We say thatv is an an α-approximation of v if for every S ⊆ [n], the relative error is at most α, i.e, v(S) ≤v(S) ≤ α · v(S). They show in BIB012 that if α < n 1/3 16 log(n) , then an exponential dependency on n is still necessary for the size of the representation. Their is based on the following matroid construction: Theorem 11.1 (Balcan and Harvey BIB012 ). For every n and > 0, there is a subset X ⊆ 2 [n] with |X | ≥ 2 16 log(n) , any approximationv must be able to distinguish between the case v(S) ≥ n 1/3 and v(S) ≤ 8 log(n) for each S ∈ X . The size of the representation must therefore be at least 2 The negative results on the representability of gross substitute functions arise from studying the subclass of matroid rank functions. A natural question that arises is: does the subclass of matroid rank functions encapsulate all difficulty in dealing with gross substitutes. Ostrovsky and Paes Leme propose to approach this question in the following way: defined the weighted matroid associated with matroid M ⊆ 2 [n] (represented my means of its independent sets) and weight vector w ∈ R n + as the function v(S) = max X⊆S,X∈M j∈X w j . Weighted matroids are gross substitute functions by Theorem 3.2. Now, define the class of Matroid Based Valuations as the smallest class that contains all weighted matroids and is closed under two operations: convolution (see Section 8) and endowment, which is the operation that given a valuation function v : 2 [n] → R + and a subset S ⊂ [n], definesṽ : 2 [n]\S → R asṽ(X) = v(X ∪ S|S). Since the class of gross substitutes is closed under convolution and endowment, the class of matroid based valuations is a subclass of gross substitutes. It is an open question whether this inclusion is strict. The collapse of those two classes would offer an explanation of why the difficulty in dealing with gross substitute valuations seem to reside in the subclass of matroid rank function. Another way to approach this question is via the notion of approximation: are there simple (or simpler) families of valuation functions that provide good approximations to gross substitute valuations ? Recall that a valuation functionv is said to be an α-approximation for v if v(S) ≤v(S) ≤ α · v(S) for all S ⊆ [n]. It is convenient at this point to restrict our attention to non-negative valued valuations, i.e., valuations in V + = {v : 2 [n] → R + }. Given two classes of valuation functions V 1 , V 2 ⊆ V + , we say that V 2 is α-approximated by V 1 if for all v ∈ V 2 , there existsv ∈ V 1 such thatv is an α-approximation of v. Typically, V 1 ⊆ V 2 or at least, is in some sense 'simpler' then V 2 . In that framework we can make the first question in this paragraph precise: what is the smallest α such that weighted matroids are an α-approximation to the class of gross substitute functions. More broadly one could ask if there exist simple and natural classes of valuations that α-approximate the class of gross substitutes for small 6 values of α. The class of endowed assignment valuations proposed by Hatfield and Milgrom (and discussed in Section 2) captures most practical examples of gross substitute valuations, but was recently shown by Paes Leme and Ostrovsky to be a strict subclass. A natural question to ask if endowed assignment valuations provides a good approximation to the class of gross substitutes. One can also ask the same question in the opposite direction, how well do gross substitute valuations approximate more complex class of substitutes such as submodular and subadditive functions ? The approximability between other classes of valuations has been extensively studied BIB008 BIB010 BIB011 BIB013 , yet very little is known about gross substitutes.

Gross substitutability: An algorithmic survey <s> Algorithms for other classes of valuation functions. <s> In most of microeconomic theory, consumers are assumed to exhibit decreasing marginal utilities. This paper considers combinatorial auctions among such submodular buyers. The valuations of such buyers are placed within a hierarchy of valuations that exhibit no complementarities, a hierarchy that includes also OR and XOR combinations of singleton valuations, and valuations satisfying the gross substitutes property. Those last valuations are shown to form a zero-measure subset of the submodular valuations that have positive measure. While we show that the allocation problem among submodular valuations is NP-hard, we present an efficient greedy 2-approximation algorithm for this case and generalize it to the case of limited complementarities. No such approximation algorithm exists in a setting allowing for arbitrary complementarities. Some results about strategic aspects of combinatorial auctions among players with decreasing marginal utilities are also presented. <s> BIB001 </s> Gross substitutability: An algorithmic survey <s> Algorithms for other classes of valuation functions. <s> We introduce a new framework for designing and analyzing algorithms. Our framework applies best to problems that are inapproximable according to the standard worst-case analysis. We circumvent such negative results by designing guarantees for classes of instances, parameterized according to properties of the optimal solution. Given our parameterized approximation, called PArametrized by the Signature of the Solution (PASS) approximation, we design algorithms with optimal approximation ratios for problems with additive and submodular objective functions such as the capacitated maximum facility location problems. We consider two types of algorithms for these problems. For greedy algorithms, our framework provides a justification for preferring a certain natural greedy rule over some alternative greedy rules that have been used in similar contexts. For LP-based algorithms, we show that the natural LP relaxation for these problems is not optimal in our framework. We design a new LP relaxation and show that this LP relaxation coupled with a new randomized rounding technique is optimal in our framework. <s> BIB002

The various positive algorithmic results described in this survey provide encouragement to look at other valuation classes, in particular, wider valuation classes that also express the idea of substitutability (such as submodular, subadditive and fractionally subadditive valuations BIB001 ) and try to give algorithmic definitions for such classes, in the spirit of Theorems 3.2 and 3.4. The following paragraph hints that this is not be an easy task beyond gross substitutes. For submodular valuations, it is known that both the greedy algorithm (Algorithm 2) and local search (Algorithm 3) can have arbitrarily bad performance for the demand oracle problem. Consider the following example: consider n items, among which n r are red and n b are blue. Now, given a set S ⊆ [n], let S r and S b be the red and blue items in the set respectively. The following valuation function is submodular (but not gross substitutes): v(S) = min{n b · |S r | + n r · |S b |, n r · n b } Let n r n b and consider prices p i = 0 for the red items and p i = n r − n b − for the blue items for some small > 0. The greedy demand oracle chooses the set of all blue items which provide total utility of n b (n b + ), while the demanded set under those prices is the set of all red items, which provides utility n b · n r . For → 0, the ratio between the utility chosen by the greedy algorithm and then optimal utility is n r /n b which can be made as large as we want. The same example shows that local search can be equally as bad, since the set of all blue items is a local maximum with respect to the neighborhood defined in Algorithm 3. The situation is more severe: Feige, Immorlica, Mirrokni and Nazerzadeh BIB002 argue that no polynomial time algorithm can provide a constant factor approximation to the demand oracle problem for submodular functions (via a reduction to uniform set cover problem). In the previous paragraph we argued that no polynomial time approximation algorithm is possible for the problem of finding a set of items maximizing v(S) − p(S) for a generic submodular function v. Lehmann, Lehmann and Nisan BIB001 look at the problem of finding a set S maximizing v(S) + p([n] \ S) which is equivalent to the demand oracle problem from the perspective of exact algorithms. From the perspective of approximation, however, this is a completely different problem. They show that the greedy algorithm is a 2-approximation. In other words, they show that if S is the output of the greedy algorithm and S * is a set in the demand set, then v(S) + p([n] \ S) ≥

A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> A transducer array has been designed according to underlying theory originally applied to broadside electromagnetic antenna arrays by Dolph. The properties of Tschebyscheff polynomials have been employed to obtain an optimized relationship between minor‐lobe level and main beam width.The shading design of a 130 element circular array is carried out utilizing a distance between stack centers of 5λ/8. Experimental measurements of the resulting transducer give a peak sensitivity of −70 db below 1 volt for a field of 1 dyne per cm2, 11 degrees main beam width at the 3‐db down points, and suppression of side lobes to more than 32 db below peak sensitivity. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Abstract A system has been developed for real-time sound source location on full-size jet engines. It consists of an array of microphones connected to a small digital computer, via a sequence of preamplifiers, analog filters and analog to digital converters and multiplexer. Microphone signals can be processed on-line to give displays of time varying source distributions or statistical averages with respect to position and frequency, by using a colour television as well as a display screen and printer/plotter. The whole system can be transported in a small estate car and can be used on both model scale and full size engine test rigs. A theoretical analysis of system performance is in terms of a line source of generally correlated omni-directional sound radiators, which shares the measurable far-field properties of a jet engine noise source. The general properties of the system are described, including its use to correlate spatially separated sound sources, application in the presence of ground reflections and use in a moving airstream. The statistical properties of apparent source distributions are also discussed. A series of experiments on a Rolls-Royce/SNECMA Olympus engine is described, in order to illustrate application of the system. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Abstract The technique described in this paper eliminates the Doppler effect from aircraft flyover noise measurements and generates narrow band spectra at required angles. Such a capability allows more accurate interpretation of flight data, and is necessary for a detailed comparison with predictions and static measurements, since 1 3 octave or narrow band levels, before de-Dopplerization, yield limited information on tonal content. The paper first explains how a single microphone output is de-Dopplerized, and includes details of aircraft tracking and computer simulation of flyover measurements. The technique is especially relevant to the analysis of noise from counter-rotating propeller driven aircraft, and results are shown for an Avro Shackleton. It is also applied to a Boeing 757, with high bypass ratio turbofan engines. Narrow band spectra at selected angles, density plots of complete flyovers, and field shapes at constant frequencies are all presented. Acoustic imaging, achieved by focussing the de-Dopplerized signals from an array of microphones, is also described, and results from a Lockheed TriStar graphically illustrate its capability. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Abstract Radiated noise generated by a high-speed electric train travelling at speeds up to 250 km/h has been measured with a line array of microphones mounted along the wayside in two different orientations. The test train comprised a 103 electric locomotive, four Intercity coaches, and a dynamo coach. Some of the wheels were fitted with experimental wheel-noise absorbers. By using the directional capabilities of the array, the locations of the dominant sources of wheel/rail radiated noise were identified on the wheels. For conventional wheels, these sources lie near or on the rim at an average height of about 0·2 m above the railhead. The effect of wheel-noise absorbers and freshly turned treads on radiated noise were also investigated. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> An overview of beamforming from a signal-processing perspective is provided, with an emphasis on recent research. Data-independent, statistically optimum, adaptive, and partially adaptive beamforming are discussed. Basic notation, terminology, and concepts are included. Several beamformer implementations are briefly described. > <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> 1. Introduction 2. Signals in Space and Time 3. Apertures and Arrays 4. Conventional Array Processing 5. Detection Theory 6. Estimation Theory 7. Adaptive Array Processing 8. Tracking Appendices References List of Symbols Index. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Abstract This paper gives an overview of the results and conclusions from measurements made with various configurations of microphone arrays designed to locate sound sources on trains operated by the Deutsche Bahn AG at speeds up to 280 km/h. These arrays comprised one- and two-dimensional arrangements of up to 29 microphones. The characteristics peculiar to microphone arrays will be mentioned only briefly here, because the theoretical background of array measurements is available in other publications. Within the context of the present paper, the sound sources located are those generated mechanically, principally by wheel/rail interactions, and those produced by aerodynamic fluctuations. In addition, noise radiation from cooling systems was also measured during passages of the trains. Examples are given of results obtained with different kinds of arrays during pass-bys of dedicated ICEs and goods trains. These examples demonstrate the usefulness of array measurements for producing detailed information pertaining to the characteristics of individual sound sources. With respect to these individual sound sources particularly on moving vehicles, array technology provides a much more efficient and capable tool for investigating these sources than does the more conventional technique employing single microphones. <s> BIB007 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> A Panavia Tornado aircraft was flown to investigate airframe and jet noise in high-speed flight. The lo- cations of the noise sources were examined with a line array consisting of 29 microphones. The flights were performed in an altitude of about 35 m above ground. The influences of various parameters were studied by flying with three different airspeeds (220, 250, 275 m/s) in unaccelerated flight and three dif- ferent engine power settings (flight idle, normal, max dry) with identical airspeeds (250 m/s) over the mea- suring position. The influence of external stores on airframe noise was studied by flying with stores first and repeating all tests with an operationally clean air- craft. Two surprising results were found, (i) airframe noise is louder with the aircraft in the operationally clean configuration, and (ii) a strong noise source ex- ists close to the nozzle exit plane that dominates the noise radiation toward the side and into the forward arc. <s> BIB008 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under the glide path on the ground. It is shown that important airframe noise sources can be identified in spite of the presence of engine noise, i.e., landing-gear noise, flap side-edge noise, flap-gap noise, jet-flap interaction noise, slat-horn noise, slat-track noise. A surprising finding is a noise source near the wing tips of some aircraft which is tentatively called wake-vortex wing interaction noise. It is shown to be the by far strongest noise source (6 dB(A) louder than the engines) on a regional jet aircraft. <s> BIB009 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Acoustic source two-dimensional localization was performed during flyover tests with an Airbus A340 in Tarbes, France, in 1997. The method combined a cross-shaped array of 29 microphones developed by ONERA to a D-GPS tracking system operated by Aerospatiale. These tests aimed to validate the method and evaluate its precision and performance. The signal processing used a 32-channel digital tape recorder, where both D-GPS synchronization tops and the acoustic signals were recorded on the ground while DGPS information was recorded on board. The aircraft performed flyovers above the array at altitudes between 40 and 180 meters and for different flight configurations. An acoustic emitter producing tone noise placed on the aircraft nose has helped for the method calibration. The source is clearly localized with a bias on position that can be lower than 50 cm. The method interest for extended and multiple sources is also confirmed : a few examples of airframe noise source maps are presented for configurations with landing gears and high-lift devices extended. <s> BIB010 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> When an aero-acoustic source is close to a reflecting wall, results from conventional phased array beamforming techniques are disturbed by the nearby mirror source, which is coherent with the original source. The recalculated source spectrum tends to deviate from the spectrum of the same source obtained in an anechoic environment. A periodic modulation of the spectrum occurs, which is most prominent at low frequencies. In this paper, a number of non-conventional beamforming techniques to correct for this spectral modulation is investigated. First, a technique is discussed which adds the mirror source to the transmission model. It was found that this technique is not very suitable because of its lack of robustness. Then, a more robust beamforming technique is proposed that minimises the influence of a given mirror source. By this technique, much better results were found. Nevertheless, at low frequencies the method still suffers from lack of robustness. Finally, a modification to this minimisation technique is proposed which preserves the robustness. Using this “controlled minimisation” technique, the best agreement was found between the recalculated spectra of a source close to a wall and the same source in anechoic conditions. The beamforming techniques were applied to array measurements on a calibration source in the DNW Low Speed Wind Tunnel LST. <s> BIB011 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> 782] The phased microphone array is becoming a standard method for the investigation of the sound emission of railways, aircraft, and road vehicles. It is the only method that allows an identification of the noise sources when the vehicle is in motion. Wheel-rail noise and aerodynamic noise of railways, airframe noise and installed engine noise of aircraft, tyre-road noise of automobiles are typical examples. The method has improved in the past years with the appearance of data acquisition systems capable of sampling a large number of signals and the use of relatively cheap electret microphones. In addition, the data reduction computing times have decreased substantially with modern fast personal computers. The requirements for source localisation on aircraft are especially high. The large flyover altitudes require large array dimensions and large number of microphones. The system that has been developed by DLR for this application will be presented. The spatial resolution and the requirements for the data acquisition are derived. The limits and possibilities for obtaining quantitative results are also demonstrated. Various examples for the distribution of noise sources on aircraft and motor vehicles will be shown and discussed. <s> BIB012 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> At the 2004 AIAA/CEAS Aeroacoustic Conference, a breakthrough in acoustic microphone array technology was reported by the authors. A Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) was developed which decouples the array design and processing influence from the noise being measured, using a simple and robust algorithm. For several prior airframe noise studies, it was shown to permit an unambiguous and accurate determination of acoustic source position and strength. As a follow-on effort, this paper examines the technique for three-dimensional (3D) applications. First, the beamforming ability for arrays, of different size and design, to focus longitudinally and laterally is examined for a range of source positions and frequency. Advantage is found for larger array designs with higher density microphone distributions towards the center. After defining a 3D grid generalized with respect to the array’s beamforming characteristics, DAMAS is employed in simulated and experimental noise test cases. It is found that spatial resolution is much less sharp in the longitudinal direction in front of the array compared to side-to-side lateral resolution. 3D DAMAS becomes useful for sufficiently large arrays at sufficiently high frequency. But, such can be a challenge to computational capabilities, with regard to the required expanse and number of grid points. Also, larger arrays can strain basic physical modeling assumptions that DAMAS and all traditional array methodologies use. An important experimental result is that turbulent shear layers can negatively impact attainable beamforming resolution. Still, the usefulness of 3D DAMAS is demonstrated by the measurement of landing gear noise source distributions in a difficult hard-wall wind tunnel environment. <s> BIB013 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Acoustic array measurements were performed on a three-bladed GAMESA G58 wind turbine with a rotor diameter of 58 m and a tower height of 53.5 m. The goal was to characterize the noise sources on this turbine, and to verify whether aerodynamic noise from the blades is dominant. In order to assess the effect of blade roughness, one blade was cleaned, one blade was tripped, and one blade was left untreated. The acoustic array consisted of 152 microphones mounted on a horizontal wooden platform (15 by 18 m 2 ), which was positioned about 58 m upwind from the rotor. In parallel to the acoustic measurements, a number of turbine parameters were monitored, such as wind speed, power, turbine orientation, RPM, and blade pitch angle. In total more than 100 measurements were taken at wind speeds between 6 and 10 m/s. Two array processing methods were used to characterise the noise from the turbine. First, the noise sources in the rotor plane were localised using conventional beamforming. These results clearly show that, besides a minor source at the rotor hub, practically all noise (radiated to the ground) is produced during the downward movement of the blades. The noise is produced by the outer part of the blades (but not by the very tip), and blade noise levels scale with the 5 th power of the local flow speed. The second processing method employed rotating scan planes to localise the noise sources on the individual blades. It turns out that the tripped blade is significantly noisier than the clean and untreated blades, which is a strong indication of trailing edge noise (rather than inflow turbulence noise). The similar noise levels for the clean and untreated blades suggest that the untreated blade was aerodynamically clean. <s> BIB014 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> The development of the beamforming method (also called microphone antenna, phased array of microphones, acoustic telescope, or acoustic camera) is reviewed in this paper. The microphone antenna was invented by Billingsley (1974) and has since seen dramatic improvements due to the availability of better data acquisition and computing hardware. Recent mathematical and software developments invert the beamforming process and allow a quantitative determination of the sources. Beamforming is indispensable for the localization of sound sources on moving objects, on flying aircraft, on high-speed trains, on motor cars in motion, on open rotors like helicopter and wind turbine rotors. In these applications, the ability to follow the motion of the sources is important. The second important applications are source localization tests in the test sections of open and closed wind tunnels. The background noise suppression capability of the beamforming method is required here. The various applications are discussed with a long list of references. 1 Berlin Beamforming Conference <s> BIB015 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Current processing of acoustic array data is burdened with considerable uncertainty. This study reports an original methodology that serves to demystify array results, reduce misinterpretation, and accurately quantify position and strength of acoustic sources. Traditional array results represent noise sources that are convolved with array beamform response functions, which depend on array geometry, size (with respect to source position and distributions), and frequency. The Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) method removes beamforming characteristics from output presentations. A unique linear system of equations accounts for reciprocal influence at different locations over the array survey region. It makes no assumption beyond the traditional processing assumption of statistically independent noise sources. A new robust iterative method seamlessly introduces a positivity constraint (due to source independence) that makes the equation system sufficiently deterministic. DAMAS is quantitatively validated using archival data from a variety of prior high-lift airframe component noise studies, including flap edge/cove, trailing edge, leading edge, slat, and calibration sources. Presentations are explicit and straightforward, as the noise radiated from a region of interest is determined by simply summing the mean-squared values over that region. DAMAS can fully replace existing array processing and presentations methodology in most applications. It appears to dramatically increase the value of arrays to the field of experimental acoustics. <s> BIB016 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> To obtain higher resolution acoustic source plots from microphone array measurements, deconvolution techniques are becoming increasingly popular. Deconvolution algorithms aim at identifying Point Spread Functions (PSF)in source plots, and may therefore fall short when actual beam patterns of measured noise sources are not similar to synthetically obtained PSF's. To overcome this, a new version of the classical deconvolution method CLEAN is proposed here: CLEAN-SC. By this new method, which is based on spatial source coherence, side lobes can be removed of actually measured beam patterns. Essentially, CLEAN-SC iteratively removes the part of the source plot which is spatially coherent with the peak source. A feature of CLEAN-SC is its ability to extract absolute sound power levels from the source plots. The merits of CLEAN-SC were demonstrated using array measurements of airframe noise on a scale model of the Airbus A340 in the 8 ×6 m2 closed test section of DNW-LLF. <s> BIB017 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> A spectral estimation method based on measurements by a phased array was developed to estimate absolute power levels radiated by compact or extended sources. After a brief recall of its principle, numerical simulations involving several source regions are presented to illustrate the accuracy and the high-resolution performances of this new method. Tests were carried out with a 1:11th scale A320/A321 Airbus model in the anechoic wind tunnel CEPRA 19. The characteristics of source regions from inboard to outboard of the wing slat and flap, and the main and nose landing gear subregions are examined. The dependence in level and frequency of the model subregions with the flow speed, the effects of the attack angle, and the effects of the flap or slat deployment are studied. The different trends revealed by the integrated power levels are also discussed. Examples of the localization of noise sources identified with conventional beam forming and the new method are presented. The consistency of the results is examined by comparing the integrated power levels for the complete wing and the measured spectra with the phased array. Finally, we show that the spectral estimation method is quite reliable in computing the far-field directivity of airframe noise sources. <s> BIB018 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> A full-scale test on a turbofan engine has been carried out in the framework of a European project to assess the acoustic benefits of new exhausts and novel hot-stream streamliners in the reduction of the overall jet noise level radiatedinthe far field. Thepurpose ofthetestsisalsoto provideabetterunderstanding ofcombustionnoise,which is one of the main issues for turbofan engines because of the significant noise reduction obtained for other acoustic sources such as the fan and jet by using innovative efficient silencers and acoustics liners. This work investigates the use of the so-called spectral estimation method to separate the combustion noise from other noise sources measured withanear-fieldarray ofmicrophones.Thecomparisonsbetweentheestimated andmeasuredspectradownstream, at the location at which the combustion noise is dominant, yield promising results, suggesting that the procedure of discrimination is relevant. <s> BIB019 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Abstract To resolve coherent/incoherent, distributed/compact, and multipole aerodynamic-sound sources with phased-array pressure data, a new source-detection algorithm is developed based on L1 generalized inverse techniques. To extract each coherent signal, a cross spectral matrix is decomposed into eigenmodes. Subsequently, the complex source-amplitude distribution that recovers each eigenmode is solved using generalized inverse techniques with reference solutions which include multipoles as well as a monopole. Namely, the source distribution consisting of pre-defined source types is solved as an L1 norm problem using iteratively re-weighted least squares (IRLS). The capabilities of the proposed algorithm are demonstrated using various benchmark problems to compare the results with several existing beam-forming algorithms, and it is found that distributed sources as well as dipoles with arbitrary orientation can be identified regardless of coherency with another source. The resolution is comparable to existing deconvolution techniques, such as DAMAS or CLEAN, and the computational cost is only several times more than that of DAMAS2. The proposed algorithm is also examined using previous model-scale test data taken in an open-jet wind-tunnel for a study on jet–flap interaction, and some indication of dipole radiation is discerned near the flap edge. <s> BIB020 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> The reconstruction of acoustical sources from discrete field measurements is a difficult inverse problem that has been approached in different ways. Classical methods (beamforming, near-field acoustical holography, inverse boundary elements, wave superposition, equivalent sources, etc.) all consist--implicitly or explicitly--in interpolating the measurements onto some spatial functions whose propagation are known and in reconstructing the source field by retropropagation. This raises the fundamental question as whether, for a given source topology and array geometry, there exists an optimal interpolation basis which minimizes the reconstruction error. This paper provides a general answer to this question, by proceeding from a Bayesian formulation that is ideally suited to combining information of physical and probabilistic natures. The main findings are the followings: (1) The optimal basis functions are the M eigen-functions of a specific continuous-discrete propagation operator, with M being the number of microphones in the array. (2) The a priori inclusion of spatial information on the source field causes super-resolution according to a phenomenon coined "Bayesian focusing." (3) The approach is naturally endowed with an internal regularization mechanism and results in a robust regularization criterion with no more than one minimum. (4) It admits classical methods as particular cases. <s> BIB021 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> This paper answers the challenge as how to automatically select a good regularization parameter when solving inverse problems in acoustics. A Bayesian solution is proposed that consists either in directly finding the most probable value of the regularization parameter or, indirectly, in estimating it as the ratio of the most probable values of the noise and source expected energies. It enjoys several nice properties such as ease of implementation and low computational complexity (the proposed algorithm boils down to searching for the global minimum of a 1D cost function). Among other advantages of the Bayesian approach, it makes possible to appraise the sensitivity of the reconstructed acoustical quantities of interest with respect to regularization, a performance that would be otherwise arduous to achieve. <s> BIB022 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Variability in noise levels for flyovers of the same aircraft type can be as large as 12 dB, hampering noise assessment around airports. The variable atmosphere (affecting the acoustic propagation) and variations in the aircraft emitted noise are considered as the two main contributors to the noise level variability. This paper presents two experiments aimed at quantifying these contributions. First, the atmospheric contribution was determined with a loudspeaker (100 m height) sending signals to microphones on the ground, indicating a sound level variability of less than 2 dB. Second, noise levels from Boeing 737 flyovers (landings) were measured with an acoustic camera. The observed noise level variability was 6–8 dB. The acoustic camera imaging capabilities eliminated variations due to ground reflections and ambient noise, and identified the turbofan engines as the dominant noise source. Assuming the two contributions to be independent statistical processes, with the atmosphere contributing 2 dB maximal... <s> BIB023 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Noise assessment around airports is hampered due to the observed large variability in noise levels for fly-overs of the same aircraft type, which is not considered by the current models. This paper assumes that the noise variability is due to variations in the aircraft emitted noise, neglecting the effect of the variable atmosphere, as previous work showed that its contribution is maximally 2 dB. In order to quantify and investigate the variability of noise levels during aircraft fly-overs, 115 measurements of noise of landing aircraft were taken using a 32 microphone array. The noise levels from Boeing 737 fly-overs were analyzed and the noise level variability was determined to be approximately 16 dB. After determining the engine settings from the spectrograms, it was found that variations on the engine settings explain over 55% of the observed total noise variation. In addition, by performing beamforming on the acoustic data, it was confirmed that airframe noise (from the landing gear and deployed flaps) is dominant for several frequencies, especially for modern aircraft. <s> BIB024 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r... <s> BIB025 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> The problem of localizing and quantifying acoustic sources from a set of acoustic measurements has been addressed, in the last decades, by a huge number of scientists, from different communities (s... <s> BIB026 </s> A review of acoustic imaging methods using phased microphone arrays <s> Historical background <s> Snowmobile manufacturers invest important efforts to reduce the noise emission of their products. The noise sources of snowmobiles are multiple and closely spaced, leading to difficult source separation in practice. In this study, source imaging results for snowmobile pass-by noise are discussed. The experiments involve a 193-microphone Underbrink array, with synchronization of acoustic with video data provided by a high-speed camera. Both conventional beamforming and Clean-SC deconvolution are implemented to provide noise source maps of the snowmobile. The results clearly reveal noise emission from the engine, exhaust, and track depending on the frequency range considered. <s> BIB027

The use of phased arrays dates back to World War II as radar antennas later developed for applications such as the sonar, radioastronomy, seismology, mobile communication, 1 3 or ultrasound medical imaging BIB015 . The theory of electromagnetic antenna arrays was already applied in the field of acoustics for determining the direction of arrival of sound sources by Davids et al. BIB001 . The phased microphone array (also known as microphone antenna, acoustic telescope, and acoustic array or acoustic camera) was introduced by Billingsley BIB002 . Using several synchronized microphones and a source localization algorithm BIB005 BIB006 , the possibility to estimate the location and strength of sound sources was enabled. Since then, significant improvements have been made, for a large part by more powerful acquisition and computing systems BIB015 , allowing higher sampling frequencies, longer acquisition times, larger numbers of microphones, and even real-time sound source localization. In the remaining of this paper, the term source localization is only related to the use of microphone arrays. Acoustic imaging algorithms BIB006 are the essential link between the sound field measured at a number of microphone positions and the assessment of useful characteristics of noise sources, such as their locations and absolute levels. The main idea is to combine the data gathered by the microphone array with a sound propagation model to infer on the source parameters BIB026 . The conventional beamforming BIB005 BIB006 (see Sect. 3.1) is, perhaps, the most basic postprocessing approach for the signals recorded by the microphones, but it normally fails to provide the satisfactory results for practical applications. The localization and quantification of sound sources are limited by the geometry of the array. Most acoustic imaging methods are exhaustive search techniques where a selected grid containing the location of potential sound sources is scanned. The development of advanced source localization algorithms has played a large role in the recent years to further improve source identification and quantitative results BIB026 . These methods normally imply a higher computational cost due to the more sophisticated approaches considered. Two main categories can be considered: • Deconvolution techniques such as DAMAS BIB016 BIB013 BIB016 (developed in 2004, see Sect. 3.5) or CLEAN-SC BIB017 (developed in 2007, see Sect. 3.4) can be seen as postprocessing methods of the results obtained using the conventional beamforming, assuming hypotheses such as source coherence or positive source powers. • Several inverse methods have been proposed, which, in contrast to beamforming algorithms, aim at solving an inverse problem accounting for the presence of all sound sources at once. This way, interferences between potentially coherent sources can be taken into account BIB026 . This inverse problem is typically underdetermined and inverse methods are often sensitive to measurement noise. Hence, regularization procedures are required. Different regularization techniques are available, depending on the source sparsity constraint set by the user BIB026 . Acoustic imaging methods such as generalized inverse beamforming BIB020 (developed in 2011, see Sect. 3.12) or the Iterative Bayesian inverse approach BIB021 BIB022 (developed in 2012, see Sect. 3.13) are included in this group. Other classifications of acoustic imaging methods considering the other criteria have been presented in the literature BIB026 . In general, measurements with phased microphone arrays provide certain advantages with respect to measurements with individual microphones when performing acoustic measurements. For experiments in wind tunnels with openand closed-test sections, as well as in engine test cells, the background noise suppression capability of the source localization algorithms is very useful BIB015 BIB018 BIB019 , as well as the removal of reflections from the walls BIB011 . Beamforming can also be applied to moving objects, such as flying aircraft or rotating blades, provided that the motion of the source is tracked accurately BIB015 . Moreover, microphone arrays are useful tools for studying the variability of the noise levels generated by different aircraft components, within the same aircraft type BIB023 BIB024 to improve the noise prediction models in the vicinity of airports. Nowadays, the microphone array has become the standard tool for analyzing noise sources on flying aircraft BIB003 BIB008 BIB009 BIB010 BIB025 , trains BIB004 BIB007 , cars BIB012 , snowmobiles BIB027 , and other machineries, such as wind turbines BIB014 .

A review of acoustic imaging methods using phased microphone arrays <s> Wind-tunnel measurements <s> Abstract The beamforming technique has become a common method to localise aeroacoustic sources. However, in the case of anechoic wind tunnel experiments, some shear flow corrections have to be taken into account in order to estimate accurately the position and power of the source. This study deals with the efficiency of such corrections. Several correction methods are presented. The first one is based on the convected wave equation and predicts that, if no flow model is used, the source is localised with an apparent source shift, that is approximately equal to the product of the Mach number by the mean flow thickness. Two other shear flow corrections, based on acoustic ray theory, are considered. They allow the modelling of both the effective source–receiver propagation time and pressure amplitude. The efficiency of such corrections is assessed by using a numerical simulation of the acoustic pressure measurement with a linear microphone array in an anechoic wind-tunnel. The simulation, based on a programme solving the linearised Euler equations, is first validated at a low Mach number (0.11) by a careful comparison, in terms of source localisation, with experimental data for several array positions. The efficiency of the shear flow corrections is then investigated numerically for a set of flow parameters. The apparent source shift is shown to be independent of the ratio of the shear flow thickness to the sound wavelength, and the shear flow corrections perform well, at low Mach numbers for thin and thick shear layers. Conversely, the mean flow thickness and the Mach number are shown to be the two main parameters having an influence on the apparent source shift. The estimate of this shift by the product of the Mach number with the mean flow thickness is shown to be a good approximation, and can be used for a posteriori corrections up to Mach number 0.4. For higher subsonic Mach numbers, the ray-based shear flow correction models associated to beamforming processing are shown to be robust enough to localise the source position in a flow, and to avoid significant source power under-estimation. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Wind-tunnel measurements <s> A subspace-based form of background subtraction is presented and applied to aeroacoustic wind tunnel data. A variant of this method has seen use in other fields such as climatology and medical imaging. The technique is based on an eigenvalue decomposition of the background noise cross-spectral matrix. Simulated results indicate similar performance to conventional background subtraction when the subtracted spectra are weaker than the true contaminating background levels. Superior performance is observed when the subtracted spectra are stronger than the true contaminating background levels, and when background data do not match between measurements. Experimental results show limited success in recovering signal behavior for data in which conventional background subtraction fails. The results also demonstrate the subspace subtraction technique’s ability to maintain a physical coherence relationship in the modified cross-spectral matrix. Deconvolution results from microphone phased array data indicate that array integration methods are largely insensitive to subtraction type, and that background subtraction with appropriate background data is an effective alternative to diagonal removal. <s> BIB002

As in the field of aerodynamics, wind-tunnel measurements offer a controlled environment to perform acoustic measurements on scaled models of aircraft or aircraft components. It is, however, difficult to replicate the exact conditions present at an aircraft in flight. As shown by Stoker et al. , differences occur when results from a standard wind-tunnel measurement with a closed-test section are compared to the results obtained from flight tests. The differences can be explained by lack of model fidelity, installation effects, a discrepancy in the Reynolds number (see Sect. 1.2.4), and the applicability of the assumptions made in phased-array processing. Depending on the size of the model, scale effects need to be taken into account for the sound-generation mechanisms . Wind-tunnel acoustic measurements feature convection of sound waves, which can be corrected for BIB001 . A major issue is the high background noise level, but mitigation techniques are available BIB002 . Wind-tunnel measurements can be performed in open jets or in closed-test sections, each of these options having different challenges: 1 3

A review of acoustic imaging methods using phased microphone arrays <s> Closed-test sections <s> The feasibility of high frequency phased array measurements on aircraft scale models in a closed wind tunnel test section was investigated. For that purpose, 100 microphones were built in a 0.6x0.5 m’ plate. which was installed in a floor panel of the 8x6 m2 test section of the Large Low-speed Facility of the German Dutch Wind tunnel (DNW-LLF). F or the microphone positions a sparse array design was used that minimises side lobes in the beamforming process. To suppress boundary layer noise, the array could optionally be covered with a 0.5 cm thick layer of acoustic foam and a 5% open perforated plate. To assess the effect of wall reflections, tests without wind were performed with a loudspeaker at several positions in the tunnel section. Furthermore, wind tunnel tests were carried out on an Airbus transport aircraft model. It is shown that location of acoustic sources is indeed possible for frequencies between 2 and 30 kHz, but their levels may differ from those measured in an anechoic environment. For the lower frequencies, application of the layer of foam and the perforated plate is beneficial. Finally, it is shown that filtering out the most dominant source can extend the array potential. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed-test sections <s> A method for reducing the flow-induced self-noise of acoustic array microphones was demonstrated in a wind tunnel. Unsteady boundary-layer flow in the wind tunnel induces large pressure fluctuations on exposed, flushmounted microphone diaphragms, reducing the signal-tonoise capability of microphone arrays. Two important steps were taken to reduce this background noise. First, the microphones were recessed to separate the microphones from the unsteady flow. Second, a porous surface material was placed above the microphones to act as an aerodynamic surface while allowing acoustic signals to pass through to the microphones. Previous attempts at this approach used perforated plates as the surface material, which tended to fatigue in the unsteady flow. The increased acoustic impedance of thicker materials caused reverberation between the surface and the microphone mounting plate. This latest attempt used a stretched sheet of Kevlar ® as the surface. The extreme strength and durability of the Kevlar ® withstood flow-induced fatigue while providing very low acoustic impedance with little attenuation of sound for most frequencies. Data is presented for two wind tunnel tests that demonstrate the capabilities of the recessed Kevlar ® array. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed-test sections <s> When an aero-acoustic source is close to a reflecting wall, results from conventional phased array beamforming techniques are disturbed by the nearby mirror source, which is coherent with the original source. The recalculated source spectrum tends to deviate from the spectrum of the same source obtained in an anechoic environment. A periodic modulation of the spectrum occurs, which is most prominent at low frequencies. In this paper, a number of non-conventional beamforming techniques to correct for this spectral modulation is investigated. First, a technique is discussed which adds the mirror source to the transmission model. It was found that this technique is not very suitable because of its lack of robustness. Then, a more robust beamforming technique is proposed that minimises the influence of a given mirror source. By this technique, much better results were found. Nevertheless, at low frequencies the method still suffers from lack of robustness. Finally, a modification to this minimisation technique is proposed which preserves the robustness. Using this “controlled minimisation” technique, the best agreement was found between the recalculated spectra of a source close to a wall and the same source in anechoic conditions. The beamforming techniques were applied to array measurements on a calibration source in the DNW Low Speed Wind Tunnel LST. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed-test sections <s> During the last decade, microphone arrays became a standard tool to study aeroacoustic sources. Nowadays, arrays are frequently applied in fly-over measurements 1 and wind tunnel testing. 2 To improve the resolution of the arrays and to reduce disturbing side lobes in the measured source maps, sophisticated deconvolution methods were proposed 3-8 in recent years. First, these methods were tested using synthetic data or data from experiments with generic configurations. 8 In a next step the methods have to be applied to real experiments. This is done in the present paper. It is tested, if the deconvolution works also under difficult conditions, and if the findings of the simulations using artificial background noise 7 can be confirmed in case of real data. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed-test sections <s> The objective of the present paper is to improve the implementation of microphone arrays on the wall of closed-section wind tunnels. A test campaign has been conducted in the ONERA F2 1.4 m × 1.8 m wind tunnel to investigate the performance of thin and smooth metallic clothes to protect the microphones from the hydrodynamic fluctuations in the turbulent boundary-layer flow and, still, to preserve the transmitted acoustic signal. Three microphone mountings have been considered : individually recessed microphones in (i) cylindrical and (ii) conical apertures and (iii) a recessed, screened, large-scale microphone mounting plate (microphones flush with the plate). It was found that by covering the top of the cylindrical and conical microphone apertures with a wire-mesh, the strong self-noise due to the interaction of the boundary-layer flow with the cavities can be suppressed. The most significant noise reduction is obtained when the large-scale microphone mounting plate is recessed behind the metallic wire-mesh. A reduction by up to 20 dB is then obtained with respect to a flush-mounted, unscreened, microphone. This microphone mounting out-performs the cone-nose-protected and Kevlar-protected microphone mountings, two conventional low-noise devices. Thanks to an original calibration technique, the acoustic response of the tested microphone mountings could be evaluated in presence of flow (� 80 m/s). The recessed, screened, microphone mounting plate has an oscillating acoustic response (in function of the frequency) by less than 5 dB deviation. This attenuation can be post-corrected by using a very simple acoustic model. <s> BIB005

Closed-test sections offer well-controlled aerodynamic properties. Acoustic measurements can be performed nonintrusively by mounting microphones flush in the floor, ceiling, or walls of the wind tunnel. However, the amplitudes of the near-field pressure fluctuations inside the turbulentboundary layer (TBL) are generally much larger than the acoustic signal from the model. Suppression of these nearfield pressure fluctuations can be realized by mounting the microphones in cavities covered by a perforated plate or wire mesh at some distance from the TBL BIB001 BIB002 BIB005 . This solution takes advantage of the fact that TBL pressure fluctuations feature short wavelengths, which decay exponentially with distance. Microphones recessed in a cavity are offered commercially too . A more radical solution for the TBL issues is to replace the wind-tunnel walls by Kevlar sheets, as in the stability tunnel of the Virginia Polytechnic Institute and State University . In addition, acoustic measurements are hampered by reflections by the test section walls BIB003 . In general, acoustic measurements in closed-test sections are dominated by high noise levels, either due to the TBL pressure fluctuations or due to the noise from the windtunnel circuit. The noise influence can be reduced substantially by subtracting the noise influence on the cross-spectral matrix (CSM) before the final-source location analysis BIB004 (see Sect. 3.1).

A review of acoustic imaging methods using phased microphone arrays <s> Open jets <s> Abstract A comparison is made between several shear layer refraction theories to determine their relationship to one another and to determine which parameters are important for an open jet wind tunnel shear layer correction. For sound transmission through a parallel sheared flow, the shear layer thickness is found to be unimportant at Mach numbers typical of open jet tunnels. The effect of reflected waves, although more significant, can usually be ignored, allowing a correction which is independent of source type and frequency. The shear layer shape (plane or cylindrical) can be important and the correction corresponding to the actual shear layer shape should be used. The numerical solutions of the Lilley equation for the limiting cases of a thick and a thin shear layer are found to agree with the algebraic expressions given for these limiting cases. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open jets <s> Abstract The beamforming technique has become a common method to localise aeroacoustic sources. However, in the case of anechoic wind tunnel experiments, some shear flow corrections have to be taken into account in order to estimate accurately the position and power of the source. This study deals with the efficiency of such corrections. Several correction methods are presented. The first one is based on the convected wave equation and predicts that, if no flow model is used, the source is localised with an apparent source shift, that is approximately equal to the product of the Mach number by the mean flow thickness. Two other shear flow corrections, based on acoustic ray theory, are considered. They allow the modelling of both the effective source–receiver propagation time and pressure amplitude. The efficiency of such corrections is assessed by using a numerical simulation of the acoustic pressure measurement with a linear microphone array in an anechoic wind-tunnel. The simulation, based on a programme solving the linearised Euler equations, is first validated at a low Mach number (0.11) by a careful comparison, in terms of source localisation, with experimental data for several array positions. The efficiency of the shear flow corrections is then investigated numerically for a set of flow parameters. The apparent source shift is shown to be independent of the ratio of the shear flow thickness to the sound wavelength, and the shear flow corrections perform well, at low Mach numbers for thin and thick shear layers. Conversely, the mean flow thickness and the Mach number are shown to be the two main parameters having an influence on the apparent source shift. The estimate of this shift by the product of the Mach number with the mean flow thickness is shown to be a good approximation, and can be used for a posteriori corrections up to Mach number 0.4. For higher subsonic Mach numbers, the ray-based shear flow correction models associated to beamforming processing are shown to be robust enough to localise the source position in a flow, and to avoid significant source power under-estimation. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open jets <s> The application of microphone arrays for aeroacoustic measurements in wind tunnels with an open test section requires to consider the sound refraction at the shear layer. Several methods are available for this that are either applicable to special scenarios such as planar or cylindrical thin shear layers or that require considerable computational effort. The paper concerns a new method for general application—ray casting—that is based on a ray tracing method. Ray tracing is briefly revisited and it is shown that the computational effort becomes quite high especially for large mapping grids. The ray casting approach with its interpolation technique is introduced. A time reversal technique is the key to make this approach very efficient and fast. Finally, the approach is demonstrated using two example results from simulation and from a practical measurement. <s> BIB003

The test chamber surrounding the jet is usually acoustically treated, so that most reflections are suppressed. Moreover, the background noise levels are lower than in a closed-test section and the microphones can be placed outside the flow, not being subject to turbulence. However, the aerodynamic conditions are less well controlled and corrections are needed to account for refraction through the shear layer, which produces some disturbances (distortion in phase) that need to be taken into account BIB002 BIB001 BIB003 . Furthermore, the turbulence in the shear layer causes spectral broadening and decorrelation , see Sect. 2.4.

A review of acoustic imaging methods using phased microphone arrays <s> Comparability of wind-tunnel measurements <s> The reliability of the phased array technique for quantifying airframe noise was assessed in the DNW-LLF open and closed wind tunnel sections. Acoustic measurements were performed on wing noise from a 1:10.6 scaled Airbus A340 model, using a 4-m diameter out-of-flow microphone array in the open jet and a 1-m diameter wall array in the closed test section. Apart from the differences in array resolution, the source characteristics are generally similar for both test sections. The open jet results show that, although the absolute integrated array level can be too low due to e.g. coherence loss, the relative sound levels determined with the array (i.e. differences between configurations) are accurate within 0.5 dB. The difference between the absolute array levels in the closed test section and the farfield levels in the open jet is smaller than 3 dB for all frequencies. The relative array levels in the closed test section agree with the open jet within 1 dB, provided that the flow conditions - and therefore t... <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Comparability of wind-tunnel measurements <s> The phased microphone array has become a widely used tool in aeroacoustic testing for the localization and quantification of sound sources. Measurements are often carried out in wind tunnels with open or closed test sections. The underlying thesis investigates the performance and comparability of microphone array measurements conducted in wind tunnels with open and closed test sections and carries out a systematic comparison. The various effects influencing the aeroacoustic results of testing in different wind tunnels can be summarized in three main groups. Deviations can result from influences which can affect the aeroacoustic sound generation. The second group concerns differences in the experimental setup. Finally, the third group consists of the varying test section boundary conditions and propagation effects. In the framework of the assessment of the comparability of microphone array measurements in open and closed wind tunnels, this thesis focuses on the third group - the influence of test section boundary conditions and propagation effects on microphone array results. Nonetheless, the first two points are also of concern and discussed in this thesis. <s> BIB002

The comparability of measurements conducted with a similar model in different wind tunnels (either of the same type of test section, but conducted at a different facility , or with different types of test sections at the same facility BIB001 BIB002 ) is still an open issue. In the work by Oerlemans et al. BIB001 , the comparability of absolute and relative source levels of microphone-array measurements in the open-and closed-test sections on a scaled Airbus A340 model has been addressed. Both measurements were conducted in the DNW-LLF wind tunnel, and both the open and the closed-test sections were used for comparison. The source maps of both measurements by Oerlemans showed a comparable source distribution. The spatial resolution at low frequencies in the open test section was higher than the resolution in the closed-test section because of the higher ratio between array diameter and distance to the scan plane. Some sources only appeared in one of the test sections, and were not present in the other. The difference in the source occurrence can most likely be explained by the different flow conditions in each of the test sections, even though the overall lift forces on the models were equal. A systematic comparison between microphone measurements in both open-and closed-test sections was performed by Kröber BIB002 . Three different types of sound sources were studied by evaluating comparable measurements in both open-and closed-test sections. In Fig. 1 , an overview of the advantages and disadvantages of both types of test sections is outlined. In general, background noise is a low-frequency issue for both cases. The importance of the reflections and the TBL influence is higher at low frequencies for closed-section wind tunnels, whereas the influence of scattering and refraction through the shear layer is more dominant at high frequencies for open-jet wind tunnels BIB002 . Therefore, open-jet wind tunnels are recommended for measuring models emitting low-frequency noise with a lowsource strength, whereas closed-section wind tunnels are preferred for measuring high-frequency noise sources BIB002 . For middle frequencies, both test sections provide comparable acoustic performance. In practice, far-field noise measurements can almost only be performed in open-jet wind tunnels, since it is typically possible to place the microphone 1 3 array further away from the source than in closed-section wind tunnels.

A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> Abstract The technique described in this paper eliminates the Doppler effect from aircraft flyover noise measurements and generates narrow band spectra at required angles. Such a capability allows more accurate interpretation of flight data, and is necessary for a detailed comparison with predictions and static measurements, since 1 3 octave or narrow band levels, before de-Dopplerization, yield limited information on tonal content. The paper first explains how a single microphone output is de-Dopplerized, and includes details of aircraft tracking and computer simulation of flyover measurements. The technique is especially relevant to the analysis of noise from counter-rotating propeller driven aircraft, and results are shown for an Avro Shackleton. It is also applied to a Boeing 757, with high bypass ratio turbofan engines. Narrow band spectra at selected angles, density plots of complete flyovers, and field shapes at constant frequencies are all presented. Acoustic imaging, achieved by focussing the de-Dopplerized signals from an array of microphones, is also described, and results from a Lockheed TriStar graphically illustrate its capability. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> A Panavia Tornado aircraft was flown to investigate airframe and jet noise in high-speed flight. The lo- cations of the noise sources were examined with a line array consisting of 29 microphones. The flights were performed in an altitude of about 35 m above ground. The influences of various parameters were studied by flying with three different airspeeds (220, 250, 275 m/s) in unaccelerated flight and three dif- ferent engine power settings (flight idle, normal, max dry) with identical airspeeds (250 m/s) over the mea- suring position. The influence of external stores on airframe noise was studied by flying with stores first and repeating all tests with an operationally clean air- craft. Two surprising results were found, (i) airframe noise is louder with the aircraft in the operationally clean configuration, and (ii) a strong noise source ex- ists close to the nozzle exit plane that dominates the noise radiation toward the side and into the forward arc. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> The noise sources of landing commercial aircraft were examined with planar arrays consisting of 96 or 111 microphones mounted on an 8 m by 8 m plate under the glide path on the ground. It is shown that important airframe noise sources can be identified in spite of the presence of engine noise, i.e., landing-gear noise, flap side-edge noise, flap-gap noise, jet-flap interaction noise, slat-horn noise, slat-track noise. A surprising finding is a noise source near the wing tips of some aircraft which is tentatively called wake-vortex wing interaction noise. It is shown to be the by far strongest noise source (6 dB(A) louder than the engines) on a regional jet aircraft. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> The development of the beamforming method (also called microphone antenna, phased array of microphones, acoustic telescope, or acoustic camera) is reviewed in this paper. The microphone antenna was invented by Billingsley (1974) and has since seen dramatic improvements due to the availability of better data acquisition and computing hardware. Recent mathematical and software developments invert the beamforming process and allow a quantitative determination of the sources. Beamforming is indispensable for the localization of sound sources on moving objects, on flying aircraft, on high-speed trains, on motor cars in motion, on open rotors like helicopter and wind turbine rotors. In these applications, the ability to follow the motion of the sources is important. The second important applications are source localization tests in the test sections of open and closed wind tunnels. The background noise suppression capability of the beamforming method is required here. The various applications are discussed with a long list of references. 1 Berlin Beamforming Conference <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> A hybrid time-frequency approach based on acoustic beamforming has been successfully developed in order to determine the absolute contribution of the aircraft noise components measured during fly-overs. The method, derived from DAMAS, 1 accounts for the fact that the sources move relative to the microphones. Indeed, the motion is responsible for a frequency shift of the sidelobes and a strong modification of the point-spread functions compared to the static case. The method developed is hybrid in the sense that the beamforming algorithm is applied in the time domain while the point-spread functions are approximated in the frequency domain. When the sound sources are characterized by broadband spectra, it becomes possible to discard the frequency coupling between the sources and the sidelobes. This enables to reduce the computing time considerably. In the present publication, the method is applied to AIRBUS A340 fly-overs with the high-lift devices deployed, landing gear up, and the engines running at idle. The study is focused on the influence of some processing parameters (grid spacing, number of iterations, cut of the focusing grid, and frequency bandwidth) on the source breakdown. It turns out that the noise spectra of the aircraft components are almost independent of reasonable variations of these parameters. This indicates that the method is relatively robust. A detailed investigation of the noise sources (flaps, slats, and engines) with respect to the variations of the airspeed, the ECAM position, the engine rating, etc. is not part of this work. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> The NASA-Gulfstream joint Airframe Noise Flight Test program was conducted at the NASA Wallops Flight Facility during October, 2006. The primary objective of the AFN flight test was to acquire baseline airframe noise data on a regional jet class of transport in order to determine noise source strengths and distributions for model validation. To accomplish this task, two measuring systems were used: a ground-based microphone array and individual microphones. Acoustic data for a Gulfstream G550 aircraft were acquired over the course of ten days. Over twenty-four test conditions were flown. The test matrix was designed to provide an acoustic characterization of both the full aircraft and individual airframe components and included cruise to landing configurations. Noise sources were isolated by selectively deploying individual components (flaps, main landing gear, nose gear, spoilers, etc.) and altering the airspeed, glide path, and engine settings. The AFN flight test program confirmed that the airframe is a major contributor to the noise from regional jets during landing operations. Sound pressure levels from the individual microphones on the ground revealed the flap system to be the dominant airframe noise source for the G550 aircraft. The corresponding array beamform maps showed that most of the radiated sound from the flaps originates from the side edges. Using velocity to the sixth power and Strouhal scaling of the sound pressure spectra obtained at different speeds failed to collapse the data into a single spectrum. The best data collapse was obtained when the frequencies were left unscaled. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> This paper presents beamforming techniques for source localization on aicraft in flight with a focus on the development at DLR in Germany. Fly-over tests with phased arrays are the only way to localize and analyze the different aerodynamic and engine sources of aircraft in flight. Many of these sources cannot be simulated numerically or in wind-tunnel tests because they they are either unknown or they cannot be resolved properly in model scale. The localization of sound sources on aircraft in flight is performed using large microphone arrays. For the data analysis, the source signals at emission time are reconstructed from the Doppler-shifted microphone data using the measured flight trajectory. Standard beamforming techniques in the frequency domain cannot be applied due transitory nature of the signals, so the data is usually analyzed using a classical beamforming algorithm in the time domain. The spatial resolution and the dynamic range of the source maps can be improved by calculating a deconvolution of the sound source maps with the point spread function of the microphone array. This compensates the imaging properties of the microphone array by eliminating side lobes and aliases. While classical beamfoming yields results that are more qualitative by nature, the deconvolution results can be used to integrate the acoustic power over the different source regions in order to obtain the powers of each source. ranking of the sources. These results can be used to rank the sources, for acoustic trouble shooting, and to assess the potential of noise abatement methods.Copyright © 2012 by ASME <s> BIB007 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> A keep-it-simple (KIS) solution is introduced that interpolates a signal up to an arbitrary accuracy. It consists of interpolating the complex envelopes at the output of a perfect-reconstruction filter-bank: if K subbands are used, the proposed interpolation scheme is shown to have a similar figure of merit as if the signal was initially oversampled by factor K at the acquisition stage, yet without the storage burden implied by the latter method. <s> BIB008 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r... <s> BIB009 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> Airframe noise is becoming increasingly important during approach, even reaching higher noise levels than the ::: engines in some cases. More people are affected due to low flight altitudes and fixed traffic routing associated with ::: typical approaches. Formost air- craft types, the landing gear system is a dominant airframe noise source. ::: However, this element can only be modeled in an approximate manner in wind tunnel experiments. In this ::: research, flyovers of landing aircraft were recorded using a 32 microphone array. Fun ctional beamforming was ::: applied to analyze the noise emissions from the landing gear system. lt was confirmed that for some aircraft types, ::: such as the Airbus A320 and the Fokker 70, the nose landing gear is a dominant noise source du ring approach. ::: The correlation between the noise levels generated by the landing gear and the aircraft velocity was found to be ::: significant, explai ning about 70% of the varia bility found in the noise levels, which is in good agreement with all ::: known theory. Moreover, the experimental resu lts for the Airbus A320 measurements were compared with those ::: obtained using the DLR system noise prediction tool PANAM. Whereas the total aircraft noise levels were in good ::: agreement. the measurements indicate a higher contribution from the nose landing gear noise compared ::: to the predictions. <s> BIB010 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> An 18% scale semispan model is used as a platform for examining the efficacy of microphone array processing using synthetic data from numerical simulations. Two hybrid Reynolds-Averaged-Navier-Stokes/Large-Eddy-Simulation (RANS/LES) codes coupled with Ffowcs Williams–Hawkings solvers are used to calculate 97 microphone signals at the locations of an array employed in the NASA Langley Research Center 14 × 22 tunnel. Conventional, DAMAS, and CLEAN-SC array processing is applied in an identical fashion to the experimental and computational results for three different configurations involving deploying and retracting the main landing gear and a part-span flap. Despite the short time records of the numerical signals, the beamform maps are able to isolate the noise sources, and the appearance of the DAMAS synthetic array maps is generally better than those from the experimental data. The experimental CLEAN-SC maps are similar in quality to those from the simulations indicating that CLEAN-SC may have less sensit... <s> BIB011 </s> A review of acoustic imaging methods using phased microphone arrays <s> Aircraft flyover measurements <s> Accurate predictions of aircraft noise levels are desirable to enforce noise control regulations around airports and to evaluate noise abatement procedures. The current best practice noise contour ... <s> BIB012

Measurements on flying aircraft provide the most reliable results of engine and airframe noise emissions of a certain aircraft type BIB004 , especially if the measurements are taken under operational conditions . However, less-controlled experimental conditions, like propagation effects , moving sources BIB001 , and localization of noise emitter, need to be considered. Therefore, the microphone signals have to be de-Dopplerized by re-sampling the original time series by linear interpolation BIB001 . Interpolation errors were shown to be small if the maximum frequency of analysis is restricted to one tenth of the sampling frequency and for flight Mach numbers up to 0.81 and flyover altitudes as low as 30 m BIB002 . Upsampling can be performed numerically before interpolation to alleviate this requirement BIB008 . Additional considerations need to be taken when applying deconvolution algorithms to moving sources . Some acoustic imaging results by Sijtsma et al. for flyover measurements are illustrated in Fig. 17 , where typical one-third-octave-band source plots for MD82, Fokker Fig. 17 is an adapted time-domain version of the conventional beamforming for moving sources . Aircraft flyover measurements with a similar acoustic array (238 microphones and a larger diameter of 35 m) were performed by DLR. The data are normally processed using a hybrid deconvolution method that postprocesses the conventional time-domain beamforming results for moving sources with a PSF in the frequency domain BIB005 . The results in Fig. 18 correspond to the DLR Advanced Technology Research Aircraft (ATRA, an Airbus A320) for an emission angle of = 90 • , and an aircraft height of 220 m. It can be observed how the deconvolution method improves the spatial resolution and dynamic range of the source plots. These results can be integrated over certain regions (see Fig. 18c ) to determine the contributions of different aircraft elements. Figure 19 shows the sound pressure-level breakdown of the engine source into the components defined in Fig. 18c : engine inlet, nozzle and jet. A separation between takeoff and climb operations is made. These results have been averaged over six flyovers in the takeoff and 5 flyovers in the climb configuration, respectively. The engine fan rotational speed (N1) BIB012 was 91% and 81% for the takeoff and the climb, respectively. Fig. 16 Trailing-edge noise spectra for an NACA 0012 airfoil using orthogonal beamforming considering different number k of eigenvalues compared to the results with DAMAS with 500 iterations Fig. 17 Conventional beamforming source plots for flyover measurements of MD82, Fokker 100, Airbus A340, and Boeing 777 aircraft. Adapted from Figures 20 and 21 present the results of different acoustic imaging methods applied to two aircraft flyovers which belong to a measurement campaign in Amsterdam Airport Schiphol by Delft University of Technology, where 115 landing aircraft were recorded with a considerable smaller array BIB009 BIB010 , featuring 32 microphones and a diameter of 1.7 m (i.e., about seven times fewer microphones and seven times smaller than for the array used in Fig. 17 ). The average aircraft height overhead was approximately 67 m. These flyovers were selected, because they presented a Fig. 18 a Conventional beamforming and b deconvolution source plots for a flyover measurement in landing approach for a one-thirdoctave-band centered at 3150 Hz. c Deconvolution source plot for a takeoff operation and for a one-third-octave-band centered at 6300 Hz with the different integration regions shown as dashed boxes strong tonal component at the presented frequencies: 1630 and 7140 Hz, respectively BIB009 . Figure 20 shows an Airbus A321 emitting sound at 1630 Hz, where the main noise source seems to be the nose-landing gear, probably due to a cavity BIB010 . Similar results were observed by Michel and Qiao . Functional beamforming (with = 100 ) and CLEAN-SC present the highest dynamic ranges. Figure 21 shows a Fokker 70 emitting sound at 7140 Hz, where the main noise sources appear to be located at the main landing gear wheels. Once again, functional beamforming (with = 100 ) and CLEAN-SC present the highest dynamic ranges, but CLEAN-SC only shows one of the two sources (left). Functional beamforming does not seem to have this problem. Figures 20 and 21 show that, even with a relatively cheap and small experimental setup, satisfactory results can be obtained. Additional measurements on aircraft flyovers can be found in the literature BIB002 BIB003 BIB005 BIB007 . Researchers from NASA Langley have developed field-deployable microphone phased arrays for flight tests BIB006 , including acoustic measurements of small Unmanned Aerial System (sUAS) vehicles . Investigations of the use of array processing techniques to cross-validate the computational and experimental results of airframe noise analysis have also been performed BIB011 .

A review of acoustic imaging methods using phased microphone arrays <s> Engine noise tests <s> The development of the beamforming method (also called microphone antenna, phased array of microphones, acoustic telescope, or acoustic camera) is reviewed in this paper. The microphone antenna was invented by Billingsley (1974) and has since seen dramatic improvements due to the availability of better data acquisition and computing hardware. Recent mathematical and software developments invert the beamforming process and allow a quantitative determination of the sources. Beamforming is indispensable for the localization of sound sources on moving objects, on flying aircraft, on high-speed trains, on motor cars in motion, on open rotors like helicopter and wind turbine rotors. In these applications, the ability to follow the motion of the sources is important. The second important applications are source localization tests in the test sections of open and closed wind tunnels. The background noise suppression capability of the beamforming method is required here. The various applications are discussed with a long list of references. 1 Berlin Beamforming Conference <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Engine noise tests <s> The directivities of all broadband sound sources of an aeroengine can be determined from measurements with a line array of microphones. A new data analysis method SODIX (SOurce DIrectivity modelling in crossspectral matriX) was developed for this purpose and applied to the experimental data obtained with a highbypass ratio turbofan engine in an open air test bed. The method is based on modelling the matrix of the crossspectra of the microphone signals with a set of contributions from point sources with unknown directivities assumed in positions along the engine axis. The source positions are defined with a separation of typically 0.4 wave lengths. The analysis has to be performed in narrow frequency bands. The influences of tones are eliminated in the cross-spectral matrix of the microphone signals by interpolation in the frequency domain. The unknown directivities of the point sources are determined with the side conditions that the values must be non-negative and that the directivities and source distributions are smooth. The results of this non-linear optimisation problem confirm that the various sources of an a eroengine have highly non-uniform directivities. The far field can be predicted from the source distributions f or positions over a large range of emission angles. The predictions on a 45.72 m radius around the engine agree within about 1 dB with experimental data. These far-field directivities can be separated into the contribut ions from inlet, casing, the two nozzles, and the jet. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Engine noise tests <s> An improved version of the inverse source location method SODIX is presented. SODIX models broadband nosie sources with a st of directive point sources by minimizing the difference between a measured and a simulated cross-spectral matrix of microphone signals. One big advantage of the method in comparison to other source location methods is, that it calculates a directive source strength from each point source towards each microphone. The new version of SODIX includes an implicit consideration of the constraint of positive source strengths in the iteration process. This improvement made the optimization process more stable and in addition independent of the initially assume source distribution. The method is also extended for applications with multidimensional source and microphone arrangements. Further, it is shown that the main diagonal of gthe cross-spectral matrix can be removed from the calculations without any mathematical violations and that the absolute strengths of the dominating sources are preserved. The improvements of the method are shown with exemplary results using measured data from a static aeroengine freefield test. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Engine noise tests <s> Methods based on microphone array measurements provide a powerful tool for determining the location and magnitude of acoustic sources. For stationary sources, sophisticated algorithms working in the frequency domain can be applied. By using circularly arranged arrays and interpolating between microphone signals it is possible to treat rotating sources, as are present in fans, as being non-moving. Measurements conducted with a four-bladed fan and analyzed with the “virtual rotating array” method show that it is not only possible to identify the main noise contributors, but also to determine a full spectrum for any rotating component of interest. <s> BIB004

Sound source localization techniques can also be applied to open air engine test beds and indoor engine test cells BIB001 BIB002 BIB003 and to ducted rotating machinery BIB004 .

A review of acoustic imaging methods using phased microphone arrays <s> Hardware requirements <s> Aircraft noise impact around airports will increase corresponding to the predicted growth in air-traffic if no measures for aircraft source noise reduction are taken or noise abatement flight procedures are developed. During the final approach phase engine noise and airframe noise are comparable in level, the latter being governed by flow noise originating from landing gears and high lift devices. Based on the results of dedicated wind tunnel studies semi-empirical/empirical airframe noise prediction schemes were developed for both high lift devices’ and landing gear noise to support the calculation of noise impact in the vicinity of airports. Within an ongoing German national research project on the development of noise abatement procedures, co-financed by the German Ministry of Education and Research (BMBF), flyover noise measurements were conducted on an Airbus A319 aiming at the validation of DLR’s airframe noise prediction schemes. In order to distinguish between airframe and engine noise sources flyovers were performed for different aircraft configurations and operational conditions. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Hardware requirements <s> A keep-it-simple (KIS) solution is introduced that interpolates a signal up to an arbitrary accuracy. It consists of interpolating the complex envelopes at the output of a perfect-reconstruction filter-bank: if K subbands are used, the proposed interpolation scheme is shown to have a similar figure of merit as if the signal was initially oversampled by factor K at the acquisition stage, yet without the storage burden implied by the latter method. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Hardware requirements <s> Microphone arrays are useful measurement devices for estimating the location and strength of sound sources. Numerous comparative studies have been conducted regarding the performance of acoustic imaging methods in the past, but literature lacks of a systematic investigation on the role of the hardware on the measurements. This research focuses on the performance differences between two 63-microphone arrays: one with digital MEMS (Micro Electro Mechanical Systems) microphones and the other with analog condenser microphones. Both systems are used on an aeroacoustic experiment performed in an anechoic open-jet wind tunnel featuring two airfoils (NACA 0012 and NACA 0018) equipped with trailing-edge serrations. Whereas both arrays provided similar frequency spectra when analyzing trailing-edge noise emissions (which are in agreement with previous research), the analog array seems to offer source maps of higher quality with a higher dynamic range (lower sidelobe level). Moreover, the results of the digital array featuring trailing-edge serrations show a noise increase at the higher frequencies (4 kHz) with respect to the straight-edge case, which is not expected from the findings of previous experimental research. The results of the analog array do not present such behavior. This manuscript is the result of a collaboration project between the University of Twente (UTwente) and Delft University of Technology (TU Delft). <s> BIB003

Processing multiple microphone signals increases the signalto-noise ratio (SNR) and the spatial resolution compared to a measurement with only one microphone . The choice of the microphones highly depends on the particular experiment to be performed BIB003 . Characteristics such as the dynamic range, the frequency range, and the sensitivity have to be selected with care. In general, smaller microphones can measure up to higher frequencies and larger microphones have higher sensitivities. The directivity of the microphones has to be taken into account, as well, especially for higher frequencies. Most of these specifications are provided by the manufacturer. All microphone signals have to be simultaneously sampled by the data acquisition system. The sampling frequency should be at least twice the maximum frequency of interest, according to the sampling theorem. As mentioned in Sect. 1.3, it is recommended to have a sampling frequency ten times the maximum frequency of interest in the case of flyover measurements or to perform upsampling BIB002 . Monitoring the data acquisition during the experiment is recommended, to check that the frequency spectra obtained are valid. Moreover, for microphone arrays installed in small plates, the sound waves scatter at the edges of the plate. Reflections from walls in wind tunnels and from the ground in flyover measurements should also be taken into account. These phenomena produce phase and amplitude errors on the measured signal BIB001 . Thus, it is recommended to either use hard plates (for complete reflection) or on an acoustically transparent structure, using sound absorbing materials, for example. The setup choice depends on the experiment to be performed and, of course, on the budget available.

A review of acoustic imaging methods using phased microphone arrays <s> Metrological determination of the microphone positions <s> L-BFGS-B is a limited-memory algorithm for solving large nonlinear optimization problems subject to simple bounds on the variables. It is intended for problems in which information on the Hessian matrix is difficult to obtain, or for large dense problems. L-BFGS-B can also be used for unconstrained problems and in this case performs similarly to its predessor, algorithm L-BFGS (Harwell routine VA15). The algorithm is implemented in Fortran 77. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Metrological determination of the microphone positions <s> The \delay and sum beamformer" algorithm (\DSB") is a powerful tool for the localisation and quantication of acoustic sources with microphone arrays. For the calculation of beamforming maps the DSB algorithm requires the following input data: time series of all microphones, a grid of focus points which includes the region of interest, parameter of the ow for boundary layer or shear layer corrections and the accurate position of all microphones. The present paper is focused on the last item: the accurate estimation of the microphone positions. Especially for aeroacoustic applications the number of microphones should be large enough in order to obtain good beamforming results. The estimation of the accurate microphone positions can mean a huge time consuming eort. The method which will be presented in this paper is similar to the well known global positioning system: distances to satellites provide information about the position of a receiver. Here, several monopole-like acoustical point sources with known positions and a reference microphone which is installed close to the sound sources are used to compute the position of the microphones of a microphone array in the three-dimensional space. After pointing out the basic concepts and algorithms a practical implementation of the test sources is described. Eight test sources and the reference microphone are integrated in a so-called calibration unit. Afterwards a calibration of a microphone array with known microphone positions is presented to verify the method and to assess the accuracy that can be achieved. Furthermore the problem is addressed how many test sources are necessary to achieve accurate results. Finally, the procedure is used to calibrate an out-of-ow microphone array with a layout of microphones where the positions are only known with some uncertainty. Investigations concerning the frequency dependence of the calibration are presented. Beamforming on a loudspeaker is performed to show in how far more accurately known microphone positions can improve beamforming results, particularly in the higher frequency range. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Metrological determination of the microphone positions <s> The growing number of commercially available beamforming systems and the quite large number of end users and application fields confronts all potential customers with the task of comparing the systems of different suppliers as well as with the problem of evaluating the quality of beamforming solutions in general. This article will point out some of the main problems connected to this practically often difficult task and it also develops some proposals for a more systematic evaluation of acoustic array beamforming systems. Some useful quantities and evaluation methods which seem appropriate for a possible classification of beamforming systems will be proposed and discussed in detail. This discussion shall serve as a first contribution for a future standardisation and hence for the further qualification of acoustic beamforming as a measurement method. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Metrological determination of the microphone positions <s> The exact positions of microphones are mandatory when using them as an array to apply beamforming algorithms. Nevertheless to obtain these positions in the three dimensional space is challenging. In this paper, further developments of the acoustic calibration unit, proposed in 2009 by Lauterbach are presented. The acoustic calibration unit consists of several speakers and a reference microphone to obtain the positions of an arbitrary amount of target microphones via triangulation. These calculated positions is not necessarily the geometric position of the microphone, it is the acoustic position, which includes possible phase shifts, directivity and mounting effects. In the new acoustic calibration unit a second reference microphone is installed to measure the actual speed of sound. A new triangulation algorithm was developed to reduce the uncertainty of the known reference positions. Furthermore the new acoustic calibration unit was tested on an traversed array with 16 microphones in an anechoic chamber and the open jet array of the ONW-LLF Windtunnel with an diameter of about 4 m. In the final paper a third measurement will be included with an array where the geometric microphone positions are known within a tolerance of 1/100 mm. <s> BIB004

A precise calibration of the microphone positions is crucial for accurate aeroacoustic measurements . Small sound sources with omnidirectional radiation patterns and with known sound pressure levels are recommended for the calibration of the microphone array in both source location and quantification. Broadband white noise signals are preferred, instead of tonal sound at single frequencies to avoid coherence problems BIB003 . Acoustic GPS is a method to determine the positions of an arbitrary number of microphones, especially for large-aperture and three-dimensional arrays. Similar to the satellite global positioning system (GPS), positions are calculated based on the time delays between the known reference positions and the microphones. The procedure requires that the pathway of acoustic waves propagating from the acoustic GPS to each microphone is not obstructed by any obstacle. However, perfect free-field conditions are not required. The basic acoustic GPS consists of N speakers speakers and one reference microphone. They are mounted at known positions on a reverberate plate. The speakers are then driven with a noise signal, one speaker at a time, and the time delays between the reference microphone and all the array microphones are calculated. To achieve the required subsample accuracy, the region around the peak in the cross correlation for delay determination is interpolated with a spline. The sampling frequency is chosen as high as possible. Using these time delays, a set of N speakers non-linear equations can be set up for every microphone. where x n is the nth unknown microphone position, j are the known speaker positions, x ref is the known reference microphone position, t n,ref,j is the measured time delay between the array microphone and reference microphone for the jth speaker, c is the speed of sound, and N speakers is the number of speakers. These equations can be solved in a least-squares sense with the classical Newton method or with the other methods, such as the L-BFGS (limited memory version of the Broyden-Fletcher-Goldfarb-Shanno algorithm) BIB001 . Since there are three coordinates to calculate, the number of speakers should be at least 3. Lauterbach et al. BIB002 chose N speakers = 8. Ernst et al. BIB004 chose N speakers = 6 and used two reference microphones to calculate the speed of sound and a more advanced algorithm to account for geometrical errors of the known speaker and reference microphone positions. The positioning accuracy decreases with increasing distance between the acoustic GPS and the array microphones. Usually, the positioning error is smaller than 1 mm. Moreover, the method is capable of compensating small individual phase errors of each array microphone and accounts for these by providing a more suitable "acoustic position" rather than a sole geometric position. Researchers from NASA Langley recently employed a hovering aerial sound source to calibrate a microphone array developed for flyover measurements , see Fig. 4 . A quality-phase calibration source should be a stable point source at a precisely known location that is free from extraneous

A review of acoustic imaging methods using phased microphone arrays <s> Microphone distribution guidelines <s> The development of the beamforming method (also called microphone antenna, phased array of microphones, acoustic telescope, or acoustic camera) is reviewed in this paper. The microphone antenna was invented by Billingsley (1974) and has since seen dramatic improvements due to the availability of better data acquisition and computing hardware. Recent mathematical and software developments invert the beamforming process and allow a quantitative determination of the sources. Beamforming is indispensable for the localization of sound sources on moving objects, on flying aircraft, on high-speed trains, on motor cars in motion, on open rotors like helicopter and wind turbine rotors. In these applications, the ability to follow the motion of the sources is important. The second important applications are source localization tests in the test sections of open and closed wind tunnels. The background noise suppression capability of the beamforming method is required here. The various applications are discussed with a long list of references. 1 Berlin Beamforming Conference <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Microphone distribution guidelines <s> Investigation of the landing gear noise of the Boeing ::: 747-400 in flight. Comparison of different fly-over ::: velocities in order to obtain a scaling law for the ::: Sound pressure Level based on microphone Array measurements. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Microphone distribution guidelines <s> Assigning proper positions to microphones within arrays is essential in order to reduce or eliminate side- and grating lobes in 2D beamform images. In this paper an objective function is derived providing a measure for the presence of artificial sources. Using the global optimization method Differential Evolution an optimized microphone configuration is obtained by minimization of this objective function. Results show that optimizing the microphone locations can significantly enhance the array performance. In a large part of the scan region surrounding the true source location, no side- or grating lobes are present, meaning that the source can be unambiguously located. From the optimization it is also found that the optimized array configuration shows the microphones distributed at almost constant distance. A linear relation is found which shows that the distance decreases with increasing frequency. This knowledge is important information for the design of an optimal microphone configuration. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Microphone distribution guidelines <s> The arrangement of microphones in an array does have a strong influence on the properties of the beamforming result. These properties are usually characterized by the beam width and the maximum side lobe level. Depending on the intended application of the array it is desirable to use microphone arrangements only that provide optimal properties using a given number of microphones. There have been several approaches in the past that rely on numerical methods to find optimal locations for all microphones in a planar array. The paper introduces a new method that does not require a numerical optimization. First, some general topological properties of any arrangement are considered. Then, the theory for continuous arrays (with infinite number of microphones) is revisited. This is used as a basis to develop a generic two-parameter approach that synthesizes array microphone arrangements. Finally, it is shown that this approach leads to Pareto-optimal arrangements and results are compared to those of other commonly used microphone arrangements. <s> BIB004

A detailed study of the optimization of array microphone distribution is out of the scope of this manuscript, but arrays consisting of spirals or of several circles with an odd number of regularly spaced microphones seem to perform best BIB001 . Interesting studies using the optimization methods BIB003 and thorough parametric approaches leading to parettooptimal arrangements BIB004 can be found in the literature. The microphone positions in the first large test campaign of aircraft flyovers were optimized with a genetic algorithm. To study moving sources, such as aircraft flyovers, the array shape can be elongated in the flight direction BIB002 to compensate for the loss of resolution due to emission angles other than 90 • . The Boeing Company also used an elliptical array shape for their flyover tests in the QTD2 program consisting of 614 microphones in five subarrays with an overall size of approximately 91 m by 76 m, likely the largest array ever used in flyover tests.

A review of acoustic imaging methods using phased microphone arrays <s> Microphone weighting and coherence loss <s> A study was conducted to examine the effects of overall size of directional (or phased) arrays on the measurement of aeroacoustic components. An airframe model was mounted in the potential core of an open-jet windtunnel, with the directional arrays located outside the flow in an anechoic environment. Two array systems were used; one with a solid measurement angle that encompasses 31.6 degrees of source directivity and a smaller one that encompasses 7.2 degrees. The arrays, and sub-arrays of various sizes, measured noise from a calibrator source and flap edge model setups. In these cases, noise was emitted from relatively small, but finite size source regions, with intense levels compared to other sources. Although the larger arrays revealed much more source region detail, the measured source levels were substantially reduced due to finer resolution compared to that of the smaller arrays. To better understand the measurements quantitatively, an analytical model was used to define the basic relationships between array to source region sizes and measured output level. Also, the effect of noise scattering by shear layer turbulence was examined using the present data and those of previous studies. Taken together, the two effects were sufficient to explain spectral level differences between arrays of different sizes. An important result of this study is that total (integrated) noise source levels are retrievable and the levels are independent of the array size as long as certain experimental and processing criteria are met. The criteria for both open and closed tunnels are discussed. The success of special purpose diagonal-removal processing in obtaining integrated results is apparently dependent in part on source distribution. Also discussed is the fact that extended sources are subject to substantial measurement error, especially for large arrays. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Microphone weighting and coherence loss <s> The use of phased array for acoustics measurement in wind tunnels is known to suffer degradation in integrated SPL and acoustic image dynamic range due to decorrelation between each pair of microphones after the acoustic ray has cros sed the wind tunnel boundary layer.. Based in the extensive works of radio propagation, underwater acoustic and atmospheric science, a physical and statiscal model for wave propagation in turbulent media is studied., including predictions of decorrelation corrections of beamforming results. The parabolic equation for acoustic wave propagation is considered and the Markov approximation is assumed. Hotwire measurements were made and the turbulence parameters, as boundary layer thickness, were inserted into the model. Theoretical turbulent models were considered and the results showed Gaussian behavior. Acoustic phased array measurement were made and the results shown to agree with the predictionss in some conditions. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Microphone weighting and coherence loss <s> Phased arrays are a common tool for detecting aero-acoustical sources in closed or open jet wind tunnels. The usual design process of a phased array can be summarized to the following steps: In the first step, an ::: ideal sound source is assumed and the theoretical array output (point-spread function, "psf ") is calculated for a given microphone distribution. General shaping parameters for the psf are the minimal distance between ::: two microphones - limiting the upper frequency resolution - and the maximum distance between two microphones (or: aperture size) - limiting the array resolution towards low frequencies. Based on the psf a cost function can be defined in order to rate the array. Possible approaches for such a cost function are to integrate the energy of the psf, to detect the highest side lobe, or to compute the deviation to a desired psf in a ::: specified frequency range. Whichever cost function is chosen, in the last step this cost function is minimized by variation of the microphone positions. The impact of a flow is usually neglected in this design process. However, due to turbulent decorrelation and/or retardation of sound waves in a shear layer and convection in the core flow, the optimum array in a case with flow might differ from an array optimised for no-flow condition. Assuming uniform flow without turbulence the sound propagation is only shifted in time due to the convective velocity. This ::: effect can be included in the array design by using the convective velocity in the calculation of travel time between source and receiver as introduced by Amiet.^1 Furthermore, on its way to the microphones, the sound wave has to pass a turbulent boundary layer in the closed test section and a turbulent shear layer in the open test section, respectively. Due to this turbulence the coherence of the sound wave at the microphone positions decreases with increasing frequency, increasing distance to other microphones and increasing turbulence level.^2 The standard beamforming algorithm in the frequency domain is based on cross correlations between different microphone positions. Therefore, a low coherence results in a loss of image resolution and a modified sound ::: level in the beamforming result. In order to optimize an array pattern in terms of decorrelation, an analytical prediction of the coherence loss is required. <s> BIB003

Different weighting or "shading" functions can be applied to the signals of each microphone to obtain better acoustic imaging results . Moreover, the beamwidth can be kept roughly constant by selecting smaller subarrays for higher frequencies to minimize coherence loss. This requires clustering the microphones in the center of the array. Shading can be applied per one-third-octave band to reduce the coherence loss, to compensate for the non-uniform microphone density, and to reduce the sidelobe levels. The impact of decorrelation of acoustic waves when passing through the shear layer in open-jet wind tunnels or through the boundary layer in closed-section wind tunnels is usually neglected. Decorrelation, however, results in both a loss of image resolution and a corruption of the sound levels in the source map. The influence of decorrelation can be estimated by means of an analytical model where the shear (or boundary) layer is modeled as a random medium with a single length scale BIB002 BIB003 . In Fig. 5 , a comparison between the theoretical predictions BIB001 assuming a Gaussian turbulence model and the measured coherence in the DNW-NWB open-jet wind tunnel BIB003 is shown. It can be seen that, as expected, the coherence loss increases with the freestream velocity, the sound frequency, and the streamwise distance of the microphone. The agreement between measurements and theoretical predictions is deemed as acceptable in most cases for microphone positions within a common distance range used in practice (below 2 m). It seems like the theoretical models overpredict the coherence values for distances close to the source, and then, after a threshold distance, they underpredict the coherence with respect to the experimental measurements.

A review of acoustic imaging methods using phased microphone arrays <s> Acoustic imaging methods <s> 1. Introduction 2. Signals in Space and Time 3. Apertures and Arrays 4. Conventional Array Processing 5. Detection Theory 6. Estimation Theory 7. Adaptive Array Processing 8. Tracking Appendices References List of Symbols Index. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Acoustic imaging methods <s> The problem of localizing and quantifying acoustic sources from a set of acoustic measurements has been addressed, in the last decades, by a huge number of scientists, from different communities (s... <s> BIB002

A vast list of acoustic imaging algorithms exists in the literature BIB002 . Some of them are based on the deconvolution of the sound sources, i.e., the removal of the effect of the PSF of the sound sources, such as CLEAN-SC, DAMAS, etc. These methods aim at enhancing the results of the conventional beamforming BIB001 , but their computational time is considerably larger. Most of the listed methods require a scan grid, where all the grid points are considered as potential sound sources. This section aims to summarize widely used acoustic imaging methods for aeroacoustic experiments.

A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> An overview of beamforming from a signal-processing perspective is provided, with an emphasis on recent research. Data-independent, statistically optimum, adaptive, and partially adaptive beamforming are discussed. Basic notation, terminology, and concepts are included. Several beamformer implementations are briefly described. > <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> 1. Introduction 2. Signals in Space and Time 3. Apertures and Arrays 4. Conventional Array Processing 5. Detection Theory 6. Estimation Theory 7. Adaptive Array Processing 8. Tracking Appendices References List of Symbols Index. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> A study was conducted to examine the effects of overall size of directional (or phased) arrays on the measurement of aeroacoustic components. An airframe model was mounted in the potential core of an open-jet windtunnel, with the directional arrays located outside the flow in an anechoic environment. Two array systems were used; one with a solid measurement angle that encompasses 31.6 degrees of source directivity and a smaller one that encompasses 7.2 degrees. The arrays, and sub-arrays of various sizes, measured noise from a calibrator source and flap edge model setups. In these cases, noise was emitted from relatively small, but finite size source regions, with intense levels compared to other sources. Although the larger arrays revealed much more source region detail, the measured source levels were substantially reduced due to finer resolution compared to that of the smaller arrays. To better understand the measurements quantitatively, an analytical model was used to define the basic relationships between array to source region sizes and measured output level. Also, the effect of noise scattering by shear layer turbulence was examined using the present data and those of previous studies. Taken together, the two effects were sufficient to explain spectral level differences between arrays of different sizes. An important result of this study is that total (integrated) noise source levels are retrievable and the levels are independent of the array size as long as certain experimental and processing criteria are met. The criteria for both open and closed tunnels are discussed. The success of special purpose diagonal-removal processing in obtaining integrated results is apparently dependent in part on source distribution. Also discussed is the fact that extended sources are subject to substantial measurement error, especially for large arrays. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Acoustic source mapping techniques using acoustic sensor arrays and delay-and-sum beamforming techniques suffer from bad spatial resolution at low-aperture-based Helmholtz numbers. This is especially a problem for three-dimensional map grids, when the sensor array is not arranged around the region spanned by the grid but on only one side of it. Then, the spatial resolution of the result map in the direction pointing away from the array is much worse than in the other lateral directions. Consequently, deconvolution techniques need to be applied. Some of the most efficient deconvolution techniques rely on the properties of the spatial beamformer filters used. As these properties ::: are governed by the steering vectors, four different steering vector formulations from the literature are examined, and their theoretical background is discussed. It is found that none of the formulations provide both the correct location and source strength. As a practical example the CLEAN-SC deconvolution methodology is applied to simulated data for a three-source scenario. It is shown that the different steering vector formulations are not equally well suited for three-dimensional application. The two preferred formulations enable the correct estimation of the source location at the cost of a negligible error in the estimated source strength. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Abstract In this paper, an alternative formulation of the time-domain beamforming is proposed using the generalized cross-correlation of measured signals. This formulation uses spatial weighting functions adapted to microphone positions and imaging points. The proposed approach is demonstrated for acoustic source localization using a microphone array, both theoretically and experimentally. An increase in accuracy of acoustic imaging results is shown for both narrow and broadband sources, while a factor of reduction up to 20 in the computation time can be achieved, allowing real-time or volumetric source localization over very large grids. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Trailing edge serrations designed to reduce airfoil self-noise are retrofitted on a NACA 0018 airfoil. An investigation of the boundary layer flow statistical properties is performed using time-resolved stereoscopic PIV. Three streamwise locations over the edge of the serrations are compared. An analysis of the results indicates that, while there is no upstream effect, the flow experiences significant changes as it convects over the serrations and toward its edges. Among the most important, a reduced shear stress and modifications of the turbulence spectra suggest beneficial changes in the unsteady surface pressure that would result in a reduction of trailing edge noise. Microphone array measurements are additionally performed to confirm that noise reduction is indeed observed by the application of the chosen serration design over the unmodified airfoil. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Abstract Millions of workers are exposed to excessive noise levels each day. Acoustic solutions have to be developed to protect workers from hearing loss. The first step of an acoustic diagnosis is the source localization which can be performed with a microphone array. Spherical microphone arrays can be used to detect the acoustic source positions in a workplace. In this study, a spherical microphone array, with polyhedral discretization, is proposed and compared with a spherical array with a slightly different geometry. The generalized cross-correlation technique is used to detect the source positions. Moreover, two criteria are introduced to improve the noise source map. The first is based on the geometric properties of the microphone array and the scan zone whereas the second is based on the energy of the spatial likelihood function. Numerical data are used to provide a systematic comparison of both geometries and criteria. Finally, experiments in a reverberant room reveal that the polyhedral microphone array associated with both criteria provides the best noise source map. <s> BIB007 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Abstract This paper addresses source localization technique in time domain for broadband acoustic sources. The objective is to accurately and quickly detect the position and amplitude of noise sources in workplaces in order to propose adequate noise control options and prevent workers hearing loss or safety risk. First, the generalized cross correlation associated with a spherical microphone array is used to generate an initial noise source map. Then a linear inverse problem is defined to improve this initial map. Commonly, the linear inverse problem is solved with an l 2 -regularization. In this study, two sparsity constraints are used to solve the inverse problem, the orthogonal matching pursuit and the truncated Newton interior-point method. Synthetic data are used to highlight the performances of the technique. High resolution imaging is achieved for various acoustic sources configurations. Moreover, the amplitudes of the acoustic sources are correctly estimated. A comparison of computation times shows that the technique is compatible with quasi real-time generation of noise source maps. Finally, the technique is tested with real data. <s> BIB008 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> The broadband noise generated by the scattering of turbulent flow at the trailing edge of a NACA 0018 airfoil with trailing edge serrations is investigated, varying both the airfoil angle of attack and serration flap angle. Acoustic emissions from the trailing edge are measured using a microphone array. The noise level is observed to be higher than that of the airfoil without serrations at frequencies beyond a crossover value. The latter is found to scale with a characteristic Strouhal number based upon the boundary layer thickness and the freestream velocity. A satisfactory collapse of the results under varying angles of attack and freestream velocities is observed. The modifications of the hydrodynamic behavior and the noise increase are linked by high-speed observations conducted with particle image velocimetry. An increase in the energy of turbulent fluctuations is also observed at the expected crossover frequency. The dominant cause of the increased noise is thereby identified at the pressure side edge of the serrations at a given flap angle. <s> BIB009 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Trailing edge serrations have been proven to work as a passive noise reduction device. Nevertheless, they have also previously been found to increase noise in a particular frequency range, argued in earlier research to be due to the misalignment of the serrations with the direction of the flow in the wake. It emerges as a high-frequency noise increase in a broadband region of the spectrum. This study investigates the effect of serration-flow misalignment on the noise emissions using acoustic beamforming, and finds a correlation with observations made on the flow using particle image velocimetry (PIV). The hydrodynamic source of the noise increase is hereby identified, and a Strouhal number relation for the high-frequency noise increase is proposed <s> BIB010 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Trailing-edge serrations are passive noise reduction add-ons widely used in wind-turbine applications. This study presents acoustic beamforming results of microphone array measurements of a cambered airfoil (DU96-W-180) in a closed-section wind-tunnel at a Reynolds number of industrial interest. Two different serration geometries with different lengths were tested and compared with the straight-edge baseline airfoil. The serrations were set at a flap angle of 6 degrees. Several flow velocities and angles of attack were tested at three chord-based Reynolds numbers ranging from 5105 to 1.5106. A phased microphone array was used to obtain source maps of the trailing-edge noise; Particle Image Velocimetry (PIV) was employed to obtain information about the turbulent boundary layer approaching the trailing edge; further, a numerical simulation using the Lattice Boltzmann Method (LBM) was performed for comparison. Far-field noise from the experimental data and computations shows a satisfactory agreement. Noise reductions of several dB were obtained, especially at lower frequencies. An increase in high-frequency noise is observed after a crossover frequency, which is assumed to be due to the set flap angle. <s> BIB011 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> Most acoustic imaging methods assume the presence of point sound sources and, hence, fail to correctly estimate the sound emissions of distributed sound sources (such as trailing– edge noise). In this contribution, three integration techniques are suggested to overcome this issue based on models considering a single point source, a line source and several line sources, respectively. Two simulated benchmark cases featuring distributed sound sources are used to compare the performance of these integration techniques with respect to other well–known methods. The considered integration methods provide the best performance in retrieving the source levels and require short computational times. In addition, the presence of unwanted noise sources, such as corner sources in wind–tunnel measurements, no longer affects the results negatively when using the last method. A sensitivity analysis shows that the integration technique based on a line source is robust with respect to the choice of the integration area (shape, position and mesh fineness). Practical recommendations are provided for the application of these methods to experimental cases. <s> BIB012 </s> A review of acoustic imaging methods using phased microphone arrays <s> Conventional beamforming (CFDBF) <s> The trailing edge of a NACA 0018 airfoil is modified through the attachment of serrations with different degrees of permeability. Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges. Different freestream velocities and angles of attack are investigated. The serration permeability is prescribed by having slits cut into the solid surface of the serrations in two different configurations. The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance. <s> BIB013

The conventional beamforming BIB001 BIB002 is a very popular method, since it is robust, fast, and intuitive. However, it suffers from the Sparrow resolution limit 1 and presents a high sidelobe level, especially at high frequencies. It can be applied both in the time domain BIB007 BIB008 BIB005 or in the frequency domain . The former is normally applied to moving sources and the latter is more commonly used for stationary sources due to the lower computational time required. The conventional frequency domain beamforming (CFDBF) algorithm considers the Fourier transforms of the recorded pressures in each of the N microphones of the array as an N-dimensional for p(f ) ∈ ℂ N×1 , with frequency (f ) dependence: Assuming a single sound source in the scan point j , the received signal is modeled as s j g j , where s j is the source strength and g j ∈ ℂ N×1 is the so-called steering vector. The steering vector has N components, g j,n , n ∈ [1 … N] , which are the modeled pressure amplitudes at the microphone locations for a sound source with unit strength at that grid point . There are several steering vector formulations in the literature BIB004 , each of them with different qualities and limitations. For simplicity, monopole sources are normally considered. For a stationary point source, the steering vector is the free-field Green's function of the Helmholtz equation: where ‖⋅‖ is the Euclidean norm of the vector, i 2 = −1 , t j,n is the time delay between the emission and the reception of the signal by the observer and x n = x n , y n , z n ∈ ℝ N×3 , n = 1 … N which are the locations of the N microphones. An estimate for the source autopower, A, at a source located at grid point j is obtained by minimizing (in a leastsquares sense) the difference between the recorded pressure vector, p , and the modeled pressures for a source at that grid point j , s j g j : In Eq. (4), an asterisk, (⋅) * , denotes the complex conjugate transpose, ⟨⋅⟩ denotes the time average of several snapshots, and w j is the weighted steering vector (once again, several different formulations for the weighted steering vector exist in the literature BIB004 ): and C is the N × N cross-spectral matrix (CSM) of the measured pressures, generated by averaging the Fourier-transformed sample blocks over time: A source map obtained with CFDBF is the summation of the PSFs of the actual sound sources, and since the strengths of the sources are always positive, because C is positive-definite (and due to the fact that noise is present in practice), the source plot represents an overestimation of the actual source levels when multiple sound sources are present. The main diagonal of C can be removed to neglect the contribution of noise which is incoherent for all the array microphones. This can be especially useful for cases with wind noise or TBL noise, such as in closed-section wind tunnels . However, precaution has to be taken when removing the main diagonal of C , because, then, C is no longer positive-definite and the PSF can get negative values (which are not physical) and the negative sidelobes of strong sources can eliminate weaker sources in some cases. This fact is explained by the appearance of negative eigenvalues in C when the main diagonal is removed, since the sum of the eigenvalues of a matrix is always equal to the sum of its diagonal elements (zero, in this case). Hence, all the methods based on the CFDBF algorithm will suffer from this issue with the diagonal removal process. When directly applied to distributed sound sources, CFDBF (and other methods assuming the presence of point sources) can lead to erroneous source levels. Therefore, integration methods, such as the source power integration (SPI) BIB003 technique, have been proposed to deal with this issue and limit the influence of the array PSF. This method can be extended to consider line sources BIB012 when distributed sound sources, such as trailing-edge noise BIB006 BIB009 BIB013 BIB010 BIB011 , are expected. This integration technique has showed the most accurate results in a simulated benchmark case BIB012 representing a trailing-edge noise measurement in a closed-section wind tunnel, with respect to the other wellknown acoustic imaging methods.

A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Summary ::: ::: The estimation of spectra of random stationary processes is an important part of the statistics of random processes. There are several books on spectral analysis, e.g. Blackman & Tukey, Hannan, and Jenkins & Watts. As a rule, spectral estimators are quadratic functions of the realizations. Recently Capon suggested a new method for estimation of spectra of random fields, in which a non-quadratic function of the realization is used: he considered a homogeneous random field ζ(t,x1,x2), i.e. one which is stationary in time and space and a random function of the time and space co-ordinates t, x1, x2. For the sake of expository convenience we shall consider ordinary stationary processes of time only, ζ(t); the generalization of our results to the case of random fields is easy. ::: ::: ::: ::: Comparison of the conventional spectral estimator and the ‘high-resolution’ estimator for an artificial example showed that the latter has less smoothing effect on the true spectrum (Capon). This was later confirmed by examples using real data (Capon). However, it was not clear whether for a finite realization the high-resolution estimator distorted the true spectrum, i.e. whether it behaved for example like a conventional estimator raised to some power. ::: ::: ::: ::: In the present paper we introduce and study a new class of spectral estimators which are generally non-linear and non-quadratic functionals of the realizations. These estimators include the conventional and high-resolution ones, for which we shall give the approximate distributions. We derive under rather general conditions the limiting distribution of the new class of estimators, and illustrate them with several examples. As a matter of fact, these new estimators are weighted means of the eigenvalues of the covariance matrix, e.g. the arithmetic mean, geometric mean, and so on. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r... <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Airframe noise is becoming increasingly important during approach, even reaching higher noise levels than the ::: engines in some cases. More people are affected due to low flight altitudes and fixed traffic routing associated with ::: typical approaches. Formost air- craft types, the landing gear system is a dominant airframe noise source. ::: However, this element can only be modeled in an approximate manner in wind tunnel experiments. In this ::: research, flyovers of landing aircraft were recorded using a 32 microphone array. Fun ctional beamforming was ::: applied to analyze the noise emissions from the landing gear system. lt was confirmed that for some aircraft types, ::: such as the Airbus A320 and the Fokker 70, the nose landing gear is a dominant noise source du ring approach. ::: The correlation between the noise levels generated by the landing gear and the aircraft velocity was found to be ::: significant, explai ning about 70% of the varia bility found in the noise levels, which is in good agreement with all ::: known theory. Moreover, the experimental resu lts for the Airbus A320 measurements were compared with those ::: obtained using the DLR system noise prediction tool PANAM. Whereas the total aircraft noise levels were in good ::: agreement. the measurements indicate a higher contribution from the nose landing gear noise compared ::: to the predictions. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Microphone array techniques are an efficient tool to detect acoustic source positions. The delay and sum beamforming is the standard method. In the time domain, the generalized cross-correlation can be used to compute the noise source map. This technique is based on the arithmetic mean of the spatial likelihood functions. In this study, the classical arithmetic mean is replaced by the more standard generalized mean. The noise source maps provide by the arithmetic, geometric and harmonic means are compared in the case of numerical and experimental data obtained in a reverberant room. The geometric and harmonic means provide the best noise source maps with no side lobes and a better source level estimation. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Microphone arrays have become efficient tools to localize sound acoustic sources. The standard method is the delay and sum beamforming which can be used in the frequency or time domain. In the frequency domain, the cross spectral matrix of the microphone signals is used to generate the noise source map. Therefore, the processing has to be done frequency by frequency. To improve noise source maps, deconvolution or inverse methods can be utilized. However, these methods increase the computation time and may require user-defined parameters. In the time domain, the generalized cross-correlation of the microphone signals is used to compute the noise source map. The generalized cross-correlation of a microphone pair signal provides the spatial likelihood functions. Then, these spatial likelihood functions are averaged to generate the noise source map. This map is composed of a main lobe which leads to the source position together with side and spurious lobes which may prevent the localization of the source. The spatial likelihood functions averaging process is commonly based on the arithmetic mean. In this study, averaging based on the generalized mean is considered to improve the source localization. Several source-microphone array configurations are numerically computed. Noise source maps obtained with geometric and harmonic means are compared with that based on the classical arithmetic mean. The geometric and harmonic means are shown to provide the best noise source maps with few side lobes. The effect of the number of microphones on the noise source map quality is also investigated. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Measured cross-spectral matrices (CSMs) from a microphone array will in some cases be contaminated by severe incoherent noise signals in the individual channels. A typical example is flow noise generated in the individual microphones when measuring in a wind tunnel. Assuming stationary signals and performing long-time averaging, the contamination will be concentrated on the CSM diagonal. When the CSM is used for traditional frequency-domain beamforming, diagonal removal (DR) will avoid use of the diagonal. DR is effective at suppressing the contamination effects, but it also has some side effects. With other beamforming algorithms and in connection with acoustic holography, however, the diagonal of the CSM is needed. The present paper describes a method for removal of incoherent noise contamination from the CSM diagonal. The method formulates the problem as a semidefinite program, which is a convex optimization problem that can be solved very efficiently and with guaranteed convergence properties. A first numerical study investigates the question, whether the semidefinite program formulation will provide in all cases the desired output. A second numerical study investigates the limitations introduced by off-diagonal noise contributions due to finite averaging time. The results of that study are backed up by results from a practical measurement. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> Functional beamforming <s> Trailing-edge serrations are passive noise reduction add-ons widely used in wind-turbine applications. This study presents acoustic beamforming results of microphone array measurements of a cambered airfoil (DU96-W-180) in a closed-section wind-tunnel at a Reynolds number of industrial interest. Two different serration geometries with different lengths were tested and compared with the straight-edge baseline airfoil. The serrations were set at a flap angle of 6 degrees. Several flow velocities and angles of attack were tested at three chord-based Reynolds numbers ranging from 5105 to 1.5106. A phased microphone array was used to obtain source maps of the trailing-edge noise; Particle Image Velocimetry (PIV) was employed to obtain information about the turbulent boundary layer approaching the trailing edge; further, a numerical simulation using the Lattice Boltzmann Method (LBM) was performed for comparison. Far-field noise from the experimental data and computations shows a satisfactory agreement. Noise reductions of several dB were obtained, especially at lower frequencies. An increase in high-frequency noise is observed after a crossover frequency, which is assumed to be due to the set flap angle. <s> BIB007

Functional beamforming is a method developed by Dougherty which is a modification of the CFDBF algorithm. A similar formulation was first proposed by Pisarenko BIB001 . Since the CSM is Hermitian and positive semidefinite, it can be expressed as its eigenvalue decomposition: where U is a unitary matrix whose columns are the eigenvectors ( u 1 , … , u N ) of C , and is a diagonal matrix whose diagonal elements are the real-valued eigenvalues The expression for the functional beamformer is as follows: with ≥ 1 a parameter which needs to be set by the user. It can be easily proven that, for the case of = 1 , the CFDBF method is obtained and that = −1 provides the adaptive beamforming formula (see Sect. 3.8). For single sound sources, the PSF, which has a value of one at the correct source locations and alias points and a value less than one elsewhere, is powered to the exponent . Therefore, powering the PSF at a sidelobe will lower its level, leaving the true source value virtually identical if an adequate grid is used BIB002 . For ideal conditions, the dynamic range of functional beamforming should increase linearly with the exponent value, . Thus, for an appropriate exponent value, the dynamic range is significantly increased. The computational time for the functional beamforming is basically identical to the CFDBF one, since the only relevant operation added is the eigenvalue decomposition of C. The application of the diagonal removal method aforementioned to functional beamforming is even more prone to errors, since this algorithm is based on the eigenvalue decomposition of the CSM. Mitigation of the diagonal removal issue is possible with CSM diagonal reconstruction methods BIB006 . In the previous work, functional beamforming has been applied to numerical simulations BIB002 , controlled experiments with components in a laboratory , and to full-scale aircraft flyover measurements under operational conditions BIB002 BIB003 . A similar integration method as the SPI technique was used for quantifying noise sources in flyover measurements BIB007 . A somewhat similar time-domain technique based on the generalized mean of the generalized cross correlation has been developed recently BIB004 BIB005 .

A review of acoustic imaging methods using phased microphone arrays <s> Orthogonal beamforming <s> 1 Introduction.- 1.1 Introduction.- 1.2 Organization of the Book.- 1.3 Notations and Preliminaries.- 2 Detection of Multiple Signals.- 2.1 Signals and Noise.- 2.2 Conventional Techniques.- 2.2.1 Beamformer.- 2.2.2 Capon's Minimum Variance Estimator.- 2.2.3 Linear Prediction Method.- 2.3 Eigenvector-Based Techniques.- 2.3.1 Completely Coherent Case.- 2.3.2 Symmetric Array Scheme: Coherent Sources in a Correlated Scene.- 2.3.3 Spatial Smoothing Schemes: Direction Finding in a Coherent Environment.- 2.4 Augmentation and Other Techniques.- 2.4.1 Augmentation Technique.- 2.4.2 ESPRIT, TLS-ESPRIT and GEESE.- 2.4.3 Direction Finding Using First Order Statistics.- Appendix 2.A Coherent and Correlated Signal Scene.- Appendix 2.B Program Listings.- Problems.- References.- 3 Performance Analysis.- 3.1 Introduction.- 3.2 The Maximum Likelihood Estimate of the Covariance Matrix and Some Related Distributions.- 3.3 Performance Analysis of Covariance Based Eigenvector Techniques: MUSIC and Spatial Smoothing Schemes.- 3.3.1 Asymptotic Distribution of Eigenparameters Associated with Smoothed Sample Covariance Matrices.- 3.3.2 Two-Source Case - Uncorrelated and Coherent Scene.- 3.4 Performance Evaluation of GEESE Scheme.- 3.4.1 The Least Favorable Configuration (J = K).- 3.4.2 The Most Favorable Configuration (J = M - 1).- 3.5 Estimation of Number of Signals.- Appendix 3.A The Complex Wishart Distribution.- Appendix 3.B Equivalence of Eigenvectors.- Appendix 3.C Eigenparameters in a Two Source Case.- Problems.- References.- 4 Estimation of Multiple Signals.- 4.1 Introduction.- 4.2 Optimum Processing: Steady State Performance and the Wiener Solution.- 4.3 Implementation of the Wiener Solution.- 4.3.1 The Method of Steepest Descent.- 4.3.2 The Least Mean Square (LMS) Algorithm.- 4.3.3 Direct Implementation by Inversion of the Sample Covariance Matrix.- Problems.- References. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Orthogonal beamforming <s> The development of the beamforming method (also called microphone antenna, phased array of microphones, acoustic telescope, or acoustic camera) is reviewed in this paper. The microphone antenna was invented by Billingsley (1974) and has since seen dramatic improvements due to the availability of better data acquisition and computing hardware. Recent mathematical and software developments invert the beamforming process and allow a quantitative determination of the sources. Beamforming is indispensable for the localization of sound sources on moving objects, on flying aircraft, on high-speed trains, on motor cars in motion, on open rotors like helicopter and wind turbine rotors. In these applications, the ability to follow the motion of the sources is important. The second important applications are source localization tests in the test sections of open and closed wind tunnels. The background noise suppression capability of the beamforming method is required here. The various applications are discussed with a long list of references. 1 Berlin Beamforming Conference <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Orthogonal beamforming <s> Phased microphone arrays are used in a variety of applications for the estimation of acoustic source location and spectra. The popular conventional delay-and-sum beamforming methods used with such arrays suffer from inaccurate estimations of absolute source levels and in some cases also from low resolution. Deconvolution approaches such as DAMAS have better performance, but require high computational effort. A fast beamforming method is proposed that can be used in conjunction with a phased microphone array in applications with focus on the correct quantitative estimation of acoustic source spectra. This method bases on an eigenvalue decomposition of the cross spectral matrix of microphone signals and uses the eigenvalues from the signal subspace to estimate absolute source levels. The theoretical basis of the method is discussed together with an assessment of the quality of the estimation. Experimental tests using a loudspeaker setup and an airfoil trailing edge noise setup in an aeroacoustic wind tunnel show that the proposed method is robust and leads to reliable quantitative results. <s> BIB003

Orthogonal beamforming BIB003 , similar to functional beamforming, is also based on the eigenvalue decomposition of the CSM. It builds on the idea of separating the signal and the noise subspace BIB001 . In a setup with K < N incoherent sources, it is reasonable to assume that the (N − K) smallest eigenvalues are attributed to noise and are all equal to n 2 . The CSM eigenvalue decomposition can be written as follows: where n 2 contains the power from uncorrelated sound sources (e.g., generated by non-acoustic pressure fluctuations, the microphone electronics, and data acquisition hardware). Hence, the eigenvectors in U S span the signal subspace of U , whereas the remaining eigenvectors span the noise subspace. Let the N × K matrix G contain the transfer functions (i.e., the steering vectors) between the K sources and N microphones g 1 … g K (see Eq. 3). As shown in BIB003 , the matrix S is mathematically similar to (G * G)C S and, therefore, has the same eigenvalues. Here, C S is the CSM of the source signals. The main idea behind orthogonal beamforming is that each eigenvalue of S can be used to estimate the absolute source level of one source, from the strongest sound source within the map to the weakest, assuming orthogonality between steering vectors. In a second step, these sources are mapped to specific locations. This is done by assigning the eigenvalues to the location of the highest peak in a special beamforming sound map, which is purposely constructed from a rank-one CSM that is synthesized only from the corresponding eigenvector. Hence, the map is the output of the spatial beamforming filter for only one single source and the highest peak in this map is an estimate of the source location. The main diagonal of the reduced CSM for each eigenvalue may be removed to reduce uncorrelated noise. The beamforming map can be constructed on the basis of vector-vector products and is, therefore, computationally very fast. BIB002 3 An important parameter in the eigenvalue decomposition, which has to be adjusted by the user, is the number of eigenvalues k that span the signal subspace U S . In a practical measurement, a reliable approach is to estimate the number of sources K and choose a value k > K . If the last eigenvalues represent sources that only marginally contribute to the overall sound level, the most important sound sources will be correctly estimated using a value of k considerably smaller than N. The influence of the choice of k is illustrated in Fig. 16 for a trailing-edge noise experiment in an open-jet wind tunnel. Since the eigenvalue decomposition of the CSM results in a reduced number of point sources in the map, which is always less than or equal to the number of microphones, the sum of all source strengths within the map is never greater than the sum of the microphone auto-spectral densities. Hence, the sum of the acoustic source strengths is never overestimated.

A review of acoustic imaging methods using phased microphone arrays <s> CLEAN-SC <s> To obtain higher resolution acoustic source plots from microphone array measurements, deconvolution techniques are becoming increasingly popular. Deconvolution algorithms aim at identifying Point Spread Functions (PSF)in source plots, and may therefore fall short when actual beam patterns of measured noise sources are not similar to synthetically obtained PSF's. To overcome this, a new version of the classical deconvolution method CLEAN is proposed here: CLEAN-SC. By this new method, which is based on spatial source coherence, side lobes can be removed of actually measured beam patterns. Essentially, CLEAN-SC iteratively removes the part of the source plot which is spatially coherent with the peak source. A feature of CLEAN-SC is its ability to extract absolute sound power levels from the source plots. The merits of CLEAN-SC were demonstrated using array measurements of airframe noise on a scale model of the Airbus A340 in the 8 ×6 m2 closed test section of DNW-LLF. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> CLEAN-SC <s> Abstract An advanced phased array system, OptiNav Array 48, and a new deconvolution algorithm, TIDY, have been used to make octave band images of supersonic and subsonic jet noise produced by the NASA Glenn Small Hot Jet Rig (SHJAR). The results are much more detailed than previous jet noise images. Shock cell structures and the production of screech in an underexpanded supersonic jet are observed directly. Some trends are similar to observations using spherical and elliptic mirrors that partially informed the two-source model of jet noise, but the radial distribution of high frequency noise near the nozzle appears to differ from expectations of this model. The beamforming approach has been validated by agreement between the integrated image results and the conventional microphone data. Introduction Jet noise remains interesting after many years of study, despite a complete lack of hardware in the noise-producing region. The simplest case, a single-flow round jet with no forward flight, can produce a range of noise source phenomena: turbulent shear flow, mixing noise from large scale coherent structures, broadband shock associated noise, and screech tones (Ref. 1). These have been studied by measuring directivity in the far field, the acoustic pressure distribution in the near field, microphone two-point correlation, and spherical and elliptic mirror imaging (Ref. 2). Shock cells occur in the supersonic case when the jet is not ideally expanded. Flow disturbances convecting through the shock cells give rise to broadband shock-associated noise and screech tones (Refs. 3 and 4). Shock-associated noise can be identified in far-field spectra as an excess above the mixing noise that follows a well-defined correlation (Ref. 5). The shock cell structure can be imaged with Schlieren photography and its fluctuations have been studied with optical deflectrometry (Ref. 6) and numerical methods (Refs. 7 and 8). Several studies of jet noise using phased arrays and related systems of microphone arrays have been made (Refs. 9 to 18). These measurements have frequently been limited to subsonic jets, and have had several drawbacks that prevent the structure of the jet noise source from being fully revealed. One-dimensional arrays (Refs. 9, 14, and 18) do not readily show the radial structure of the source. Cage arrays (Refs. 10, 12, and 15) are designed for three-dimensional imaging, but they have several drawbacks. They subtend an angle that is too large to illustrate the source for different directivity angles. Their location in the near field can give confusing results. Finally, the large separation of the microphones produces severe problems with turbulent decorrelation. Phased array studies of jet noise have been criticized on the basis that the monopole source model used in beamforming is too simplistic. This issue has been addressed by Michel (Ref. 19), who showed that jet source convection and correlation length effects can be reduced to simple sources with directivity patterns. The use of a phased array is therefore justified, provided that it subtends a reasonably small range of solid angle. A similar argument was made to justify mirror studies of jets in the 1970s (Ref. 20). A straightforward but conservative specification would be to require that the array is small enough that its Rayleigh resolution spot size is larger than the presumed correlation length of the turbulence. The Rayleigh resolution of imaging system of this type (essentially a microscope) is <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> CLEAN-SC <s> In this article, a high-resolution extension of CLEAN-SC is proposed: high-resolution-CLEAN-SC. Where CLEAN-SC uses peak sources in ‘dirty maps’ to define so-called source components, high-resolution-CLEAN-SC takes advantage of the fact that source components can likewise be derived from points at some distance from the peak, as long as these ‘source markers’ are on the main lobe of the point spread function. This is very useful when sources are closely spaced together, such that their point spread functions interfere. Then, alternative markers can be sought in which the relative influence by point spread functions of other source locations is minimised. For those markers, the source components agree better with the actual sources, which allows for better estimation of their locations and strengths. This article outlines the theory needed to understand this approach and discusses applications to 2D and 3D microphone array simulations with closely spaced sources. An experimental validation was performed with two closely spaced loudspeakers in an anechoic chamber. <s> BIB003

CLEAN-SC BIB001 is a frequency domain deconvolution technique developed by Sijtsma and based on the radioastronomy method CLEAN-PSF . CLEAN-SC starts from the assumption that the CSM can be written as a summation of contributions from K incoherent sources: Herein, p k are the N-dimensional acoustic "source vectors" representing the Fourier components of the signals from the kth source. The assumption of Eq. (10) is valid under the following conditions: • The CSM is calculated from a large number of time blocks, so that the ensemble averages of the cross-products p k p * l , k ≠ l , can be neglected. • There is no decorrelation of signals from the same source between different microphones (e.g., due to sound propagation through turbulence). • All sound sources present are incoherent. • There is no additional incoherent noise. The CLEAN-SC algorithm starts with finding the steering vector yielding the maximum value of the beamforming source plot (Eq. 4), say at scan point j : The corresponding "source component" h j is defined by the following: Insertion of Eq. (10) yields the following: If the sources are well separated, then the term between parentheses in Eq. (13) is large when there is a close match between w j and the peak source and small for the other sources. Then, the source component provides a good estimate of the loudest source vector, even if this vector is not exactly proportional to the corresponding steering vector. Thus, we arrive at the following estimate: This expression (or a fraction of it) is subtracted from the CSM and the source is given an amplitude which is related to the average autopower of the microphone array. Then, the same procedure is repeated for the remaining CSM, until a certain stop criterion is fulfilled BIB001 . Ideally, the remaining CSM is "empty" after the iteration process. In other words, its norm should be small compared the one from the original CSM. This method works well for the case of a well-located sound source and it is especially suitable for closed-section wind-tunnel measurements. Another algorithm called TIDY BIB002 is similar to CLEAN-SC but works in the time domain, using the crosscorrelation matrix instead of the CSM. TIDY has been used for imaging jet noise BIB002 and motor vehicle pass-by tests . Recently, a higher resolution version of CLEAN-SC (HR-CLEAN-SC) has been developed and applied successfully to simulated data and to experimental data using two speakers in an anechoic chamber BIB003 .

A review of acoustic imaging methods using phased microphone arrays <s> DAMAS <s> At the 2004 AIAA/CEAS Aeroacoustic Conference, a breakthrough in acoustic microphone array technology was reported by the authors. A Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) was developed which decouples the array design and processing influence from the noise being measured, using a simple and robust algorithm. For several prior airframe noise studies, it was shown to permit an unambiguous and accurate determination of acoustic source position and strength. As a follow-on effort, this paper examines the technique for three-dimensional (3D) applications. First, the beamforming ability for arrays, of different size and design, to focus longitudinally and laterally is examined for a range of source positions and frequency. Advantage is found for larger array designs with higher density microphone distributions towards the center. After defining a 3D grid generalized with respect to the array’s beamforming characteristics, DAMAS is employed in simulated and experimental noise test cases. It is found that spatial resolution is much less sharp in the longitudinal direction in front of the array compared to side-to-side lateral resolution. 3D DAMAS becomes useful for sufficiently large arrays at sufficiently high frequency. But, such can be a challenge to computational capabilities, with regard to the required expanse and number of grid points. Also, larger arrays can strain basic physical modeling assumptions that DAMAS and all traditional array methodologies use. An important experimental result is that turbulent shear layers can negatively impact attainable beamforming resolution. Still, the usefulness of 3D DAMAS is demonstrated by the measurement of landing gear noise source distributions in a difficult hard-wall wind tunnel environment. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS <s> Current processing of acoustic array data is burdened with considerable uncertainty. This study reports an original methodology that serves to demystify array results, reduce misinterpretation, and accurately quantify position and strength of acoustic sources. Traditional array results represent noise sources that are convolved with array beamform response functions, which depend on array geometry, size (with respect to source position and distributions), and frequency. The Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) method removes beamforming characteristics from output presentations. A unique linear system of equations accounts for reciprocal influence at different locations over the array survey region. It makes no assumption beyond the traditional processing assumption of statistically independent noise sources. A new robust iterative method seamlessly introduces a positivity constraint (due to source independence) that makes the equation system sufficiently deterministic. DAMAS is quantitatively validated using archival data from a variety of prior high-lift airframe component noise studies, including flap edge/cove, trailing edge, leading edge, slat, and calibration sources. Presentations are explicit and straightforward, as the noise radiated from a region of interest is determined by simply summing the mean-squared values over that region. DAMAS can fully replace existing array processing and presentations methodology in most applications. It appears to dramatically increase the value of arrays to the field of experimental acoustics. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS <s> The present study reports a new development of the DAMAS microphone phased array processing methodology that allows the determination and separation of coherent and incoherent noise source distributions. In 2004, a Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) was developed which decoupled the array design and processing influence from the noise being measured, using a simple and robust algorithm. In 2005, three-dimensional applications of DAMAS were examined. DAMAS has been shown to render an unambiguous quantitative determination of acoustic source position and strength. However, an underlying premise of DAMAS, as well as that of classical array beamforming methodology, is that the noise regions under study are distributions of statistically independent sources. The present development, called DAMAS-C, extends the basic approach to include coherence definition between noise sources. The solutions incorporate cross-beamforming array measurements over the survey region. While the resulting inverse problem can be large and the iteration solution computationally demanding, it solves problems no other technique can approach. DAMAS-C is validated using noise source simulations and is applied to airframe flap noise test results. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS <s> A new approach to the deconvolution of acoustic sources has been developed and is presented in this work. The goal of this postprocessing is to simplify the beamforming output by suppressing the side lobes and reducing the sources' main lobes to single (or a small number of) points that accurately identify the noise sources' positions and their actual levels. In this work, a new modeling technique for the beamforming output is proposed. The idea is to use an image-processing-like approach to identify the noise sources from the beamforming maps: that is, recognizing lobe patterns in the beamformed output and relating them to the noise sources that would produce that map. For incoherent sources, the beamforming output is modeled as a superposition of point-spread functions and a linear system is posted. For coherent sources, the beamforming output is modeled as a superposition of complex point-spread functions and a nonlinear system of equations in terms of the sources' amplitudes is posted. As a first approach to solving this difficult problem, the system is solved using a new two-step procedure. In the first step, an approximated linear problem is solved. In the second step, an optimization is performed over the nonzero values obtained in the previous step. The solution to this system of equations renders the sources' positions and amplitudes. In the case of coherent sources, their relative phase can also be recovered. The technique is referred as noise source localization and optimization of phased-array results. A detailed analytical formulation, numerical simulations, and sample experimental results are shown for the proposed postprocessing. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS <s> A jet noise measurement study was conducted using a phased microphone array system for a range of jet nozzle configurations and flow conditions. The test effort included convergent and convergent/divergent single flow nozzles, as well as conventional and chevron dual-flow core and fan configurations. Cold jets were tested with and without wind tunnel co-flow, whereas, hot jets were tested only with co-flow. The intent of the measurement effort was to allow evaluation of new phased array technologies for their ability to separate and quantify distributions of jet noise sources. In the present paper, the array post-processing method focused upon is DAMAS (Deconvolution Approach for the Mapping of Acoustic Sources) for the quantitative determination of spatial distributions of noise sources. Jet noise is highly complex with stationary and convecting noise sources, convecting flows that are the sources themselves, and shock-related and screech noise for supersonic flow. The analysis presented in this paper addresses some processing details with DAMAS, for the array positioned at 90ϒ (normal) to the jet. The paper demonstrates the applicability of DAMAS and how it indicates when strong coherence is present. Also, a new approach to calibrating the array focus and position is introduced and demonstrated. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS <s> Microphone arrays and beamforming have become a standard method to localize aeroacoustic sources. Deconvolution techniques have been developed to improve spatial resolution of beamforming maps. The deconvolution approach for the mapping of acoustic sources (DAMAS) is a standard deconvolution technique, which has been enhanced via a sparsity approach called sparsity constrained deconvolution approach for the mapping of acoustic sources (SC-DAMAS). In this paper, the DAMAS inverse problem is solved using the orthogonal matching pursuit (OMP) and compared with beamforming and SC-DAMAS. The resulting noise source maps show that OMP-DAMAS is an efficient source localization technique in the case of uncorrelated or correlated acoustic sources. Moreover, the computation time is clearly reduced as compared to SC-DAMAS. <s> BIB006

The deconvolution approach for the mapping of acoustic sources (DAMAS) is a tool for quantitative analysis of beamforming results that was developed in the NASA Langley Research Center by Brooks and Humphreys BIB002 BIB001 BIB002 . This method solves the following inverse problem in an attempt to remove the influence of the array geometry and aperture on the output of CFDBF: where y ∈ ℝ J×1 is a column vector whose elements are the source autopowers of the J grid points of the source map obtained with CFDBF, à ∈ ℝ J×J is the propagation matrix whose columns contain the PSF at each of the J grid (15) y =Ãx, 1 3 points, and x ∈ ℝ J×1 is a column vector containing the actual unknown source autopowers. Due to the finite resolution and the presence of sidelobes, x ≠ y . For practical cases, the number of grid points J is high and much larger than the number of actual sources K. DAMAS considers incoherent sound source distributions and attempts to determine each source power by solving the inverse problem of Eq. (15), subject to the constraint that source powers are non-negative. The problem is commonly solved using a Gauss-Seidel iterative method, which typically requires thousands of iterations to provide a "clean" source map. Moreover, for practical grids, the large size of à can become an issue. The computation time of DAMAS employed this way is proportional to the third power of the number of grid points, J 3 . In most applications, à is singular and not diagonal dominant and the convergence towards the exact solution is not guaranteed. DAMAS has no mechanism to let the iteration converge towards a welldefined result and the solution may depend on how the grid points and the initial values are ordered . The inverse problem in Eq. (15) can also be evaluated using efficient non-negative least-squares (NNLS) solvers . In case sparsity of the vector x is considered, the inverse problem can also be solved with greedy algorithms such as the Orthogonal Matching Pursuit (OMP), which approximates the solution in a considerably lower computation time, proportional to J 2 instead of J 3 . The different steps of OMP are summarized in BIB006 . The Least Angle Regression Lasso algorithm (LarsLasso) also benefits of the sparsity of x , but requires the choice of a regularization factor by the user, which can be quite complicated for non-experienced users . Herold et al. compared the NNLS, OMP, and LarsLasso approaches on an aeroacoustic experiment, and found that only NNLS and LarsLasso (using an appropriate regularization factor) surpass the classic DAMAS algorithm in terms of overall performance. One of the most famous results of DAMAS is depicted in Fig. 6 , where simulated incoherent monopoles were distributed to form the word NASA BIB002 . The grid spacing x was chosen to be 2.54 cm (1 in.), which, normalized by the array beamwidth B, provides x∕B = 0.25 . Apart from considerably improving the results of CFDBF (top left), the integrated sound levels rapidly converged to the correct value (within 0.05 dB after 100 iterations) BIB002 . DAMAS was later extended to allow for source coherence (DAMAS-C) BIB003 . Whereas the computational challenges in the use of DAMAS-C have limited its widespread application, the conventional DAMAS has shown its potential with coherent source distributions in jet-noise analyses BIB005 . This jet-noise study also demonstrated the use of in situ point-source measurements for the calibration of array results. A similar method to DAMAS-C called noise source localization and optimization of phased-array results (LORE) was proposed by Ravetta et al. BIB004 BIB004 . LORE first solves an equivalent linear problem as DAMAS using an NNLS solver, but considering the complex point spread function BIB004 BIB004 , which contains information about the relative source phase. The output obtained is optimized solving a non-linear problem. Satisfactory results were obtained in the simulated and experimental cases in a laboratory featuring incoherent and coherent sound sources BIB004 BIB004 . Whereas this method is faster than DAMAS-C, it does not provide accurate results when using diagonal removal and when calibration errors are present in the microphone array BIB004 BIB004 .

A review of acoustic imaging methods using phased microphone arrays <s> DAMAS2 <s> The DAMAS deconvolution algorithm represents a breakthrough in phased array imaging for aeroacoustics, potentially eliminating sidelobles and array resolution effects from beamform maps . DAMAS is an iterative non-negative least squares solver. The original algorithm is too slow and lacks an explicit regularization method to prevent noise amplification. Two extensions are proposed, DAMAS2 and DAMAS3. DAMAS2 provides a dramatic speedup of each iteration and adds regularization by a low pass filter. DAMAS3 also provides fast iterations, and additionally, reduces the required number of iterations. It uses a different regularization technique from DAMAS2, and is partially based on the Wiener filter. Both DAMAS2 and DAMAS3 restrict the point spread function to a translationally-invariant, convolutional, form. This is a common assumption in optics and radio astronomy, but may be a serious limitation in aeroacoustic beamforming. This limitation is addressed with a change of variables from (x,y,z) to a new set, (u,v,w). The concepts taken together, along with appropriate array design, may permit practical 3D beamforming in aeroacoustics. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS2 <s> T HE resolution of noise-source maps processed using phasedarray data has been substantially improved by introducing a deconvolution approach for the mapping of acoustic sources (DAMAS [1]), but its practicality is limited due to high computational cost. Subsequently, an improved algorithm, known as DAMAS2 [2], was developed to reduce the computational cost dramatically. Assuming that the monopole response (i.e., the pointspread function) on the source map is invariant over the entire map, referred to as shift invariant, a discrete Fourier transform and its inverse are introduced for deconvolution in DAMAS2. The drawback is that noise-source maps tend to deteriorate away from the point where we defined the point-spread function. Distortion of the monopole response is often severe in wind-tunnel tests, where there are constraints on the location of the microphone array. This note proposes amethod that suppresses such deterioration associatedwith the shift-invariant assumption for DAMAS2 with minimal extra computational cost. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS2 <s> Abstract Microphone arrays have become a standard technique to localize and quantify source in aeroacoustics. The simplest approach is the beamforming that provides noise source maps with large main lobe and strong side lobes at low frequency. Since a decade, the focus is set on deconvolution techniques such as DAMAS or Clean-SC. While the source map is clearly improved, these methods require a large computation time. In this paper, we propose a sound source localization technique based on an inverse problem with beamforming regularization matrix called Hybrid Method. With synthetic data, we show that the side lobes are removed and the main lobe is narrower. Moreover, if the sound noise source map provided by this method is used as input in the DAMAS process, the number of DAMAS iterations is highly reduced. The Hybrid Method is applied to experimental data obtained in a closed wind-tunnel. In both cases of acoustic or aeroacoustic data, the source is correctly detected. The proposed Hybrid Method is found simple to implement and the computation time is low if the number of scan points is reasonable. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS2 <s> When using deconvolution algorithms for the improvement of beamforming results, the family of DAMAS-algorithms has shown to be very useful. One member of this family – the DAMAS2 algorithm – comes into focus when one is concerned with computational speed and efficient memory usage. The algorithm is especially suited for wavenumber beamforming, since it requires a shift-invariant point-spread function for best results. During application of this method, it was noticed that the results can be even improved ::: when increasing the size of the point-spread function. This further enhancement of the algorithm will be presented in examples and discussed. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS2 <s> Abstract Deconvolution of aeroacoustic data acquired with microphone phased arrays is a computationally challenging task for distributed sources with arbitrary coherence. A new technique for performing such deconvolution is proposed. This technique relies on analysis of the array data in the wavenumber–frequency domain, allowing for fast convolution and reduced storage requirements when compared to traditional coherent deconvolution. A positive semidefinite constraint for the iterative deconvolution procedure is implemented and shows improved behavior in terms of quantifiable convergence metrics when compared to a standalone covariance inequality constraint. A series of simulations validates the method׳s ability to resolve coherence and phase angle relationships between partially coherent sources, as well as determines convergence criteria for deconvolution analysis. Simulations for point sources near the microphone phased array show potential for handling such data in the wavenumber–frequency domain. In particular, a physics-based integration boundary calculation is described, and can successfully isolate sources and track the appropriate integration bounds with and without the presence of flow. Magnitude and phase relationships between multiple sources are successfully extracted. Limitations of the deconvolution technique are determined from the simulations, particularly in the context of a simulated acoustic field in a closed test section wind tunnel with strong boundary layer contamination. A final application to a trailing edge noise experiment conducted in an open-jet wind tunnel matches best estimates of acoustic levels from traditional calculation methods and qualitatively assesses the coherence characteristics of the trailing edge noise source. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> DAMAS2 <s> Abstract Phased microphone arrays have become a standard technique for acoustic source localization. Compared with beamforming algorithms such as the conventional beamforming, deconvolution approaches such as DAMAS successfully improve the spatial resolution. However deconvolution approaches usually require high computational effort compared to beamforming algorithms. Without optimizing deconvolution algorithm, recently DAMAS with compression computational grid based on the conventional beamforming (denoted by DAMAS-CG2) has reduced computational run time of DAMAS in applications (Ma and Liu, 2017). This paper proposes a novel algorithm that DAMAS with a novel compression computational grid based on an advance beamforming algorithm functional beamforming (denoted by DAMAS-CG3). This new algorithm takes advantages of functional beamforming to obtain large compression ratio. Simulated applications and experimental applications of benchmark test DLR1 show that DAMAS-CG3 is one order of magnitude faster than DAMAS-CG2 in most cases. In addition, the advantage of DAMAS-CG3 compared to DAMAS-CG2 is particularly more obvious with the threshold decreasing. However for some extreme situations that very complicated sources distribute to a larger extent relative to the scanning plane, the advantage of DAMAS-CG3 compared to DAMAS-CG2 may disappear. In order to get a large compression ratio in any application, the authors highly recommend compressing computational grid based on not only conventional beamforming but also functional beamforming, and then choosing the compression grid with larger compression ratio. <s> BIB006

Further versions of DAMAS have been proposed in the literature BIB001 BIB003 BIB006 , especially for reducing the high computational cost that it implies. For example, DAMAS2 assumes that the array's PSF is shift invariant. The term shift invariant describes the property that the characteristics of the PSF do not vary relative to the source position even if the absolute position of that source in the steering grid is changed. Thus, if a source is translated by a certain offset, the entire PSF will also translate with it. The error involved in this assumption is small in astronomy applications where the distance between the source and the observer is huge compared to the size of the array or of the source itself, but, in aeroacoustic measurements, the PSF can vary significantly within the source region . The distortion of the PSF away from the center of the scanning domain can be alleviated by including spatial-differentiation terms BIB002 . When applied, the size of the PSF has to be chosen large enough to prohibit wrapping, which has been implemented in DAMAS2.1 BIB004 . In addition, a fast implementation of DAMAS-C based on similar techniques to reduce computational costs as DAMAS2 has been introduced BIB005 , exploiting the benefits of convolution using Fourier transforms. Embedded versions of DAMAS2 and a Fourier-based nonnegative least-squares (NNLS) approach of DAMAS were proposed by Ehrenfried and Koop which do account for the shift variation of the PSF and are potential faster alternatives compared to the original DAMAS algorithm.

A review of acoustic imaging methods using phased microphone arrays <s> Wavenumber beamforming <s> During the last decade, microphone arrays became a standard tool to study aeroacoustic sources. Nowadays, arrays are frequently applied in fly-over measurements 1 and wind tunnel testing. 2 To improve the resolution of the arrays and to reduce disturbing side lobes in the measured source maps, sophisticated deconvolution methods were proposed 3-8 in recent years. First, these methods were tested using synthetic data or data from experiments with generic configurations. 8 In a next step the methods have to be applied to real experiments. This is done in the present paper. It is tested, if the deconvolution works also under difficult conditions, and if the findings of the simulations using artificial background noise 7 can be confirmed in case of real data. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Wavenumber beamforming <s> The wall-pressure fluctuations beneath a compressible turbulent boundary layer are measured using a sensor array. The boundary layer is realized on a flat plate in a wind tunnel with closed test section. Adaptive side walls are used to adjust a zero gradient of mean pressure. The array consists of 48 piezo-resistive pressure transducers using a threaded mounting. The array data are processed using beamforming and a deconvolution algorithm to obtain the wavenumber spectrum. As example a test case at Mach number M = 0.83 is considered. The convective ridge and acoustic waves are found in the wavenumber spectrum. The form of the convective ridge is consistent with empirical models developed for low Mach numbers. The results show the dominance of convective fluctuations at higher frequencies, while acoustic fluctuations are prevailing at low frequencies. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Wavenumber beamforming <s> This paper concerns the direct measurement of wavenumber-frequency spectra beneath a two-dimensional Turbulent Boundary Layer. The main contribution of this work is the proposed setup that should help achieving a compromise between a small sensor diameter (to avoid spatial averaging) and small sensor spacing (to gain high spatial resolution). The effect of different pinhole sizes on quarter inch microphones is first studied using an acoustical coupler to perform relative calibrations. Single-point measurements of wall pressure fluctuations in a wind-tunnel confirm that a 0.5 mm pinhole quarter inch microphone correctly captures wall pressure statistics up to a frequency of 5 kHz. Under the assumption of stationnary pressure fields, it is then suggested how rotative Archimedean spirals mainly composed of pinhole microphones can provide a uniform coverage of pressure measurements on a disc, or high resolution measurements in selected directions. Using 57 of these pinhole pressure sensors, a probe microphone as a central sensor reference and 3 Knowles pressure sensor, one of the suggested designs of spiral-shaped rotative arrays was instrumented. Measurements have been performed at low Mach numbers (M < 0:1) in a closed-loop wind-tunnel, with the array flush mounted on a side of the wind tunnel. The TBL is characterized with velocity measurements, and 2D wavenumber spectra of the wall pressure fluctuations are obtained which reveal the convective peak at low frequencies. Even preliminary and still contaminated by a strong background noise, these first results indicate that the experimental setup behaves as expected but needs additional refinements. The next steps of this on-going work are finally detailed. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Wavenumber beamforming <s> The fluctuating pressure field underneath a turbulent boundary layer can lead to excitation of the surface structure of airplanes, trains and cars. At high flow speeds, this excitation becomes a major noise source in the vehicle interior. In the past, the characteristics of this excitation have been measured by the DLR in wind tunnels and in flight tests. A beam forming technique using planar waves was used as wavenumber decomposition in order to analyze the pressure fluctuations present over an installed microphone array. The information contained in the wavenumber spectrum resulting from the analysis is essential when empirically modeling the excitation behavior of a surface exposed to a pressure field. Speed, direction, and propagation mechanism (acoustic or hydrodynamic) of pressure waves can be determined from the position of sources in the wavenumber domain. This makes the wavenumber decomposition a helpful tool for the characterization of source mechanisms. In this paper, a summary of the test results is given. The attributes and potential of the wavenumber decomposition are emphasized in particular. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Wavenumber beamforming <s> When using deconvolution algorithms for the improvement of beamforming results, the family of DAMAS-algorithms has shown to be very useful. One member of this family – the DAMAS2 algorithm – comes into focus when one is concerned with computational speed and efficient memory usage. The algorithm is especially suited for wavenumber beamforming, since it requires a shift-invariant point-spread function for best results. During application of this method, it was noticed that the results can be even improved ::: when increasing the size of the point-spread function. This further enhancement of the algorithm will be presented in examples and discussed. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Wavenumber beamforming <s> Abstract Boundary layer measurements at high subsonic Mach number are evaluated in order to obtain the dominant phase velocities of boundary layer pressure fluctuations. The measurements were performed in a transonic wind tunnel which had a very strong background noise. The phase velocity was taken from phase inclination and from the convective peak in one- and two-dimensional wavenumber spectra. An approach was introduced to remove the acoustic noise from the data by applying a method based on CLEAN-SC on the two-dimensional spectra, thereby increasing the frequency range where information about the boundary layer was retrievable. A comparison with prediction models showed some discrepancies in the low-frequency range. Therefore, pressure data from a DNS calculation was used to substantiate the results of the analysis in this frequency range. Using the measured data, the DNS results and a review of the models used for comparison it was found that the phase velocity decreases at low frequencies. <s> BIB006

A different steering vector representation can be used to display the sources in terms of propagation characteristic, rather than source position (Eq. 3). The uniformly weighted steering vector can be written as follows: which is the representation of a planar wave. In contrast to monopole sources, which cast a near-field-like pressure pattern on the array, the sources used for wavenumber beamforming are located at infinite distance and can, therefore, conveniently be characterized by their wavenumbers k x and k y as a plane wave. This type of beamforming is particularly useful when mechanisms resulting in different propagation speeds and directions are present, provided that far-field conditions apply. This technique has been used to distinguish between noise from a model and duct modes in a closedsection wind tunnel BIB001 and to characterize the turbulentboundary-layer propagation and the acoustic disturbances of a high-speed wind-tunnel flow BIB002 BIB006 BIB003 and of an aircraft boundary-layer flow BIB004 . (16) g j,n = exp −i k x,j x n + k y,j y n , The planar wave approach produces beamforming maps based on a shift-invariant PSF which makes them very suitable for further processing with DAMAS2.1 BIB005 . Exemplary plots of the wavenumber domain for f = 1480 Hz are shown in Fig. 7 . Due to the relation c = 2 f ∕‖k‖ with c being the propagation velocity, and k = (k x , k y ) the wavevector of a source, each position in the map represents a different propagation velocity. Sources with a propagation speed equal to or higher than the speed of sound are located in the elliptic-shaped acoustic domain shown in both plots in Fig. 7 with a solid black line. In the closed-section wind-tunnel test of Fig. 7 (left) , acoustic sources are present, while the flight test data of Fig. 7 (right) appear to be free of dominant acoustic content at the frequency shown. The elongated spot on the right-hand side outside the acoustic domain is a representation of the pressure fluctuations caused by the subsonic TBL flow. In the wind-tunnel data, this elongated spot is seen to be parallel to the k y -axis, indicating a flow component only in the x-direction. In the flight test plot, the elongated shape is rotated slightly about the origin, which indicates a flow direction that is not aligned with the array's x-and y-axes. The wavenumber domain can be used to easily separate between different propagation mechanisms.

A review of acoustic imaging methods using phased microphone arrays <s> Linear programming deconvolution (LPD) <s> A new deconvolution strategy for aeroacoustic beamforming based on linear programming (LP) is introduced. Results of an aeroacoustic test show that it produces virtually the same results as DAMAS in much less time. DAMAS, DAMAS2 and LP display superresolution, which is expressed in terms of the Sparrow resolution limit from optics. CLEAN-SC and TIDY did not achieve superresolution in the test, but did produce higher dynamic range than DAMAS and LP at high frequency. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Linear programming deconvolution (LPD) <s> This paper answers the challenge as how to automatically select a good regularization parameter when solving inverse problems in acoustics. A Bayesian solution is proposed that consists either in directly finding the most probable value of the regularization parameter or, indirectly, in estimating it as the ratio of the most probable values of the noise and source expected energies. It enjoys several nice properties such as ease of implementation and low computational complexity (the proposed algorithm boils down to searching for the global minimum of a 1D cost function). Among other advantages of the Bayesian approach, it makes possible to appraise the sensitivity of the reconstructed acoustical quantities of interest with respect to regularization, a performance that would be otherwise arduous to achieve. <s> BIB002

Linear programming deconvolution (LPD) BIB001 is basically a faster alternative than DAMAS to solve the inverse problem introduced in Eq. BIB002 . It considers an additional constraint that no correct model of the beamform map Ãx would exceed the beamform source map obtained by CFDBF y anywhere. This difference ( y −Ãx ) represents the effect of uncorrelated sound sources that were present in the measurement but not in the model, such as background noise, microphone self noise, and long-range reflections BIB001 . Hence, an approach to obtain x is to maximize the model Ãx , subject to the constraint that it nowhere exceeds y . Defining the 1 × J propagation vector c as follows: Therefore, the proposed problem is to maximize the product c ⋅x , subject to Ãx ≤ y and x ≥ 0 . This linear programming problem can be solved, for example, by the simplex algorithm, which guarantees finding an optimal solution in a finite number of steps, provided that a feasible vector exists and that the objective function is bounded. Unlike DAMAS (see Sect. 3.5), LPD has a definite result with no uncertainty about whether a sufficient number of iterations have been performed BIB001 . However, a disadvantage of LPD is that it does not work well with diagonal removal BIB001 . An alternative approach is to add an extra element to x which represents the incoherent noise level. The matrix à would now be J × (J + 1) and the extra column is filled with ones. This method has been applied to a distribution of aeroacoustic point sources in a laboratory BIB001 and was shown to provide a better resolution than the Sparrow resolution limit. A combination of LPD and functional beamforming has been reported by Dougherty showing better results due to the higher dynamic range offered by functional beamforming. The application of LPD, however, breaks up continuous source distributions into spots. This method is thus appropriate for discrete sources and for situations where spatial resolution is more important than dynamic range.

A review of acoustic imaging methods using phased microphone arrays <s> Robust adaptive beamforming (RAB) <s> Adaptive beamforming algorithms can be extremely sensitive to slight errors in array characteristics. Errors which are uncorrelated from sensor to sensor pass through the beamformer like uncorrelated or spatially white noise. Hence, gain against white noise is a measure of robustness. A new algorithm is presented which includes a quadratic inequality constraint on the array gain against uncorrelated noise, while minimizing output power subject to multiple linear equality constraints. It is shown that a simple scaling of the projection of tentative weights, in the subspace orthogonal to the linear constraints, can be used to satisfy the quadratic inequality constraint. Moreover, this scaling is equivalent to a projection onto the quadratic constraint boundary so that the usual favorable properties of projection algorithms apply. This leads to a simple, effective, robust adaptive beamforming algorithm in which all constraints are satisfied exactly at each step and roundoff errors do not accumulate. The algorithm is then extended to the case of a more general quadratic constraint. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Robust adaptive beamforming (RAB) <s> Tests have been performed in a conventional low speed wind tunnel using a linear array of microphones to measure the received source level of a speaker projecting within the test section. Conventional, or delay and sum beamforming (CBF), and adaptive beamforming (ABF) have been employed to determine the source bearing and the received source level. Both narrow‐band and broadband sources were used, with two different array configurations and several array orientations. The results show that ABF can be used in such an environment and that it rejects much of the multipath. Consequently more accurate broadband measurements are obtained with ABF than with CBF. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Robust adaptive beamforming (RAB) <s> A full-scale test on a turbofan engine has been carried out in the framework of a European project to assess the acoustic benefits of new exhausts and novel hot-stream streamliners in the reduction of the overall jet noise level radiatedinthe far field. Thepurpose ofthetestsisalsoto provideabetterunderstanding ofcombustionnoise,which is one of the main issues for turbofan engines because of the significant noise reduction obtained for other acoustic sources such as the fan and jet by using innovative efficient silencers and acoustics liners. This work investigates the use of the so-called spectral estimation method to separate the combustion noise from other noise sources measured withanear-fieldarray ofmicrophones.Thecomparisonsbetweentheestimated andmeasuredspectradownstream, at the location at which the combustion noise is dominant, yield promising results, suggesting that the procedure of discrimination is relevant. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Robust adaptive beamforming (RAB) <s> Phased microphone arrays have become an important tool in the localization of noise sources for aeroacoustic applications. In most practical aerospace cases the conventional beamforming algorithm of the delay-and-sum type has been adopted. Conventional beamforming cannot take advantage of knowledge of the noise field, and thus has poorer resolution in the presence of noise and interference. Adaptive beamforming has been used for more than three decades to address these issues and has already achieved various degrees of success in areas of communication and sonar. In this work an adaptive beamforming algorithm designed specifically for aeroacoustic applications is discussed and applied to practical experimental data. It shows that the adaptive beamforming method could save significant amounts of post-processing time for a deconvolution method. For example, the adaptive beamforming method is able to reduce the DAMAS computation time by at least 60% for the practical case considered in this work. Therefore, ... <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Robust adaptive beamforming (RAB) <s> Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r... <s> BIB005

Adaptive beamforming BIB001 , also known as Capon or minimum variance distortionless response beamforming, can produce acoustic images with a higher spatial resolution than CFDBF and has been used in array signal processing for sonar and radar applications. This method uses a weighted steering vector formulation that maximizes the SNR: It is natural to expect that adaptive beamforming could be helpful to locate aeroacoustic noise sources more accurately and better minimize the convolution effects BIB002 , which, in turn, could produce array outputs of higher quality saving the computational efforts of the following deconvolution methods. However, Huang et al. BIB004 showed that adaptive beamforming is quite sensitive to any perturbations and its BIB003 performance quickly deteriorates below an acceptable level, preventing the direct application of present adaptive beamforming methods for aeroacoustic measurements. To improve the performance, a robust adaptive beamforming (RAB) method has been proposed BIB004 specifically for aeroacoustic applications. To mitigate any potential illconditioning of the CSM, diagonal loading BIB004 is applied as follows: where I is the N × N identity matrix. The value of the diagonal loading factor is usually determined empirically. One method proposed by Huang et al. BIB004 consists of calculating the maximum eigenvalue of the CSM and multiply it by a diagonal loading parameter 0 : The value of 0 is typically between 0.001 and 0.5, but needs to be iteratively determined considering a quality threshold in the difference between the obtained results and the CFDBF results, usually 3 dB. In general, smaller values of 0 provide acoustic images with a better array resolution, but the computation can fail due to numerical instability. On the other hand, a larger value of 0 generates results more similar to the CFDBF. RAB can save significant amounts of postprocessing time compared with the deconvolution methods BIB004 . A different approach to calculate , based on the white noise gain constraint idea from Cox et al. BIB001 , can be found in . In this approach, however, a different value of is calculated for each grid point, consequently, increasing the computational cost considerably. An application of this method for aircraft flyover measurements BIB005 can be found in Figs. 20 and 21. Capon beamforming can be extended to treat potentially coherent sources .

A review of acoustic imaging methods using phased microphone arrays <s> Spectral estimation method (SEM) <s> Array processing is a powerful tool to extract the noise source characteristics from the acoustic signals measured during wind-tunnels tests. For many years, this measurement technology is commonly being used to study airframe noise. Conventional beam forming methods have proven their efficiency in source localization, but it is well known that they are limited in the estimation of the power levels of extended sources. An approach based on a spectral estimation method (SEM) is proposed in order to determine the actual sound-pressure levels of the acoustic sources found by the localization method. The SEM is based on the array cross-spectral matrix, which contains an indirect measurement of the source power levels of interest and on a modeling of this matrix. Optimal estimates of the actual source power levels can be obtained by minimizing the mean square error between the measurement and model matrices. However, the direct minimization suffers from the sensitivity to noise inherent in the ill-posed nature of the problem. To obtain meaningful results, the problem is regularized by using a priori information. Numerical simulations involving multiple source areas are provided to illustrate the effectiveness of the estimation procedure. The method is successfully applied to compute the power levels of the main sources on a 1/11th-scaled model of the Airbus A320/A321 tested in CEPRA 19 wind tunnel. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Spectral estimation method (SEM) <s> A spectral estimation method based on measurements by a phased array was developed to estimate absolute power levels radiated by compact or extended sources. After a brief recall of its principle, numerical simulations involving several source regions are presented to illustrate the accuracy and the high-resolution performances of this new method. Tests were carried out with a 1:11th scale A320/A321 Airbus model in the anechoic wind tunnel CEPRA 19. The characteristics of source regions from inboard to outboard of the wing slat and flap, and the main and nose landing gear subregions are examined. The dependence in level and frequency of the model subregions with the flow speed, the effects of the attack angle, and the effects of the flap or slat deployment are studied. The different trends revealed by the integrated power levels are also discussed. Examples of the localization of noise sources identified with conventional beam forming and the new method are presented. The consistency of the results is examined by comparing the integrated power levels for the complete wing and the measured spectra with the phased array. Finally, we show that the spectral estimation method is quite reliable in computing the far-field directivity of airframe noise sources. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Spectral estimation method (SEM) <s> A full-scale test on a turbofan engine has been carried out in the framework of a European project to assess the acoustic benefits of new exhausts and novel hot-stream streamliners in the reduction of the overall jet noise level radiatedinthe far field. Thepurpose ofthetestsisalsoto provideabetterunderstanding ofcombustionnoise,which is one of the main issues for turbofan engines because of the significant noise reduction obtained for other acoustic sources such as the fan and jet by using innovative efficient silencers and acoustics liners. This work investigates the use of the so-called spectral estimation method to separate the combustion noise from other noise sources measured withanear-fieldarray ofmicrophones.Thecomparisonsbetweentheestimated andmeasuredspectradownstream, at the location at which the combustion noise is dominant, yield promising results, suggesting that the procedure of discrimination is relevant. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Spectral estimation method (SEM) <s> Microphone arrays are commonly used for noise source localization and power estimation in aeroacoustic measurements. The delay-and-sum (DAS) beamformer, which is the most widely used beamforming algorithm in practice, suffers from low resolution and high sidelobe level problems. Therefore, deconvolution approaches, such as the deconvolution approach for the mapping of acoustic sources (DAMAS), are often used for extracting the actual source powers from the contaminated DAS results. However, most deconvolution approaches assume that the sources are uncorrelated. Although deconvolution algorithms that can deal with correlated sources, such as DAMAS for correlated sources, do exist, these algorithms are computationally impractical even for small scanning grid sizes. This paper presents a covariance fitting approach for the mapping of acoustic correlated sources (MACS), which can work with uncorrelated, partially correlated or even coherent sources with a reasonably low computational complexity. MACS minimize... <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Spectral estimation method (SEM) <s> This paper presents a systematic comparison of several prominent beamforming algorithms developed for aeroacoustic measurements. The most widely used delay-and-sum (DAS) beamformer is known to suffer from high sidelobe level and low resolution problems. Therefore, more advanced methods, in particular the deconvolution approach for the mapping of acoustic sources (DAMAS), sparsity constrained DAMAS (SC-DAMAS), covariance matrix fitting (CMF) and CLEAN based on spatial source coherence (CLEAN-SC), have been considered to achieve improved resolution and more accurate signal power estimates. The performances of the aforementioned algorithms are evaluated via experiments involving a 63-element logarithmic spiral microphone array in the presence of a single source, two incoherent sources with similar strengths and with different strengths, and two coherent sources. It is observed that DAMAS, SC-DAMAS and CMF provide the most reliable source location estimates, even at relatively low frequencies. Furthermore, th... <s> BIB005

The spectral estimation method (SEM) BIB001 is intended for the location of distributed sound sources. It is based on the idea of describing sound sources by the mathematical models that depend on several unknown parameters. It is assumed that the power spectral density (PSD) of the sources can then be expressed in terms of these parameters. This method is also known in the literature as covariance matrix fitting (CMF) BIB004 BIB005 . The choice of the source model is based on the fact that an extended sound source may only be viewed as an equivalence class between source functions radiating the same pressure field on a phased microphone array. In many applications, a majority of sound sources have smooth directivity patterns. This means that if the aperture angle of a microphone array seen from the overall source region is not too large, the directivity pattern of each region may be considered as isotropic within this aperture, neglecting its directivity. Therefore, SEM models the array CSM C m,n with a set of J uncorrelated monopoles with unknown autopowers x j (see Sect. 3.5), which is a quite appropriate model to study airframe noise, by minimizing the following cost function (mean least-squares error between the array and modeled matrices): The constraint of a non-negative PSD is satisfied by introducing a new unknown s j defined by x = s 2 . This allows the use of efficient optimization methods for solving unconstrained problems. The resulting non-linear minimization problem is solved using an iterative procedure based on a conjugate gradient method algorithm. The solution generally converges fast with only a few hundred iterations. SEM has shown its efficiency in noiseless environments, on numerical simulations BIB001 and on data measured during experiments performed with an aircraft model in the open-jet anechoic wind tunnel CEPRA 19 (see Fig. 13 ) BIB002 BIB003 BIB001 . One big advantage of SEM over deconvolution methods using beamforming is that the main diagonal of the CSM can be excluded from the optimization without violating any assumptions. SEM can even be used to reconstruct the diagonal without the influence of the spurious contributions. The resulting source distribution is relatively independent on the array pattern and the assumed source positions can be restricted to the known regions on an aircraft. To take into account the inevitable background noise in practical applications, an extension of SEM has been proposed: the spectral estimation method with additive noise (SEMWAN) . This method is based on a prior knowledge of the noise signal and it has the advantage of being able to reduce the smearing effect due to the array response and, at the same time, the inaccuracy of the results caused by noise sources, which can be coherent as well as incoherent, with high or low levels. This technique is well suited for applications in wind tunnels, since, for example, a noise reference or record of the environmental noise can be obtained prior to the installation of the model in the test section . Figures 10 and 11 present the results of SEM and SEM-WAN applied in a closed-section wind-tunnel experiment. Another important issue arises when the acoustic measurements are performed in a non-anechoic closed-section wind tunnel. In this situation, the pressures collected by the microphones are not only due to the direct paths of the acoustic sources, but are also due to their unwanted reflections on the unlined walls, thereby losing accuracy when calculating the power spectra. To remove this drawback, a multi-microphone cepstrum method, aiming at removing spurious echoes in the power spectra, has been developed and tested successfully with the numerical and experimental data .

A review of acoustic imaging methods using phased microphone arrays <s> SODIX <s> The directivities of all broadband sound sources of an aeroengine can be determined from measurements with a line array of microphones. A new data analysis method SODIX (SOurce DIrectivity modelling in crossspectral matriX) was developed for this purpose and applied to the experimental data obtained with a highbypass ratio turbofan engine in an open air test bed. The method is based on modelling the matrix of the crossspectra of the microphone signals with a set of contributions from point sources with unknown directivities assumed in positions along the engine axis. The source positions are defined with a separation of typically 0.4 wave lengths. The analysis has to be performed in narrow frequency bands. The influences of tones are eliminated in the cross-spectral matrix of the microphone signals by interpolation in the frequency domain. The unknown directivities of the point sources are determined with the side conditions that the values must be non-negative and that the directivities and source distributions are smooth. The results of this non-linear optimisation problem confirm that the various sources of an a eroengine have highly non-uniform directivities. The far field can be predicted from the source distributions f or positions over a large range of emission angles. The predictions on a 45.72 m radius around the engine agree within about 1 dB with experimental data. These far-field directivities can be separated into the contribut ions from inlet, casing, the two nozzles, and the jet. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> SODIX <s> It is demonstrated that the source strengths and directivit ies of all sound sources of a high bypass ratio aeroengine can be determined from measurements in an open air test bed with a line array of microphones, which is laid out parallel to the e ngine axis in the geometric near field of the engine. The method is based on modelling the matri x of the cross-spectra of the microphones with a set of contributions from point monopoles assumed in positions along the engine axis and by solving the resulting set of linear equations for the unknown source strengths. The directivities of the sources are estimated b y performing the analysis with a subarray of microphones, sliding from the front to the rear of the engine. The set of linear equations is solved with the side condition that the source s trengths must be non-negative. The results demonstrate that the various sources of an aeroengine have a highly non-uniform directivity. The source positions are resolved with a separ ation of 0.4 wave lengths. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> SODIX <s> An improved version of the inverse source location method SODIX is presented. SODIX models broadband nosie sources with a st of directive point sources by minimizing the difference between a measured and a simulated cross-spectral matrix of microphone signals. One big advantage of the method in comparison to other source location methods is, that it calculates a directive source strength from each point source towards each microphone. The new version of SODIX includes an implicit consideration of the constraint of positive source strengths in the iteration process. This improvement made the optimization process more stable and in addition independent of the initially assume source distribution. The method is also extended for applications with multidimensional source and microphone arrangements. Further, it is shown that the main diagonal of gthe cross-spectral matrix can be removed from the calculations without any mathematical violations and that the absolute strengths of the dominating sources are preserved. The improvements of the method are shown with exemplary results using measured data from a static aeroengine freefield test. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> SODIX <s> Noise assessment around airports is hampered due to the observed large variability in noise levels for fly-overs of the same aircraft type, which is not considered by the current models. This paper assumes that the noise variability is due to variations in the aircraft emitted noise, neglecting the effect of the variable atmosphere, as previous work showed that its contribution is maximally 2 dB. In order to quantify and investigate the variability of noise levels during aircraft fly-overs, 115 measurements of noise of landing aircraft were taken using a 32 microphone array. The noise levels from Boeing 737 fly-overs were analyzed and the noise level variability was determined to be approximately 16 dB. After determining the engine settings from the spectrograms, it was found that variations on the engine settings explain over 55% of the observed total noise variation. In addition, by performing beamforming on the acoustic data, it was confirmed that airframe noise (from the landing gear and deployed flaps) is dominant for several frequencies, especially for modern aircraft. <s> BIB004

SODIX (Source Directivity Modeling in the Cross-Spectral Matrix) is an extension of SEM that can model sound sources with arbitrary directivities. The method was initially developed for noise tests with engines in open static test beds to separate the various broadband sound sources of turbofan engines, which are known to have sizable directivities. However, the method can be applied to any problem where the directivities of the sound sources are of interest. The source model of SODIX extends the point-source model of SEM by replacing the omnidirectional source amplitudes s j with individual source amplitudes D j,m , which are proportional to the sound pressure radiated from a source j to a microphone m. The least-squares optimization problem features the cost function: A conjugate gradient method is used to determine the source amplitudes that minimize the cost function in Eq. BIB004 . Equation BIB004 was first published by Michel and Funke BIB001 BIB002 in 2008. The constraint of positive source amplitudes was considered similarly as in SEM (see Sect. 3.9) using D =d 2 and solving for d BIB003 . This modification also increases the robustness of SODIX and makes it converge from simple starting solutions, e.g., an energy-equivalent, constant source distribution that can be directly derived from the microphone signals. To reduce the number of possible solutions and to support physical results, smoothing terms can be added to the cost function BIB001 BIB003 BIB002 . The smoothing terms prevent spurious peaks in the source amplitudes and can, therefore, also improve the dynamic range of the results. The source model of SODIX is based on the two assumptions: 1. The amplitudes d 2 j,m of each point source may have different values for every microphone.

A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Array processing is a powerful tool to extract the noise source characteristics from the acoustic signals measured during wind-tunnels tests. For many years, this measurement technology is commonly being used to study airframe noise. Conventional beam forming methods have proven their efficiency in source localization, but it is well known that they are limited in the estimation of the power levels of extended sources. An approach based on a spectral estimation method (SEM) is proposed in order to determine the actual sound-pressure levels of the acoustic sources found by the localization method. The SEM is based on the array cross-spectral matrix, which contains an indirect measurement of the source power levels of interest and on a modeling of this matrix. Optimal estimates of the actual source power levels can be obtained by minimizing the mean square error between the measurement and model matrices. However, the direct minimization suffers from the sensitivity to noise inherent in the ill-posed nature of the problem. To obtain meaningful results, the problem is regularized by using a priori information. Numerical simulations involving multiple source areas are provided to illustrate the effectiveness of the estimation procedure. The method is successfully applied to compute the power levels of the main sources on a 1/11th-scaled model of the Airbus A320/A321 tested in CEPRA 19 wind tunnel. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> A spectral estimation method based on measurements by a phased array was developed to estimate absolute power levels radiated by compact or extended sources. After a brief recall of its principle, numerical simulations involving several source regions are presented to illustrate the accuracy and the high-resolution performances of this new method. Tests were carried out with a 1:11th scale A320/A321 Airbus model in the anechoic wind tunnel CEPRA 19. The characteristics of source regions from inboard to outboard of the wing slat and flap, and the main and nose landing gear subregions are examined. The dependence in level and frequency of the model subregions with the flow speed, the effects of the attack angle, and the effects of the flap or slat deployment are studied. The different trends revealed by the integrated power levels are also discussed. Examples of the localization of noise sources identified with conventional beam forming and the new method are presented. The consistency of the results is examined by comparing the integrated power levels for the complete wing and the measured spectra with the phased array. Finally, we show that the spectral estimation method is quite reliable in computing the far-field directivity of airframe noise sources. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> The directivities of all broadband sound sources of an aeroengine can be determined from measurements with a line array of microphones. A new data analysis method SODIX (SOurce DIrectivity modelling in crossspectral matriX) was developed for this purpose and applied to the experimental data obtained with a highbypass ratio turbofan engine in an open air test bed. The method is based on modelling the matrix of the crossspectra of the microphone signals with a set of contributions from point sources with unknown directivities assumed in positions along the engine axis. The source positions are defined with a separation of typically 0.4 wave lengths. The analysis has to be performed in narrow frequency bands. The influences of tones are eliminated in the cross-spectral matrix of the microphone signals by interpolation in the frequency domain. The unknown directivities of the point sources are determined with the side conditions that the values must be non-negative and that the directivities and source distributions are smooth. The results of this non-linear optimisation problem confirm that the various sources of an a eroengine have highly non-uniform directivities. The far field can be predicted from the source distributions f or positions over a large range of emission angles. The predictions on a 45.72 m radius around the engine agree within about 1 dB with experimental data. These far-field directivities can be separated into the contribut ions from inlet, casing, the two nozzles, and the jet. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> It is demonstrated that the source strengths and directivit ies of all sound sources of a high bypass ratio aeroengine can be determined from measurements in an open air test bed with a line array of microphones, which is laid out parallel to the e ngine axis in the geometric near field of the engine. The method is based on modelling the matri x of the cross-spectra of the microphones with a set of contributions from point monopoles assumed in positions along the engine axis and by solving the resulting set of linear equations for the unknown source strengths. The directivities of the sources are estimated b y performing the analysis with a subarray of microphones, sliding from the front to the rear of the engine. The set of linear equations is solved with the side condition that the source s trengths must be non-negative. The results demonstrate that the various sources of an aeroengine have a highly non-uniform directivity. The source positions are resolved with a separ ation of 0.4 wave lengths. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Conventional approaches to sampling signals or images follow Shannon's theorem: the sampling rate must be at least twice the maximum frequency present in the signal (Nyquist rate). In the field of data conversion, standard analog-to-digital converter (ADC) technology implements the usual quantized Shannon representation - the signal is uniformly sampled at or above the Nyquist rate. This article surveys the theory of compressive sampling, also known as compressed sensing or CS, a novel sensing/sampling paradigm that goes against the common wisdom in data acquisition. CS theory asserts that one can recover certain signals and images from far fewer samples or measurements than traditional methods use. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> A full-scale test on a turbofan engine has been carried out in the framework of a European project to assess the acoustic benefits of new exhausts and novel hot-stream streamliners in the reduction of the overall jet noise level radiatedinthe far field. Thepurpose ofthetestsisalsoto provideabetterunderstanding ofcombustionnoise,which is one of the main issues for turbofan engines because of the significant noise reduction obtained for other acoustic sources such as the fan and jet by using innovative efficient silencers and acoustics liners. This work investigates the use of the so-called spectral estimation method to separate the combustion noise from other noise sources measured withanear-fieldarray ofmicrophones.Thecomparisonsbetweentheestimated andmeasuredspectradownstream, at the location at which the combustion noise is dominant, yield promising results, suggesting that the procedure of discrimination is relevant. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> This paper presents a systematic comparison of several prominent beamforming algorithms developed for aeroacoustic measurements. The most widely used delay-and-sum (DAS) beamformer is known to suffer from high sidelobe level and low resolution problems. Therefore, more advanced methods, in particular the deconvolution approach for the mapping of acoustic sources (DAMAS), sparsity constrained DAMAS (SC-DAMAS), covariance matrix fitting (CMF) and CLEAN based on spatial source coherence (CLEAN-SC), have been considered to achieve improved resolution and more accurate signal power estimates. The performances of the aforementioned algorithms are evaluated via experiments involving a 63-element logarithmic spiral microphone array in the presence of a single source, two incoherent sources with similar strengths and with different strengths, and two coherent sources. It is observed that DAMAS, SC-DAMAS and CMF provide the most reliable source location estimates, even at relatively low frequencies. Furthermore, th... <s> BIB007 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> The DLR department of engine acoustics performed acoustic measurements of a contra-rotating open-rotor (CROR) model with a linear microphone array in the T-104 open wind-tunnel at TsAGI. The microphone data is corrected for shear-layer effects using the model of an infinitely thin shear layer by Amiet. Investigations of the room-acoustics of the test hall show strong reflections from the ceiling and two side walls, which cause strong interference patterns at the microphone positions. The acoustic change between different rotor confgurations, like different rotor spacings and an upstream mounted mock pylon, is shown with spatially averaged spectra. The evaluation of the source directivities is strongly influenced by the interference patterns of the reflections in the test hall. The rotor tones can be localized at the correct position with classical beamforming. The inverse method SODIX is used to model the directivities of the broadband sound-sources of the CROR. Green's functions that take the shear layer and the first-order reflections into account were ::: implemented. The accuracy of these correction methods could be investigated with the directive SODIX results. The shear-layer model gives accurate results for emission angles < 100° while all modeled sources move downstream for larger emission angles. Corrections for the first-order reflections of the test hall show an increased spatial resolution of the SODIX results and a suppression of artificial sources. <s> BIB008 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Abstract To resolve coherent/incoherent, distributed/compact, and multipole aerodynamic-sound sources with phased-array pressure data, a new source-detection algorithm is developed based on L1 generalized inverse techniques. To extract each coherent signal, a cross spectral matrix is decomposed into eigenmodes. Subsequently, the complex source-amplitude distribution that recovers each eigenmode is solved using generalized inverse techniques with reference solutions which include multipoles as well as a monopole. Namely, the source distribution consisting of pre-defined source types is solved as an L1 norm problem using iteratively re-weighted least squares (IRLS). The capabilities of the proposed algorithm are demonstrated using various benchmark problems to compare the results with several existing beam-forming algorithms, and it is found that distributed sources as well as dipoles with arbitrary orientation can be identified regardless of coherency with another source. The resolution is comparable to existing deconvolution techniques, such as DAMAS or CLEAN, and the computational cost is only several times more than that of DAMAS2. The proposed algorithm is also examined using previous model-scale test data taken in an open-jet wind-tunnel for a study on jet–flap interaction, and some indication of dipole radiation is discerned near the flap edge. <s> BIB009 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> An improved version of the inverse source location method SODIX is presented. SODIX models broadband nosie sources with a st of directive point sources by minimizing the difference between a measured and a simulated cross-spectral matrix of microphone signals. One big advantage of the method in comparison to other source location methods is, that it calculates a directive source strength from each point source towards each microphone. The new version of SODIX includes an implicit consideration of the constraint of positive source strengths in the iteration process. This improvement made the optimization process more stable and in addition independent of the initially assume source distribution. The method is also extended for applications with multidimensional source and microphone arrangements. Further, it is shown that the main diagonal of gthe cross-spectral matrix can be removed from the calculations without any mathematical violations and that the absolute strengths of the dominating sources are preserved. The improvements of the method are shown with exemplary results using measured data from a static aeroengine freefield test. <s> BIB010 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Acoustic measurements of a scaled model of a contra-rotating open rotor engine were performed in an open wind tunnel facility. The motivation for the work presented here was to analyse the possibilities for quantitative acoustic measurements in an environment that was not designed for acoustic measurements and where reverberations, shear layer refraction and turbulence distort the sound field. Extensive acoustic measurements were performed using a linear microphone array with 104 microphones on the floor of the hall. Room acoustics effects were investigated with a reference sound source and the sound field of the rotors was measured for different configurations. The results show that reflections from the ceiling and side walls interfere with the direct sound field and influence the measured directivity due to strong local variations of the sound pressure level from one microphone position to the next. General trends and differences between the configurations can be shown from spatially averaged frequency ... <s> BIB011 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> The reconstruction of acoustical sources from discrete field measurements is a difficult inverse problem that has been approached in different ways. Classical methods (beamforming, near-field acoustical holography, inverse boundary elements, wave superposition, equivalent sources, etc.) all consist--implicitly or explicitly--in interpolating the measurements onto some spatial functions whose propagation are known and in reconstructing the source field by retropropagation. This raises the fundamental question as whether, for a given source topology and array geometry, there exists an optimal interpolation basis which minimizes the reconstruction error. This paper provides a general answer to this question, by proceeding from a Bayesian formulation that is ideally suited to combining information of physical and probabilistic natures. The main findings are the followings: (1) The optimal basis functions are the M eigen-functions of a specific continuous-discrete propagation operator, with M being the number of microphones in the array. (2) The a priori inclusion of spatial information on the source field causes super-resolution according to a phenomenon coined "Bayesian focusing." (3) The approach is naturally endowed with an internal regularization mechanism and results in a robust regularization criterion with no more than one minimum. (4) It admits classical methods as particular cases. <s> BIB012 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> a, b = integers arg min = the argument of the minimum B = blade number C0 = speed of sound, m∕s CM = a constant E = approximation error f = a test function g = approximation of f ĝmin = arg min kĝk1 J = Bessel function of the first kind k = wave number K = compressive sample number M = number of nonzero entries Mj = Mach number m = circumferential mode N = dimension of a signal n = integer p = pressure, Pa r = radial axis u, v, w = velocity, m∕s V = vane number x = axial axis θ = circumferential angle, rad θK = circumferential position of Kth microphone, rad ρ = density, kg∕m ω = angular frequency, rad∕s <s> BIB013 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> The noise emission of a V2500 engine during ground operation was investigated with the A320 research aircraft ATRA of the German aerospace center DLR. Acoustic measurements with a microphone array parallel to the engine shaft axis were performed with the engines running at different speeds between idle and maximum continuous thrust. The main measurements took place in a noise-protection hangar, because optical measurents with laser devices were performed in parallel. In order to assess the impact of the room acoustics on the sound field, reference measurements were performed under free-field conditions, using the same microphone array set up on the ground. The array data were analysed using classical spectral methods and the SODIX (source directivity modelling in the cross-spectral matrix) method. SODIX fits equivalent source distributions, for different frequency bands and emission directions, to the measured cross-spectral matrices. The results show sources at the engine inlet, nozzle and along the jet. When the spectra and source distributions from the main measurements in the hangar are compared to the free-field data, the deviations are acceptable. This confirms that this type of noise protection hangar is a suitable environment to perform acoustic measurements of aircraft engines. <s> BIB014 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Abstract To identify multiple acoustic duct modes, conventional beam-forming, CLEAN as well as L 2 (i.e. pseudo-inverse) and L 1 generalized-inverse beam-forming are applied to phased-array pressure data. A tone signal of a prescribed mode or broadband signal is generated upstream of a curved rectangular duct, and acoustic fields formed in both upstream and downstream stations of the test section are measured with identical wall-mounted microphone arrays. Sound-power distributions of several horizontal and vertical modes including upstream- and downstream-propagating waves can be identified with phased-array techniques, and the results are compared among the four approaches. The comparisons using synthetic data demonstrate that the L 2 generalized-inverse algorithm can sufficiently suppress undesirable noise levels and detect amplitude distributions accurately in over-determined cases (i.e. the number of microphones is more than the number of cut-on modes) with minimum computational cost. As the number of cut-on modes exceeds the number of microphones (i.e. under-determined problems), the L 1 algorithm is necessary to retain better accuracy. The comparison using test data acquired in the curved duct test rig (CDTR) at NASA Langley Research Center suggests that the L 1 / L 2 generalized-inverse approach as well as CLEAN can improve the dynamic range of the detected mode by as much as 10 dB relative to conventional beam-forming even with mean flow of M =0.5. <s> BIB015 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> This paper answers the challenge as how to automatically select a good regularization parameter when solving inverse problems in acoustics. A Bayesian solution is proposed that consists either in directly finding the most probable value of the regularization parameter or, indirectly, in estimating it as the ratio of the most probable values of the noise and source expected energies. It enjoys several nice properties such as ease of implementation and low computational complexity (the proposed algorithm boils down to searching for the global minimum of a 1D cost function). Among other advantages of the Bayesian approach, it makes possible to appraise the sensitivity of the reconstructed acoustical quantities of interest with respect to regularization, a performance that would be otherwise arduous to achieve. <s> BIB016 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Establishing microphone array methods as tool for precise acoustic measurements necessitates assessing their reliability depending on given boundary conditions. In this contribution, an approach to evaluate the performance of microphone array methods is proposed. Instead of simply testing an algorithm on distinct scenarios, Monte Carlo simulations are performed to allow for a statistical analysis. In a simulation scenario, number of sources, source positions, and source levels are varied. The objective is set to correctly reconstruct the level and position of the sound sources, and a rating criterion is dened. The microphone array methods DAMAS, CLEAN-SC, Orthogonal Beamforming and Covariance Matrix Fitting are tested for their performance on 12 600 generated data sets. The results are then statistically evaluated regarding the precision of the reconstruction abilities depending on the varied parameters. It is shown that this approach helps deducing information about the characteristic performance of the methods that could not easily have been revealed otherwise. <s> BIB017 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> This paper presents a compressive sensing based experimental method for detecting spinning modes of sound waves propagating inside a cylindrical duct system. This method requires fewer dynamic pressure sensors than the number required by the Shannon–Nyquist sampling theorem so long as the incident waves are sparse in spinning modes. In this work, the proposed new method is firstly validated by preparing some of the numerical simulations with representative set-ups. Then, a duct acoustic testing rig with a spinning mode synthesiser and an in-duct microphone array is built to experimentally demonstrate the new approach. Both the numerical simulations and the experiment results are satisfactory, even when the practical issue of the background noise pollution is taken into account. The approach is beneficial for sensory array tests of silent aeroengines in particular and some other engineering systems with duct acoustics in general. <s> BIB018 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> A new approach for the Azimuthal Mode Analysis (AMA) of over- and subsampled sound fields in flow ducts of turbomachines using circumferential sensor arrays is presented which is based on Compressed Sensing. Compressed Sensing is a framework to solve underdetermined systems of linear equations under the assumption that the solution vector is sparse. In the context of AMA, such sparse mode spectra often occur in the analysis of tonal sound field components generated by rotor-stator interaction. Important features such as stability and accuracy are investigated by analysis of a wide range of simulated and measured pressure data and comparison with commonly applied methods. Also two different optimisation approaches for the design of circumferential sensor arrays are introduced which allows a reduction of the number of sensors for large modal ranges if only a limited number of modes are dominant. Furthermore, the remaining mode amplitudes are estimated exploiting the deconvolution property of the applied Compressed Sensing algorithm. <s> BIB019 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r... <s> BIB020 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> The source distribution in the dual jets of a 1:10 scale jet engine model was investigated in a large anechoic facility. A linear microphone array was used to record the data and the analysis was performed with the SODIX method, an optimization technique that fits a model distribution of sound sources with non-uniform directivity to the measured cross spectral matrix. Configurations with a plain secondary nozzle and a secondary nozzle with a serrated trailing edge were investigated with and without a wing installed above the engine. The results show that the SODIX method is able to resolve the different source distributions of the jet with the plain and the serrated nozzle as well as the additional sources that occur when a wing is installed. <s> BIB021 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> The problem of localizing and quantifying acoustic sources from a set of acoustic measurements has been addressed, in the last decades, by a huge number of scientists, from different communities (s... <s> BIB022 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> In this werk, various microphone phased array data processing techniques are applied to two existing datasets from aeroacoustic wind tunnel tests. The first of these is from a !arge closed-wall facility, DLR's Kryo-Kanal Koln (DNW·KKK), and is a measurement cf the high-lift noise cf a semispan model. The second is from a small-scale open-jet facility, the NASA Langley Quiet Flow Facility (QFF), and is a measurement cf a clean airfoil selfnoise. The data had been made publicly available in 2015, and were analyzed by several research groups using multiple analysis techniques. This procedure allows the assessment of the variability of individual methods across various organizational implementations, as weil as the variability cf results produced by different array analysis methods. This paper summarizes the results presented at panel sessions held at AIAA conferences in 2015 and 2016. Results show that with appropriate handling of background noise, all advanced methods can identify dominant acoustic sources for a broad range cf frequencies. Lowerlevel sources may be masked er underpredicted. lntegrated levels are more robust and in closer agreement between methods than narrowband maps for individual frequencies. Overall there is no obvious best method, though multiple methods may be used to bound expected behavior. <s> BIB023 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> Conventional beamforming with a microphone array is a well-established method for localizing and quantifying sound sources. It provides estimates for the source strengths on a predefined grid by determining the agreement between the pressures measured and those modeled for a source located at the grid point under consideration. As such, conventional beamforming can be seen as an exhaustive search for those locations that provide a maximum match between measured and modeled pressures. In this contribution, the authors propose to, instead of the exhaustive search, use an efficient global optimization method to search for the source locations that maximize the agreement between model and measurement. Advantages are two-fold. First, the efficient optimization allows for inclusion of more unknowns, such as the source position in three-dimensional or environmental parameters such as the speed of sound. Second, the model for the received pressure field can be readily adapted to reflect, for example, the presence of more sound sources or environmental parameters that affect the received signals. For the work considered, the global optimization method, Differential Evolution, is selected. Results with simulated and experimental data show that sources can be accurately identified, including the distance from the source to the array. <s> BIB024 </s> A review of acoustic imaging methods using phased microphone arrays <s> 3 <s> In 2015, the Japan Aerospace Exploration Agency launched the Flight demonstration of QUiet technology to Reduce nOise from High-lift configurations project to verify by flight demonstration the feasibility of practical noise-reducing aircraft modification concepts. In order to serve as a baseline for comparison before modification, airframe noise sources of the JAXA Jet Flying Test Bed “Hisho” were measured with a 30 m diameter array of 195 microphones mounted on a wooden platform built temporary beside the runway of Noto Satoyama Airport in Japan. A classical Delay and Sum in the time domain beamforming algorithm was adapted for the present study, with weight factors introduced to improve the low-frequency resolution and autocorrelations eliminated to suppress wind noise at high frequencies. In the landing configuration at idle thrust, the main landing gear, nose landing gear, and side edges of the six extended flap panels were found to be the dominant “Hisho” airframe noise sources. Deconvolution by the DAMAS and CLEAN-SC algorithms provided clearer positions of these sound sources at low frequencies. Integration of acoustical maps agreed well with the sound pressure level measured by a microphone placed at the center of the microphone array and gave detailed information about the contribution of each noise source. <s> BIB025

2. The phase of a sound wave radiated by a point source spreads spherically, i.e., according to the complex argument in the steering vector g j,m . Both assumptions are valid for the sound propagation within a single lobe in the acoustic far field of a multipole source. The analysis of broadband noise during static engine tests with large linear microphone arrays demonstrated that SODIX can calculate reliable results over a wide range of angles around the engine BIB010 . Phase jumps between different radiation lobes become a problem mainly for tonal noise. They violate the second assumption in the source model and, therefore, lead to source distributions and directivities that are not representative. However, the outstanding capability of SODIX is the free modeling of the directivities of the sound sources: SODIX can not only account for the multipole characteristics of the sound sources, but also for directivities due to source interference within coherent sources, such as jet mixing noise. As in the case of SEM (see Sect. 3.9), the main diagonal of the CSM can be removed from the calculations without violating any assumptions. This makes SODIX, like SEMWAN (see Sect. 3.9), suitable for closed-section wind-tunnel applications. The spatial resolution of SODIX was compared to that of CFDBF and various deconvolution methods in . The results showed that SODIX overcomes the Sparrow resolution limit and, therefore, provides super-resolution. However, the computational effort for SODIX is rather high, because the number of unknowns to be determined, i.e., J × N , can be very large. SODIX was also successfully applied to engine indoor tests , ground tests with an Airbus A320 aircraft BIB014 , measurements of a counter rotating open rotor (CROR) model in an open-jet wind tunnel BIB008 BIB011 , and measurements of model-scale turbofan-nozzles in an open-jet wind tunnel BIB021 . This method can be extended to treat potentially coherent sources . • Spectral estimation method (SEM): BIB001 is intended for the location of distributed sound sources. It offers a better dynamic range and spatial resolution than CFDBF. Removing the main diagonal of the CSM does not violate any assumption for this method. The effect of background noise and reflections can also be taken into account using SEMWAN or cepstrum , respectively. This method has been applied in open-jet and closed-section wind-tunnel experiments BIB002 BIB006 BIB001 . • SODIX: BIB003 BIB010 BIB004 is an extension of SEM for experiments where the directivities of the sound sources are of interest. It provides a better resolution than the Sparrow resolution limit. SODIX has been mostly applied to static engine tests on free-field and indoor test beds, BIB003 BIB010 and measurements in open-jet wind tunnels BIB008 BIB011 BIB021 . • Compressive-sensing beamforming: BIB005 BIB013 assumes spatially sparse distributions of sound sources and requires a lower number of microphones. This inverse technique has been used to identify spinning modes of turbofan engines BIB018 BIB019 . • Generalized inverse beamforming: BIB009 considers partially coherent sound sources using inversion techniques. Multipoles in an arbitrary direction can be considered as well. An application to duct acoustics was studied in BIB015 and to airfoil noise in . • Iterative Bayesian inverse approach (IBIA): BIB022 BIB012 BIB016 is based on a user-defined level of sparsity of the sound sources. The results are somewhat between those of CFDBF (no sparsity) and CLEAN-SC (strong sparsity). It has been applied to measurements in closed-section wind tunnels BIB023 . • Global optimization methods: BIB024 can be used for searching the locations and amplitudes of sound sources without using a scan grid. Other parameters, such as the sound speed, can be added to the optimization problem to obtain more information about the sound field. Differential evolution was applied to a experiment with a speaker in an anechoic room BIB024 . A summary of the required parameters and typical applications of all the methods described in this manuscript is presented in Table 1 , as well as additional comments if needed. Except when explicitly stated, a scan grid is also required for most of the acoustic imaging methods listed. The reader is warned to consider that the information indicated in Table 1 is just indicative and not at all restrictive. The beamforming methods that determine only the incoherent source strengths are conventional, functional, orthogonal, wavenumber, and robust adaptive beamforming, as well as CLEAN-SC, DAMAS, linear programming deconvolution, and SEM. SODIX adds source directivity to the results, and DAMAS-C adds source coherence as an output (CLEAN-SC considers it implicitly). The remaining methods attempt to find the amplitude, phase, and, in some cases, the partial coherence of sources using different regularization schemes. A summary of all these characteristics is presented in Table 1 . In general, more complex methods require considerably more computational time than CFDBF. Hence, depending on the experiment requirements, this can pose some constraints when selecting the most suitable method. Other benchmark cases analyzing the performance of some of these methods for specific acoustic applications can be found in the literature BIB020 BIB007 BIB017 BIB025 . Moreover, a broad effort in the aeroacoustics community was started by NASA Langley researchers to establish a common set of benchmark problems for the purpose of testing and validation of new analysis techniques . This ongoing working group has met at several forums, shared the initial results for the chosen test cases, and has recently presented the results from the first release of benchmark problems BIB023 .

A review of acoustic imaging methods using phased microphone arrays <s> Compressive-sensing beamforming <s> Conventional approaches to sampling signals or images follow Shannon's theorem: the sampling rate must be at least twice the maximum frequency present in the signal (Nyquist rate). In the field of data conversion, standard analog-to-digital converter (ADC) technology implements the usual quantized Shannon representation - the signal is uniformly sampled at or above the Nyquist rate. This article surveys the theory of compressive sampling, also known as compressed sensing or CS, a novel sensing/sampling paradigm that goes against the common wisdom in data acquisition. CS theory asserts that one can recover certain signals and images from far fewer samples or measurements than traditional methods use. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Compressive-sensing beamforming <s> a, b = integers arg min = the argument of the minimum B = blade number C0 = speed of sound, m∕s CM = a constant E = approximation error f = a test function g = approximation of f ĝmin = arg min kĝk1 J = Bessel function of the first kind k = wave number K = compressive sample number M = number of nonzero entries Mj = Mach number m = circumferential mode N = dimension of a signal n = integer p = pressure, Pa r = radial axis u, v, w = velocity, m∕s V = vane number x = axial axis θ = circumferential angle, rad θK = circumferential position of Kth microphone, rad ρ = density, kg∕m ω = angular frequency, rad∕s <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Compressive-sensing beamforming <s> Compressive sensing, a newly emerging method from information technology, is applied to array beamforming and associated acoustic applications. A compressive sensing beamforming method (CSB-II) is developed based on sampling covariance matrix, assuming spatially sparse and incoherent signals, and then examined using both simulations and aeroacoustic measurements. The simulation results clearly show that the proposed CSB-II method is robust to sensing noise. In addition, aeroacoustic tests of a landing gear model demonstrate the good performance in terms of resolution and sidelobe rejection. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Compressive-sensing beamforming <s> This paper presents a compressive sensing based experimental method for detecting spinning modes of sound waves propagating inside a cylindrical duct system. This method requires fewer dynamic pressure sensors than the number required by the Shannon–Nyquist sampling theorem so long as the incident waves are sparse in spinning modes. In this work, the proposed new method is firstly validated by preparing some of the numerical simulations with representative set-ups. Then, a duct acoustic testing rig with a spinning mode synthesiser and an in-duct microphone array is built to experimentally demonstrate the new approach. Both the numerical simulations and the experiment results are satisfactory, even when the practical issue of the background noise pollution is taken into account. The approach is beneficial for sensory array tests of silent aeroengines in particular and some other engineering systems with duct acoustics in general. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Compressive-sensing beamforming <s> A new approach for the Azimuthal Mode Analysis (AMA) of over- and subsampled sound fields in flow ducts of turbomachines using circumferential sensor arrays is presented which is based on Compressed Sensing. Compressed Sensing is a framework to solve underdetermined systems of linear equations under the assumption that the solution vector is sparse. In the context of AMA, such sparse mode spectra often occur in the analysis of tonal sound field components generated by rotor-stator interaction. Important features such as stability and accuracy are investigated by analysis of a wide range of simulated and measured pressure data and comparison with commonly applied methods. Also two different optimisation approaches for the design of circumferential sensor arrays are introduced which allows a reduction of the number of sensors for large modal ranges if only a limited number of modes are dominant. Furthermore, the remaining mode amplitudes are estimated exploiting the deconvolution property of the applied Compressed Sensing algorithm. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Compressive-sensing beamforming <s> A novel signal processing approach is proposed for sensor arrays to reconstruct spinning modal noise source from scattered sound wave measurements outside the jet flow downstream of an unflanged duct. The approach relies on the forward propagation model based on the Wiener-Hopf method and the key contribution is the development of the inverse acoustic scattering approach for a sensor array by combining compressive sensing and beamforming strategies in a non-classical way. In particular, the beamforming and compressive sensing methods are considered to improve the performance of the inverse reconstruction, especially in terms of array imagining resolutions and the number of sensors. Finally, numerical examples are prepared to demonstrate the performance of the proposed approach and the results show that the classical-beamforming-based method is more robust for numerical noise while the compressive-sensing-based method is more efficient. Overall, the proposed testing approach should be able to extensively expand the current acoustic array testing capability and will be especially useful for aerospace engineering applications, such as the design and evaluation of low-noise aircraft engines. <s> BIB006

Compressive sensing BIB001 is a new paradigm of signal processing used in the field of information technology which reduces sampling efforts extensively by conducting L 1 optimization. Reference BIB002 provides a tutorial example to demonstrate this method for potential aeroacoustic applications. A compressive-sensing-based beamforming method has been developed recently for aeroacoustic applications BIB003 , by assuming a spatially sparse distribution of flow-induced sound sources, S . In particular, a narrow-band compressive-sensing beamforming can be performed using a non-linear optimization algorithm, such as: where, here, ( ⋅) denotes the estimation and is a noise parameter for which = 0 if the measurements are free of (23) arg min ||Ŝ|| 1 , subject to ||p − GŜ|| 2 ≤ , ≥ 0, noise. A more robust (but more complicated) form can be found in reference BIB003 . This method has been adopted in the identification of spinning modes for turbofan noise using microphone-array measurements. The required number of sensors can be much less than the number required by the sampling theorem as long as the incident fan noise is sparse in spinning modes BIB004 BIB005 BIB006 . It should be noted that compressive sensing, unlike the beamforming methods discussed, is an inverse method that aims to determine the phase and amplitude of the source distribution. The linear algebra problems at the core of inverse methods are often severely under-determined. Selecting a particular solution to display requires that the undetermined components of the solution be established somehow. The conventional approach of minimizing the L 2 norm of the solution can give solutions that look incorrect, because they have many small non-zero elements. Compressive sensing can be considered as a cosmetic improvement that tends to cluster the non-zero sources. If there is reason to believe that the true sources have this kind of distribution, then the compressive-sensing result may be more accurate than the conventional inverse solution. Several of the techniques discussed below are also inverse methods that handle the illconditioned problem in slightly different ways.

A review of acoustic imaging methods using phased microphone arrays <s> Generalized inverse beamforming (GIBF) <s> Abstract To resolve coherent/incoherent, distributed/compact, and multipole aerodynamic-sound sources with phased-array pressure data, a new source-detection algorithm is developed based on L1 generalized inverse techniques. To extract each coherent signal, a cross spectral matrix is decomposed into eigenmodes. Subsequently, the complex source-amplitude distribution that recovers each eigenmode is solved using generalized inverse techniques with reference solutions which include multipoles as well as a monopole. Namely, the source distribution consisting of pre-defined source types is solved as an L1 norm problem using iteratively re-weighted least squares (IRLS). The capabilities of the proposed algorithm are demonstrated using various benchmark problems to compare the results with several existing beam-forming algorithms, and it is found that distributed sources as well as dipoles with arbitrary orientation can be identified regardless of coherency with another source. The resolution is comparable to existing deconvolution techniques, such as DAMAS or CLEAN, and the computational cost is only several times more than that of DAMAS2. The proposed algorithm is also examined using previous model-scale test data taken in an open-jet wind-tunnel for a study on jet–flap interaction, and some indication of dipole radiation is discerned near the flap edge. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Generalized inverse beamforming (GIBF) <s> The acoustic holography/generalized inverse method of microphone array processing is improved in two ways: the linear algebra is simplified to avoid separate processing of cross spectral matrix eigenvalues, and a new regularization technique is introduced to produce reasonable results when the problem is underdetermined. Model-scale testing has shown that the improved method can map coherent sources and extrapolate directivity patterns using a modest 2D nearfield array and one data acquisition in a warehouse with no acoustic treatment. Tests with shop air jets show that rectangular jets can have asymmetric radiation patterns and the flight deck and jet blast deflector dramatically alter the radiation pattern in a configuration representing an aircraft carrier launch operation. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Generalized inverse beamforming (GIBF) <s> Abstract To identify multiple acoustic duct modes, conventional beam-forming, CLEAN as well as L 2 (i.e. pseudo-inverse) and L 1 generalized-inverse beam-forming are applied to phased-array pressure data. A tone signal of a prescribed mode or broadband signal is generated upstream of a curved rectangular duct, and acoustic fields formed in both upstream and downstream stations of the test section are measured with identical wall-mounted microphone arrays. Sound-power distributions of several horizontal and vertical modes including upstream- and downstream-propagating waves can be identified with phased-array techniques, and the results are compared among the four approaches. The comparisons using synthetic data demonstrate that the L 2 generalized-inverse algorithm can sufficiently suppress undesirable noise levels and detect amplitude distributions accurately in over-determined cases (i.e. the number of microphones is more than the number of cut-on modes) with minimum computational cost. As the number of cut-on modes exceeds the number of microphones (i.e. under-determined problems), the L 1 algorithm is necessary to retain better accuracy. The comparison using test data acquired in the curved duct test rig (CDTR) at NASA Langley Research Center suggests that the L 1 / L 2 generalized-inverse approach as well as CLEAN can improve the dynamic range of the detected mode by as much as 10 dB relative to conventional beam-forming even with mean flow of M =0.5. <s> BIB003

The idea behind generalized inverse beamforming (GIBF) BIB001 is to reconstruct the CSM by a collection of partially coherent sources. Although a method exists which directly recovers the entire CSM , its computational cost drastically increases with the number of microphones. To reduce the computational time substantially, use of an eigenvalue decomposition can be made [see Eq. (7)]. This decomposition breaks the problem into N decoupled equations as follows: where G ∈ ℂ N×J denotes an N × J matrix consisting of steering vectors g 1 … g J and S n represents a column vector of the complex source-amplitude distribution s 1 , … , s J ⊺ n corresponding to the nth eigenvector. Here, (⋅) ⊺ represents the transpose. In general, the number of grid points J is larger than the number of microphones N. Therefore, Eq. (24) can be solved as an under-determined problem using generalized inverse techniques. The simplest method is to minimize an L 2 norm, and to generate a source map for each eigenvector by solving the following equation: where generally ranges from 0.1 to 10% of the maximum eigenvalue of to numerically stabilize the matrix inversion (a systematic optimization method was proposed in ). This solution actually serves as an initial condition for the iterative method given below. The resolution of source maps generated by Eq. (25) is, however, still comparable to CFDBF BIB002 . To increase the resolution, it is crucial to minimize an L 1 norm by iteratively solving the following equation: where the superscript i denotes the iteration counter, and n is a J × J diagonal matrix whose components are The convergence can be accelerated by reducing the number of grid points through the iteration. Likewise, over-determined cases can be solved using an analogous generalized inverse technique BIB001 . Benefits of this algorithm are not only the treatment of the source coherence, but also to include any types of prescribed sources in G . Hence, in principle, multipoles in an arbitrary orientation can be detected and different types of sources can be collocated at the same grid point. An application to duct acoustics, in which over-determined problems are typically solved, was also studied in BIB003 . Several regularization methods for GIBF have been recently applied to airfoil-noise measurements in open-jet wind tunnels .

A review of acoustic imaging methods using phased microphone arrays <s> Iterative Bayesian inverse approach (IBIA) <s> The reconstruction of acoustical sources from discrete field measurements is a difficult inverse problem that has been approached in different ways. Classical methods (beamforming, near-field acoustical holography, inverse boundary elements, wave superposition, equivalent sources, etc.) all consist--implicitly or explicitly--in interpolating the measurements onto some spatial functions whose propagation are known and in reconstructing the source field by retropropagation. This raises the fundamental question as whether, for a given source topology and array geometry, there exists an optimal interpolation basis which minimizes the reconstruction error. This paper provides a general answer to this question, by proceeding from a Bayesian formulation that is ideally suited to combining information of physical and probabilistic natures. The main findings are the followings: (1) The optimal basis functions are the M eigen-functions of a specific continuous-discrete propagation operator, with M being the number of microphones in the array. (2) The a priori inclusion of spatial information on the source field causes super-resolution according to a phenomenon coined "Bayesian focusing." (3) The approach is naturally endowed with an internal regularization mechanism and results in a robust regularization criterion with no more than one minimum. (4) It admits classical methods as particular cases. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Iterative Bayesian inverse approach (IBIA) <s> This paper answers the challenge as how to automatically select a good regularization parameter when solving inverse problems in acoustics. A Bayesian solution is proposed that consists either in directly finding the most probable value of the regularization parameter or, indirectly, in estimating it as the ratio of the most probable values of the noise and source expected energies. It enjoys several nice properties such as ease of implementation and low computational complexity (the proposed algorithm boils down to searching for the global minimum of a 1D cost function). Among other advantages of the Bayesian approach, it makes possible to appraise the sensitivity of the reconstructed acoustical quantities of interest with respect to regularization, a performance that would be otherwise arduous to achieve. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Iterative Bayesian inverse approach (IBIA) <s> The problem of localizing and quantifying acoustic sources from a set of acoustic measurements has been addressed, in the last decades, by a huge number of scientists, from different communities (s... <s> BIB003

Inverse methods are based on a global formulation and are used to identify the source strength at all the points of the scan grid at once. The output of the inverse formulation is no longer described as single-source autopowers A( j ) at each point j of the grid, as in Eq. (4), but rather as a global source CSM, noted B ∈ ℂ J×J , obtained as follows: The matrix (∈ ℂ J×N ) is a regularized inverse of G , C is the CSM of the signals at the microphones, and the diagonal terms of B represent the actual source autopowers x j for j ∈ [1 … J] . The difficulty in applying inverse methods is to correctly build the matrix . This is not an easy task, because the problem is often under-determined ( J ≫ N ), and ill-conditioned (steering vectors g j are far from being orthogonal to each other, especially at low frequencies). Iterative Bayesian Inverse Approaches (IBIA) are based on a Bayesian regularization process assuming a userdefined level of sparsity. The first step of the iterative process is to calculate the pseudo-inverse of G with Tikhonov regularization: where the regularization parameter 2 is estimated using a Bayesian criterion BIB003 BIB001 BIB002 . The optimal value of 2 is defined as the one minimizing the following cost function: where ̃n and ũ n (n ∈ [1 … N]) are the singular values and left singular vectors of G , respectively. The sparsity constraint is then enforced through an iterative estimation of : where R i is a right diagonal-weighting matrix balancing the weight of each source in the regularization process (for the ith iteration), whose jth diagonal entry is calculated as follows: where q is the parameter defined by the user controlling the sparsity with 0 ≤q ≤ 2 . In Eq. (30), 2 is estimated during each iteration using Eq. (29), in which ̃n and ũ n are now the singular values and left singular vectors of (GR i ) , respectively. Several conditions can be used as a stopping criterion, like the maximum number of iterations or a distance between B i+1 and B i . The parameter q , defining the shape of the a priori probability density function of sources, determines the power of the solution norm used in the regularization process. A value of q = 2 keeps the initial value 0 (Eq. (28)), which means no sparsity. Thus, the amount of sparsity increases as q decreases, down to 0 for which a strong sparsity is requested. This method has been tested with the experimental data from a half-aircraft model in a closed-section wind tunnel, as shown in Fig. 12.

A review of acoustic imaging methods using phased microphone arrays <s> Global optimization methods <s> For a selected number of shallow water test cases of the 1997 Geoacoustic Inversion Workshop we have applied Matched-Field Inversion to determine the geoacoustic and geometric (source location, water depth) parameters. A genetic algorithm has been applied for performing the optimization, whereas the replica fields have been calculated using a standard normal-mode model. The energy function to be optimized is based on the incoherent multi-frequency Bartlett processor. We have used the data sets provided at a few frequencies in the band 25-500 Hz for a vertical line array positioned at 5 km from the source. A comparison between the inverted and true parameter values is made. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Global optimization methods <s> This article presents an overview of recent work on ant algorithms, that is, algorithms for discrete optimization that took inspiration from the observation of ant colonies' foraging behavior, and introduces the ant colony optimization (ACO) metaheuristic. In the first part of the article the basic biological findings on real ants are reviewed and their artificial counterparts as well as the ACO metaheuristic are defined. In the second part of the article a number of applications of ACO algorithms to combinatorial optimization and routing in communications networks are described. We conclude with a discussion of related work and of some of the most important aspects of the ACO metaheuristic. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Global optimization methods <s> Having available efficient global optimization methods is of high importance when going to a practical application of geo-acoustic inversion, where fast processing of the data is an essential requirement. A series of global optimization techniques are available and have been described in literature. In this paper three optimization techniques are considered, being a genetic algorithm (GA), differential evolution (DE), and the downhill simplex algorithm (DHS). The performance of these three methods is assessed using a test function, demonstrating superior performance of DE. Additionally, the DE optimal setting is determined. As a next step DE is applied for determining the geo-acoustic properties of the upper seabed sediments from simulated seabed reflection loss, indicating good DE performance also for real geo-acoustic inversion problems. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Global optimization methods <s> Conventional beamforming with a microphone array is a well-established method for localizing and quantifying sound sources. It provides estimates for the source strengths on a predefined grid by determining the agreement between the pressures measured and those modeled for a source located at the grid point under consideration. As such, conventional beamforming can be seen as an exhaustive search for those locations that provide a maximum match between measured and modeled pressures. In this contribution, the authors propose to, instead of the exhaustive search, use an efficient global optimization method to search for the source locations that maximize the agreement between model and measurement. Advantages are two-fold. First, the efficient optimization allows for inclusion of more unknowns, such as the source position in three-dimensional or environmental parameters such as the speed of sound. Second, the model for the received pressure field can be readily adapted to reflect, for example, the presence of more sound sources or environmental parameters that affect the received signals. For the work considered, the global optimization method, Differential Evolution, is selected. Results with simulated and experimental data show that sources can be accurately identified, including the distance from the source to the array. <s> BIB004

In Ref. BIB004 a method is presented where the search for the locations and amplitudes of sound sources is treated as a global optimization problem. The search can be easily extended to more unknowns, such as additional geometrical parameters, and more complex situations with, for example, multiple sound sources or reflections being present. The method is essentially grid-free and can overcome the Sparrow resolution limit when sources are positioned close together. The presence of sidelobes will, however, hamper the optimization as they act as local optima against which the global optimum needs to be found. In the literature, a number of mathematical methods are presented which allow for optimization problems with many unknowns and with the capability to escape from local optima, in contrast to local search techniques, e.g., gradient methods. These methods are generally denoted as global optimization methods. Wellknown examples are genetic algorithms BIB001 , simulated annealing , and ant colony optimization BIB002 . In BIB004 , a variation of the genetic algorithm, called differential evolution BIB003 , is proposed as a global optimization method. This type of optimization method mimics natural evolution. They use populations of solutions, where promising solutions are given a high probability to reproduce and worse solutions have a lower probability to reproduce. This work can be seen as an alternative approach to DAMAS and SEM. Whereas DAMAS assesses the performance of a solution based on the agreement between modeled and measured beamformed outputs, SEM is based on the comparison between the modeled and measured pressure fields. In contrast, in this technique, the locations of the sources are sought using a global optimization method, instead of considering a predefined grid of potential source locations. This way, estimates for source positions and source strengths are obtained as a solution of the optimization and do not need to be obtained from a source plot. To use global optimization methods, a cost function F has to be defined. This can be done by constructing a CSM from a signal model, C model , and comparing it to the measured CSM, C meas . For example, the objective function can have the following form: where is the parameter vector for the optimization method containing the spatial positions and strengths of the sources. This objective function can then, in turn, be used in an optimizer, such as differential evolution. By minimizing the objective function over many generations, the parameter vector will converge to the actual source positions and strengths. Some first experimental results of this technique with the simulated data and a single speaker in an anechoic room are presented in reference BIB004 .

A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> An overview of beamforming from a signal-processing perspective is provided, with an emphasis on recent research. Data-independent, statistically optimum, adaptive, and partially adaptive beamforming are discussed. Basic notation, terminology, and concepts are included. Several beamformer implementations are briefly described. > <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> 1 Introduction.- 1.1 Introduction.- 1.2 Organization of the Book.- 1.3 Notations and Preliminaries.- 2 Detection of Multiple Signals.- 2.1 Signals and Noise.- 2.2 Conventional Techniques.- 2.2.1 Beamformer.- 2.2.2 Capon's Minimum Variance Estimator.- 2.2.3 Linear Prediction Method.- 2.3 Eigenvector-Based Techniques.- 2.3.1 Completely Coherent Case.- 2.3.2 Symmetric Array Scheme: Coherent Sources in a Correlated Scene.- 2.3.3 Spatial Smoothing Schemes: Direction Finding in a Coherent Environment.- 2.4 Augmentation and Other Techniques.- 2.4.1 Augmentation Technique.- 2.4.2 ESPRIT, TLS-ESPRIT and GEESE.- 2.4.3 Direction Finding Using First Order Statistics.- Appendix 2.A Coherent and Correlated Signal Scene.- Appendix 2.B Program Listings.- Problems.- References.- 3 Performance Analysis.- 3.1 Introduction.- 3.2 The Maximum Likelihood Estimate of the Covariance Matrix and Some Related Distributions.- 3.3 Performance Analysis of Covariance Based Eigenvector Techniques: MUSIC and Spatial Smoothing Schemes.- 3.3.1 Asymptotic Distribution of Eigenparameters Associated with Smoothed Sample Covariance Matrices.- 3.3.2 Two-Source Case - Uncorrelated and Coherent Scene.- 3.4 Performance Evaluation of GEESE Scheme.- 3.4.1 The Least Favorable Configuration (J = K).- 3.4.2 The Most Favorable Configuration (J = M - 1).- 3.5 Estimation of Number of Signals.- Appendix 3.A The Complex Wishart Distribution.- Appendix 3.B Equivalence of Eigenvectors.- Appendix 3.C Eigenparameters in a Two Source Case.- Problems.- References.- 4 Estimation of Multiple Signals.- 4.1 Introduction.- 4.2 Optimum Processing: Steady State Performance and the Wiener Solution.- 4.3 Implementation of the Wiener Solution.- 4.3.1 The Method of Steepest Descent.- 4.3.2 The Least Mean Square (LMS) Algorithm.- 4.3.3 Direct Implementation by Inversion of the Sample Covariance Matrix.- Problems.- References. <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> 1. Introduction 2. Signals in Space and Time 3. Apertures and Arrays 4. Conventional Array Processing 5. Detection Theory 6. Estimation Theory 7. Adaptive Array Processing 8. Tracking Appendices References List of Symbols Index. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> At the 2004 AIAA/CEAS Aeroacoustic Conference, a breakthrough in acoustic microphone array technology was reported by the authors. A Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) was developed which decouples the array design and processing influence from the noise being measured, using a simple and robust algorithm. For several prior airframe noise studies, it was shown to permit an unambiguous and accurate determination of acoustic source position and strength. As a follow-on effort, this paper examines the technique for three-dimensional (3D) applications. First, the beamforming ability for arrays, of different size and design, to focus longitudinally and laterally is examined for a range of source positions and frequency. Advantage is found for larger array designs with higher density microphone distributions towards the center. After defining a 3D grid generalized with respect to the array’s beamforming characteristics, DAMAS is employed in simulated and experimental noise test cases. It is found that spatial resolution is much less sharp in the longitudinal direction in front of the array compared to side-to-side lateral resolution. 3D DAMAS becomes useful for sufficiently large arrays at sufficiently high frequency. But, such can be a challenge to computational capabilities, with regard to the required expanse and number of grid points. Also, larger arrays can strain basic physical modeling assumptions that DAMAS and all traditional array methodologies use. An important experimental result is that turbulent shear layers can negatively impact attainable beamforming resolution. Still, the usefulness of 3D DAMAS is demonstrated by the measurement of landing gear noise source distributions in a difficult hard-wall wind tunnel environment. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> The DAMAS deconvolution algorithm represents a breakthrough in phased array imaging for aeroacoustics, potentially eliminating sidelobles and array resolution effects from beamform maps . DAMAS is an iterative non-negative least squares solver. The original algorithm is too slow and lacks an explicit regularization method to prevent noise amplification. Two extensions are proposed, DAMAS2 and DAMAS3. DAMAS2 provides a dramatic speedup of each iteration and adds regularization by a low pass filter. DAMAS3 also provides fast iterations, and additionally, reduces the required number of iterations. It uses a different regularization technique from DAMAS2, and is partially based on the Wiener filter. Both DAMAS2 and DAMAS3 restrict the point spread function to a translationally-invariant, convolutional, form. This is a common assumption in optics and radio astronomy, but may be a serious limitation in aeroacoustic beamforming. This limitation is addressed with a change of variables from (x,y,z) to a new set, (u,v,w). The concepts taken together, along with appropriate array design, may permit practical 3D beamforming in aeroacoustics. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Current processing of acoustic array data is burdened with considerable uncertainty. This study reports an original methodology that serves to demystify array results, reduce misinterpretation, and accurately quantify position and strength of acoustic sources. Traditional array results represent noise sources that are convolved with array beamform response functions, which depend on array geometry, size (with respect to source position and distributions), and frequency. The Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) method removes beamforming characteristics from output presentations. A unique linear system of equations accounts for reciprocal influence at different locations over the array survey region. It makes no assumption beyond the traditional processing assumption of statistically independent noise sources. A new robust iterative method seamlessly introduces a positivity constraint (due to source independence) that makes the equation system sufficiently deterministic. DAMAS is quantitatively validated using archival data from a variety of prior high-lift airframe component noise studies, including flap edge/cove, trailing edge, leading edge, slat, and calibration sources. Presentations are explicit and straightforward, as the noise radiated from a region of interest is determined by simply summing the mean-squared values over that region. DAMAS can fully replace existing array processing and presentations methodology in most applications. It appears to dramatically increase the value of arrays to the field of experimental acoustics. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> The present study reports a new development of the DAMAS microphone phased array processing methodology that allows the determination and separation of coherent and incoherent noise source distributions. In 2004, a Deconvolution Approach for the Mapping of Acoustic Sources (DAMAS) was developed which decoupled the array design and processing influence from the noise being measured, using a simple and robust algorithm. In 2005, three-dimensional applications of DAMAS were examined. DAMAS has been shown to render an unambiguous quantitative determination of acoustic source position and strength. However, an underlying premise of DAMAS, as well as that of classical array beamforming methodology, is that the noise regions under study are distributions of statistically independent sources. The present development, called DAMAS-C, extends the basic approach to include coherence definition between noise sources. The solutions incorporate cross-beamforming array measurements over the survey region. While the resulting inverse problem can be large and the iteration solution computationally demanding, it solves problems no other technique can approach. DAMAS-C is validated using noise source simulations and is applied to airframe flap noise test results. <s> BIB007 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> To obtain higher resolution acoustic source plots from microphone array measurements, deconvolution techniques are becoming increasingly popular. Deconvolution algorithms aim at identifying Point Spread Functions (PSF)in source plots, and may therefore fall short when actual beam patterns of measured noise sources are not similar to synthetically obtained PSF's. To overcome this, a new version of the classical deconvolution method CLEAN is proposed here: CLEAN-SC. By this new method, which is based on spatial source coherence, side lobes can be removed of actually measured beam patterns. Essentially, CLEAN-SC iteratively removes the part of the source plot which is spatially coherent with the peak source. A feature of CLEAN-SC is its ability to extract absolute sound power levels from the source plots. The merits of CLEAN-SC were demonstrated using array measurements of airframe noise on a scale model of the Airbus A340 in the 8 ×6 m2 closed test section of DNW-LLF. <s> BIB008 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> During the last decade, microphone arrays became a standard tool to study aeroacoustic sources. Nowadays, arrays are frequently applied in fly-over measurements 1 and wind tunnel testing. 2 To improve the resolution of the arrays and to reduce disturbing side lobes in the measured source maps, sophisticated deconvolution methods were proposed 3-8 in recent years. First, these methods were tested using synthetic data or data from experiments with generic configurations. 8 In a next step the methods have to be applied to real experiments. This is done in the present paper. It is tested, if the deconvolution works also under difficult conditions, and if the findings of the simulations using artificial background noise 7 can be confirmed in case of real data. <s> BIB009 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> The wall-pressure fluctuations beneath a compressible turbulent boundary layer are measured using a sensor array. The boundary layer is realized on a flat plate in a wind tunnel with closed test section. Adaptive side walls are used to adjust a zero gradient of mean pressure. The array consists of 48 piezo-resistive pressure transducers using a threaded mounting. The array data are processed using beamforming and a deconvolution algorithm to obtain the wavenumber spectrum. As example a test case at Mach number M = 0.83 is considered. The convective ridge and acoustic waves are found in the wavenumber spectrum. The form of the convective ridge is consistent with empirical models developed for low Mach numbers. The results show the dominance of convective fluctuations at higher frequencies, while acoustic fluctuations are prevailing at low frequencies. <s> BIB010 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> A new approach to the deconvolution of acoustic sources has been developed and is presented in this work. The goal of this postprocessing is to simplify the beamforming output by suppressing the side lobes and reducing the sources' main lobes to single (or a small number of) points that accurately identify the noise sources' positions and their actual levels. In this work, a new modeling technique for the beamforming output is proposed. The idea is to use an image-processing-like approach to identify the noise sources from the beamforming maps: that is, recognizing lobe patterns in the beamformed output and relating them to the noise sources that would produce that map. For incoherent sources, the beamforming output is modeled as a superposition of point-spread functions and a linear system is posted. For coherent sources, the beamforming output is modeled as a superposition of complex point-spread functions and a nonlinear system of equations in terms of the sources' amplitudes is posted. As a first approach to solving this difficult problem, the system is solved using a new two-step procedure. In the first step, an approximated linear problem is solved. In the second step, an optimization is performed over the nonzero values obtained in the previous step. The solution to this system of equations renders the sources' positions and amplitudes. In the case of coherent sources, their relative phase can also be recovered. The technique is referred as noise source localization and optimization of phased-array results. A detailed analytical formulation, numerical simulations, and sample experimental results are shown for the proposed postprocessing. <s> BIB011 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Phased microphone arrays are used in a variety of applications for the estimation of acoustic source location and spectra. The popular conventional delay-and-sum beamforming methods used with such arrays suffer from inaccurate estimations of absolute source levels and in some cases also from low resolution. Deconvolution approaches such as DAMAS have better performance, but require high computational effort. A fast beamforming method is proposed that can be used in conjunction with a phased microphone array in applications with focus on the correct quantitative estimation of acoustic source spectra. This method bases on an eigenvalue decomposition of the cross spectral matrix of microphone signals and uses the eigenvalues from the signal subspace to estimate absolute source levels. The theoretical basis of the method is discussed together with an assessment of the quality of the estimation. Experimental tests using a loudspeaker setup and an airfoil trailing edge noise setup in an aeroacoustic wind tunnel show that the proposed method is robust and leads to reliable quantitative results. <s> BIB012 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Owls are commonly known for their quiet flight, enabled by three adaptions of their wings and plumage: leading edge serrations, trailing edge fringes and a soft and elastic downy upper surface of the feathers. In order to gain a better understanding of the aeroacoustic effects of the third property that is equivalent to an increased permeability of the plumage to air, an experimental survey on a set of airfoils made of different porous materials was carried out. Several airfoils with the same shape and size but made of different porous materials characterized by their flow resistivities and one non-porous reference airfoil were subject to the flow in an aeroacoustic open jet wind tunnel. The flow speed has been varied between approximately 25 and 50 m/s. The geometric angle of attack ranged from −16° to 20° in 4°-steps. The results of the aeroacoustic measurements, made with a 56-microphone array positioned out of flow, and of the measurements of lift and drag are given and discussed. <s> BIB013 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> permeable) materials. The objective of the research is the analysis of the turbulent boundary layer properties of porous airfoils and, subsequently, of the noise generated at the trailing edge. The inuence of the porous materials, characterized by their air ow resistivity, is discussed. The acoustic measurements were performed using a planar 56{channel microphone array and the boundary layer properties were measured using constant temperature anemometry. The recorded acoustic data underwent further processing by application of an advanced beamforming algorithm. A noticeable reduction of the emitted trailing edge noise was measured for the porous airfoils over a large range of frequencies. At high frequencies, some of the porous airfoils were found to generate more noise than the reference airfoil which might be due to the surface roughness noise contribution. It is found that the turbulent boundary layer thickness and the boundary layer displacement thickness of the airfoils increase with decreasing ow resistivities for both suction and pressure side. Both boundary layer thickness and displacement thickness of the non{porous airfoil are below those of the porous airfoils. <s> BIB014 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> T HE resolution of noise-source maps processed using phasedarray data has been substantially improved by introducing a deconvolution approach for the mapping of acoustic sources (DAMAS [1]), but its practicality is limited due to high computational cost. Subsequently, an improved algorithm, known as DAMAS2 [2], was developed to reduce the computational cost dramatically. Assuming that the monopole response (i.e., the pointspread function) on the source map is invariant over the entire map, referred to as shift invariant, a discrete Fourier transform and its inverse are introduced for deconvolution in DAMAS2. The drawback is that noise-source maps tend to deteriorate away from the point where we defined the point-spread function. Distortion of the monopole response is often severe in wind-tunnel tests, where there are constraints on the location of the microphone array. This note proposes amethod that suppresses such deterioration associatedwith the shift-invariant assumption for DAMAS2 with minimal extra computational cost. <s> BIB015 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> A jet noise measurement study was conducted using a phased microphone array system for a range of jet nozzle configurations and flow conditions. The test effort included convergent and convergent/divergent single flow nozzles, as well as conventional and chevron dual-flow core and fan configurations. Cold jets were tested with and without wind tunnel co-flow, whereas, hot jets were tested only with co-flow. The intent of the measurement effort was to allow evaluation of new phased array technologies for their ability to separate and quantify distributions of jet noise sources. In the present paper, the array post-processing method focused upon is DAMAS (Deconvolution Approach for the Mapping of Acoustic Sources) for the quantitative determination of spatial distributions of noise sources. Jet noise is highly complex with stationary and convecting noise sources, convecting flows that are the sources themselves, and shock-related and screech noise for supersonic flow. The analysis presented in this paper addresses some processing details with DAMAS, for the array positioned at 90ϒ (normal) to the jet. The paper demonstrates the applicability of DAMAS and how it indicates when strong coherence is present. Also, a new approach to calibrating the array focus and position is introduced and demonstrated. <s> BIB016 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Phased microphone arrays have become an important tool in the localization of noise sources for aeroacoustic applications. In most practical aerospace cases the conventional beamforming algorithm of the delay-and-sum type has been adopted. Conventional beamforming cannot take advantage of knowledge of the noise field, and thus has poorer resolution in the presence of noise and interference. Adaptive beamforming has been used for more than three decades to address these issues and has already achieved various degrees of success in areas of communication and sonar. In this work an adaptive beamforming algorithm designed specifically for aeroacoustic applications is discussed and applied to practical experimental data. It shows that the adaptive beamforming method could save significant amounts of post-processing time for a deconvolution method. For example, the adaptive beamforming method is able to reduce the DAMAS computation time by at least 60% for the practical case considered in this work. Therefore, ... <s> BIB017 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> This paper concerns the direct measurement of wavenumber-frequency spectra beneath a two-dimensional Turbulent Boundary Layer. The main contribution of this work is the proposed setup that should help achieving a compromise between a small sensor diameter (to avoid spatial averaging) and small sensor spacing (to gain high spatial resolution). The effect of different pinhole sizes on quarter inch microphones is first studied using an acoustical coupler to perform relative calibrations. Single-point measurements of wall pressure fluctuations in a wind-tunnel confirm that a 0.5 mm pinhole quarter inch microphone correctly captures wall pressure statistics up to a frequency of 5 kHz. Under the assumption of stationnary pressure fields, it is then suggested how rotative Archimedean spirals mainly composed of pinhole microphones can provide a uniform coverage of pressure measurements on a disc, or high resolution measurements in selected directions. Using 57 of these pinhole pressure sensors, a probe microphone as a central sensor reference and 3 Knowles pressure sensor, one of the suggested designs of spiral-shaped rotative arrays was instrumented. Measurements have been performed at low Mach numbers (M < 0:1) in a closed-loop wind-tunnel, with the array flush mounted on a side of the wind tunnel. The TBL is characterized with velocity measurements, and 2D wavenumber spectra of the wall pressure fluctuations are obtained which reveal the convective peak at low frequencies. Even preliminary and still contaminated by a strong background noise, these first results indicate that the experimental setup behaves as expected but needs additional refinements. The next steps of this on-going work are finally detailed. <s> BIB018 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> A new deconvolution strategy for aeroacoustic beamforming based on linear programming (LP) is introduced. Results of an aeroacoustic test show that it produces virtually the same results as DAMAS in much less time. DAMAS, DAMAS2 and LP display superresolution, which is expressed in terms of the Sparrow resolution limit from optics. CLEAN-SC and TIDY did not achieve superresolution in the test, but did produce higher dynamic range than DAMAS and LP at high frequency. <s> BIB019 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Abstract Microphone arrays have become a standard technique to localize and quantify source in aeroacoustics. The simplest approach is the beamforming that provides noise source maps with large main lobe and strong side lobes at low frequency. Since a decade, the focus is set on deconvolution techniques such as DAMAS or Clean-SC. While the source map is clearly improved, these methods require a large computation time. In this paper, we propose a sound source localization technique based on an inverse problem with beamforming regularization matrix called Hybrid Method. With synthetic data, we show that the side lobes are removed and the main lobe is narrower. Moreover, if the sound noise source map provided by this method is used as input in the DAMAS process, the number of DAMAS iterations is highly reduced. The Hybrid Method is applied to experimental data obtained in a closed wind-tunnel. In both cases of acoustic or aeroacoustic data, the source is correctly detected. The proposed Hybrid Method is found simple to implement and the computation time is low if the number of scan points is reasonable. <s> BIB020 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> The fluctuating pressure field underneath a turbulent boundary layer can lead to excitation of the surface structure of airplanes, trains and cars. At high flow speeds, this excitation becomes a major noise source in the vehicle interior. In the past, the characteristics of this excitation have been measured by the DLR in wind tunnels and in flight tests. A beam forming technique using planar waves was used as wavenumber decomposition in order to analyze the pressure fluctuations present over an installed microphone array. The information contained in the wavenumber spectrum resulting from the analysis is essential when empirically modeling the excitation behavior of a surface exposed to a pressure field. Speed, direction, and propagation mechanism (acoustic or hydrodynamic) of pressure waves can be determined from the position of sources in the wavenumber domain. This makes the wavenumber decomposition a helpful tool for the characterization of source mechanisms. In this paper, a summary of the test results is given. The attributes and potential of the wavenumber decomposition are emphasized in particular. <s> BIB021 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Functional beamforming is a state-of-the-art nonlinear algorithm based on the conventional frequency domain beamformer. In general, it is found to provide improved array spatial resolution and dynamic range. The computational time required for the functional beamforming is approximately the same as that for the conventional frequency domain beamformer and, in general, notably shorter than those of the deconvolution methods. In this paper, several simulations are presented comparing the performance of this algorithm with other imaging methods. Moreover, this beamforming technique is applied to 115 flyover measurements performed with a 32 microphone array on landing aircraft. The simulated and experimental results show good agreement. It is found that, for both synthetic and experimental data, functional beamforming offers better quality acoustic images, with a dynamic range (i.e., the difference in decibels between the main lobe and the highest sidelobe) approximately 30 times larger and an array spatial r... <s> BIB022 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Airframe noise is becoming increasingly important during approach, even reaching higher noise levels than the ::: engines in some cases. More people are affected due to low flight altitudes and fixed traffic routing associated with ::: typical approaches. Formost air- craft types, the landing gear system is a dominant airframe noise source. ::: However, this element can only be modeled in an approximate manner in wind tunnel experiments. In this ::: research, flyovers of landing aircraft were recorded using a 32 microphone array. Fun ctional beamforming was ::: applied to analyze the noise emissions from the landing gear system. lt was confirmed that for some aircraft types, ::: such as the Airbus A320 and the Fokker 70, the nose landing gear is a dominant noise source du ring approach. ::: The correlation between the noise levels generated by the landing gear and the aircraft velocity was found to be ::: significant, explai ning about 70% of the varia bility found in the noise levels, which is in good agreement with all ::: known theory. Moreover, the experimental resu lts for the Airbus A320 measurements were compared with those ::: obtained using the DLR system noise prediction tool PANAM. Whereas the total aircraft noise levels were in good ::: agreement. the measurements indicate a higher contribution from the nose landing gear noise compared ::: to the predictions. <s> BIB023 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> When using deconvolution algorithms for the improvement of beamforming results, the family of DAMAS-algorithms has shown to be very useful. One member of this family – the DAMAS2 algorithm – comes into focus when one is concerned with computational speed and efficient memory usage. The algorithm is especially suited for wavenumber beamforming, since it requires a shift-invariant point-spread function for best results. During application of this method, it was noticed that the results can be even improved ::: when increasing the size of the point-spread function. This further enhancement of the algorithm will be presented in examples and discussed. <s> BIB024 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Abstract Deconvolution of aeroacoustic data acquired with microphone phased arrays is a computationally challenging task for distributed sources with arbitrary coherence. A new technique for performing such deconvolution is proposed. This technique relies on analysis of the array data in the wavenumber–frequency domain, allowing for fast convolution and reduced storage requirements when compared to traditional coherent deconvolution. A positive semidefinite constraint for the iterative deconvolution procedure is implemented and shows improved behavior in terms of quantifiable convergence metrics when compared to a standalone covariance inequality constraint. A series of simulations validates the method׳s ability to resolve coherence and phase angle relationships between partially coherent sources, as well as determines convergence criteria for deconvolution analysis. Simulations for point sources near the microphone phased array show potential for handling such data in the wavenumber–frequency domain. In particular, a physics-based integration boundary calculation is described, and can successfully isolate sources and track the appropriate integration bounds with and without the presence of flow. Magnitude and phase relationships between multiple sources are successfully extracted. Limitations of the deconvolution technique are determined from the simulations, particularly in the context of a simulated acoustic field in a closed test section wind tunnel with strong boundary layer contamination. A final application to a trailing edge noise experiment conducted in an open-jet wind tunnel matches best estimates of acoustic levels from traditional calculation methods and qualitatively assesses the coherence characteristics of the trailing edge noise source. <s> BIB025 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Abstract Microphone array methods aim at the characterization of multiple simultaneously operating sound sources. However, existing data processing algorithms have been shown to yield different results when applied to the same input data. The present paper introduces a method for estimating the reliability of such algorithms. Using Monte Carlo simulations, data sets with random variation of selected parameters are generated. Four different microphone array methods are applied to analyze the simulated data sets. The calculated results are compared with the expected outcome, and the dependency of the reliability on several parameters is quantified. It is shown not only that the performance of a method depends on the given source distribution, but also that the methods differ in terms of their sensitivity to imperfect input data. <s> BIB026 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> In this article, a high-resolution extension of CLEAN-SC is proposed: high-resolution-CLEAN-SC. Where CLEAN-SC uses peak sources in ‘dirty maps’ to define so-called source components, high-resolution-CLEAN-SC takes advantage of the fact that source components can likewise be derived from points at some distance from the peak, as long as these ‘source markers’ are on the main lobe of the point spread function. This is very useful when sources are closely spaced together, such that their point spread functions interfere. Then, alternative markers can be sought in which the relative influence by point spread functions of other source locations is minimised. For those markers, the source components agree better with the actual sources, which allows for better estimation of their locations and strengths. This article outlines the theory needed to understand this approach and discusses applications to 2D and 3D microphone array simulations with closely spaced sources. An experimental validation was performed with two closely spaced loudspeakers in an anechoic chamber. <s> BIB027 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Abstract Boundary layer measurements at high subsonic Mach number are evaluated in order to obtain the dominant phase velocities of boundary layer pressure fluctuations. The measurements were performed in a transonic wind tunnel which had a very strong background noise. The phase velocity was taken from phase inclination and from the convective peak in one- and two-dimensional wavenumber spectra. An approach was introduced to remove the acoustic noise from the data by applying a method based on CLEAN-SC on the two-dimensional spectra, thereby increasing the frequency range where information about the boundary layer was retrievable. A comparison with prediction models showed some discrepancies in the low-frequency range. Therefore, pressure data from a DNS calculation was used to substantiate the results of the analysis in this frequency range. Using the measured data, the DNS results and a review of the models used for comparison it was found that the phase velocity decreases at low frequencies. <s> BIB028 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Most acoustic imaging methods assume the presence of point sound sources and, hence, fail to correctly estimate the sound emissions of distributed sound sources (such as trailing– edge noise). In this contribution, three integration techniques are suggested to overcome this issue based on models considering a single point source, a line source and several line sources, respectively. Two simulated benchmark cases featuring distributed sound sources are used to compare the performance of these integration techniques with respect to other well–known methods. The considered integration methods provide the best performance in retrieving the source levels and require short computational times. In addition, the presence of unwanted noise sources, such as corner sources in wind–tunnel measurements, no longer affects the results negatively when using the last method. A sensitivity analysis shows that the integration technique based on a line source is robust with respect to the choice of the integration area (shape, position and mesh fineness). Practical recommendations are provided for the application of these methods to experimental cases. <s> BIB029 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> The noise emissions of a full-scale nose landing gear, measured in a wind tunnel and obtained from computational simulations, are compared with those of three regional aircraft types recorded in flyover measurements. The results from these three approaches are also compared with the predictions of two airframe noise models (Fink and Guo). The geometries of the nose landing gears in all cases were similar. Microphone arrays and acoustic imaging algorithms were employed to estimate the sound emissions of the nose landing gears. A good agreement was found between the overall trends of the frequency spectra in all cases. Moreover, the expected 6th power law with the flow velocity was confirmed. On the other hand, strong tonal peaks (at around 2200 Hz) were only found for the flyover tests and computational simulations and are not present in typical noise prediction models. As the frequencies of the tones did not depend on the flow velocity, they are likely to be caused by cavities found in structural components of the nose landing gear. Removing these tones would cause overall noise reductions up to 2 dB in the frequency range examined. The noise emissions in the side direction did not present tonal peaks. The acoustic source maps showed that the dominant noise sources were located in the middle of the wheel axle, followed by the main strut and the bay doors. It is, therefore, recommended to further investigate this phenomenon, to include cavity-noise estimations in the current noise prediction models, and to eliminate such cavities where possible with the use of cavity caps, for example. <s> BIB030 </s> A review of acoustic imaging methods using phased microphone arrays <s> Applications <s> Abstract Phased microphone arrays have become a standard technique for acoustic source localization. Compared with beamforming algorithms such as the conventional beamforming, deconvolution approaches such as DAMAS successfully improve the spatial resolution. However deconvolution approaches usually require high computational effort compared to beamforming algorithms. Without optimizing deconvolution algorithm, recently DAMAS with compression computational grid based on the conventional beamforming (denoted by DAMAS-CG2) has reduced computational run time of DAMAS in applications (Ma and Liu, 2017). This paper proposes a novel algorithm that DAMAS with a novel compression computational grid based on an advance beamforming algorithm functional beamforming (denoted by DAMAS-CG3). This new algorithm takes advantages of functional beamforming to obtain large compression ratio. Simulated applications and experimental applications of benchmark test DLR1 show that DAMAS-CG3 is one order of magnitude faster than DAMAS-CG2 in most cases. In addition, the advantage of DAMAS-CG3 compared to DAMAS-CG2 is particularly more obvious with the threshold decreasing. However for some extreme situations that very complicated sources distribute to a larger extent relative to the scanning plane, the advantage of DAMAS-CG3 compared to DAMAS-CG2 may disappear. In order to get a large compression ratio in any application, the authors highly recommend compressing computational grid based on not only conventional beamforming but also functional beamforming, and then choosing the compression grid with larger compression ratio. <s> BIB031

A short summary of the aforementioned acoustic imaging methods and their most suitable applications is presented in the following list: • Conventional beamforming (CFDBF): BIB001 BIB003 should be a standard procedure in all cases, since it provides a fast overview of the sound sources characteristics. However, its spatial resolution and dynamic range are usually not suitable for several applications. Integration methods can be applied for distributed sources BIB029 . • Functional beamforming: greatly increases the dynamic range compared with CFDBF in a comparable computational time. It works well in wind-tunnel (both in open-jet and closed-section) and in aircraft flyover measurements BIB022 BIB023 BIB030 . However, diagonal removal produces considerable errors and diagonal denoising methods are recommended . • Orthogonal beamforming: BIB012 BIB002 is based on the eigenvalue decomposition of the CSM, has a low computational cost, and never overestimates the strength of the acoustic sources. This method has been applied in trailing-edge noise measurements in an open-jet windtunnel BIB013 BIB014 BIB026 . • CLEAN-SC: BIB008 is a widely used deconvolution technique that cleans the source map obtained with CFDBF iteratively, removing the parts of it that are coherent with the real sources. Thus, the dynamic range is greatly improved. The spatial resolution can be increased beyond the Rayleigh resolution limit using the new high-resolution version HR-CLEAN-SC BIB027 . This technique has been applied to wind-tunnel BIB008 and aircraft flyover experiments BIB022 . • DAMAS: BIB006 BIB004 BIB006 is a deconvolution method that solves an inverse problem iteratively to remove the influence of the array geometry from the obtained results. Enhancements in dynamic range and spatial resolution are obtained, but it has a high computational cost. The extension DAMAS-C BIB007 is suitable for analyzing source coherence, but it requires even higher computational resources. Further and similar versions of DAMAS have been proposed BIB011 BIB011 BIB005 BIB020 BIB031 BIB015 BIB024 BIB025 to reduce the computational time. This technique is normally used in jet-noise analyses BIB016 and in wind-tunnel tests BIB006 BIB004 BIB006 . • Wavenumber beamforming: BIB009 BIB010 BIB028 BIB018 BIB021 is a useful technique when mechanisms resulting in different propagation speeds and directions are present, provided that far-field conditions apply. This method has been used in wind-tunnel experiments BIB009 BIB010 BIB028 BIB018 and to characterize an aircraft boundary-layer flow BIB021 . • Linear programming deconvolution: BIB019 is a faster alternative to DAMAS. It has been applied to aeroacoustic point sources in a laboratory BIB019 , showing a better resolution than the Sparrow resolution limit. It can be combined with the functional beamforming providing even better results . • Robust adaptive beamforming: BIB017 attempts to maximize the SNR. It works well for clean data, but it is quite sensitive to noise and errors in the data. It has been used for aeroacoustic sources and flyover measurements BIB022 .

A review of acoustic imaging methods using phased microphone arrays <s> Closed wind tunnels <s> To obtain higher resolution acoustic source plots from microphone array measurements, deconvolution techniques are becoming increasingly popular. Deconvolution algorithms aim at identifying Point Spread Functions (PSF)in source plots, and may therefore fall short when actual beam patterns of measured noise sources are not similar to synthetically obtained PSF's. To overcome this, a new version of the classical deconvolution method CLEAN is proposed here: CLEAN-SC. By this new method, which is based on spatial source coherence, side lobes can be removed of actually measured beam patterns. Essentially, CLEAN-SC iteratively removes the part of the source plot which is spatially coherent with the peak source. A feature of CLEAN-SC is its ability to extract absolute sound power levels from the source plots. The merits of CLEAN-SC were demonstrated using array measurements of airframe noise on a scale model of the Airbus A340 in the 8 ×6 m2 closed test section of DNW-LLF. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed wind tunnels <s> This paper presents a systematic comparison of several prominent beamforming algorithms developed for aeroacoustic measurements. The most widely used delay-and-sum (DAS) beamformer is known to suffer from high sidelobe level and low resolution problems. Therefore, more advanced methods, in particular the deconvolution approach for the mapping of acoustic sources (DAMAS), sparsity constrained DAMAS (SC-DAMAS), covariance matrix fitting (CMF) and CLEAN based on spatial source coherence (CLEAN-SC), have been considered to achieve improved resolution and more accurate signal power estimates. The performances of the aforementioned algorithms are evaluated via experiments involving a 63-element logarithmic spiral microphone array in the presence of a single source, two incoherent sources with similar strengths and with different strengths, and two coherent sources. It is observed that DAMAS, SC-DAMAS and CMF provide the most reliable source location estimates, even at relatively low frequencies. Furthermore, th... <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed wind tunnels <s> The measurement of airframe noise on small-scale models is well known and common practice in conventional wind tunnels. Since conventional wind tunnels cannot generally achieve full-scale Reynolds numbers, measurements during the development process of modern aircraft are often performed in cryogenic and/or pressurized wind tunnels which are capable of higher Reynolds number flows. Thus, the characteristics of the moving fluid are better adapted to the scale model. At the DLR Institute of Aerodynamics and Flow Technology the microphone array measurement technique was further developed to perform measurements in a cryogenic wind tunnel at temperatures down to 100 K. A microphone array consisting of 144 microphones was designed and constructed for this purpose. In this paper, acoustic array measurements performed in a cryogenic wind tunnel are described for various Reynolds numbers using a 9.24% Dornier-728 half model. Additionally, the background noise of the empty test section was measured within the range of the measurements performed on the Dornier-728 half model. Our results seems to indicate a Reynolds number dependency of the measured sound power for various sources. <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed wind tunnels <s> In this werk, various microphone phased array data processing techniques are applied to two existing datasets from aeroacoustic wind tunnel tests. The first of these is from a !arge closed-wall facility, DLR's Kryo-Kanal Koln (DNW·KKK), and is a measurement cf the high-lift noise cf a semispan model. The second is from a small-scale open-jet facility, the NASA Langley Quiet Flow Facility (QFF), and is a measurement cf a clean airfoil selfnoise. The data had been made publicly available in 2015, and were analyzed by several research groups using multiple analysis techniques. This procedure allows the assessment of the variability of individual methods across various organizational implementations, as weil as the variability cf results produced by different array analysis methods. This paper summarizes the results presented at panel sessions held at AIAA conferences in 2015 and 2016. Results show that with appropriate handling of background noise, all advanced methods can identify dominant acoustic sources for a broad range cf frequencies. Lowerlevel sources may be masked er underpredicted. lntegrated levels are more robust and in closer agreement between methods than narrowband maps for individual frequencies. Overall there is no obvious best method, though multiple methods may be used to bound expected behavior. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Closed wind tunnels <s> Trailing-edge serrations are passive noise reduction add-ons widely used in wind-turbine applications. This study presents acoustic beamforming results of microphone array measurements of a cambered airfoil (DU96-W-180) in a closed-section wind-tunnel at a Reynolds number of industrial interest. Two different serration geometries with different lengths were tested and compared with the straight-edge baseline airfoil. The serrations were set at a flap angle of 6 degrees. Several flow velocities and angles of attack were tested at three chord-based Reynolds numbers ranging from 5105 to 1.5106. A phased microphone array was used to obtain source maps of the trailing-edge noise; Particle Image Velocimetry (PIV) was employed to obtain information about the turbulent boundary layer approaching the trailing edge; further, a numerical simulation using the Lattice Boltzmann Method (LBM) was performed for comparison. Far-field noise from the experimental data and computations shows a satisfactory agreement. Noise reductions of several dB were obtained, especially at lower frequencies. An increase in high-frequency noise is observed after a crossover frequency, which is assumed to be due to the set flap angle. <s> BIB005

As mentioned before in Sect. 3.1, removing the main diagonal of the CSM can lead to cleaner results in closed wind tunnels. An example of this technique can be observed in Fig. 8 for a half-model of a Fokker 100 aircraft tested in the DNW-LST closed wind tunnel . A successful example of CLEAN-SC is presented in Fig. 9 for a scaled model of an Airbus A340 in the closedtest section of the DNW-LLF wind tunnel BIB001 . The application of the CFDBF algorithm at a frequency of 12360 Hz shows a dominant outer wing slat noise source (which was found to be due to the low Reynolds number of the flow), but its sidelobes mask useful information about the other noise sources. CLEAN-SC reveals additional leading-edge noise 1 3 1 3 sources between the main source and the outer engine, which are not visible with CFDBF. The performances of SEM and SEMWAN are evaluated in Fig. 10 for an Airbus half-model for a one-third-octave frequency band centered at 2500 Hz in a closed-section wind tunnel in Bremen compared to CFDBF with diagonal removal. SEMWAN offers even better results than SEM compared to CFDBF, reducing the influence of the background noise of the closed wind tunnel and showing two main noise sources at the leading edges of the wing root and the wing tip. Additional results for the same model for a one-third-octave frequency band centered at 3150 Hz are shown in Fig. 11 , where results for CFDBF (with diagonal removal), DAMAS (with diagonal removal), and SEM-WAN are presented. Once again, SEMWAN improves the results with respect to CFDBF and renders distributed sound sources, in comparison with DAMAS, which only shows discrete point sources. The integration area and the integrated spectra using SPI on the results obtained by DAMAS and SEMWAN for an area covering the whole aircraft half-model are depicted in the bottom part of Fig. 11 . The measured spectrum by the microphone array is also plotted for reference. The SEMWAN spectrum presents a peak at around 4 kHz which is also shown in the measured spectrum by the array, but is not captured by DAMAS. Moreover, SEMWAN provides higher noise-reduction capabilities than DAMAS, especially at low frequencies. Overall, both spectra (DAMAS and SEMWAN) show comparable trends. Figure 12 presents some results of the IBIA (see Sect. 3.13) applied to a scaled half-model of a Dornier-728 at M = 0.2 in a closed-section wind tunnel at the DLR Cologne site. Full details of this experimental application are available in BIB003 . This data set has been studied in the frame of the AIAA benchmark on array methods BIB004 . Figure 12 shows the effect of the sparsity parameter q in Eq. (31) on the source map at 8500 Hz, and for q = 2 (no sparsity, left), 1.2 (soft sparsity, center) and 0 (strong sparsity, right). The main sources are identified on the whole wing leading edge, and on the flap-side edge. The source map presents an almost continuous source along the leading edge for q = 2 , while point sources clearly appear when q decreases (precisely on the slat tracks). An interesting remark is that the results for q = 2 and q = 0 are very similar to results obtained with CFDBF and CLEAN-SC, respectively BIB004 . Airfoil noise can also be studied at closed-section wind tunnels BIB005 . Figure 13 depicts the source plots of an Airbus A320 halfaircraft model tested in the CEPRA 19 anechoic open-jet wind tunnel at ONERA in France. An improved spatial resolution and a reduction of the sidelobe level are observed using SEM. The comparison of SEM with the other acoustic imaging methods in open-jet wind-tunnel measurements can be found in the literature BIB002 .

A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> A study was conducted to examine the effects of overall size of directional (or phased) arrays on the measurement of aeroacoustic components. An airframe model was mounted in the potential core of an open-jet windtunnel, with the directional arrays located outside the flow in an anechoic environment. Two array systems were used; one with a solid measurement angle that encompasses 31.6 degrees of source directivity and a smaller one that encompasses 7.2 degrees. The arrays, and sub-arrays of various sizes, measured noise from a calibrator source and flap edge model setups. In these cases, noise was emitted from relatively small, but finite size source regions, with intense levels compared to other sources. Although the larger arrays revealed much more source region detail, the measured source levels were substantially reduced due to finer resolution compared to that of the smaller arrays. To better understand the measurements quantitatively, an analytical model was used to define the basic relationships between array to source region sizes and measured output level. Also, the effect of noise scattering by shear layer turbulence was examined using the present data and those of previous studies. Taken together, the two effects were sufficient to explain spectral level differences between arrays of different sizes. An important result of this study is that total (integrated) noise source levels are retrievable and the levels are independent of the array size as long as certain experimental and processing criteria are met. The criteria for both open and closed tunnels are discussed. The success of special purpose diagonal-removal processing in obtaining integrated results is apparently dependent in part on source distribution. Also discussed is the fact that extended sources are subject to substantial measurement error, especially for large arrays. <s> BIB001 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> The use of a directional array of microphones for the measurement of trailing edge (TE) noise is described. The capabilities of this method are evaluated via measurements of TE noise from a NACA 63-215 airfoil model and from a cylindrical rod. This TE noise measurement approach is compared to one that is based on the cross spectral analysis of output signals from a pair of microphones (COP method). Advantages and limitations of both methods are examined. It is shown that the microphone array can accurately measures TE noise and captures its two-dimensional characteristic over a large frequency range for any TE configuration as long as noise contamination from extraneous sources is within bounds. The COP method is shown to also accurately measure TE noise but over a more limited frequency range that narrows for increased TE thickness. Finally, the applicability and generality of an airfoil self-noise prediction method was evaluated via comparison to the experimental data obtained using the COP and array me... <s> BIB002 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> An extensive experimental study has been conducted to acquire detailed noise spectra and directivity data for a high-fidelity, 6.3%-scale, Boeing 777 main landing gear. The measurements were conducted in the NASA Langley Quiet Flow Facility using a 41-microphone directional array system positioned at a range of polar and azimuthal observer angles with respect to the model. DAMAS (Deconvolution Approach for the Mapping of Acoustic Sources) array processing as well as straightforward individual microphone processing were employed to compile unique flyover and sideline directivity databases for a range of freestream Mach numbers (0.11 - 0.17) covering typical approach conditions. Comprehensive corrections were applied to the test data to account for shear layer ray path and amplitude variations. This allowed proper beamforming at different measurement orientations, as well as directivity presentation in free-field emission coordinates. Four different configurations of the landing gear were tested: a baseline... <s> BIB003 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> Owls are commonly known for their quiet flight, enabled by three adaptions of their wings and plumage: leading edge serrations, trailing edge fringes and a soft and elastic downy upper surface of the feathers. In order to gain a better understanding of the aeroacoustic effects of the third property that is equivalent to an increased permeability of the plumage to air, an experimental survey on a set of airfoils made of different porous materials was carried out. Several airfoils with the same shape and size but made of different porous materials characterized by their flow resistivities and one non-porous reference airfoil were subject to the flow in an aeroacoustic open jet wind tunnel. The flow speed has been varied between approximately 25 and 50 m/s. The geometric angle of attack ranged from −16° to 20° in 4°-steps. The results of the aeroacoustic measurements, made with a 56-microphone array positioned out of flow, and of the measurements of lift and drag are given and discussed. <s> BIB004 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> permeable) materials. The objective of the research is the analysis of the turbulent boundary layer properties of porous airfoils and, subsequently, of the noise generated at the trailing edge. The inuence of the porous materials, characterized by their air ow resistivity, is discussed. The acoustic measurements were performed using a planar 56{channel microphone array and the boundary layer properties were measured using constant temperature anemometry. The recorded acoustic data underwent further processing by application of an advanced beamforming algorithm. A noticeable reduction of the emitted trailing edge noise was measured for the porous airfoils over a large range of frequencies. At high frequencies, some of the porous airfoils were found to generate more noise than the reference airfoil which might be due to the surface roughness noise contribution. It is found that the turbulent boundary layer thickness and the boundary layer displacement thickness of the airfoils increase with decreasing ow resistivities for both suction and pressure side. Both boundary layer thickness and displacement thickness of the non{porous airfoil are below those of the porous airfoils. <s> BIB005 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> Trailing edge serrations designed to reduce airfoil self-noise are retrofitted on a NACA 0018 airfoil. An investigation of the boundary layer flow statistical properties is performed using time-resolved stereoscopic PIV. Three streamwise locations over the edge of the serrations are compared. An analysis of the results indicates that, while there is no upstream effect, the flow experiences significant changes as it convects over the serrations and toward its edges. Among the most important, a reduced shear stress and modifications of the turbulence spectra suggest beneficial changes in the unsteady surface pressure that would result in a reduction of trailing edge noise. Microphone array measurements are additionally performed to confirm that noise reduction is indeed observed by the application of the chosen serration design over the unmodified airfoil. <s> BIB006 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> The broadband noise generated by the scattering of turbulent flow at the trailing edge of a NACA 0018 airfoil with trailing edge serrations is investigated, varying both the airfoil angle of attack and serration flap angle. Acoustic emissions from the trailing edge are measured using a microphone array. The noise level is observed to be higher than that of the airfoil without serrations at frequencies beyond a crossover value. The latter is found to scale with a characteristic Strouhal number based upon the boundary layer thickness and the freestream velocity. A satisfactory collapse of the results under varying angles of attack and freestream velocities is observed. The modifications of the hydrodynamic behavior and the noise increase are linked by high-speed observations conducted with particle image velocimetry. An increase in the energy of turbulent fluctuations is also observed at the expected crossover frequency. The dominant cause of the increased noise is thereby identified at the pressure side edge of the serrations at a given flap angle. <s> BIB007 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> Trailing edge serrations have been proven to work as a passive noise reduction device. Nevertheless, they have also previously been found to increase noise in a particular frequency range, argued in earlier research to be due to the misalignment of the serrations with the direction of the flow in the wake. It emerges as a high-frequency noise increase in a broadband region of the spectrum. This study investigates the effect of serration-flow misalignment on the noise emissions using acoustic beamforming, and finds a correlation with observations made on the flow using particle image velocimetry (PIV). The hydrodynamic source of the noise increase is hereby identified, and a Strouhal number relation for the high-frequency noise increase is proposed <s> BIB008 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> Trailing-edge serrations are passive noise reduction add-ons widely used in wind-turbine applications. This study presents acoustic beamforming results of microphone array measurements of a cambered airfoil (DU96-W-180) in a closed-section wind-tunnel at a Reynolds number of industrial interest. Two different serration geometries with different lengths were tested and compared with the straight-edge baseline airfoil. The serrations were set at a flap angle of 6 degrees. Several flow velocities and angles of attack were tested at three chord-based Reynolds numbers ranging from 5105 to 1.5106. A phased microphone array was used to obtain source maps of the trailing-edge noise; Particle Image Velocimetry (PIV) was employed to obtain information about the turbulent boundary layer approaching the trailing edge; further, a numerical simulation using the Lattice Boltzmann Method (LBM) was performed for comparison. Far-field noise from the experimental data and computations shows a satisfactory agreement. Noise reductions of several dB were obtained, especially at lower frequencies. An increase in high-frequency noise is observed after a crossover frequency, which is assumed to be due to the set flap angle. <s> BIB009 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> Abstract Microphone array methods aim at the characterization of multiple simultaneously operating sound sources. However, existing data processing algorithms have been shown to yield different results when applied to the same input data. The present paper introduces a method for estimating the reliability of such algorithms. Using Monte Carlo simulations, data sets with random variation of selected parameters are generated. Four different microphone array methods are applied to analyze the simulated data sets. The calculated results are compared with the expected outcome, and the dependency of the reliability on several parameters is quantified. It is shown not only that the performance of a method depends on the given source distribution, but also that the methods differ in terms of their sensitivity to imperfect input data. <s> BIB010 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> The trailing edge of a NACA 0018 airfoil is modified through the attachment of serrations with different degrees of permeability. Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges. Different freestream velocities and angles of attack are investigated. The serration permeability is prescribed by having slits cut into the solid surface of the serrations in two different configurations. The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance. <s> BIB011 </s> A review of acoustic imaging methods using phased microphone arrays <s> Open-jet wind tunnels <s> Most acoustic imaging methods assume the presence of point sound sources and, hence, fail to correctly estimate the sound emissions of distributed sound sources (such as trailing– edge noise). In this contribution, three integration techniques are suggested to overcome this issue based on models considering a single point source, a line source and several line sources, respectively. Two simulated benchmark cases featuring distributed sound sources are used to compare the performance of these integration techniques with respect to other well–known methods. The considered integration methods provide the best performance in retrieving the source levels and require short computational times. In addition, the presence of unwanted noise sources, such as corner sources in wind–tunnel measurements, no longer affects the results negatively when using the last method. A sensitivity analysis shows that the integration technique based on a line source is robust with respect to the choice of the integration area (shape, position and mesh fineness). Practical recommendations are provided for the application of these methods to experimental cases. <s> BIB012

An example of acoustic imaging for rotating noise sources is presented in Fig. 14 and do not resolve the individual noise sources successfully. In case the Rotating Source Identifier (ROSI) technique is applied, the five noise sources located at the tip of each blade are properly identified, see the right plot in Fig. 14. One typical aeroacoustic experiment in open-jet wind tunnels is the measurement of airfoil trailing-edge noise. Moreover, the performance of noise-reduction mechanisms for this source, such as trailing-edge serrations BIB006 BIB007 BIB011 BIB008 BIB009 or the use of porous inserts BIB004 BIB005 can be assessed. An illustrative example of the use of trailing-edge serrations is depicted in Fig. 15 for a tripped NACA 0018 airfoil tested in the open-jet wind tunnel at Delft University of Technology BIB006 . The results correspond to CFDBF and flow speeds of 30, 35 and 40 m/s (left to right), a zero angle of attack and a frequency range between 1 and 5 kHz. The CFDBF results can be integrated over an area (see dashed rectangles in Fig. 15 ) for obtaining the radiated sound spectrum BIB012 . The influence of the number of eigenvalues k considered for orthogonal beamforming can be observed in Fig. 16 for trailing-edge noise measurements of a tripped NACA 0012 airfoil with a span of 0.15 m with a freestream velocity of 81.5 m/s and zero angle of attack. The results obtained with DAMAS with 500 iterations are also plotted for comparison purposes. Ray tracing was applied to the results to account for refraction at the shear layer and diagonal removal was applied for both methods. The results in Fig. 16 represent the integrated source maps over a region covering a part of the trailing edge. As expected, the orthogonal beamforming results converge to a final solution when the value of k is increased. The agreement with the results obtained by DAMAS is good after 3 kHz, but there seems to be an offset of about 6 dB between both methods below that frequency. Further comparisons of orthogonal beamforming with the other acoustic imaging methods can be found in the literature BIB004 BIB005 BIB010 . Additional studies about the noise-generation mechanisms for trailing-edge noise, flaps, slats, and landing gears involving microphone arrays were performed at the Quiet Flow Facility in NASA Langley BIB001 BIB002 BIB003

A Review on Wireless Grid Computing <s> II. WIRELESS GRID ARCHITECTURES <s> One application domain the mobile computing community has not yet entered is that of grid computing -- the aggregation of network-connected computers to form a large-scale, distributed system used to tackle complex scientific or commercial problems. In this paper we present the challenge of harvesting the increasingly widespread availability of Internet connected wireless mobile devices such as PDAs and laptops to be beneficially used within the emerging national and global computational grid. The integration of mobile wireless consumer devices into the Grid initially seems unlikely due to the inherent limitations typical of mobile devices, such as reduced CPU performance, small secondary storage, heightened battery consumption sensitivity, and unreliable low-bandwidth communication. However, the millions of laptops and PDAs sold annually suggest that this untapped abundance should not be prematurely dismissed. Given that the benefits of combining the resources of mobile devices with the computational grid are potentially enormous, one must compensate for the inherent limitations of these devices in order to successfully utilise them in the Grid. In this paper we identify the research challenges arising from this problem and propose our vision of a potential architectural solution. We suggest a proxy based, clustered system architecture with favourable deployment, interoperability, scalability, adaptivity, and fault-tolerance characteristics as well as an economic model to stimulate future research in this emerging field. <s> BIB001 </s> A Review on Wireless Grid Computing <s> II. WIRELESS GRID ARCHITECTURES <s> The explosive growth in the number of mobile devices such as Internet-enabled cellular phones, wireless handheld devices, wireless laptops, and tablet PCs has driven the corresponding growth in applications for mobile computing. These applications usually belong to one of two classes: collaborative applications and individual application. While collaborative applications require several mobile devices to work together and include peer-to-peer computing and grid computing, individual applications are local to the mobile device and its user. In this paper, we present a framework that allows mobile devices to collaboratively work on a computationally-expensive problem. Such a problem is decomposed into smaller tasks and distributed across other mobile devices willing to share their computational power with others. This framework is based on the paradigm of grid computing applied to the domain of wireless mobile devices. This paper presents our current implementation of the framework architecture and simulation of its functionality. Also, we focus on the issues of mobility, QoS and network stability and their effect on the performance of our collaborative problem-solving framework. <s> BIB002 </s> A Review on Wireless Grid Computing <s> II. WIRELESS GRID ARCHITECTURES <s> Greedy geographic routing is attractive in wireless sensor networks due to its efficiency and scalability. However, greedy geographic routing may incur long routing paths or even fail due to routing voids on random network topologies. We study greedy geographic routing in an important class of wireless sensor networks that provide sensing coverage over a geographic area (e.g., surveillance or object tracking systems). Our geometric analysis and simulation results demonstrate that existing greedy geographic routing algorithms can successfully find short routing paths based on local states in sensing-covered networks. In particular, we derive theoretical upper bounds on the network dilation of sensing-covered networks under greedy geographic routing algorithms. Furthermore, we propose a new greedy geographic routing algorithm called Bounded Voronoi Greedy Forwarding (BVGF) that allows sensing-covered networks to achieve an asymptotic network dilation lower than 4:62 as long as the communication range is at least twice the sensing range. Our results show that simple greedy geographic routing is an effective routing scheme in many sensing-covered networks. <s> BIB003 </s> A Review on Wireless Grid Computing <s> II. WIRELESS GRID ARCHITECTURES <s> In this paper, we discuss power save mechanisms that allow hosts to go to sleep to conserve energy. When sleeping hosts need to receive packets from other hosts, these hosts must somehow wake up the sleeping hosts. The paper discusses a busy-tone mechanism for this purpose, and an approach to improve the mechanism by using multiple busy tones. We generalize this protocol to develop the notion of multi-level power save mechanisms, and present some examples to illustrate this notion. <s> BIB004 </s> A Review on Wireless Grid Computing <s> II. WIRELESS GRID ARCHITECTURES <s> Advances in grid computing are stimulating the emergence of novel types of grids, such as accessible, manageable, interactive, and personal grids. More and more researchers are realizing emerging grids' potential to bridge the gap between grid technologies and users. This review of emerging grids sets out to develop a comprehensive classification of both traditional and emerging grid systems, with an aim to motivate further research and to help establish a solid foundation in this rapidly developing field. <s> BIB005

Wireless grid architectures can be broadly classified into the following four categories BIB005 based on the devices predominant in the grid and the relative mobility of the devices in the grid. • Fixed Wireless Grids • Mobile or Dynamic Wireless Grids • Ad Hoc grids • Sensor Network Grids Fixed Wireless grids: The wireless grid extends grid resources to wireless devices of varying sizes and capabilities such as sensors, laptops, special instruments, and edge devices, where these devices are usually static. In wireless grids, wireless devices can act as real grid nodes where part of data processing and storage is taking place. In a special type of wireless grid, all wireless devices are considered pure access devices without processing or storage capabilities BIB005 ; required resources are obtained from a wired, resource-rich backbone grid. Many technical concerns arise when integrating wireless devices into a grid. These include low bandwidth and high security risks, power consumption, and latency. So, several communities, including the Interdisciplinary Wireless Grid Team (www.wireless grids.net) are exploring these new issues to ensure that future grid peers can be wireless devices. Mobile Or Dynamic Wireless Grids: Mobile grids make grid services accessible through mobile devices such as PDAs and smart phones. As the processing power and other capabilities of these mobile devices increases, researchers and commercial organizations are discovering new ways to use and share their resources. When the large numbers of available wireless devices is considered, the potential of these dynamic ad hoc connections becomes vast BIB001 . Through wireless grid connections, these devices are able to connect to the Internet, provide peer-to-peer networking, take advantage of the resources of wired grid networks and make their own resources available to the wired grids . In emergency situations, such as during natural disasters and on battle fields, wireless mobile devices might be the only available communication and computation services. The mobile devices integrated into grid systems can actively participate and provide computational or data services . These mobile devices also serve as an interface to a stationary grid for sending requests and receiving results . Sometimes this approach is labeled mobile access to grid infrastructure, or simply mobile access grids. Recently, researchers have made numerous efforts toward establishing mobile grids. Researchers have proposed various techniques for implementing the mobile grid vision, including centralized and P2P structure, intelligent mobile agents, mobile grid middleware, and many more - BIB002 . Existing mobile grid projects include Akogrimo (www.mobilegrids.org), and MADAM (www.intermedia.uio.no/display/madam/Home). Ad Hoc grids : Mobile ad hoc network consists of devices with a high degree of heterogeneity. These mobile devices range from relatively powerful computing systems carried by a vehicle, to very tiny, low-power sensors that can be implanted in the human body. Although they may know little about the identities and capabilities of each other, a group of mobile devices are able to organize a highly dynamic and infrastructure-less ad-hoc network, in which nodes can communicate in a hop-by-hop manner. We have to integrate the resource aggregation model of grid with mobile ad hoc networks, so as to build a mobile ad-hoc Grid platform that can be instantly constructed anytime, anywhere . Having been constructed from a group of mobile devices, an ad-hoc grid would allow the networked devices to accomplish a specific mission that maybe beyond an individuals computing or communication capacity. Examples of applications of mobile ad-hoc grids can be disaster management, wildfire fighting, and e-healthcare emergency, etc. The general attributes of mobile ad hoc networks can be extended to ad hoc grids and hence ad hoc grids are attributed by bandwidth and power constraints, multi-hop delivery, network partitioning, and infrastructure unpredictability. Sensor Network Grids : Sensor networks are composed of tiny devices that are generally dedicated to a single purpose. Though each device in the grid is dedicated to a single purpose, the grid itself may be comprised of a number of different types of devices in order to accomplish the goals of the particular grid. As described in BIB003 , wireless sensor networks integrate detection, processing and communication into the grid. The sensors taking part in a sensor grid may be stationary once they have been deployed BIB004 or they may be mobile. Sensor networks are currently in use monitoring environmental factors such as temperature or humidity change, motion and light intensity. The developments in sensor networks is leading to advances in agriculture, physical building security, firefighting, warfare and a number of industrial areas. Wireless sensor network that can be deployed in residential and commercial buildings to monitor human presence and turn off the lights when no people are detected , is under development by Dust Networks of Berkeley, California. RFID technology to track products from manufacture through distribution and delivery and, potentially, to the consumers shopping cart leaving the store , is developed.

A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> One application domain the mobile computing community has not yet entered is that of grid computing -- the aggregation of network-connected computers to form a large-scale, distributed system used to tackle complex scientific or commercial problems. In this paper we present the challenge of harvesting the increasingly widespread availability of Internet connected wireless mobile devices such as PDAs and laptops to be beneficially used within the emerging national and global computational grid. The integration of mobile wireless consumer devices into the Grid initially seems unlikely due to the inherent limitations typical of mobile devices, such as reduced CPU performance, small secondary storage, heightened battery consumption sensitivity, and unreliable low-bandwidth communication. However, the millions of laptops and PDAs sold annually suggest that this untapped abundance should not be prematurely dismissed. Given that the benefits of combining the resources of mobile devices with the computational grid are potentially enormous, one must compensate for the inherent limitations of these devices in order to successfully utilise them in the Grid. In this paper we identify the research challenges arising from this problem and propose our vision of a potential architectural solution. We suggest a proxy based, clustered system architecture with favourable deployment, interoperability, scalability, adaptivity, and fault-tolerance characteristics as well as an economic model to stimulate future research in this emerging field. <s> BIB001 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> This paper proposes S-MAC, a medium-access control (MAC) protocol designed for wireless sensor networks. Wireless sensor networks use battery-operated computing and sensing devices. A network of these devices will collaborate for a common application such as environmental monitoring. We expect sensor networks to be deployed in an ad hoc fashion, with individual nodes remaining largely inactive for long periods of time, but then becoming suddenly active when something is detected. These characteristics of sensor networks and applications motivate a MAC that is different from traditional wireless MACs such as IEEE 802.11 in almost every way: energy conservation and self-configuration are primary goals, while per-node fairness and latency are less important. S-MAC uses three novel techniques to reduce energy consumption and support self-configuration. To reduce energy consumption in listening to an idle channel, nodes periodically sleep. Neighboring nodes form virtual clusters to auto-synchronize on sleep schedules. Inspired by PAMAS, S-MAC also sets the radio to sleep during transmissions of other nodes. Unlike PAMAS, it only uses in-channel signaling. Finally, S-MAC applies message passing to reduce contention latency for sensor-network applications that require store-and-forward processing as data move through the network. We evaluate our implementation of S-MAC over a sample sensor node, the Mote, developed at University of California, Berkeley. The experiment results show that, on a source node, an 802.11-like MAC consumes 2-6 times more energy than S-MAC for traffic load with messages sent every 1-10 s. <s> BIB002 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> We introduce ad hoc grids as a hierarchy of mobile devices with different computing and communication capabilities. An ad hoc grid allows a group of individuals to accomplish a mission, often in a hostile environment; examples of applications of ad hoc grids are disaster management, wild-fire prevention, and peacekeeping operations. We are concerned with the interplay between computing and communication in the power-constrained environment of an ad hoc grid. <s> BIB003 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> The paradigm of grid computing has been successfully applied in the domain of computationally-intensive applications supporting scientific research using server-class computers interconnected by wired network. The users of the existing and growing number of wireless mobile devices often demand more computational power than their devices can currently provide. We propose using the computational grid paradigm to build a problem-solving environment for wireless mobile devices. <s> BIB004 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> The explosive growth in the number of mobile devices such as Internet-enabled cellular phones, wireless handheld devices, wireless laptops, and tablet PCs has driven the corresponding growth in applications for mobile computing. These applications usually belong to one of two classes: collaborative applications and individual application. While collaborative applications require several mobile devices to work together and include peer-to-peer computing and grid computing, individual applications are local to the mobile device and its user. In this paper, we present a framework that allows mobile devices to collaboratively work on a computationally-expensive problem. Such a problem is decomposed into smaller tasks and distributed across other mobile devices willing to share their computational power with others. This framework is based on the paradigm of grid computing applied to the domain of wireless mobile devices. This paper presents our current implementation of the framework architecture and simulation of its functionality. Also, we focus on the issues of mobility, QoS and network stability and their effect on the performance of our collaborative problem-solving framework. <s> BIB005 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> Grid systems follow a new paradigm of distributed computing that enables the coordination of resources and services that are not subject to centralized control, can dynamically join and leave virtual pools, and are assigned to users by means of an explicit assignment functionality. The monitoring of a Grid is a multi-institutional and Virtual Organization (VO)-oriented service. It must deal with the dynamics, diversity, and geographical distribution of the resources available to Virtual Organizations, and the various levels of abstraction for modeling them. This paper presents the requirements, architecture and implementation of GridICE, a monitoring service for Grid systems. The suitability of this tool in real-life scenarios is analyzed and discussed. <s> BIB006 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> Monitoring is an issue of primary concern in current and next generation networked systems. For ex, the objective of sensor networks is to monitor their surroundings for a variety of different applications like atmospheric conditions, wildlife behavior, and troop movements among others. Similarly, monitoring in data networks is critical not only for accounting and management, but also for detecting anomalies and attacks. Such monitoring applications are inherently continuous and distributed, and must be designed to minimize the communication overhead that they introduce. In this context we introduce and study a fundamental class of problems called "thresholded counts" where we must return the aggregate frequency count of an event that is continuously monitored by distributed nodes with a user-specified accuracy whenever the actual count exceeds a given threshold value.In this paper we propose to address the problem of thresholded counts by setting local thresholds at each monitoring node and initiating communication only when the locally observed data exceeds these local thresholds. We explore algorithms in two categories: static and adaptive thresholds. In the static case, we consider thresholds based on a linear combination of two alternate strategies, and show that there exists an optimal blend of the two strategies that results in minimum communication overhead. We further show that this optimal blend can be found using a steepest descent search. In the adaptive case, we propose algorithms that adjust the local thresholds based on the observed distributions of updated information. We use extensive simulations not only to verify the accuracy of our algorithms and validate our theoretical results, but also to evaluate the performance of our algorithms. We find that both approaches yield significant savings over the naive approach of centralized processing. <s> BIB007 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> Grids are systems that involve coordinate resource sharing and problem solving in heterogeneous dynamic environments. Resource allocation is central to service provisioning in grids. In this paper, a brief survey of resource allocation of twelve existing systems is presented. Moreover, directions for future research are suggested. <s> BIB008 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> Wireless mobile grids are one of the emerging grid types, which help to pool the resources of several willing and cooperative mobile devices to resolve computationally intensive tasks. These tasks are more sensitive to processor availability than are to memory or bandwidth. The mobile grids exhibit stronger challenges like mobility management of devices, providing transparent access to grid resources, handling of limited resources so that resources are shared efficiently, and monitoring of dynamically changing resources. Device resource status is affected by number of applications it is executing, amount of data it is communicating, battery level, mobility and others. In order to keep up-to-date resource’s status, a continuous monitoring is needed. Such monitoring will definitely give most recent status information but increases monitoring cost. To solve this problem, we propose a Device Resource Monitoring System in Wireless Grids. Regular, Skew and Interleaved monitoring mechanisms are proposed and and analyzed. In skewed and interleaving monitoring mechanisms, different importance factors for different resources of a device are considered and accordingly assigns different relative frequencies for resource observation. The scheme monitors critical resources more frequently, maintain high degree of accuracy of status, and reduces monitoring load. <s> BIB009 </s> A Review on Wireless Grid Computing <s> III. CHALLENGES IN WIRELESS GRIDS <s> Grid computing is a technology for distributed computing. To manage a large scale of Grid resources for dynamic access, resource management is a key component. In this paper, a Grid Resource Information Monitoring (GRIM) prototype is introduced. To support the constantly changing resource states in the GRIM prototype, the push-based data delivery protocol named Grid Resource Information Retrieving (GRIR) is provided. There is a trade-off between information fidelity and updating transmission cost. The more frequent the reporting is, the more precise the information will be. But there will be more overheads. The offset-sensitive mechanism, the time-sensitive mechanism, and the hybrid mechanism in GRIR are used to achieve a high degree of data accuracy while decreasing the cost of updating messages. Experimental results show that the proposal alleviates both the update transmission cost and the loss of data accuracy compared to prior methods. <s> BIB010

Due to mobile nature of wireless devices and limitations of wireless communications a number of unique challenges must be overcome when building a grid application for such devices. Applications must be capable of adjusting to frequent disconnections of specific devices from the grid. Large distributed grid systems pose new challenges in job scheduling due to complex workload characteristics and system characteristics. Due to the numerous parameters that must be considered and the complex interactions that can occur between different resource allocation policies, it encounters lot many challenges. Some of the requirements and challenges of wireless grids are discussed in brief as follows. • Resource status monitoring: Monitoring is the act of collecting information concerning the characteristics and status of resources of interest. Monitoring system must be able to provide information about the current state of various resources such as static, and dynamic resources, and generate alarms whenever certain important events occur. It is responsible for monitoring the overall health of grid. To provide an up-to-date resources status information a device can be monitored either continuously or periodically or when some event occurs. Some works are done to address this issue. Globus toolkit consists of monitoring software called Monitoring and Discovery System (MDS) . The information sources are registered to index servers in MDS BIB006 . The users can query directory servers to obtain detailed descriptions about resources. Three different methods called as regular, skewed, and interleaved methods are proposed in BIB009 to monitor resource status of a device in wireless grid. • Resource status updation and communication : In order to keep up-to-date resource's status, a continuous monitoring is needed. The increase in number of status delivery of such monitored observations will consume lot much of bandwidth, making the database size of monitoring server to grow continuously over a period of time. So it is very much essential to keep track of only relevant changes and communicate instantly. In the paper BIB010 , a Grid Resource Information Monitoring (GRIM) prototype is introduced. To support the constantly changing resource states in the GRIM prototype, the push-based data delivery protocol named Grid Resource Information Retrieving is provided. To monitor grid infrastructure and to maintain system-wide invariants and detect abnormal events with minimal communication overhead, paper presents threshold based distributed triggering mechanism. The work given in , BIB007 solved the problem of detecting threshold violations with specified accuracy while minimizing communication overhead, as well as providing the flexibility for users to trade off communication overhead with detection accuracy. • Authentication and Authorization of device/user: Since the system is built upon a wireless and pervasive network, it will be more difficult to perform authentications and to provide general security mechanisms. The authentication of devices and users as they enter the grid presents additional complexities in wireless grids. To access any grid resources or services, each device and user has to be authorized. Though a number of researchers have recognized this difficulty of identity verification in ad hoc wireless grids, few solutions have been forthcoming. The current accepted identity management mechanism relies on the same digital signature certificates in use on the wired Internet. The works given in BIB001 , discuss the need of this issue. • Resource description : It is the formal process to describe the resources to be shared. The heterogeneity of grid devices, operating systems they use, services they provide, data they represent needs a standard to describe the resources so that any wireless grid can utilize the services. Some standards for resource description include XML, RDF, and OWL. • Resource discovery: The concept of resource sharing in grid computing is realized by service discovery mechanisms which transparently and seamlessly locate available resources/services throughout the grid infrastructure upon request. Since user wants the service as faster as possible at reasonable cost, service discovery becomes more important issue in grid computing , . Hence proper publishing, indexing, and cataloguing of shared resources become very essential. Standards such as GRDL, WSDL etc. are available for resource discovery. • Resource allocation: Since these wireless devices are more constrained in their processing power, memory, and bandwidth, many users compete for these scarce resources. Some economic market model is preferred to allocate the resources on price basis. Proper job scheduling and/or resource allocation becomes very crucial in order to provide efficient service to grid users. A survey on resource allocation in service oriented grids is presented in BIB008 . Resource allocation has to be done following the administrative policies, and the conditions under which devices will extend access to other devices. • Routing of messages through the grid: This area is concerned with the reduction of power consumption but through efficiency in the routing of messages. A second concern is the reduction of message latency. • Power consumption : Since mobile devices are resource constrained, identifying techniques to reduce power consumption to extend battery life while retaining acceptable levels of service is an issue. This is particularly important in wireless sensor grids due to the small size of the sensors and the difficulty of replacing those that fail. The work presented in BIB002 - BIB003 discusses about reducing the power consumption and routing the information via the paths that reduces power consumption. • Mobility: If the devices in the grid are highly mobile, it will adversely impact on the time taken to perform a computationally-intensive task. The mobility of the devices causes network instability leading to poor QoS. In a resource-sharing environment such as the wireless grid, mobility causes loss of data resulting in potentially erroneous outcomes. The work in BIB004 , BIB005 described an impact of mobility in wireless grid environment and presented an architecture that tries to minimize the negative effects of mobility. • Information Security: Security of the information processed by the application is another main issue. Due to their wireless nature, devices in the grid communicate and pass information over standard radio frequencies that can be easily tapped. A number of encryption standards such as WEP and WPA have been devised to ensure data security and integrity over these otherwise insecure transmission frequencies. • Fault management: The frequent failures of mobile nodes or communication links would always negatively impact the performance of the whole system. The issue of managing the fault that may occur because of device mobility, battery failure, or sudden switch off of a device is of major concern. • Energy-efficient medium access: The media access schemes must use low-power communications and must be able to maintain higher throughput and lower delays. An energy efficient MAC protocol for wireless sensor networks is presented in BIB002 . Energy consumption is crucial for the lifetime of battery-limited mobile devices as well as the whole system. To make Grid functionalities feasible in the network of mobile devices, energy efficiency should be the major concern for their implementation in each node. • Communication Paradigms: The complex environment and the mobility of the devices can lead to frequent disconnections in the wireless grids degrading the performance. Hence a proper communication methodology becomes more essential. The traditional communication approach called Socket programming is not suitable for complex environments in which security, heterogeneity issues are to be considered. Remote Procedure Call (RPC), is a classical approach for client server communication. While RPC is well suited to the procedural programming paradigm, it is not directly applicable to the object-oriented programming style that has gained much popularity in recent years. So Remote Method Invocation (RMI) is evolved that integrates the distributed object model into the Java language in a natural way. In RMI Remote objects can be passed as parameters in remote method calls.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Symmetry Transformations of Differential Equations. <s> An investigation is made of the magnetic Rayleigh problem where a semi-infinite plate is given an impulsive motion and thereafter moves with constant velocity in a non-Newtonian power law fluid of infinite extent. The solution of this highly non-linear problem is obtained by means of the transformation group theoretic approach. The one-parameter group transformation reduces the number of independent variables by one and the governing partial differential equation with the boundary conditions reduce to an ordinary differential equation with the appropriate boundary conditions. Effect of the some parameters on the velocity u (y, t) has been studied and the results are plotted. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Symmetry Transformations of Differential Equations. <s> Abstract We classify all similarity solutions of a boundary-value problem for a nonlinear diffusion equation arising in the study of a charged power-law non-Newtonian fluid through a time-dependent transverse magnetic field. Two families of exact invariant solutions are obtained. <s> BIB002

A transformation under which a differential equation remains invariant (unchanged) is called a symmetry transformation of the differential equation. Consider a th order ( ≥ 1) system of differential equations ( , , BIB001 , . . . , ( ) ) = 0, = 1, . . . , , where = ( 1 , . . . , ), called the dependent variable, is a function of the independent variable = ( 1 , . . . , ) and BIB001 , BIB002 up to ( ) are the collection of all first-order and second-order up to th order derivatives of . A transformation of the variables and , namely. is called a symmetry transformation of system (1) if (1) is form-invariant in the new variables and ; that is, ( , , BIB001 , . . . , ( ) ) = 0, = 1, . . . , , whenever ( , , BIB001 , . . . , ( ) ) = 0, = 1, . . . , . For example, the first-order Abel equation of the second kind has symmetry transformations = ,

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> = + ( , ) + ( <s> An investigation is made of the magnetic Rayleigh problem where a semi-infinite plate is given an impulsive motion and thereafter moves with constant velocity in a non-Newtonian power law fluid of infinite extent. The solution of this highly non-linear problem is obtained by means of the transformation group theoretic approach. The one-parameter group transformation reduces the number of independent variables by one and the governing partial differential equation with the boundary conditions reduce to an ordinary differential equation with the appropriate boundary conditions. Effect of the some parameters on the velocity u (y, t) has been studied and the results are plotted. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> = + ( , ) + ( <s> Abstract We classify all similarity solutions of a boundary-value problem for a nonlinear diffusion equation arising in the study of a charged power-law non-Newtonian fluid through a time-dependent transverse magnetic field. Two families of exact invariant solutions are obtained. <s> BIB002

2 ) , . . . Transformations ( where = ( 1 , . . . , ) and = ( 1 , . . . , ). Transformations (10) can be denoted by the Lie symmetry generator where the functions ( = 1, . . . , ) and ( = 1, . . . , ) are the coefficient functions of the operator . Operator (14) is a symmetry generator of (7) if where [ ] represents the th prolongation of the operator and is given by BIB001 = + ∑ =1 , BIB002 [ ] = BIB001 + ⋅ ⋅ ⋅ + 4

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> Abstract Using the machinery of Lie group-analysis we determine an exact steady state solution for the plane flow of an incompressible micropolar fluid. It is then shown how this solution, and in general any other time-independent solution, generates an infinite number of time-dependent solutions. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> Two dimensional equations of steady motion for third order fluids are expressed in a special coordinate system generated by the potential flow corresponding to an inviscid fluid. For the inviscid flow around an arbitrary object, the streamlines are the phicoordinates and velocity potential lines are psi-coordinates which form an orthogonal curvilinear set of coordinates. The outcome, boundary layer equations, is then shown to be independent of the body shape immersed into the flow. As a first approximation, assumption that second grade terms are negligible compared to viscous and third grade terms. Second grade terms spoil scaling transformation which is only transformation leading to similarity solutions for third grade fluid. By using Lie group methods, infinitesimal generators of boundary layer equations are calculated. The equations are transformed into an ordinary differential system. Numerical solutions of outcoming nonlinear differential equations are found by using combination of a Runge-Kutta algorithm and shooting technique. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> In this paper, the exact solutions for an unsteady flow of an incompressible fluid of third grade occupying space over an infinite porous plate are obtained for a given velocity component. Translational type symmetries are employed to reduce the non-linear equations governing the flow under discussion to two ordinary differential equations (with and without dispersion terms). Graphs representing the solutions are discussed, and appropriate conclusions drawn. <s> BIB003 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> An analysis is made of the unsteady flow of a third-grade fluid in a porous medium. A modified Darcy's law is considered in the flow modelling. Reduction and solutions are obtained by employing similarity and numerical methods. The effects of pertinent parameters on the flow velocity are studied through graphs. <s> BIB004 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> The magnetohydrodynamics (MHD) convection flow and heat transfer of an incompressible viscous nanofluid past a semi-infinite vertical stretching sheet in the presence of thermal stratification are examined. The partial differential equations governing the problem under consideration are transformed by a special form of the Lie symmetry group transformations, i.e., a one-parameter group of transformations into a system of ordinary differential equations which are numerically solved using the Runge-Kutta-Gill-based shooting method. It is concluded that the flow field, temperature, and nanoparticle volume fraction profiles are significantly influenced by the thermal stratification and the magnetic field. <s> BIB005 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> Solar power plants with surface receivers have low overall energy conversion efficiencies due to large emissive losses at high temperatures. Alternatively, volumetric receivers promise increased performance because solar radiation can be transferred into a fluid medium, which subsequently reduces the concentrated heat at the surface. Copper nanofluid-based direct solar receivers, where nanoparticles in a liquid medium can scatter and absorb solar radiation, have recently received interest to efficiently distribute and store the thermal energy. The objective of the present work is to investigate theoretically the unsteady Hiemenz flow of an incompressible viscous Cu-nanofluid past a porous wedge due to solar energy (incident radiation). The partial differential equations governing the problem under consideration are transformed by a special form of Lie symmetry group transformations viz. one-parameter group of transformation into a system of ordinary differential equations which are solved numerically using Runge Kutta Gill based shooting method. The conclusion is drawn that the flow field and temperature are significantly influenced by magnetic strength, convective radiation, thermal stratification, buoyancy force and porosity of the wedge sheet. <s> BIB006 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> Effects of chemical reaction on Nanofluid flow over a convectively heated stretching sheet have been studied.Magnetic field decreases the thickness of momentum boundary layer.The surface convection parameter is able to enhance heat transfer rate at the wall.Brownian motion and thermophoresis play an important role on fluid flow in the boundary layer region.The concentration boundary layer becomes thin for larger chemical reaction parameter. The steady MHD boundary layer flow of an electrically conducting nanofluid past a vertical convectively heated permeable stretching surface with variable stream conditions in presence of chemical reaction has been investigated numerically. The transport model includes the effect of Brownian motion with thermophoresis in the presence of chemical reaction. The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie group transformations viz. scaling group of transformations. The influences of various relevant parameters on the flow field, temperature and nanoparticle volume fraction as well as wall heat flux and wall mass flux are elucidated through graphs and tables. <s> BIB007 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> The objective of this work is to study the Williamson fluid flow with a chemically reactive species.The governing equations of Williamson model in two dimensional flows are constructed by using scaling transformation under a Reynolds and Weissenberg numbers approximation. The analytic solution of the system of nonlinear ordinary differential equations (ODEs) is constructed in the series form by using homotopy analysis method (HAM). The features of various physical parameters have been discussed graphically on flow and concentration profiles.The result came up with the ou tcome that the Reynolds number step up the fluid motion but slo w down the concentration of the fluid. The Weissenberg number show t he distinct effects on the velocity and concentration of the Williamson fluid model. <s> BIB008 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering <s> The governing nonlinear equation for unidirectional flow of a Sisko fluid in a cylindrical tube due to translation of the tube wall is modelled in cylindrical polar coordinates. The exact steady-state solution for the nonlinear problem is obtained. The reduction of the nonlinear initial value problem is carried out by using a similarity transformation. The partial differential equation is transformed into an ordinary differential equation, which is integrated numerically taking into account the influence of the exponent n and the material parameter b of the Sisko fluid. The initial approximation for the fluid velocity on the axis of the cylinder is obtained by matching inner and outer expansions for the fluid velocity. A comparison of the velocity, vorticity, and shear stress of Newtonian and Sisko fluids is presented. <s> BIB009

In the above equations, the additional coefficient functions satisfy the following relations: . . . where denotes the total derivative operator and is given by The determining equation BIB009 results in a polynomial in terms of the derivatives of the dependent variable . After separation of (15) with respect to the partial derivatives of and their powers, one obtains an overdetermined system of linear homogeneous partial differential equations for the coefficient functions 's and 's. By solving the overdetermined system, one has the following cases: (i) There is no symmetry, which means that the Lie point symmetry generators given by and are all zero. (ii) The point symmetry has ̸ = 0 arbitrary constants; in this case, we obtain generators of symmetry which forms an -dimensional Lie algebra of point symmetries. (iii) The point symmetry admits some finite number of arbitrary constants and arbitrary functions, in which case we obtain an infinite-dimensional Lie algebra. The dimensionless forms of the governing equations of the flow model are The boundary conditions specified to solve the above system of PDEs are The scaling symmetry operator for the system of PDEs (65) is calculated as = ( The corresponding similarity transformations are Transformation (67) transforms the system of PDEs (65) into a nonlinear system of ODEs. The reduced ordinary differential equations are solved numerically. The equations governing the model BIB008 are where is the Weissenberg number and is a Reynolds number. The boundary conditions for the problem are The Lie point symmetries of the system of PDEs (118) are BIB008 (120) The corresponding similarity transformations are The similarity transformations (121) are utilized in BIB008 to reduce the above PDE system into a system of nonlinear ordinary differential equations. The reduced ordinary differential system was solved analytically by homotopy analysis method. The system of PDEs (165) is invariant under the algebra spanned by the following generators : The operators given in (167) are used to reduce the above system of PDEs into a system of nonlinear ODEs. The reduced ordinary differential system was solved numerically using a fourth-order Runge-Kutta algorithm and the shooting technique. BIB001 . The study dealing with the plane micropolar flows from the viewpoint of Lie groups was made by Saccomandi BIB001 . An exact steady state solution for the plane flow of an incompressible micropolar fluid was deduced by employing the theory of Lie groups. The equations governing the motion of an incompressible two-dimensional micropolar fluid are With the use of the symmetries given in (179), the similarity transformations used for the reduction of above system of PDEs are The similarity transformation given in (180) transforms the above system of PDEs into nonlinear system of ODEs. The reduced ordinary differential equations were solved numerically by using a Keller-box method. The equation governing the flow model is We know that from basics of Lie symmetry methods that if a differential equation is explicitly independent of any dependent or independent variable, then this differential equation remains invariant under the translation symmetry corresponding to that particular variable. The above PDE (195) admits Lie point symmetry generators, / (time translation) and / (space-translation in ). Thus, a travelling wave solution was obtained in corresponding to the generator / + / . After performing the successive symmetry reductions, the governing PDE (195) admits exact solution of the form ( , ) = 1 where The same problem was investigated later by Fakhar . In , translational type symmetries were utilized to perform the travelling wave reduction on the governing model and the reduced model was solved using a power series method. Fakhar et al. BIB003 investigated the same model again. They made use of translational symmetries to reduce the governing model and presented some numerical solutions for a particular choice of boundary conditions. BIB002 . Yürüsoy BIB002 derived the boundary layer equations for a third-grade non-Newtonian fluid. By using Lie group methods, infinitesimal generators of the boundary layer equations were calculated. The equations were transformed into an ordinary differential system and then the numerical solutions of the resultant nonlinear differential equations were found by using a combination of a Runge-Kutta algorithm and a shooting technique. With the use of the symmetries given above, two types of group invariant solutions have been found in BIB004 and are ) }] . (211) The corresponding similarity transformations are given by BIB006 = , = , = ( ) . Transformations (302) are employed to reduce the PDE system (300) to a nonlinear ODE system which was solved numerically by Runge-Kutta Gill based shooting technique. Rosmila et al. BIB005 studied the magnetohydrodynamics convection flow and heat transfer of an incompressible viscous nanofluid past a semi-infinite vertical stretching. The partial differential equations governing the problem under consideration were transformed by a special form of the Lie symmetry group transformations, that is, a oneparameter group of transformations into a system of ordinary differential equations which was numerically solved using the Runge-Kutta-Gill-based shooting method. Recently, Das BIB007 has numerically investigated the steady MHD boundary layer flow of an electrically conducting nanofluid past a vertical convectively heated permeable stretching surface with variable stream conditions. The symmetry groups admitted by the corresponding boundary value problem were obtained by using scaling group of transformations which have been utilized to reduce the governing PDE system to a nonlinear ODE system.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Example on the Lie Symmetry Method. <s> Abstract The two-dimensional equations of motions for the slowly flowing and heat transfer in second grade fluid are written in cartesian coordinates neglecting the inertial terms. When the inertia terms are simply omitted from the equations of motions the resulting solutions are valid approximately for Re⪡1. This fact can also be deduced from the dimensionless form of the momentum and energy equations. By employing Lie group analysis, the symmetries of the equations are calculated. The Lie algebra consist of four finite parameter and one infinite parameter Lie group transformations, one being the scaling symmetry and the others being translations. Two different types of solutions are found using the symmetries. Using translations in x and y coordinates, an exponential type of exact solution is presented. For the scaling symmetry, the outcoming ordinary differential equations are more involved and only a series type of approximate solution is presented. Finally, some boundary value problems are discussed. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Example on the Lie Symmetry Method. <s> Abstract Two-dimensional unsteady boundary layer equations of non-Newtonian fluids are treated. Flow of a thin fluid film of a power-law caused by stretching of surface is investigated by using a similarity transformation. By using this transformation, we reduce the unsteady boundary layer equations to a non-linear ordinary differential equation system. Numerical solutions of outcoming nonlinear differential equations are found by using a combination a Runge–Kutta algorithm and shooting technique. Boundary layer thickness is explored numerically for different values of power-law index. <s> BIB002

Here we illustrate the use of the Lie symmetry method on the well-known Korteweg-de Vries equation given by We seek for an operator of the form Equation (21) is a symmetry generator of BIB001 if BIB002 ( + + ) =− − = 0. The third prolongation in this case is Therefore, the determining equation (22) becomes Using the definitions of , , and into lead to an overdetermined system of linear homogenous system of partial differential equations given by By solving system (25), we find four Lie point symmetries which are generated by the following generators:

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Solution of the Rayleigh Problem for a Power-Law NonNewtonian Conducting Fluid via Group <s> An investigation is made of the magnetic Rayleigh problem where a semi-infinite plate is given an impulsive motion and thereafter moves with constant velocity in a non-Newtonian power law fluid of infinite extent. The solution of this highly non-linear problem is obtained by means of the transformation group theoretic approach. The one-parameter group transformation reduces the number of independent variables by one and the governing partial differential equation with the boundary conditions reduce to an ordinary differential equation with the appropriate boundary conditions. Effect of the some parameters on the velocity u (y, t) has been studied and the results are plotted. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Solution of the Rayleigh Problem for a Power-Law NonNewtonian Conducting Fluid via Group <s> Abstract We classify all similarity solutions of a boundary-value problem for a nonlinear diffusion equation arising in the study of a charged power-law non-Newtonian fluid through a time-dependent transverse magnetic field. Two families of exact invariant solutions are obtained. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Solution of the Rayleigh Problem for a Power-Law NonNewtonian Conducting Fluid via Group <s> The present work deals with the modeling and solution of the unsteady flow of an incompressible third grade fluid over a porous plate within a porous medium. The flow is generated due to an arbitrary velocity of the porous plate. The fluid is electrically conducting in the presence of a uniform magnetic field applied transversely to the flow. Lie group theory is employed to find symmetries of the modeled equation. These symmetries have been applied to transform the original third order partial differential equation into third order ordinary differential equations. These third order ordinary differential equations are then solved analytically and numerically. The manner in which various emerging parameters have an effect on the structure of the velocity is discussed with the help of several graphs. <s> BIB003 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Solution of the Rayleigh Problem for a Power-Law NonNewtonian Conducting Fluid via Group <s> Abstract In this communication some non-linear flow problems dealing with the unsteady flow of a third grade fluid in porous half-space are analyzed. A new class of closed-form conditionally invariant solutions for these flow models are constructed by using the conditional or non-classical symmetry approach. All possible non-classical symmetries of the model equations are obtained and various new classically invariant solutions have been constructed. The solutions are valid for a half-space and also satisfy the physically relevant initial and the boundary conditions. <s> BIB004

Method BIB001 . Abdel-Malek et al. BIB001 studied the magnetic Rayleigh problem where a semi-infinite plate is given an impulsive motion and thereafter moves with constant velocity in a non-Newtonian power-law fluid of infinite extent. The governing nonlinear model was solved by means of the Lie group approach. The governing problem describing the flow model BIB001 is given by with the boundary and initial conditions The method of solution employed in BIB001 depends on the application of a one-parameter group of transformations to the partial differential equation BIB003 . The one-parameter group, which transforms the PDE BIB003 and the boundary conditions BIB004 , is of the form [1] Under transformations (50), the two independent variables reduce by one and the partial differential equation (48) is transformed into an ordinary differential equation. The reduced ordinary differential equation was then solved numerically. BIB002 . Wafo Soh BIB002 investigated a boundary value problem for a nonlinear diffusion equation arising in the study of a charged power-law non-Newtonian fluid through a time-dependent transverse magnetic field. Two families of exact invariant solutions were obtained by use of the Lie symmetry method. The governing equation describing the flow model is given by BIB002 − ( )

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering 7 <s> Abstract Two-dimensional unsteady boundary layer equations of non-Newtonian fluids are treated. Flow of a thin fluid film of a power-law caused by stretching of surface is investigated by using a similarity transformation. By using this transformation, we reduce the unsteady boundary layer equations to a non-linear ordinary differential equation system. Numerical solutions of outcoming nonlinear differential equations are found by using a combination a Runge–Kutta algorithm and shooting technique. Boundary layer thickness is explored numerically for different values of power-law index. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering 7 <s> Separable solutions admitted by a nonlinear partial differential equation modeling the axisymmetric spreading under gravity of a thin power-law fluid on a horizontal plane are investigated. The model equation is reduced to a highly nonlinear second-order ordinary differential equation for the spatial variable. Using the techniques of Lie group analysis, the nonlinear ordinary differential equation is linearized and solved. As a consequence of this linearization, new results are obtained. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering 7 <s> In this article, a similarity solution of the steady boundary layer flow near the stagnation-point flow on a permeable stretching sheet in a porous medium saturated with a nanofluid and in the presence of internal heat generation/absorption is theoretically studied. The governing partial differential equations with the corresponding boundary conditions are reduced to a set of ordinary differential equations with the appropriate boundary conditions via Lie-group analysis. Copper (Cu) with water as its base fluid has been considered and representative results have been obtained for the nanoparticle volume fraction parameter \({\phi}\) in the range \({0\leq \phi \leq 0.2}\) with the Prandtl number of Pr = 6.8 for the water working fluid. Velocity and temperature profiles as well as the skin friction coefficient and the local Nusselt number are determined numerically. The influence of pertinent parameters such as nanofluid volume fraction parameter, the ratio of free stream velocity and stretching velocity parameter, the permeability parameter, suction/blowing parameter, and heat source/sink parameter on the flow and heat transfer characteristics is discussed. Comparisons with published results are also presented. It is shown that the inclusion of a nanoparticle into the base fluid of this problem is capable to change the flow pattern. <s> BIB003 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering 7 <s> The problem of laminar fluid flow which results from the stretching of a vertical surface with variable stream conditions in a nanofluid has been investigated numerically. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis in the presence of magnetic field. The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie group transformations viz. scaling group of transformations. An exact solution is obtained for translation symmetry and numerical solutions for scaling symmetry. This solution depends on a Lewis number, magnetic field, Brownian motion parameter and thermophoretic parameter. The conclusion is drawn that the flow field and temperature and nanoparticle volume fraction profiles are significantly influenced by these parameters. A nanofluid is a new class of heat transfer fluids that contain a base fluid and nanoparticles. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. In nanofluid systems, thermophoresis particle deposition in the presence of magnetic field with Brownian motion has a substantial effect on the flow field. <s> BIB004

The governing equations describing the flow model BIB001 are where and V are the velocity components inside the boundary layer and ( , ) is the velocity outside the boundary layer. The boundary conditions for flow over a stretching sheet are By use of the Lie group method, the similarity transformations for the reduction of the above system of PDEs are given by BIB001 Transformation (59) The equation modelling the height of a thin power-law fluid film on a horizontal plane in presence of gravity is given by BIB002 Here ℎ( , ) is the film height and is the power-law fluid parameter. The Lie point symmetry generator for the PDE (60) is BIB002 The invariant solution of PDE BIB003 corresponding to the symmetry generator BIB004 found in BIB002 is

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Symmetry Reductions of a Flow with Power-Law Fluid <s> In this study symmetry analysis is carried out for a system dealing with nonreactive pollutant transport along a single channel. Constitutive equations obeying a power law fluid are used in the description of the mathematical problem. We obtain forms of the source term for which the governing system admits extra point symmetries. Invariant solutions which satisfy physical boundary conditions are constructed. In some cases we resort to numerical methods to obtain approximate solutions. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Symmetry Reductions of a Flow with Power-Law Fluid <s> Solar power plants with surface receivers have low overall energy conversion efficiencies due to large emissive losses at high temperatures. Alternatively, volumetric receivers promise increased performance because solar radiation can be transferred into a fluid medium, which subsequently reduces the concentrated heat at the surface. Copper nanofluid-based direct solar receivers, where nanoparticles in a liquid medium can scatter and absorb solar radiation, have recently received interest to efficiently distribute and store the thermal energy. The objective of the present work is to investigate theoretically the unsteady Hiemenz flow of an incompressible viscous Cu-nanofluid past a porous wedge due to solar energy (incident radiation). The partial differential equations governing the problem under consideration are transformed by a special form of Lie symmetry group transformations viz. one-parameter group of transformation into a system of ordinary differential equations which are solved numerically using Runge Kutta Gill based shooting method. The conclusion is drawn that the flow field and temperature are significantly influenced by magnetic strength, convective radiation, thermal stratification, buoyancy force and porosity of the wedge sheet. <s> BIB002

and Contaminant-Modified Viscosity BIB001 . Moitsheki et al. BIB001 have analyzed a system dealing with nonreactive pollutant transport along a single channel. Constitutive equations obeying a power-law fluid are utilized in the description of the mathematical problem. Invariant solutions which satisfy physical boundary conditions have been constructed using the Lie group approach. The dimensionless governing equations that describe the flow model are BIB001 = + [ ( ) Here is the Schmidt number and is the imposed constant pressure axial gradient. The Lie point symmetries of the above system corresponding to different forms of the source term ( , ) are given in Table 1 of BIB001 . The invariant solutions of system BIB002 found in BIB001 are of the form . An analysis has been conducted to study the problem of heat transfer of a power-law fluid over a vertical stretching sheet with slip boundary condition by Mutlag et al. . The partial differential equations governing the physical model have been converted into a set of nonlinear coupled ordinary differential equations using scaling group of transformations. These reduced equations are then solved numerically using the Runge-Kutta-Fehlberg fourthfifth order numerical method.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Rayleigh Problem for a MHD Sisko Fluid <s> The problem of unsteady unidirectional flow of an incompressible Sisko fluid bounded by a suddenly moved plate is studied. The fluid is magnetohydrodynamic (MHD) in the presence of a time-dependent magnetic field applied transversely to the flow. The non-linear flow problem arising from the laws of momentum, mass and suitable boundary and initial conditions is solved analytically using Lie symmetry method. The manner in which the various emerging parameters affect the structure of the velocity is delineated. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Rayleigh Problem for a MHD Sisko Fluid <s> We investigate the time-dependent flow of an incompressible Sisko fluid over a wall with suction or blowing. The flow is caused by sudden motion of the wall in its own plane. The magnetodynamic nature of the fluid is taken into account by applying a variable magnetic field. The resulting nonlinear problem is solved by invoking a symmetry approach and numerical techniques. The essential features of the embedded key parameters are described. Particularly the significance of the rheological effects is studied. <s> BIB002

with The symmetry Lie algebra of the PDE (80) is three-dimensional and spanned by the operators BIB001 1 = , where The similarity solution from the invariants of 2 assumes the form BIB001 ( , ) = (2 + ) 1/2 ( ) Invariant (84) The relevant boundary and initial conditions are The above PDE admits the Lie point symmetry generators 1 = , (87) The travelling wave solutions of the PDE (85) were constructed corresponding to the symmetry generators (87) and is given by ( , ) = exp BIB002 . The study of time-dependent flow of an incompressible Sisko fluid over a wall with suction or blowing was performed by Hayat et al. BIB002 . The magnetohydrodynamic nature of the fluid was taken into account by applying a variable magnetic field. The resulting nonlinear problem was solved by invoking the symmetry approach. The problem governing the flow model BIB002 in a nondimensional form is given by

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Boundary Layer Equations and Lie Group Analysis of a <s> Boundary layer equations are derived for the Sisko fluid. Using Lie group theory, a symmetry analysis of the equations is performed. A partial differential system is transferred to an ordinary differential system via symmetries. Resulting equations are numerically solved. Effects of non-Newtonian parameters on the solutions are discussed. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Boundary Layer Equations and Lie Group Analysis of a <s> Purpose – The purpose of this paper is to find analytic approximate solutions for time-dependent flow and heat transfer of a Sisko fluid. Design/methodology/approach – The homotopy analysis method is used to find a family of travelling wave solutions of the governing non-linear problem. Findings – The effects of different parameters on the velocity and temperature profiles are shown graphically. Originality/value – The analytic solutions of the system of non-linear ordinary differential equations are constructed in the series form for various values of the power index. <s> BIB002

Sisko Fluid BIB001 . Sari et al. BIB001 recently derived the boundary layer equations for a Sisko fluid. Using Lie group theory, a symmetry analysis of the equations was performed. A partial differential system is transferred to an ordinary differential system using symmetries and the resulting reduced equations were numerically solved. The dimensionless form of the boundary layer equations for a Sisko fluid is BIB001 The classical boundary conditions for the problem are BIB001 ( , 0) = 0, The infinitesimals of the above system of PDEs are BIB001 The corresponding similarity transformations are Transformations (97) are used to reduce the above PDE system to an ordinary differential system. The reduced ordinary differential system was solved by using a numerical method. BIB002 . The purpose of this study was to find analytic approximate solutions for unsteady flow and heat transfer of a Sisko fluid. Translational symmetries were utilized in BIB002 to find a family of travelling wave solutions of the governing nonlinear problem. In dimensionless form, the governing problem takes the form BIB002

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering 15 <s> The two-dimensional equations of motion for the slowly flowing second grade fluid are written in cartesian coordinates neglecting the inertial terms. By employing Lie group analysis, the symmetries of the equations are calculated. The Lie algebra consists of four finite parameter Lie group transformations, one being the scaling symmetry and the others being translations. Two different types of solutions are found using the symmetries. Using the translations in x and y coordinates, an exponential type of exact solution is constructed. For the scaling symmetry, the outcoming ordinary differential equations are more involved and only a series type of approximate solution is presented. Finally, some boundary value problems are discussed. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Mathematical Problems in Engineering 15 <s> Abstract The two-dimensional equations of motions for the slowly flowing and heat transfer in second grade fluid are written in cartesian coordinates neglecting the inertial terms. When the inertia terms are simply omitted from the equations of motions the resulting solutions are valid approximately for Re⪡1. This fact can also be deduced from the dimensionless form of the momentum and energy equations. By employing Lie group analysis, the symmetries of the equations are calculated. The Lie algebra consist of four finite parameter and one infinite parameter Lie group transformations, one being the scaling symmetry and the others being translations. Two different types of solutions are found using the symmetries. Using translations in x and y coordinates, an exponential type of exact solution is presented. For the scaling symmetry, the outcoming ordinary differential equations are more involved and only a series type of approximate solution is presented. Finally, some boundary value problems are discussed. <s> BIB002

With the use of Lie point symmetries (125), the group invariant solutions for the system of PDEs (123) are BIB001 ( , ) BIB002 . The steady plane creeping flow and heat transfer equations of a second-grade fluid in Cartesian coordinates are modelled by Yürüsoy BIB002 . Lie group theory was employed for the equations of motion. The symmetries of the equations were deduced. The equations admit a scaling symmetry, translation symmetries, and an infinite parameter dependent symmetry. New exact analytical solutions are found for the model equations. The equations of the flow model BIB002 are

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Similarity Solutions for Creeping Flow and Heat Transfer in Second-Grade Fluids <s> Abstract The two-dimensional equations of motions for the slowly flowing and heat transfer in second grade fluid are written in cartesian coordinates neglecting the inertial terms. When the inertia terms are simply omitted from the equations of motions the resulting solutions are valid approximately for Re⪡1. This fact can also be deduced from the dimensionless form of the momentum and energy equations. By employing Lie group analysis, the symmetries of the equations are calculated. The Lie algebra consist of four finite parameter and one infinite parameter Lie group transformations, one being the scaling symmetry and the others being translations. Two different types of solutions are found using the symmetries. Using translations in x and y coordinates, an exponential type of exact solution is presented. For the scaling symmetry, the outcoming ordinary differential equations are more involved and only a series type of approximate solution is presented. Finally, some boundary value problems are discussed. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Similarity Solutions for Creeping Flow and Heat Transfer in Second-Grade Fluids <s> The Lie symmetry analysis and the basic similarity reductions are performed for the rotating flow of a second-order fluid on a porous plate. The porous character of the plate and non-Newtonian effect of the fluid increase the order of the governing partial differential equations up to order third. Two new exact solutions generated from the similarity transformation to these equations, are provided. A discussion on physical aspects of these solutions is also presented. <s> BIB002

The infinitesimals for the system of PDEs (127) are BIB001 1 = + , The invariant solutions for the system of PDEs (127) deduced in BIB001 are BIB002 . The Lie symmetry analysis and the basic similarity reductions are performed for the rotating flow of a secondorder fluid on a porous plate by Fakhar et al. BIB002 . Two new exact solutions to these equations were generated from the similarity transformations.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Some New Exact Solutions for MHD Aligned Creeping Flow and Heat Transfer in Second-Grade Fluids by Using Lie Group <s> The Lie group analysis and the basic similarity reductions are performed for the MHD aligned slowly flowing and heat transfer in second grade fluid with neglecting the inertial terms. When the inertia terms are simply omitted from the equations of motions the resulting solutions are valid approximately for Re≺≺1. This fact can also be deduced from the dimensionless form of the momentum and energy equations. By employing Lie group theory, the full one-parameter infinitesimal transformation group leaving the equations of motion invariant is derived along with its associated Lie algebra. Subgroups of the full group are then used to obtain a reduction by one in the number of independent variable in the system. Two new exact solutions generated from the similarity transformation to these equations, are provided. Comparisons with previously work is performed and the result is found to be inexcellent. Finally, some boundary value problems are discussed. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Some New Exact Solutions for MHD Aligned Creeping Flow and Heat Transfer in Second-Grade Fluids by Using Lie Group <s> Abstract In this work, an approach is implemented for finding exact solutions of an incompressible MHD aligned flow with heat transfer in a second grade fluid. This approach based on travelling wave phenomenon. The partial differential equations (PDEs) are reduced to ordinary differential equations (ODEs) by using wave parameter. The methodology used in this work is independent of perturbation, symmetry consideration and other restrictive assumption. Comparison is made with the results obtained previously. <s> BIB002

Analysis BIB001 . Afify BIB001 carried out the Lie group analysis and the basic similarity reductions for the MHD aligned flow and heat transfer in a second-grade fluid. Two new exact solutions were constructed for the model equations. The equations governing the model BIB001 are The symmetries for the system of PDEs (133) are BIB001 1 = + , With the use of the symmetries given in (134), the invariant solutions for the system of PDEs (133) are BIB001 ( , ) The same model was investigated again very recently by Khan et al. BIB002 . The travelling wave symmetry reduction was performed in BIB002 to reduce the governing system of PDEs (133) and thereafter the same family of exact solutions was found as in BIB001 . . Abd-el-Malek and Hassan studied the steady two-dimensional boundary layer stagnation-point flow of Rivlin-Ericksen fluid of second grade with a uniform suction that is carried out via symmetry approach. By using the Lie group method for the given system of nonlinear partial differential equations, the symmetries of the equations were obtained. Using these symmetries, the solution of the given equations was constructed. The dimensionless form of the governing equations is [24]

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Symmetries and Solution of a Micropolar Fluid Flow through a Cylinder <s> The system of equations of motion for a micropolar fluid inside a circular cylinder subjected to longitudinal and rotational motion is considered. Classical Lie symmetries of the system of equations are studied. Classes of invariant solutions corresponding to different symmetry subgroups are obtained. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Symmetries and Solution of a Micropolar Fluid Flow through a Cylinder <s> The time-independent equations for the two dimensional incompressible micropolar fluid have been considered. Using group method the equations have been reduced to ordinary differential equations and then solved analytically. Finally the boundary value problem has been discussed, and the graphical results are in good agreement with the numerical solution. <s> BIB002

where the dimensionless micropolar parameters are The system of PDEs (156)- (157) is invariant under the algebra of the following operators BIB001 : The symmetries given in (158) have been employed to construct the group invariant solutions given by BIB001 BIB002 . Shahzad et al. BIB002 discussed the time-independent equations for the two-dimensional incompressible micropolar fluid. Using group methods, the equations are reduced to ordinary differential equations and then solved analytically. The governing problem of the flow model BIB002 is

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Analytic Solution for Flow of a Micropolar Fluid <s> Abstract A modified second grade non-Newtonian fluid model is considered. The model is a combination of power-law and second grade fluids in which the fluid may exhibit normal stresses, shear thinning or shear thickening behaviors. The equations of motion are derived for two dimensional incompressible flows. The boundary layer equations are derived from the equations. Symmetries of the boundary layer equations are calculated using Lie Group theory. For a special power law index of m = −1, the principal Lie algebra extends. Using one of the symmetries, the partial differential system is transferred to an ordinary differential system. The ordinary differential equations are numerically integrated for the stretching sheet boundary conditions. Effects of power-law index and second grade coefficient on the boundary layers are shown and solutions are contrasted with the usual second grade fluid solutions. The shear stress on the boundary is also calculated. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Analytic Solution for Flow of a Micropolar Fluid <s> The time-independent equations for the two dimensional incompressible micropolar fluid have been considered. Using group method the equations have been reduced to ordinary differential equations and then solved analytically. Finally the boundary value problem has been discussed, and the graphical results are in good agreement with the numerical solution. <s> BIB002

where 0 is the velocity of the plate, 0 is the magnetic fluid penetrating into the plate, and 0 is the pressure deep in the magmatic fluid. The infinitesimals for the system of PDEs (160) are BIB001 1 = , The infinitesimals given in (162) have been used to construct the group invariant solutions given as BIB002 where

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Couette Flow of a Third-Grade Fluid with Variable <s> This investigation deals with the analytic solution for the time-dependent flow of an incompressible third-grade fluid which is under the influence of a magnetic field of variable strength. The fluid is in an annular region between two coaxial cylinders. The motion is induced due to an inner cylinder with arbitrary velocity. Group theoretic methods are employed to analyse the nonlinear problem and a solution for the velocity field is obtained analytically. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Couette Flow of a Third-Grade Fluid with Variable <s> Abstract This paper deals with the boundary layer flow and heat transfer of Powell-Eyring fluid over a continuously moving permeable surface in a parallel free stream. The governing boundary layer equations are transformed to self similar nonlinear ordinary differential equations using scaling group of transformations. Numerical results of the resulting equations are obtained using the Keller-box method. The effects of various governing parameters on the flow field are discussed and analyzed. A comparison is made with the existing literature. <s> BIB002

Magnetic Field BIB001 . The study dealing with the analytic solution for the time-dependent flow of an incompressible third-grade fluid under the influence of a magnetic field of variable strength was made by Jalil et al. BIB002 . Group theoretic methods were employed to analyze the nonlinear problem and a solution for the velocity field was obtained analytically.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The governing equation of the problem [41] is <s> This investigation deals with the analytic solution for the time-dependent flow of an incompressible third-grade fluid which is under the influence of a magnetic field of variable strength. The fluid is in an annular region between two coaxial cylinders. The motion is induced due to an inner cylinder with arbitrary velocity. Group theoretic methods are employed to analyse the nonlinear problem and a solution for the velocity field is obtained analytically. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The governing equation of the problem [41] is <s> The unsteady magnetohydrodynamic flow of an electrically conducting viscous incompressible third grade fluid bounded by an infinite porous plate is studied with the Hall effect. An external uniform magnetic field is applied perpendicular to the plate and the fluid motion is subjected to a uniform suction and injection. Similarity transformations are employed to reduce the non-linear equations governing the flow under discussion to two ordinary differential equations (with and without dispersion terms). Using the finite difference scheme, numerical solutions represented by graphs with reference to the various involved parameters of interest are discussed and appropriate conclusions are drawn. <s> BIB002

where , and ] is the kinematic viscosity. The Lie point symmetries for the PDE (201) are BIB001 With the use of the symmetries in (202), the group invariant solution for the PDE (201) was calculated in BIB001 and is given by BIB002 . The model of unsteady MHD flow of an incompressible third-grade fluid bounded by an infinite porous plate in the presence of Hall current was investigated by Fakhar et al. BIB002 . Similarity transformations were employed to reduce the governing partial differential equation into two nonlinear ordinary differential equations.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Hall Effects on Unsteady Magnetohydrodynamic Flow of a Third-Grade Fluid <s> The unsteady magnetohydrodynamic flow of an electrically conducting viscous incompressible third grade fluid bounded by an infinite porous plate is studied with the Hall effect. An external uniform magnetic field is applied perpendicular to the plate and the fluid motion is subjected to a uniform suction and injection. Similarity transformations are employed to reduce the non-linear equations governing the flow under discussion to two ordinary differential equations (with and without dispersion terms). Using the finite difference scheme, numerical solutions represented by graphs with reference to the various involved parameters of interest are discussed and appropriate conclusions are drawn. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Hall Effects on Unsteady Magnetohydrodynamic Flow of a Third-Grade Fluid <s> An analysis is made of the unsteady flow of a third-grade fluid in a porous medium. A modified Darcy's law is considered in the flow modelling. Reduction and solutions are obtained by employing similarity and numerical methods. The effects of pertinent parameters on the flow velocity are studied through graphs. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Hall Effects on Unsteady Magnetohydrodynamic Flow of a Third-Grade Fluid <s> In this article, a combination of Lie symmetry and homotopy analysis methods (HAM) are used to obtain solutions for the unsteady magnetohydrodynamics flow of an incompressible, electrically conducting third grade fluid, bounded by an infinite porous plate in the presence of Hall current. In particular, similarity reductions are performed on the governing equations in its complex scalar and corresponding vector system forms. Also, nontrivial conservation laws, using the multiplier approach, are constructed for the complex scalar equation. Furthermore, a comparison of the results with numerical results already existing in the literature is done. The analytical solutions are presented through graphs by choosing a range of the relevant physical parameters. The underlying calculations were obtained via a combination of software packages in Mathematica and Maple, in particular, for the Lie symmetry generators, Euler Lagrange operators and homotopy operators; the latter being towards the construction of the conserved flows. <s> BIB003

where = ( /(1 − )) and = + . In BIB001 , the travelling wave symmetries were utilized to reduce the above PDE into two nonlinear ODEs. The reduced ODEs were solved for the particular choice of boundary conditions using the finite difference method. Fakhar et al. BIB003 recently revisited the above problem. In BIB003 , they performed the complete Lie group analysis of (204). The Lie point symmetries of PDE (204) found in BIB003 ] , ] . The symmetry Lie generators given in (205) were used to reduce the above PDE into two nonlinear ODEs. The reduced ODEs were solved by making use of the homotopy analysis method. BIB002 . An analysis was made of the unsteady flow of a third-grade fluid in a porous medium by Hayat et al. BIB002 .

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The Rayleigh Problem for a Third-Grade Electrically Conducting Fluid in a Magnetic <s> Abstract The influence of a magnetic field on the flow of an incompressible third grade electrically conducting fluid bounded by a rigid plate is investigated. The flow is induced due by the motion of a plate in its own plane with an arbitrary velocity. The solution of the equations of conservation of mass and momentum balance are obtained analytically. We also present numerical solutions with particular choices of the magnetic field and boundary conditions. Emphasis has been given to the study of the effects of the magnetic field and non-Newtonian fluid parameters. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The Rayleigh Problem for a Third-Grade Electrically Conducting Fluid in a Magnetic <s> We investigate the invariance properties, nontrivial conservation laws and interplay between these notions that underly the equations governing Stokes’ first problem for third-grade rotating fluids. We show that a knowledge of this leads to a number of different reductions of the governing equations and, thus, a number of exact solutions can be obtained and a spectrum of further analyses may be pursued. <s> BIB002

Field BIB001 . The influence of a magnetic field on the flow of an incompressible third-grade electrically conducting fluid bounded by a rigid plate was investigated by Hayat et al. BIB001 . The Lie group approach was employed to perform the reduction of the model equation and thereafter numerical solutions were obtained. The governing problem describing the flow model BIB001 is The symmetry Lie algebra of the PDE (212) is spanned by the following operators BIB001 : where The point symmetries given in (214) were used to reduce the PDE (212) into a nonlinear ODE. The reduced ODE was solved numerically with suitable invariant boundary conditions. BIB002 . Fakhar et al. BIB002 studied the equations governing the Stokes' first problem for a thirdgrade rotating fluid. Lie symmetry formulation was used to perform various reductions of the governing system. The system of equations describing the Stokes' first problem for a third-grade rotating fluid is BIB002

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> A Note on the Interplay between Symmetries, Reduction, and Conservation Laws of Stokes' First Problem for ThirdGrade Rotating Fluids <s> An investigation is made of the magnetic Rayleigh problem where a semi-infinite plate is given an impulsive motion and thereafter moves with constant velocity in a non-Newtonian power law fluid of infinite extent. The solution of this highly non-linear problem is obtained by means of the transformation group theoretic approach. The one-parameter group transformation reduces the number of independent variables by one and the governing partial differential equation with the boundary conditions reduce to an ordinary differential equation with the appropriate boundary conditions. Effect of the some parameters on the velocity u (y, t) has been studied and the results are plotted. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> A Note on the Interplay between Symmetries, Reduction, and Conservation Laws of Stokes' First Problem for ThirdGrade Rotating Fluids <s> We investigate the invariance properties, nontrivial conservation laws and interplay between these notions that underly the equations governing Stokes’ first problem for third-grade rotating fluids. We show that a knowledge of this leads to a number of different reductions of the governing equations and, thus, a number of exact solutions can be obtained and a spectrum of further analyses may be pursued. <s> BIB002

The symmetries for the above system of PDEs were calculated in BIB002 and are given by BIB001 = , The five Lie symmetries given in (217) were used to reduce the PDE system (216) to a system of ODEs.

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> MHD Flow of a Third-Grade Fluid in a Porous <s> An analysis is made of the unsteady flow of a third-grade fluid in a porous medium. A modified Darcy's law is considered in the flow modelling. Reduction and solutions are obtained by employing similarity and numerical methods. The effects of pertinent parameters on the flow velocity are studied through graphs. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> MHD Flow of a Third-Grade Fluid in a Porous <s> We investigate the invariance properties, nontrivial conservation laws and interplay between these notions that underly the equations governing Stokes’ first problem for third-grade rotating fluids. We show that a knowledge of this leads to a number of different reductions of the governing equations and, thus, a number of exact solutions can be obtained and a spectrum of further analyses may be pursued. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> MHD Flow of a Third-Grade Fluid in a Porous <s> The present work deals with the modeling and solution of the unsteady flow of an incompressible third grade fluid over a porous plate within a porous medium. The flow is generated due to an arbitrary velocity of the porous plate. The fluid is electrically conducting in the presence of a uniform magnetic field applied transversely to the flow. Lie group theory is employed to find symmetries of the modeled equation. These symmetries have been applied to transform the original third order partial differential equation into third order ordinary differential equations. These third order ordinary differential equations are then solved analytically and numerically. The manner in which various emerging parameters have an effect on the structure of the velocity is discussed with the help of several graphs. <s> BIB003 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> MHD Flow of a Third-Grade Fluid in a Porous <s> Abstract This work describes the time-dependent flow of an incompressible non-Newtonian fluid over an infinite rigid plate. The flow is induced due to the arbitrary velocity V(t) of the plate. The fluid occupies the porous half space y > 0 and is also electrically conducting in the presence of a constant applied magnetic field in the transverse direction to the flow. Analytical solutions of the governing non-linear partial differential equation for the unidirectional flow of a third grade fluid are established using the symmetry approach. We construct three types of analytical solutions by employing the Lie symmetry method and the better solution from the physical point of view is shown to be the non-travelling wave solution. We also present numerical solutions of the governing PDE and the reduced ODE and compare with the analytical results. Finally, the influence of emerging parameters are studied through several graphs with emphasis on the study of the effects of the magnetic field and non-Newtonian fluid parameters. <s> BIB004 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> MHD Flow of a Third-Grade Fluid in a Porous <s> Abstract In this communication some non-linear flow problems dealing with the unsteady flow of a third grade fluid in porous half-space are analyzed. A new class of closed-form conditionally invariant solutions for these flow models are constructed by using the conditional or non-classical symmetry approach. All possible non-classical symmetries of the model equations are obtained and various new classically invariant solutions have been constructed. The solutions are valid for a half-space and also satisfy the physically relevant initial and the boundary conditions. <s> BIB005

Half Space with Plate Suction or Injection BIB003 . The modelling and solution of the unsteady flow of an incompressible thirdgrade fluid over a porous plate within a porous medium were performed by A. Aziz and T. Aziz BIB003 . Lie group theory was employed to find the symmetries of the model equation. These symmetries were applied to transform the original third-order partial differential equation to thirdorder ordinary differential equations. These third-order ordinary differential equations were solved analytically. The governing problem describing the flow model BIB003 is where The symmetry Lie algebra of the PDE (225) is spanned by the following generators BIB003 . Case 1 (when ( * + 2 * ) ̸ = * / * ). For this case, we find a two-dimensional Lie algebra generated by Case 2 (when ( * + 2 * ) = * / * ). Here we obtain a threedimensional Lie algebra generated by The generators given above were utilized to obtain three different types of group invariant solutions BIB003 ( , ) = exp (− √ * ( − ) √ ) . Remark 1. A revisit has been made by Aziz et al. BIB005 for three flow problems BIB001 BIB004 BIB003 related to a third-grade fluid model and discussed earlier. A conditional symmetry approach was employed in BIB005 to construct some new exact solutions of these models. All possible nonclassical symmetry generators were calculated in BIB005 for these problems. The concept of conditional/nonclassical symmetry was not previously used to find conditionally invariant solutions of non-Newtonian fluid flow problems. This is the first time that a complete nonclassical symmetry analysis was performed to tackle a nonlinear problem dealing with the flow models of nonNewtonian fluids. For the flow model BIB001 , the following nonclassical symmetry generators were found in BIB005 where ( ) is any function of time . Thus, infinitely many nonclassical symmetry generators were obtained for any arbitrary value of the function ( ). With the use of the nonclassical symmetries given in (231), the following new invariant solutions were obtained in BIB005 for the model BIB001 For the model BIB002 , the following nonclassical symmetry generators were found in BIB005 where ( ) is any arbitrary function of time. With the use of the symmetries given in (233), the following new exact solutions were found in BIB005 for the model BIB002 ( where ( ) = ∫ ( ) , For the flow model BIB003 , the only nonclassical symmetry operator found in BIB005 is With the use of the symmetry given above, the new group invariant solution of the flow model BIB003 found in BIB005 is given by (236)

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The Unsteady Flow of a Fourth-Grade Fluid Past a Porous <s> The unsteady flow of a hydrodynamic fluid past a porous plate is examined. The constitutive equations of the fluid are modelled by those for a fourth-grade fluid. They give rise to a boundary value problem consisting of a fifth-order differential equation but there are only two boundary conditions. With the augmentation of boundary conditions at infinity, it is possible to obtain a solution by implementation of the Lie group method. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The Unsteady Flow of a Fourth-Grade Fluid Past a Porous <s> A non-linear partial differential equation modelling the flow of a fourth-grade fluid is derived. Both analytical and numerical travelling wave solutions admitted by the model equation are obtained. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> The Unsteady Flow of a Fourth-Grade Fluid Past a Porous <s> The unsteady unidirectional flow of an incompressible fourth grade fluid over an infinite porous plate is studied. The flow is induced due to the motion of the plate in its own plane with an arbitrary velocity. Symmetry reductions are performed to transform the governing nonlinear partial differential equations into ordinary differential equations. The reduced equations are then solved analytically and numerically. The influence of various physical parameters of interest on the velocity profile are shown and discussed through several graphs. A comparison of the present analysis shows excellent agreement between analytical and numerical solutions. <s> BIB003

Plate BIB001 . Hayat et al. BIB001 examined the unsteady flow of a hydrodynamic fluid past a porous plate. The solution of the governing nonlinear problem was obtained by the implementation of the Lie group method. The problem governing the unsteady flow model of a fourth-grade fluid past a porous plate is BIB001 [ − ] = ] + Γ [( ) where Γ = 2(3 2 + 3 + 4 + 5 + 3 7 + 8 ). The relevant initial and boundary conditions are Equation (239) admits the Lie point symmetry generators = / (time translation) and = / (translation in ). The invariant solution corresponding to the generator + which represents a travelling wave solution with constant wave speed has been considered in BIB001 . The final form of the travelling wave solution found in BIB001 is The same model was recently discussed by Aziz and Mahomed BIB003 by considering a different set of boundary and initial conditions and they found some more physically meaningful exact solutions. The closed-form travelling wave and the steady state solutions found in BIB003 are BIB002 . A nonlinear partial differential equation modelling the flow of a fourth-grade fluid was derived by Hayat et al. BIB002 .

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Travelling Wave Solutions to Stokes' Problem for a FourthGrade Fluid <s> A non-linear partial differential equation modelling the flow of a fourth-grade fluid is derived. Both analytical and numerical travelling wave solutions admitted by the model equation are obtained. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Travelling Wave Solutions to Stokes' Problem for a FourthGrade Fluid <s> The unsteady unidirectional flow of an incompressible fourth grade fluid bounded by a suddenly moved rigid plate is studied. The governing nonlinear higher order partial differential equation for this flow in a semiinfinite domain is modelled. Translational symmetries in variables and are employed to construct two different classes of closed-form travelling wave solutions of the model equation. A conditional symmetry solution of the model equation is also obtained. The physical behavior and the properties of various interesting flow parameters on the structure of the velocity are presented and discussed. In particular, the significance of the rheological effects are mentioned. <s> BIB002

where * = ( 2 + 3 ) and Γ = 2(3 2 + 3 + 4 + 5 + 3 7 + 8 ). It can be seen that the PDE (243) admits Lie point symmetry generators With the use of symmetry generators given in (244), the exact travelling wave solutions found in BIB001 are given by The same PDE (243) was investigated recently by Aziz and Mahomed BIB002 with a suitable choice of physically realistic 32 Mathematical Problems in Engineering boundary conditions. Both forward and the backward wave front type travelling wave solutions were found in BIB002 with the aid of the Lie group approach and are given by A conditional/nonclassical symmetry solution of PDE (243) was also found by Aziz and Mahomed BIB002 and is given by . (247)

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> A Note on Some Solutions for the Flow of a Fourth-Grade <s> Abstract This paper investigates the time independent unidirectional flow of a fourth grade fluid filling the porous half space. Flow modelling is based upon a modified Darcy’s law. Travelling wave and conditional symmetry solutions are developed. The graphs are plotted and discussed for the sundry parameters. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> A Note on Some Solutions for the Flow of a Fourth-Grade <s> An analysis is carried out to study the time-dependent flow of an incompressible electrically conducting fourth-grade fluid over an infinite porous plate. The flow is caused by the motion of the porous plate in its own plane with an impulsive velocity V(t). The governing nonlinear problem is solved by invoking the Lie group theoretic approach and a numerical technique. Travelling wave solutions of the forward and backward type, together with a steady state solution, form the basis of our analytical analysis. Further, the closed-form solutions are also compared against numerical results. The essential features of the embedded parameters are described. In particular, the physical significance of the plate suction/injection and magnetic field is studied. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> A Note on Some Solutions for the Flow of a Fourth-Grade <s> Abstract In this study, an incompressible time-dependent flow of a fourth-grade fluid in a porous half space is investigated. The flow is generated due to the motion of the flat rigid plate in its own plane with an impulsive velocity. The partial differential equation governing the motion is reduced to ordinary differential equations by means of the Lie group theoretic analysis. A complete group analysis is performed for the governing nonlinear partial differential equation to deduce all possible Lie point symmetries. One-dimensional optimal systems of subalgebras are also obtained, which give all possibilities for classifying meaningful solutions in using the Lie group analysis. The conditional symmetry approach is also utilised to solve the governing model. Various new classes of group-invariant solutions are developed for the model problem. Travelling wave solutions, steady-state solution, and conditional symmetry solutions are obtained as closed-form exponential functions. The influence of pertinent parameters on the fluid motion is graphically underlined and discussed. <s> BIB003

Fluid in a Porous Space BIB001 . Hayat et al. BIB001 where The relevant initial and boundary conditions are With the use of translational symmetry generators, the travelling wave solution for the PDE (253) found in BIB001 is given by The nonclassical symmetry generator for the PDE (253) found in BIB001 is given by With the use of nonclassical symmetry operator (257), the conditionally invariant solution found in BIB001 is given by where with (0) = 1, + 2Γ [( ) The relevant boundary and initial conditions are BIB002 (0, ) = ( ) , > 0, With the use of translational symmetry generators, the backward type travelling wave solution for the PDE (261) found in BIB002 is given by The forward type travelling wave solution for the PDE (261) is given by Finally the stead-state solution of the problem found in BIB002 is + 2Γ * [( ) The relevant space and time-dependent velocity boundary and initial conditions are BIB003 (0, ) = ( ) , > 0, where ( Here 1 is translation in time, 2 is translation in space, and 3 has path curves which are equivalent to a combination of translations in and scaling in . With the use of the symmetries given in (268), the following exact solutions were found in BIB003 for the model BIB003 ( , ) The nonclassical symmetry generator for the PDE (266) found in BIB003 is given by With the use of nonclassical symmetry operator (271), the conditionally invariant solutions found in BIB003 are given by (272)

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Scaling Group Transformation for MHD Boundary Layer Flow of a Nanofluid Past a Vertical Stretching Surface in the <s> The problem of laminar fluid flow which results from the stretching of a vertical surface with variable stream conditions in a nanofluid has been investigated numerically. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis in the presence of magnetic field. The symmetry groups admitted by the corresponding boundary value problem are obtained by using a special form of Lie group transformations viz. scaling group of transformations. An exact solution is obtained for translation symmetry and numerical solutions for scaling symmetry. This solution depends on a Lewis number, magnetic field, Brownian motion parameter and thermophoretic parameter. The conclusion is drawn that the flow field and temperature and nanoparticle volume fraction profiles are significantly influenced by these parameters. A nanofluid is a new class of heat transfer fluids that contain a base fluid and nanoparticles. The model used for the nanofluid incorporates the effects of Brownian motion and thermophoresis. In nanofluid systems, thermophoresis particle deposition in the presence of magnetic field with Brownian motion has a substantial effect on the flow field. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Scaling Group Transformation for MHD Boundary Layer Flow of a Nanofluid Past a Vertical Stretching Surface in the <s> Abstract In this paper, we discuss similarity reductions for problems of magnetic field effects on free convection flow of a nanofluid past a semi-infinite vertical flat plate. The application of a one-parameter group reduces the number of independent variables by 1, and consequently the governing partial differential equation with the auxiliary conditions to an ordinary differential equation with the appropriate corresponding conditions. The differential equations obtained are solved numerically and the effects of the parameters governing the problem are discussed. Different kinds of nanoparticles were tested. <s> BIB002

Presence of Suction/Injection BIB001 . The similarity transformation used to reduce the PDE system (293) of BIB001 is given by With the use of transformations (295), the above system of PDEs was reduced to a nonlinear system of ODEs which was then solved using a numerical method. BIB002 . The similarity reductions for problems of magnetic field effects on free convection flow of a nanofluid past a semi-infinite vertical flat plate were studied by Hamad et al. BIB002 . The application of a one-parameter group reduced the number of independent variables by one and consequently the governing partial differential equation with the auxiliary conditions to an ordinary differential equation with the appropriate corresponding conditions. The differential equations obtained were solved numerically. The governing problem describing the flow model BIB002

Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Concluding Remarks <s> Abstract The conservation laws for second order scalar partial differential equations and systems of partial differential equations which occur in fluid mechanics are constructed using different approaches. The direct method, Noether’s theorem, the characteristic method, the variational approach (multiplier approach) for arbitrary functions as well as on the solution space, symmetry conditions on the conserved quantities, the direct construction formula approach, the partial Noether approach and the Noether approach for the equation and its adjoint are discussed and explained with the help of an illustrative example on a non-linear field equation describing the relaxation to a Maxwellian distribution. The conservation laws for the non-linear diffusion equation for the spreading of an axisymmetric thin liquid drop, the system of two partial differential equations governing flow in a laminar two-dimensional jet and the system of two partial differential equations governing flow in a laminar radial jet are discussed via these approaches. <s> BIB001 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Concluding Remarks <s> Abstract A systematic way to derive the conserved quantities for the liquid jet, free jet and wall jet using conservation laws is presented. Both two-dimensional and radial jets are considered. The jet flows are described by Prandtl’s momentum boundary layer equation and the continuity equation. The multiplier approach (also know as variational derivative approach) is first applied to construct a basis of conserved vectors for the system. The basis consists of two conserved vectors. By integrating the corresponding conservation laws across the jet and imposing the boundary conditions, conserved quantities are derived for the liquid jet and the free jet. The multiplier approach is then applied to construct a basis of conserved vectors for the third-order partial differential equation for the stream function. The basis consists of two local conserved vectors one of which is a non-local conserved vector for the system. The conserved quantities for the free jet and the wall jet are derived from the corresponding conservation laws and boundary conditions. The approach gives a unified treatment to the derivation of conserved quantities for jet flows and may lead to a new classification of jets through conserved vectors and their multipliers. <s> BIB002 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Concluding Remarks <s> Abstract The conservation laws for Prandtl’s boundary layer equations for an incompressible fluid governing the flow in radial and two-dimensional jets are investigated. For both radial and two-dimensional jets the partial Lagrangian method is used to derive conservation laws for the system of two differential equations for the velocity components. The Lie point symmetries are calculated for both cases and a symmetry is associated with the conserved vector that is used to establish the conserved quantity for the jet. This associated symmetry is then used to derive the group invariant solution for the system governing the flow in the free jet. <s> BIB003 </s> Applications of group theoretical methods to non-newtonian fluid flow models: survey of results <s> Concluding Remarks <s> Abstract In this communication some non-linear flow problems dealing with the unsteady flow of a third grade fluid in porous half-space are analyzed. A new class of closed-form conditionally invariant solutions for these flow models are constructed by using the conditional or non-classical symmetry approach. All possible non-classical symmetries of the model equations are obtained and various new classically invariant solutions have been constructed. The solutions are valid for a half-space and also satisfy the physically relevant initial and the boundary conditions. <s> BIB004

In this topical survey, we have classified all those studies which are related to the flow models of different nonNewtonian fluids and solved using Lie symmetry and conditional symmetry approaches. The mathematical modelling, the symmetries found, and the solutions obtained for each of the studies under investigation were presented. We have observed that the symmetry methods used to solve different non-Newtonian fluid flow models are useful for a wide range of nonlinear problems in fluids given the paucity of known exact solutions especially in non-Newtonian fluids. In the majority of the models that we have reviewed, the Lie symmetry methods have been successfully applied to obtain exact solutions. Another notable significant feature of the Lie theory analysis that we have observed is that it has been used to construct the similarity transformations which are used for the reductions of the governing equations into reduced (these reduced equations are sometimes solved numerically) or exactly solvable equations. The Lie approach is not only applicable to many nonlinear and complicated scalar ODEs and PDEs but also to a nonlinear system of ODEs and PDEs in the context of non-Newtonian fluid mechanics. In recent years, the ansatz method has been used to construct exact solutions of nonlinear differential equations arising in the study of non-Newtonian fluids. In the ansatz method, different forms of the solution are assumed and different techniques are used to develop analytical results. We pinpoint one of the central points of this survey which is the usefulness of systematic group theoretic approach. Other methods such as homotopy approach, Adomian method, and similar iterative methods have been applied for number of times for similar class of problems discussed in this survey. However, their precise application is still unclear and the results they produce are not dictated by the physical problem at hand. The applications of Lie symmetry methods for nonNewtonian fluid flow models as discussed here are more systematic, rigorous, and general treatment for such type of problems. In other words, the group theoretical methods provide a unified treatment to classify exact solutions of models of Newtonian and non-Newtonian fluids which are solved in the literature using different techniques. Another important aspect of this survey we have observed is that many researchers have employed Lie group methods to investigate different interesting features of non-Newtonian fluids in different situations but very limited studies have been reported in the literature in which researchers have utilized the nonclassical symmetry method to solve non-Newtonian fluid flow models. In particular, only a single study BIB004 has been found in which the authors have performed the nonclassical symmetry analysis of a particular model of a non-Newtonian third-grade fluid. Thus, the concept of nonclassical symmetry has not been widely used to find conditionally invariant solutions of non-Newtonian fluid flow problems. This also applies to the use of weak and higher symmetries in the context of fluids. We believe that these deserve further importance in tackling non-Newtonian fluid flow problems. Since the problems dealing with the flow of non-Newtonian fluids have received much attention in recent years, the present survey is intended to provide a platform for researchers to apply group methods to tackle nonlinear problems in the fertile field of fluid mechanics. Moreover, we did not focus on symmetry associated with conservation law works such as that of BIB001 BIB002 BIB003 . This necessitated giving notions on conservation laws of partial differential equations and the group approaches related to these which are extensive.

Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> Abstract Vehicle-to-grid power (V2G) uses electric-drive vehicles (battery, fuel cell, or hybrid) to provide power for specific electric markets. This article examines the systems and processes needed to tap energy in vehicles and implement V2G. It quantitatively compares today's light vehicle fleet with the electric power system. The vehicle fleet has 20 times the power capacity, less than one-tenth the utilization, and one-tenth the capital cost per prime mover kW. Conversely, utility generators have 10–50 times longer operating life and lower operating costs per kWh. To tap V2G is to synergistically use these complementary strengths and to reconcile the complementary needs of the driver and grid manager. This article suggests strategies and business models for doing so, and the steps necessary for the implementation of V2G. After the initial high-value, V2G markets saturate and production costs drop, V2G can provide storage for renewable energy generation. Our calculations suggest that V2G could stabilize large-scale (one-half of US electricity) wind power with 3% of the fleet dedicated to regulation for wind, plus 8–38% of the fleet providing operating reserves or storage for wind. Jurisdictions more likely to take the lead in adopting V2G are identified. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> The benefits and costs of increasing solar electricity generation depend on the scale of the increase and on the time frame over which it occurs. Short-run analyses focus on the cost-effectiveness of incremental increases in solar capacity, holding the rest of the power system fixed. Solar’s variability adds value if its power occurs at high-demand times and displaces relatively carbon-intensive generation. Medium-run analyses consider the implications of nonincremental changes in solar capacity. The cost of each installation may fall through experience effects, but the cost of grid integration increases when solar requires ancillary services and fails to displace investment in other types of generation. Long-run analyses consider the role of solar in reaching twenty-first-century carbon targets. Solar’s contribution depends on the representation of grid integration costs, on the availability of other low-carbon technologies, and on the potential for technological advances. By surveying analyses for different time horizons, this article begins to connect and integrate a fairly disjointed literature on the economics of solar energy. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> Electric vehicles (EVs) and renewable energy sources offer the potential to substantially decrease carbon emissions from both the transportation and power generation sectors of the economy. Mass adoption of EVs will have a number of impacts and benefits, including the ability to assist in the integration of renewable energy into existing electric grids. This paper reviews the current literature on EVs, the electric grid, and renewable energy integration. Key methods and assumptions of the literature are discussed. The economic, environmental and grid impacts of EVs are reviewed. Numerous studies assessing the ability of EVs to integrate renewable energy sources are assessed; the literature indicates that EVs can significantly reduce the amount of excess renewable energy produced in an electric system. Studies on wind–EV interaction are much more detailed than those on solar photovoltaics (PV) and EVs. The paper concludes with recommendations for future research. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> This paper presents a comprehensive review and assessment of the latest research and advancement of electric vehicles (EVs) interaction with smart grid portraying the future electric power system model. The concept goal of the smart grid along with the future deployment of the EVs puts forward various challenges in terms of electric grid infrastructure, communication and control. Following an intensive review on advanced smart metering and communication infrastructures, the strategy for integrating the EVs into the electric grid is presented. Various EV smart charging technologies are also extensively examined with the perspective of their potential, impacts and limitations under the vehicle-to-grid (V2G) phenomenon. Moreover, the high penetration of renewable energy sources (wind and photovoltaic solar) is soaring up into the power system. However, their intermittent power output poses different challenges on the planning, operation and control of the power system networks. On the other hand, the deployment of EVs in the energy market can compensate for the fluctuations of the electric grid. In this context, a literature review on the integration of the renewable energy and the latest feasible solution using EVs with the insight of the promising research gap to be covered up are investigated. Furthermore, the feasibility of the smart V2G system is thoroughly discussed. In this paper, the EVs interactions with the smart grid as the future energy system model are extensively discussed and research gap is revealed for the possible solutions. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> In order to reach significant CO2 emissions reductions, future energy systems will require a large share of renewable energies, such as wind and solar photovoltaic power. However, relying on such renewable energy sources is expected to generate considerable excess power during certain periods of the day, in particular during night time for wind and daytime for solar power. This excess power may be conveniently used to power electric mobility. This paper explores the possible complementarities between wind and solar power and electric vehicles charging, based on 2050 scenarios for the case study of Portugal. Model results show that CO2 emissions targets can only be achieved with high levels of photovoltaics penetration and electric vehicles, reinforcing the need for daytime charging infrastructures, presumably at or near work facilities. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> Electric vehicles (EVs) represent one of the most promising technologies to green the transportation systems. An important issue is that high penetration of EVs brings heavy electricity demand to the power grid. One effective way to alleviate the impact is to integrate local power generation such as renewable energy sources (RESs) into charging infrastructure. Because of the intermittent and indispatchable nature of RESs, it becomes very challenging to coordinate EVs charging with other grid load and renewable generation. In this paper, EVs charging problem in the presence of smart grid technologies is investigated, and the interaction with renewable energy is reviewed. An overview about EVs and RESs is first presented, which mainly introduces major types of EVs and renewable energy estimation methods. Then, according to the objectives, the existing research works are divided into three categories: cost-aware, efficiency-aware, and emissions-aware interactions between EVs and RESs. Each category׳s discussion includes the description of core ideas, summarization of solutions, and comparison between different works. Finally, some key open issues about EVs interacting with RESs are given and some possible solutions are also discussed. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> The next-generation electricity grid (“smart grid”) is expected to integrate interoperable technologies – particularly in the energy, transport, information and communication fields – with the aim to increase reliability, affordability and sustainability considering at the same time, distribution systems and market operation. In order to gauge the implications of the anticipated paradigm shift for the electricity system, new reference architectures and assessment methodologies shall be developed to properly capture the interactions between the different actors (especially utilities, operators, energy aggregators, end-users, etc.) and technologies to value and allocate the costs and benefits of such transformation. Against this background, this paper proposes a conceptual architecture and an assessment framework to explore how high penetration scenarios of electric vehicles and intermittent renewable generation can complement each other in emerging distribution networks. We start from the identification of the smart grid functionalities to be implemented in a system with distributed power injections under the need to supervise and coordinate myriads of decentralized and interoperable energy sources and actors. Relying upon the proposed smart grid conceptual architecture, we develop an assessment framework to maximize the renewable electricity and electric vehicle penetration for given electricity and transport systems. The application of the proposed assessment framework to a realistic case study, representing the distribution and mobility systems of a typical mid-size Italian city, illustrates how some of the limitations and constraints of the current electricity network operation and design approaches can be addressed and overcome. We show how integration of substantial amounts of energy production and electric-based transport technologies can be achieved while improving the reliability and sustainability performances of the emerging power systems. <s> BIB007 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> This paper proposes a novel electric vehicle (EV) classification scheme for a photovoltaic (PV) powered EV charging station (CS) that reduces the effect of intermittency of electricity supply as well as reducing the cost of energy trading of the CS. Since not all EV drivers would like to be environmentally friendly, all vehicles in the CS are divided into three categories: 1) premium, 2) conservative, and 3) green, according to their charging behavior. Premium and conservative EVs are considered to be interested only in charging their batteries, with noticeably higher rate of charging for premium EVs. Green vehicles are more environmentally friendly, and thus assist the CS to reduce its cost of energy trading by allowing the CS to use their batteries as distributed storage. A different charging scheme is proposed for each type of EV, which is adopted by the CS to encourage more EVs to be green. A basic mixed integer programming (MIP) technique is used to facilitate the proposed classification scheme. It is shown that the uncertainty in PV generation can be effectively compensated, along with minimization of total cost of energy trading to the CS, by consolidating more green EVs. Real solar and pricing data are used for performance analysis of the system. It is demonstrated that the total cost to the CS reduces considerably as the percentage of green vehicles increases, and also that the contributions of green EVs in winter are greater than those in summer. <s> BIB008 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> The concept of solar parking lots aims at coupling the development of clean solar electricity and electric mobility. Solar panels provide shade and generate electricity to charge parked electric vehicles. In a vehicle-to-grid approach, the vehicles may also feed the grid and support it with ancillary services. <s> BIB009 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> Rising greenhouse gas emissions raise the risk of severe climate change. The household sector׳s greenhouse gas emissions have increased over time as more people drive gasoline cars and consume electricity generated using coal and natural gas. The household sector׳s emissions would decline if more households drove electric vehicles and owned solar panels. In recent years automobile manufacturers have been producing high-performance electric vehicles, and solar panels are becoming more efficient and less expensive. Using several data sets from California, we document evidence of the growth of the joint purchase of electric and hybrid vehicles and solar panels. We discuss pricing and quality trends for these green durable goods. <s> BIB010 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Introduction <s> Abstract Electric vehicle demands have increased rapidly since 2010, and depend on renewables and socioeconomic factors. Using panel data from fourteen countries between 2010 and 2015, we study impacts of seven factors in a multiple linear regression model. The factors include percentage of renewable energies in electricity generation, number of charging stations, education level, population density, gasoline price, GDP per capita and urbanization. The first four factors have apparent and positive impacts on the demands, and the last two factors don’t. The gasoline price affects the demands for BEVs (battery electric vehicles) more than that for PHEVs (plug-in hybrid electric vehicles). One percent increase in renewables would lead to approximately 2–6% increase in EV demands. Based on the results, policy implications are discussed. <s> BIB011

Electric vehicles (EV) and photovoltaic generation (PV) 1 are two independent technologies disrupting their respective sectors. In 2017 for instance, world EV fleet has reached two million units, while solar energy has got the largest growth of all energy sources 2] . Both technologies meet in power grids, in a distributed way, and tend to develop geographically close BIB010 BIB011 . Many works have pointed out that the flexibility of EV battery would be an excellent complement for renewable energy integration BIB001 . This potential synergy should provide individual and systemic benefits, by decreasing both technology costs and ecological footprints BIB007 BIB005 BIB008 . Eventually, it could even stimulate the development of each technology. Nevertheless, several barriers exist for mutual beneficial interactions. EV and PV bring various technical and economic problems on power grids. These issues mainly result in additional costs on power grids managements, which increase with the penetration of each technology BIB002 . Moreover, a disorganized EV/PV system can even reinforce these costs. Therefore, a growing literature has studied the combined integration of EV and PV (or other renewable energies) in power grids. Several authors have written reviews on the subject BIB003 BIB004 BIB006 BIB009 and have identified main technical features of such integration. Assessing properties of EV/PV systems is a complex task as they depend on various technical, meteorological, behavioral, economic and political parameters. Moreover, the benefits of EV/PV synergy are often mixed with other benefits belonging to one single technology (such as energy trading outside the system for instance). Therefore, the purpose of this paper is to build a systematic framework in order to assess EV/PV systems properties and the conditions for synergy. This framework allowed us to identify the main conditions for synergy, that lead to better economic and technical efficiency and reduced ecological footprint of EV/PV systems. This synergy is determined by a smart control strategy aiming at optimizing the power flows in the system, mostly by adapting the EV charging to variations from photovoltaic generation. The spatial configuration of the EV/PV system is a major aspect of the synergy. EV/PV couplings were particularly efficient in intermediate scales (large workplace buildings and charging station). During the day, these configurations host a medium number of EV, which facilitates the predictability of charging demand. In addition, this demand is adapted to the PV generation profile and therefore, smart control is very efficient. On the other hand, the specificity of EV/PV couplings is less obvious on other scales (households, territories) and technologically diversified system. Generally, the literature did not assess the economic conditions of EV/PV synergy in a first intention. Nevertheless, we were able to identify the key-aspects of those conditions. First, it appears that the efficiency of most EV/PV systems was enforced by a certain level of cooperation between the different participants of the system. Cooperation is even more crucial as it is needed where the potential of EV/PV synergy is the highest. Then, that the interactions of EV/PV systems and power grids could lead to additional efficiency gains for the EV/PV system and for the different grids. Finally, the economic context (electricity pricing, entry barriers, support mechanisms) in which EV/PV systems were operating has a major role in the synergy. The remainder of the paper begins by exhibiting the motivation and the components of our systematic framework. Then we detail the elements of our analytic framework with a third section exploring the different technical aspects, followed by a fourth section exploring the economic aspects of EV/PV systems. We will conclude the review by summarizing key-elements of EV/PV interactions and giving potential leads for future research.

Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> In this paper, a new energy management strategy (EMS) for a dc distribution system in buildings is being proposed. The dc distribution system is considered as a prospective system according to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and fuel cells. Since the dc distribution system has many advantages such as feasible connection of DERs and electric vehicles (EVs), reduction of conversion losses between dc output sources and loads, no reactive power issues, it is very suitable for industrial and commercial buildings interfaced with DERs and EVs. The establishment of an appropriate EMS based on the economic point of view can reduce energy costs of buildings and provide benefits to participants in energy management. Applicable elements for the dc distribution system are identified and the real-time decision-making-based algorithm for minimizing operating costs is proposed in this paper. The EV service model for the EMS to offer incentive to EV owners who participate in battery discharging is described. To verify the performances of the proposed algorithm, computer simulation and economic analysis are being performed where the results show that the proposed EMS reduces energy costs, motivates EV owners, and can be applied to the dc distribution buildings. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> Abstract This paper proposes a collaborative strategy between the photovoltaic (PV) participants and electric vehicle (EV) owners to reduce the forecast uncertainties and improve the predictability of PV power. The PV generation is predicted using an auto regressive moving average (ARMA) time series model. Fuzzy C-means (FCM) clustering is used to group the EVs into fleets with similar daily driving patterns. Uncertainties of the PV power and stochastic nature of driving patterns are characterized by a Monte Carlo simulation (MCS) technique. A particle swarm optimization (PSO) algorithm is developed to optimally use the vehicle-to-grid (V2G) capacities of EVs and minimize the penalty cost for PV power imbalances between the predicted power and actual output. The proposed method provides a coordinated charging/discharging scheme to realize the full potential of V2G services and increase the revenues and incentives for both PV producers and EV drivers. An economic model is developed to include the V2G expenses and revenues to provide a complete picture of the cost–benefit analysis. The proposed model is used to evaluate the economic feasibility of V2G services for PV power integration. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> Commercial building microgrids will play an important role in the smart energy city. Stochastic and uncoordinated electric vehicle (EV) charging activities, which may cause performance degradations and overloads, have put great stress on the distribution system. In order to improve the self-consumption of PV energy and reduce the impact on the power grid, a heuristic operation strategy for commercial building microgrids is proposed. The strategy is composed of three parts: the model of EV feasible charging region, the mechanism of dynamical event triggering, and the algorithm of real-time power allocation for EVs. Furthermore, in order to lower the cost of computation resource, the strategy is designed to operate without forecasting on photovoltaic output or EV charging demand. A comprehensive result obtained from simulation tests has shown that the proposed strategy has both satisfactory results and high efficiency, which can be utilized in embedded systems for real-time allocation of EV charging rate. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> This paper addresses a two-stage framework for the economic operation of a microgrid-like electric vehicle (EV) parking deck with on-site renewable energy generation (roof-top photovoltaic panel). This microgrid-like EV parking deck is a localized grouping of distributed generation (solar), energy storage (EV batteries), and load (EV charging load). Although EV parking decks can enable greater adoption of renewable energy sources by scheduling charging loads to coincide with periods of strong sun, the inherent intermittency of renewable energy resources and variable EV parking behaviors complicates the economic operation. In this paper, the proposed first stage of this framework provides the parking deck operators with a stochastic approach for dealing with the uncertainty of solar energy so as to make an optimal price decision (marginal electricity sale price and parking fee rebate) at the day-ahead time scale. The second stage introduces a model predictive control-based operation strategy of EV charging dealing with the uncertainty of parking behaviors within the real-time operation. Case studies demonstrate the better performance of the proposed framework, offering an effective day-ahead marginal electricity price for tomorrow's operation and increasing the microgrid-like EV parking deck's revenue during the real-time operation. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> The magnitude of building- and transportation-related GHG (greenhouse gas) emissions makes the adoption of all-EVs (electric vehicles) powered with renewable power as one of the most effective strategies to reduce emission of GHGs. This paper formulates the problem of GHG mitigation strategy under uncertain conditions and optimizes the strategies in which EVs are powered by solar energy. Under a pre-specified budget, the objective is to determine the type of EV and power generation capacity of the solar system in such a way as to maximize GHG emissions reductions. The model supports the three primary solar systems: off-grid, grid-tied, and hybrid. First, a stochastic optimization model using probability distributions of stochastic variables and EV and solar system specifications is developed. The model is then validated by comparing the estimated values of the optimal strategies and actual values. It is found that the mitigation strategies in which EVs are powered by a hybrid solar system lead to the best cost-expected reduction of CO2 emissions ratio. The results show an accuracy of about 4% for mitigation strategies in which EVs are powered by a grid-tied or hybrid solar system and 11% when applied to estimate the CO2 emissions reductions of an off-grid system. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> Abstract In order to maximize the operation profit while maintaining the service quality, an effective energy management scheme is highly needed for the Photovoltaic-assisted Charging Station (PVCS). Considering the uncertainty of Electric Vehicle (EV) charging demand and PV power output, it will be challenging to determine the charging power for EVs to make informed real-time decisions. In this study, an online energy management method leveraging both offline optimization and online learning is proposed. In order to maximize the self-consumption of Photovoltaic (PV) energy and decide the power supplied from the power grid with Time-of-Use (TOU) pricing, here online learning is coupled with the rule-based decision-making to obtain a real-time online algorithm. The knowledge base for online learning is derived and updated from the results of offline optimization after every operation day. The PVCS located at workplace parking lots is used as an example to test the proposed method. The simulation results show that the method can be implemented without the information on future PV power and charging demand. The obtained results are close to the optimal results from offline optimization. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> A real-time price (RTP)-based automatic demand response (ADR) strategy for PV-assisted electric vehicle (EV) Charging Station (PVCS) without vehicle to grid is proposed. The charging process is modeled as a dynamic linear program instead of the normal day-ahead and real-time regulation strategy, to capture the advantages of both global and real-time optimization. Different from conventional price forecasting algorithms, a dynamic price vector formation model is proposed based on a clustering algorithm to form an RTP vector for a particular day. A dynamic feasible energy demand region (DFEDR) model considering grid voltage profiles is designed to calculate the lower and upper bounds. A deduction method is proposed to deal with the unknown information of future intervals, such as the actual stochastic arrival and departure times of EVs, which make the DFEDR model suitable for global optimization. Finally, both the comparative cases articulate the advantages of the developed methods and the validity in reducing electricity costs, mitigating peak charging demand, and improving PV self-consumption of the proposed strategy are verified through simulation scenarios. <s> BIB007 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Coordination methods <s> Corrections were made to author information for the paper, “Energy management system with PV power forecast to optimally charge EVs at the workplace,” (van Der Meer, D., et al), IEEE Trans. Ind. Informat., vol. 14, no. 1, pp. 311–320, Jan. 2018. <s> BIB008

A coordination method is the mathematical formulation of the control strategy. It gathers together the strategy objective, the control modes with the diverse constraints on EV (charging demand needs, battery availability), PV generation, other technologies availability and other technical constraints (e.g. from power grids). We distinguish coordination between optimization methods, heuristic methods and hybrid methods. Optimization methods. One of the most common methods for energy management in EV/PV systems is to optimize an objective function (based on cost, energy efficiency or ecological footprint). Typically, this function can be the imported power from the grid (in cost or energy) over a given time. It can be also: power losses, system total lifetime cost, and direct and indirect pollutants emissions... This function can also be a combination of different objectives. As we will see later, multi-objective optimization methods are an adequate way to simulate cooperation between different actors. As an illustration, we formulate a simple problem for a system with an independent load, several charging EVs and PV power. The system manager would want to minimize its grid power costs over a certain time under the following constraints: conservation between energy produced (PV and grid) and consumed (load, EVs and losses), EV charging dynamics, EV battery limits (minimal and maximal state of charge), power limits for charging, possibility of bidirectional EV flows (vehicle-to-grid) and the state of charge required by drivers. Such problem would require data on power grid prices, PV generation profiles, load profiles and charging efficiencies. Solving an optimization problem depends mostly on the functional forms. Linear, quadratic and more generally, most convex problems are easily manageable with numerical solvers. When functions are not convex, genetic algorithms and particle swarm optimization algorithms can achieve nearoptimal results in a reasonable computation time. As the problems contain several uncertainty sources (for solar irradiance, EV charging demand, electricity prices...), stochastic modeling can be used. An objective function would then be the expected value of the objective functions over different scenarios. Several authors have incorporated stochasticity in their smart strategies. BIB004 shows that a microgrid revenue of a difference between a strategy with perfect forecast and a stochastic optimized strategy to 1-9%. In this case study, the range was comparable to improvement of a stochastic optimized strategy over the uncontrolled strategy. Similarly, BIB005 show that a stochastic algorithm decreases forecast error by 15-30% compared to a deterministic algorithm. Finally it is possible to use statistical methods to get prediction of EV arrivals, PV power or grid electricity prices BIB008 BIB002 . Heuristic methods. Energy management systems of EV/PV systems often need simple logical rules to react instantaneously to new events (plugging/unplugging of an EV, decrease of PV power, electricity price). Ruled-based decision making provide a simple heuristic method to manage the system with little data requirements for prediction. Typical example of such method can be found in references BIB001 BIB003 . BIB003 shows that it can even provide results almost as good as optimization problems, while reducing the energy management system computation time by a factor 300. Hybrid methods. Optimization and heuristic can be combined as hybrid approaches. BIB006 define a smart control strategy by a two level approach. Before every day of service, an offline deterministic optimization gather data on EV charging needs, PV power and grid prices and a database of past optimal strategies. A second step is a real-time approach that uses a machine learning algorithm (based on the database of past events) and rule-based decision making. The authors demonstrate that such methods do not require any forecasts on PV power and charging demand while getting near-optimal strategies. The hybrid strategy of BIB007 consists in a dynamic price forecasting algorithm and a rule-based decision making algorithm.

Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> Renewable energy sources and plug-in hybrid electric vehicles (PHEVs) are becoming very popular in research areas, as well as in the market. The aim of this paper is to demonstrate how a solar powered building interacts with energy storage and how it can be used to power a PHEV and to support the grid with peak shaving, load shifting, and reducing annual energy usage. A net zero energy house (ZEH5) is selected as the base house for this experiment. Oak Ridge National Laboratory (ORNL) is developing simulation models and energy management scenarios using the actual solar production and residential energy usage data, and a PHEV. The system interaction with the grid is evaluated after getting all the data from PHEV charging, photovoltaic (PV) power production, and residential load. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> When charged with renewable energy, electric vehicles can reduce the CO2 emissions of the transportation sector. As a result, the German government has set the target of one million electric vehicles by 2020. At the same time, it is predicted that renewable energies will contribute 35 % to the national energy consumption by 2020. Because photovoltaic systems can be connected to the same grid node as electric vehicles, they seem to be the first choice for charging electric vehicles with renewable energies. In this paper, the synergies between photovoltaic systems and electric vehicles will be analyzed from a technical as well as from an economic point of view. Specifically, a model has been developed in order to determine the amount of solar energy used by the electric vehicle, the grid load/feed-in and the cost savings for owners of an electric vehicle and a photovoltaic system. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> Photovoltaic (PV) power production and residential power demand are negatively correlated at high latitudes on both annual and diurnal basis. If PV penetration levels increase, methods to deal with power overproduction in the local distribution grids are needed to avoid costly grid reinforcements. Increased local consumption is one such option. The introduction of a home-charged plug-in electric vehicle (PEV) has a significant impact on the household load and potentially changes the coincidence between household load and photovoltaic power production. This paper uses a stochastic model to investigate the effect on the coincidence between household load and photovoltaic power production when including a PEV load. The investigation is based on two system levels: (I) individual household level and (II) aggregate household level. The stochastic model produces theoretical high-resolution load profiles for household load and home charged PEV load over time. The photovoltaic power production model is based on high-resolution irradiance data for Uppsala, Sweden. It is shown that the introduction of a PEV improves the self-consumption of the photovoltaic power both on an individual and an aggregate level, but the increase is limited due to the low coincidence between the photovoltaic power production pattern and the charging patterns of the PEV. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> A (nearly) zero-energy building (ZEB) is a building that roughly consumes the same amount of energy on an annual basis that the amount of renewable energy created on site. This paper tries to investigate to which extent Plug-in Electric Vehicles (PEVs) that are able to deliver part of their battery capacity to the residential appliances can allow ZEBs to operate in a truly self-sustainable fashion, i.e., minimising the amount of energy exchanged with the outer grid. Special emphasis is given to study the effect of the limited PEV owners' cooperation that is due to the need of guaranteeing a preset driving range, as well as the capability of fixed batteries to compensate for low PEV owners' cooperation. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> Abstract In this paper we investigate household electricity use, electric vehicle (EV) home-charging and distributed photovoltaic (PV) power production in a case study for the city of Westminster, London. Since it is economically beneficial to maximize PV power self-consumption in the UK context the power consumption/production patterns with/without introducing EV home-charging on the household level is investigated. Additionally, since this might have an effect on the electricity use on an aggregate of households a large-scale introduction of EV charging and PV power production in the entire city of Westminster is also investigated. Household electricity consumption and EV home-charging are modeled with a Markov-chain model. PV power production is estimated from solar irradiation data from Meteonorm for the location of Westminster combined with a model for photovoltaic power production on tilted planes. The available rooftop area is estimated from the UK map geographic information database. EV home-charging increases the household electricity use mainly during evening with a maximum during winter whereas PV produces power during daytime with maximum during summer. On the household level this mismatch introduces variability in power consumption/production, which is shown to be less prominent for the large-scale scenario of the entire city of Westminster. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> This study compares the lifecycle costs and greenhouse gas (GHG) emissions of electric vehicle (EV) ownership to that of other popular and similar cars in Hawaii. It focuses on the interaction of EV costs with Hawaii's rapid solar PV uptake, using a scenario planning approach for future fuel and electricity prices. EVs include battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs). We find that the total cost of ownership (TCO) of EVs tends to be higher than their internal combustion engine vehicle (ICEV) or hybrid electric vehicle (HEV) counterparts. Once accounting for the federal tax credit, however, some EVs become relatively cost-effective. Moreover, access to residential solar PV makes EVs quite attractive. Layering the federal EV subsidy with solar PV charging makes the full lifecycle cost of the Nissan Leaf about $1200 less expensive than the next lowest cost vehicle, the Toyota Corolla (over a 150,000-mile lifetime). Nonetheless, it may be too early to tout EVs in Hawaii as a GHG abatement strategy. Based on today's mix of electricity generation, the best performing PHEV and BEV emit 2 and 5 MTCO2, respectively, more over their lifetime than the best performing HEV. However, many EVs become on par with the high performing HEVs when considering Hawaii's adoption of aggressive renewable energy goals for the electric sector. If the electric sector meets its 2030 Renewable Portfolio Standard (RPS) target of 40% renewables through low carbon sources like wind and solar, the Toyota Plug-in Prius, Nissan Leaf and Toyota Prius become comparable in terms of their GHG impacts. Integrating residential solar PV, even for just weekend charging, makes all EVs outperform the Toyota Prius in regards to lifetime GHG emissions. In addition, at this level of charging from renewable sources of electricity, all BEVs now outperform PHEVs. The environmental benefits of EVs depend critically on the electricity system from which they derive their power. Given the wide variation in the mix of electricity generation throughout the U.S., and even throughout the day with the adoption of intermittent sources of renewable energy, additional policy tools are needed to match places and times with high levels of renewables with EV charging. In particular, we suggest that 1) a regional approach to EV subsidies that can account for the emissions intensity of electricity systems may be more appropriate than the current blunt federal tax credit; and 2) adoption of time-of-use pricing that accounts for GHG impacts may be critical to supporting EVs as a GHG abatement tool. Currently, however, EVs are a relatively costly GHG abatement strategy. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Individual household <s> Abstract With the increasing penetration of distributed renewable energy generation and dynamic electricity pricing schemes, applications for residential demand side management are becoming more appealing. In this work, we present an optimal control model for studying the economic and grid interaction benefits of smart charging of electric vehicles (EV), vehicle-to-grid, and space heating load control for residential houses with on-site photovoltaics (PV). A case study is conducted on 1–10 net zero energy houses with detailed empirical data, resulting in 8–33% yearly electricity cost savings per household with various electric vehicle and space heating system combinations. The self-consumption of PV is also significantly increased. Additional benefits through increasing the number of cooperating households are minor and saturate already at around 3–5 households. Permitting electricity transfer between the houses and EV charging stations at workplaces increases self-sufficiency significantly, but it provides limited economic benefit. The additional cost savings from vehicle-to-grid compared to smart charging are minor due to increased battery degradation, despite a significant self-sufficiency increase. If the optimization is conducted without taking the battery degradation cost into account, the added monetary value of vehicle-to-grid can even be negative due to the unmanaged degradation. Neglecting battery degradation completely leads to overestimation of the vehicle-to-grid cost benefit. <s> BIB007

The smallest scale for EV/PV coupling to operate is in residential households. Several works have shown that EV/PV coupling could improve utilization of those two technologies aiming at leading houses towards the objective of zero net emissions over a year. Typically, EV charging becomes less costly while solar panels self-consumption increases. BIB006 exhibits the conditions, based on a case study in Hawai, that would lead EV total ownership cost to be lower than their plug-in hybrid EV or internal combustion engine vehicle competitors. Integrating EV in an electricity mix with high penetration of PV leads to much more attractive EV. BIB002 studies the domestic EV charging in house equipped with solar panels. The authors find that the effectiveness is highly dependent on the driver's mobility uses. Indeed, a long-range commuter does not take advantage of PV charging while private users with several daily travels at home benefit of almost 70% of PV energy in their EV charging. Short distance commuters have around 40% of PV energy in their EV charging. Electricity bill savings are rather low (around 100e per year) for everyone. Two case studies carried for Sweden and UK, BIB003 and BIB005 are developing a Markov chains stochastic model of energy consumption of households equipped with PV systems and owner of EV using real data. Both studies show a poor EV/PV synergy. Indeed, most EVs come back home and plug in the evening during the household peak load. As the authors do not define a smart charging, EVs only increase energy consumption and its standard deviation. In both cases, they point out that EV disturbances on local grid (measured by consumption standard deviation) are attenuated at large scale by statistical effects. For a microgrid of a residential district (from one to a dozen buildings) equipped with solar panels of 10kWp each, BIB007 seeks to minimize total operational power cost. These costs gather EV charging, V2G use, other types of electric consumption and heating. By solving the linear programming problem, the authors find a 8-33% cost reduction per building. Cooperation between buildings through direct power exchanges improves cost reductions. Moreover, V2G use allows an extension of self-consumption, but has a very limited effect on overall operational costs due to battery degradation costs. [30] formulates a mobility model (with Markov chains) and a predictive model for the house load and its PV electricity generation. The authors then develop a stochastic optimization model minimizing grid energy purchases. By considering respectively a Tesla S and a Nissan Leaf, annual cost reductions are -493% and -175%. We will note that those tremendous falls are due to vehicle-to-grid utilization, with high feed-in tariffs and without taking into account potential cost on the battery. It is also important to note that optimal charging is little coordinated with the PV generation profile. BIB004 combines a mobility model and a building electric consumption model to simulate the energy management of a "zero energy building". The authors quantify the benefits of Vehicle-to-House (V2H) for tens houses with one EV each. The authors then determine the sensibility of their results to PV capacity (20-90kWp) to the degree of cooperation of EV owners (i.e. their chosen state-of-charge fraction available for V2H). Two scenarios are simulated, with and without electric storage system (ESS). Depending on the installed capacity, solar panels provide 20-30% of houses loads. And depending on the authorized state-of-charge fraction for V2H, EV can provide up to 30% of house power. ESS reduces even more grid power imports. [32] models the electric system of a residential household (consumption, PV generation, ESS and EV). Once consumption is optimized, a rule-based decision making algorithm is used to manage EV and battery storage charging in function of the PV generation and electricity time-of-use tariffs. The proposed algorithm leads to a 62% decrease on the electricity bill. The individual effect of EV on the total strategy efficiency is difficultly tractable compared the stand-alone battery one, but its role is even greater that its initial state of charge is high. BIB001 uses empirical data of residential consumption and PV generation to assess the effect of a combination of house loads, EV recharging and solar panels. Using a commercial energy management system, the authors find a 30-50% reduction on the electricity bill. Having an EV almost double the electricity consumption of a household, so having an own energy source (as PV) makes sense. Nevertheless, it appears through the literature that a coupling of EV and PV inside households can be beneficial, but those benefits are bounded by the utilization of EV for mobility. Indeed, EV are usually away from home during the day and therefore cannot benefit from maximal generation of PV. As we will see later, additional energy flexibility source (as battery storage or thermal systems) is beneficial to the house energy management.

Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> Due to the increased penetration of electric vehicles (EVs) and photovoltaic (PV) systems, additional application for home/building energy management system (EMS) is needed to determine when and how much to charge an electric vehicle in an individual home/building. This paper presents a smart EV charging method for smart homes/buildings with a PV system. The paper consists of two parts: EV charging scheduling algorithm for smart homes/buildings and implementation of prototype application for home/building EMS. The proposed EV charging algorithm is designed to determine the optimal schedules of EV charging based on predicted PV output and electricity consumption. The implemented prototype application for home/building EMS can provide EV charging schedules according to user preferences. Numerical results are provided to demonstrate the effectiveness of the proposed smart EV charging method. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> In this paper, a new energy management strategy (EMS) for a dc distribution system in buildings is being proposed. The dc distribution system is considered as a prospective system according to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and fuel cells. Since the dc distribution system has many advantages such as feasible connection of DERs and electric vehicles (EVs), reduction of conversion losses between dc output sources and loads, no reactive power issues, it is very suitable for industrial and commercial buildings interfaced with DERs and EVs. The establishment of an appropriate EMS based on the economic point of view can reduce energy costs of buildings and provide benefits to participants in energy management. Applicable elements for the dc distribution system are identified and the real-time decision-making-based algorithm for minimizing operating costs is proposed in this paper. The EV service model for the EMS to offer incentive to EV owners who participate in battery discharging is described. To verify the performances of the proposed algorithm, computer simulation and economic analysis are being performed where the results show that the proposed EMS reduces energy costs, motivates EV owners, and can be applied to the dc distribution buildings. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> Plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) have high potential for reducing fuel consumption and emissions, and for providing a way to utilize renewable energy sources for the transportation sector. On the other hand, charging millions of PEVs could overload the power grid, increase emissions and significantly alter economic characteristics. A day-time photovoltaic (PV) based, plug-in electric vehicle charging station located in a workplace parking garage is considered in this research. The results show the impact of PV based workplace charging on the economics and emissions from the power grid. An optimal charge scheduling strategy is compared with an uncontrolled charging case to perform the economics and emissions analysis. Two locations (Columbus, OH and Los Angeles, CA) are selected such that the analysis includes different scenarios of yearly variation of solar radiation and finance structure. A high fidelity hourly simulation model for energy economic analysis is developed considering different types of vehicles, statistical data for driving distances, parking time, installation cost, tax rebates and incentives. An incremental parking rate for accessing the charging facility is considered for economic analysis for the garage owner and the vehicle owner. The analysis is extended to consider the impact of carbon tax implementation on the driver economics and shows the feasibility of such PV based charging stations. Parametric analysis for different parking rates and installed capacities show (i) the feasibility of a PV based workplace charging facility, (ii) benefits to the vehicle owner and the garage owner, and (iii) the need for an optimal charging controller. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> This paper discusses the charging of multiple plug-in hybrid electric vehicles (PHEVs) in an apartment building, equipped with a photovoltaic (PV) system. Different charging strategies and charging power ratings are examined, which are assessed in terms of their grid impact, the self-consumption of local electricity generation, and the electric driving range. The impact of a residential building, which incorporates electric vehicle (EV) charging, on the distribution grid can be significantly reduced by using simple EV charging strategies. These strategies include complementing night-time with day-time charging, peak shaving at vehicle level, and charging the surplus of local generation. Effective results are obtained using only the knowledge of the present battery state of charge, next departure time, and the instantaneous local generation surplus. The simultaneity of the EV charging and the PV production increases. The increase in electric driving range is negligible for three-phase charging. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> We present a model developed to study the increase of self-consumption of photovoltaic (PV) power by smart charging of electric vehicles (EVs) and vehicle-to-grid (V2G) technology. Whereas previous studies mostly use large EV fleets in their models, our focus is on a smaller scale. We apply the model to a microgrid in Lombok, a residential neighbourhood in the city of Utrecht, the Netherlands. The microgrid consists of a 31kWp PV installation, an office, internet servers, three households, and two EVs. Three control algorithms are presented which manage the charging profile of multiple EVs either in real-time or using linear optimisation with predictions for PV power and electricity demand. We perform one-year simulations using data for PV power, EV use, and electricity demand. Simulations results are evaluated on PV self-consumption and peak demand reduction. In addition, we make qualitative statements on battery degradation resulting from the charging strategies based on several indicators. We also simulate changes in microgrid composition, for example by including more EVs. In the simulations, self-consumption increases from 49% to 62–87% and demand peaks decrease by 27–67%. These results clearly demonstrate the benefits of smart charging EVs with PV power. Furthermore, our results give insight into the effect of different charging strategies and microgrid compositions. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> This paper addresses the problem of MicroGrid (MG) management including Electric Vehicles (EV) scheduling in a low voltage residential grid. The methodology uses a deterministic algorithm approach implemented in TOMLAB software. A realistic case study using a 25-bus MG with distributed generation, dispersed storage and EVs is provided to demonstrate the use of this application. The method runs in less than 1 second while minimizing operation costs and improving the benefits of the MG. The potential benefits of storage units are demonstrated when high PV power production is available. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> Integration of Electric Vehicles (EV) and Renewable Energy Sources (RES) with Energy Management System (EMS) in microgrid has been widely discussed. In this paper, we present a two-tier EMS system currently in operation on campus of University of California, Los Angeles. The upper level system, called Super Control Center (SCC), processes grid-wide energy coordination while the lower level systems manage local EV charging and other micro-scale services. We use the concept of Solar-to-Vehicle (S2V) to demonstrate the combined function of the EMS system. For lower level system, we present two queuing algorithms with user priority calculated from user's Solar-Friendliness Index (SFI), i.e., solar composition of user's energy consumption profile. The algorithms have shown capabilities to increase RES utilization, while actively encouraging users to change consumption behavior and promoting higher RES utilization. Simulation results have shown that SCC can reduce up to 73% of the EV load. The lower level EV coordination algorithms are able to increase the RES utilization from 0.504 to 1. <s> BIB007 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> Abstract The paper analyzes the introduction of a photovoltaic system satisfying electric, space heating and cooling demand of an office building located in southern Italy. The electric load is due to an electric heat pump, used to satisfy space heating and cooling load, a pure electric demand (personal computers, printers, lighting, etc.) and an electric vehicle charged during working hours. Dynamic simulations to evaluate the energy and environmental performance of the analyzed system considering different photovoltaic peak powers (4.5–9.0 kW), electric vehicle distance per day (40–120 km) and charging mode is carried out. The solar based system shows primary energy saving and equivalent carbon dioxide emission reduction higher than 40% in comparison to the reference conventional system based on a natural gas fired boiler, an electric chiller and a diesel car. The results highlight that the solar energy system is more competitive when DC charging system is provided. <s> BIB008 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> Abstract The electric vehicle (EV) can be utilized as a dynamically configurable dispersed energy storage in the vehicle-to-grid (V2G) and vehicle-to-building (V2B) operation mode to balance the energy demand between buildings and EVs. This paper proposes a mixed integer linear programming based collaborative decision model to study the energy sharing between a building and an EV charging station (CS). The building has its distributed generator, and electric and thermal energy storage, and the CS has its own renewable energy source. To model the V2G/V2B integration, we introduce three sets of decision variables to represent the energy exchange among building, CS and power grid. A set of parameters are introduced to model the driver behaviors, such as initial and desired state of charge level of EV battery, and available hours of EV, and sixteen different building categories (e.g., office, restaurant, hotel, warehouse, etc.) are studied. The impacts of driver behaviors and building categories on the economic performance of V2G/V2B integration are characterized and analyzed. The results from this research can recommend best V2G/V2B integration considering various driver behaviors and building categories which can provide valuable insight for smart community design. <s> BIB009 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Buildings <s> Corrections were made to author information for the paper, “Energy management system with PV power forecast to optimally charge EVs at the workplace,” (van Der Meer, D., et al), IEEE Trans. Ind. Informat., vol. 14, no. 1, pp. 311–320, Jan. 2018. <s> BIB010

In the case of individual houses, we saw that the benefits of EV/PV coupling could be limited by the absence of EV at maximal PV generation hours. EVs should be parked at this time, whether at workplaces or in other car parks. Given that those places should be able to host more EV, this would allow a better predictability, EV/PV coupling should be more efficient to improve self-consumption in commercial or workplace buildings. In a commercial building equipped with solar panels of 50kWp capacity, BIB001 seek to optimize the charging of 12 EV by minimizing total electricity costs. EV mobility is modeled simply with arrivals and departures hours, and arrival states of charge as fixed parameters. Smart charging defined on PV power predictions allows to reduce electricity purchases from 6 to 15% compared to an uncontrolled charging. BIB010 formulates an optimization problem for the charging management of 6 EV at a workplace equipped with 9kWp of PV capacity. Compared to an uncontrolled charge, optimized charging allows to improve self-consumption by 20-30%, while generating profits. BIB004 tests different EV charging strategies on a large residential building. Depending on the number of EV parked, self-consumption can be improved by 20%. BIB005 uses a linear programming algorithm to define EV smart charging on a microgrid of a residential neighborhood in a case study based on Dutch data. The microgrid has 31kWp PV capacity and 2-5 EV, which charging demand is estimated using probabilistic methods. The authors find an improvement of 13-38% of the microgrid self-consumption. BIB006 formulates an optimization problem to minimize the operational cost of an eco-district microgrid with EV and PV systems. Compared to an uncontrolled scenario, optimal management lowers operational costs by 70%. For a university campus microgrid, BIB007 develops an energy management framework in two levels: the first one deals with energy exchanges with the grid while the second one coordinates EVs charging. Such system allows to reduce imported power by 73 % and to improve selfconsumption by 50%. BIB008 develops a simulation model of power management of a workplace. By comparing a "classical" system (with an classical vehicle, gas and electricity heating) with a "green" one (with an EV, PV system and heat pump), the authors find a 40% diminution of primary energy consumption and 40% diminution of CO 2 emissions. BIB002 simulates the DC electric system of a building with 6kWp PV capacity and a parking hosting 8 EVs. Energy management follows a rule-based decision making algorithm allowing vehicle-to-grid. Compared to a dumb charging case, the smart charging leads to a decrease of 43% of charging costs. BIB003 assesses the economic feasibility of a workplace charging station with solar capacity. The authors use a stochastic dynamic optimization problem to define the smart charging EV that minimizes imported power from the grid. EV arrival and PV power are estimated using probabilistic methods . Compared to the uncontrolled charging with and without PV, smart charging with PV leads to a reduction of respectively 80 and 90% of imported power. Then, the authors develop an investment model to determine the pay-back time of the station depending on user tariff and regulatory tools. Two case studies are made in two US state with different solar irradiation and subsidy frameworks. The authors find that optimal charging matters when PV capacity is low. In BIB009 , an interesting sensibility study is made on categories of buildings electric loads. Based on sixteen building categories (e.g. offices, hotels, hospital, schools...), the authors show that the smart control strategy of a EV/PV building system bring costs reductions from 4% to 18%. Moreover, the coupling performance is highly dependent on the building type and its typical load profile. For instance warehouses, fast-food restaurants, hostels and supermarkets benefit the most of EV/PV synergy, contrary to hospitals and health care centers.

Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> An energy integration between photovoltaic (PV) systems and electric vehicles (EVs) can help to overcome problems related to the feasibility of a more sustainable mobility, most of all in urban context. This paper aims to examine the potential and the technical benefits of using PV systems as energy supplier for charging EVs. For this purpose, an urban scale integrated system is presented through a mathematical model that considers the power flows related to the PV generator, the charging station, and electric grid. The most significant result is the evaluation of the self-consumption in order to optimize the interaction between the PV system and charging station for EVs. This analysis has been conducted for different vehicles typologies and different PV installations, giving rise to more than 9000 different cases and allowing to consider the wideness of the self-consumption range for different months. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> Vehicle-to-grid (V2G) units are gaining prominence and may dominate the auto-market in the near future. The V2G batteries require corporate parking lots for charging and discharging operations. The electric power capacity of an existing parking lot can be increased by the installation of photovoltaic (PV) rooftops. This paper describes mathematical models for estimating the electric power capacity of a V2G parking lot (VPL) system with PV canopy. The electric vehicle (EV) demand/supply model was formulated as a queuing theory problem, exhibiting stochastic characteristics. New formulae were developed to address the impacts of battery charger efficiency on the amount of power demand during battery charging, and also how the latter is effected by inverter efficiency during discharging. Mathematical models for grid gain factor were developed. The proposed models were tested using Tesla Roadster EV and Nissan leaf EV. Promising simulation results are gained leading to a conclusion that vehicle parking lots with PV facilities can potentially relieve and enhance the grid capacity. Results show that 60% gain factor is possible. The effect of weather uncertainties and energy market price were studied. The study could be useful in battery-charger control studies. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> This paper aims to optimize the behavior of Plug-in Electric Vehicle (PEV) parking lots considering the interaction with renewable energy resources. In the proposed model, the PEV parking lot can participate in the day-ahead energy, spinning reserve and regulation markets. On this basis, behavior of the PEV parking lot in a distribution system is modeled consisting of wind and photovoltaic power producers. Moreover, the model consists of uncertainty characteristics of PEV behavior, wind and photovoltaic power generation. In order to tackle the mentioned uncertainties, a stochastic programming approach is utilized to maximize the profit of the parking lot operator. Numerical cases are studied to show the effectiveness of the proposed model. The results indicate that renewable energy resources have significant impacts on the behavior of PEV parking lot. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> The concept of solar parking lots aims at coupling the development of clean solar electricity and electric mobility. Solar panels provide shade and generate electricity to charge parked electric vehicles. In a vehicle-to-grid approach, the vehicles may also feed the grid and support it with ancillary services. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> This paper addresses a two-stage framework for the economic operation of a microgrid-like electric vehicle (EV) parking deck with on-site renewable energy generation (roof-top photovoltaic panel). This microgrid-like EV parking deck is a localized grouping of distributed generation (solar), energy storage (EV batteries), and load (EV charging load). Although EV parking decks can enable greater adoption of renewable energy sources by scheduling charging loads to coincide with periods of strong sun, the inherent intermittency of renewable energy resources and variable EV parking behaviors complicates the economic operation. In this paper, the proposed first stage of this framework provides the parking deck operators with a stochastic approach for dealing with the uncertainty of solar energy so as to make an optimal price decision (marginal electricity sale price and parking fee rebate) at the day-ahead time scale. The second stage introduces a model predictive control-based operation strategy of EV charging dealing with the uncertainty of parking behaviors within the real-time operation. Case studies demonstrate the better performance of the proposed framework, offering an effective day-ahead marginal electricity price for tomorrow's operation and increasing the microgrid-like EV parking deck's revenue during the real-time operation. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> Photovoltaic-assisted charging station (PVCS) is expected to be one of the important charging facilities for serving electric vehicles (EVs). In this paper, a type of solid-state transformer (SST) is introduced to the PVCS design and an autonomous energy management strategy (EMS) for SST is proposed. This study aims to develop an effective real-time EMS for PVCS participating in ancillary service of smart grid, and the rule-based decision-making method is utilized. Considering the dynamic classification of EVs, an energy-bound calculation (EBC) model is proposed to find the upper and lower bounds of flexible resources. Moreover, considering the EBC results and power command from the aggregator, a charging power allocation algorithm is designed for power distribution of flexible EVs. By case study and experiment analysis, the proposed EMS is effective in real-time energy management and suitable for practical applications. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> A real-time price (RTP)-based automatic demand response (ADR) strategy for PV-assisted electric vehicle (EV) Charging Station (PVCS) without vehicle to grid is proposed. The charging process is modeled as a dynamic linear program instead of the normal day-ahead and real-time regulation strategy, to capture the advantages of both global and real-time optimization. Different from conventional price forecasting algorithms, a dynamic price vector formation model is proposed based on a clustering algorithm to form an RTP vector for a particular day. A dynamic feasible energy demand region (DFEDR) model considering grid voltage profiles is designed to calculate the lower and upper bounds. A deduction method is proposed to deal with the unknown information of future intervals, such as the actual stochastic arrival and departure times of EVs, which make the DFEDR model suitable for global optimization. Finally, both the comparative cases articulate the advantages of the developed methods and the validity in reducing electricity costs, mitigating peak charging demand, and improving PV self-consumption of the proposed strategy are verified through simulation scenarios. <s> BIB007 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> The emerging interest in deployment of plug-in electric vehicles (PEVs) in distribution networks represents a great challenge to both system planners and owners of PEV-parking lots. The owners of PEV-parking lots might be interested in maximizing their profit via installing charging units to supply the PEV demand. However, with stringent rules of network upgrades, installing these charging units would be very challenging. Network constraints could be relaxed via controlling the net demand through integrating distributed generation (DG) and/or storage units. This paper presents an optimization model for determining the optimal mix of solar-based DG and storage units, as well as the optimal charging prices for PEVs. The main objective is to maximize the benefit of the PEV-parking lot's owner without violating system constraints. Two cases are considered in this paper: uncoordinated and coordinated PEV demand. A novel mathematical model is further developed whereby the behavior of vehicles’ drivers, in response to different charging prices, is considered in generating the energy consumption of PEVs. <s> BIB008 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> Abstract Electric vehicles are one of the concepts towards green and sustainable transportation systems. However, several uncertainties with respect to electricity demand and availability of electric vehicles as well as electricity supply by renewable energy sources influence an optimal scheduling through smart charging strategies. This paper investigates the possibilities to integrate additionally loads of uncertain renewable energy sources by smart charging strategies of three different electric vehicle fleets – namely, commercial customers, commuters, and opportunity parkers. Therefore, data from an empiric field test with a public charging infrastructure in a parking garage with a photovoltaic system is taken. Various strategies are analyzed, considering the changing individual electricity demand, restrictions and parking times of electric vehicle fleets by combining a Monte Carlo simulation approach with different methodologies like a heuristic algorithm, an optimization model and stochastic programming. The numerical results indicate that the domestic photovoltaic generation of the car park can be fully used by the electric vehicle fleets for charging and the utilization of photovoltaic can be doubled when comparing uncontrolled and optimized charging strategies. The commuter fleet has the highest CO2 emission reduction potential of all three electric vehicle fleets. Moreover, load management decreases costs, even when uncertainties are considered. <s> BIB009 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> Abstract The direct coupling of solar electricity with electric vehicles is very topical due to their many co-benefits, such as reducing the electricity flow into and from the grid. This work illustrates this solution using a park-and-ride lot outside Lisbon to study its solar energy potential to charge electric vehicles. The solar resource, its exploitation and coordination with the vehicles under different charging approaches are discussed in detail, using methods extendible to other cases. A genetic-algorithm optimisation was performed to maximise the solar energy generation. To ascertain the techno-economic feasibility of the concept, several sets of assumptions are analysed, including the use of energy storage and smart charging. For current market conditions, the payback time is found to be 14 years; a modest financial public incentive would significantly improve the project economics – making the payback drop to 7 years – enabling conditions to put this inescapable and green approach at the forefront. <s> BIB010 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Charging station <s> Workplace charging of electric vehicles (EVs) from photovoltaic (PV) panels installed on an office building can provide several benefits. This includes the local production and use of PV energy for charging the EV and making use of dynamic tariffs from the grid to schedule the energy exchange with the grid. The long parking time at the workplace provides the chance for the EV to support the grid via vehicle-to-grid technology, the use of a single EV charger for charging several EVs by multiplexing and the offer of ancillary services to the grid for up and down regulation. Further, distribution network constraints can be considered to limit the power and prevent the overloading of the grid. A single mixed integer linear programming (MILP) formulation that considers all the above applications has been proposed in this paper for a charging a fleet of EVs from PV. The MILP is implemented as a receding-horizon model predictive energy management system. Numerical simulations based on market and PV data in Austin, TX, USA, have shown 32% to 651% reduction in the net cost of EV charging from PV when compared to immediate and average rate charging policies. <s> BIB011

This section focuses on the study of EV/PV complementarity in the case of charging stations with on-site photovoltaic capacities (noted hereafter PVCS) or off-site PV powerstation. Their purpose can be commercial or professional (e.g. included in workplace). The number of vehicles involved in PVCS management are diverse, since charging station goes from a small workplace station to groups of large car parks. BIB004 provides a review of PVCS main techno-economic features with its environmental benefits. In a Monte-Carlo simulation, BIB001 shows that the potential shares of PV energy in EV charging goes from 1% to 72% without smart charging. Season and charging power are shown to be the main determinants of the efficiency of the PV-based charging. Indeed, lower power increases charging duration and so PV selfconsumption. BIB006 considers novel technology of solid-state transformers to enable a flexible management of a PVCS. The authors develop a rule-based decision making model in order to participate in ancillary services for the grid. In their case study of PVCS hosting 60 EVs with a 200kWp PV capacities, the authors find an increase of 200% of revenues from ancillary services compared to their uncontrolled case without PV. BIB007 proposes an architecture for a PVCS with a hybrid control strategy. The authors' smart control strategy leads to a reduction of 50-75% charging costs, while increasing PV self-consumption by 20%. The strategy does not particularly improve charging completion which stays stable at 85-92%. BIB008 seeks to maximize profits of a PVCS investor while considering the grid constraints. By optimizing the net present value of the PVCS revenues and costs, the authors find that a smart charging of a fleet allows to increase owners' benefits by 10% while reducing 40% of the peak load. In their study of a small charging station equipped with up to 10kWp PV capacity for a single EV (Nissan Leaf or Tesla Roadster), BIB002 develops a technical model to introduce a smart strategy. Results show that vehicle-to-grid capabilities can be enhanced by 60% for ancillary services. For a PVCS able to host 300 EVs, BIB005 develops an operational framework in two steps to maximize the PVCS profits. A first stochastic algorithm calculates day-ahead grid power need and marginal electricity cost for the PVCS. A second one optimizes the charging scheduling of EV. This smart strategy increases revenues of the PVCS operator by 3% in summer and 10% in winter. BIB009 considers a charging station hosting until 650 EVs and equipped with 100kWp of solar capacity. The EVs are divided in three categories that define their charging profile: commercial (night charging), commuter (long day charging) and opportunity (short day charging). The authors minimize the grid power purchases using different optimization algorithms (deterministic and stochastic). Compared to the uncontrolled strategy, the smart strategies double the PV utilization while reducing grid power costs by 86-95%. BIB010 studies the technical and economic feasibility of a PVCS from an investor point of view. The authors investigate various designs of the PVCS management (control strategies, storage). From real parking data, the daily correlation between occupation profile and PV generation is found to be between 36% and 59%. In every case, the PVCS is found to be profitable with a 7% interest rate on the period covering PV modules lifetime. BIB003 has been studying the management of an EV parking lot in ancillary markets (regulation and reserve). Revenues of the PVCS are defined by the addition of participation of EVs in various ancillary markets (energy, spinning reserve, regulation up and down) and the charging and parking fees of EVs. Costs gather the purchased power from the grid, penalties in markets, payment to EVs owners for their vehicle-to-grid services, and battery degradation costs. In one of their case study, the distribution system handle a 200kWp solar farm. In this situation, the authors find expected profits 5.3% higher than in the case without renewable energy. In this scenario, PV and EV systems interact indirectly in energy markets. Noticeably, regulations market revenues decrease (-2.6%), while reserve revenues remain steady and energy market revenues increase (+10%). BIB011 develops a management model in which the PVCS is able to use vehicle-to-grid capabilities, to trade energy and offer reserve services to the grid. The optimal strategy leads to a 250% decrease on the PVCS energy costs on average along the year.

Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Electric vehicles (EVs) are a key technology to reduce dependency on oil imports as well as to diminish environmental effects of individual transportation. Especially in megacities like Singapore where travel distances are moderate, this new mode of transportation is often discussed as a future option. This paper investigates possible effects of large scale EV integration on the power supply system. A unit commitment model combined with an integrated approach for smart charging is used. The mixed-integer linear programming (MILP) formulated unit commitment algorithm cooptimizes energy, regulation, and spinning reserve power. The effects of different charging strategies on the power plant scheduling are analyzed. The power system infrastructure is kept at status quo in a baseline scenario and extended to future scenarios with intermittent photovoltaics (PV) power. Effects on power plants scheduling are evaluated by measuring resulting variable cost of electricity as well as CO2-emissions. Moreover, effects of EVs providing regulation and spinning reserve by controllable charging are investigated. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Abstract A number of studies have found that plug-in hybrid electric vehicles (PHEVs) will have relatively small grid impacts if charged with overnight off-peak electricity. However the greatest petroleum displacement will occur if vehicles are allowed to charge during the day, maximizing all-electric range. Charging during the day also allows for a smaller, lower cost battery. Mid-day charging will add to peak electricity demands and may occur in locations where it is difficult to construct new generation and transmission capacity. Solar photovoltaics (PV) provide an option to provide mid-day peaking capacity. Mid-day charging of PHEVs also may absorb low value or even curtailed PV generation during periods of low demand. This study identifies possible co-benefits of large scale PV and PHEV deployment by simulating the Texas grid and identifying changes in peak capacity requirements and PV curtailment. A modest deployment of PV is able to avoid most of the increase in capacity requirements associated with very large PHEV penetrations. PHEVs are also able to reduce curtailment at high PV penetration, especially if charging can be controlled to improve the coincidence of consumer charging demand with normal PV generation patterns. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Electric vehicles (EVs) and renewable energy sources offer the potential to substantially decrease carbon emissions from both the transportation and power generation sectors of the economy. Mass adoption of EVs will have a number of impacts and benefits, including the ability to assist in the integration of renewable energy into existing electric grids. This paper reviews the current literature on EVs, the electric grid, and renewable energy integration. Key methods and assumptions of the literature are discussed. The economic, environmental and grid impacts of EVs are reviewed. Numerous studies assessing the ability of EVs to integrate renewable energy sources are assessed; the literature indicates that EVs can significantly reduce the amount of excess renewable energy produced in an electric system. Studies on wind–EV interaction are much more detailed than those on solar photovoltaics (PV) and EVs. The paper concludes with recommendations for future research. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> ABSTRACT This article presents a feasibility analysis on photovoltaic (PV)-powered electric vehicles (EVs) by considering the technological and economic aspects involved. The analysis is based on the fact that EVs can provide energy storage capacity to efficiently utilize the intermittent electricity output of a PV system, thus reducing greenhouse gas (GHG) emissions. Four models of power supply systems are assessed based on net present cost, cost of energy, and carbon emission intensity. The renewable energy optimization software HOMER is used to optimize costs of energy and carbon emission, with the outcome of energy cost ranging from $0.204/kWh to $0.372/kWh for the PV-powered EVs. Although the energy cost of PV-powered EVs is higher than that of grid-connected ones ($0.150/kWh), GHG emissions can be reduced by 47%–78%. The four models may be applicable in different development stages of EVs in China. In the short term, grid-connected EVs have cost advantages, however, considering such factors as the i... <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Abstract This paper proposes a collaborative strategy between the photovoltaic (PV) participants and electric vehicle (EV) owners to reduce the forecast uncertainties and improve the predictability of PV power. The PV generation is predicted using an auto regressive moving average (ARMA) time series model. Fuzzy C-means (FCM) clustering is used to group the EVs into fleets with similar daily driving patterns. Uncertainties of the PV power and stochastic nature of driving patterns are characterized by a Monte Carlo simulation (MCS) technique. A particle swarm optimization (PSO) algorithm is developed to optimally use the vehicle-to-grid (V2G) capacities of EVs and minimize the penalty cost for PV power imbalances between the predicted power and actual output. The proposed method provides a coordinated charging/discharging scheme to realize the full potential of V2G services and increase the revenues and incentives for both PV producers and EV drivers. An economic model is developed to include the V2G expenses and revenues to provide a complete picture of the cost–benefit analysis. The proposed model is used to evaluate the economic feasibility of V2G services for PV power integration. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Abstract Electric vehicles are expected to greatly increase their market share in the near future. Their impact on the energy system will depend also on the way electricity will be generated. Renewable energy sources and intelligent control strategies will offer relevant solutions to mitigate that impact. In this paper we study the combination of photovoltaic energy and electric vehicles under uncontrolled charging regime and under the application of smart charging and vehicle-to-grid strategies. The analysis assumes different levels of photovoltaic generation and different penetrations of the electric vehicles. The assignment is carried out by means of an open source linear optimization model named EVLS, which simulates the interactions between the electric vehicles and the upstream energy system by considering market, technical and behavioral constraints. The results show that a high photovoltaic capacity could cover only a small portion of the transportation demand, if the charge is uncontrolled. In such a case, the non-photovoltaic generation would be required to severely ramp up in the late afternoon hours. An intelligent control of the charge could better accommodate the photovoltaic energy and reduce the ramps. The vehicle-to-grid could additionally help harnessing the photovoltaic energy to shave the peaks of the conventional load profile. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> This paper presents a comprehensive review and assessment of the latest research and advancement of electric vehicles (EVs) interaction with smart grid portraying the future electric power system model. The concept goal of the smart grid along with the future deployment of the EVs puts forward various challenges in terms of electric grid infrastructure, communication and control. Following an intensive review on advanced smart metering and communication infrastructures, the strategy for integrating the EVs into the electric grid is presented. Various EV smart charging technologies are also extensively examined with the perspective of their potential, impacts and limitations under the vehicle-to-grid (V2G) phenomenon. Moreover, the high penetration of renewable energy sources (wind and photovoltaic solar) is soaring up into the power system. However, their intermittent power output poses different challenges on the planning, operation and control of the power system networks. On the other hand, the deployment of EVs in the energy market can compensate for the fluctuations of the electric grid. In this context, a literature review on the integration of the renewable energy and the latest feasible solution using EVs with the insight of the promising research gap to be covered up are investigated. Furthermore, the feasibility of the smart V2G system is thoroughly discussed. In this paper, the EVs interactions with the smart grid as the future energy system model are extensively discussed and research gap is revealed for the possible solutions. <s> BIB007 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Summary ::: ::: The combined use of clean technologies can lead amongst other benefits to reduced environmental impacts, improved system efficiencies, better management of land scarcity, and diminishment of the effect of power variability of intermittent clean energy sources. Nonetheless, private investors facing budgetary constraints will only opt to invest in the combination of technologies if the latter is more profitable than the investment in a single technology. The aim of the paper is to provide a systematic model for decision makers that allows them to evaluate the profitability of any random combination of technologies under budgetary constraints, and to compare this profitability with that of the individual projects in isolation. This research goes beyond the state of art in the field of financial management and more specifically in the field of the rationing of capital amongst interdependent projects, by developing a method to calculate the payoff of interdependent projects undertaken together. Moreover, this paper develops a computational model from the investor's point of view, of which the purpose is threefold: First, the model allows to directly compare the economic payoff of individual complementary technologies with the economic payoff of their integrated combination, under budgetary constraints. Second, the model calculates economic synergies labeled ‘benefits of combined technologies’ (BOCT) when combining complementary technologies. Third, the model explains the rationalization behind the presence of BOCT. The model exemplifies an ex ante cost benefit analysis developed for business and non-governmental use. A four step methodology is proposed and illustrated by means of a case study of PV solar power and battery electric vehicles (BEVs) for a small Belgian enterprise. Results show that at low electricity prices ( €0.134/kWh), investment in exclusively PV becomes most attractive. In all other cases, it is more profitable to invest in the combination of both technologies. Copyright © 2015 John Wiley & Sons, Ltd. <s> BIB008 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> Electric vehicles (EVs) represent one of the most promising technologies to green the transportation systems. An important issue is that high penetration of EVs brings heavy electricity demand to the power grid. One effective way to alleviate the impact is to integrate local power generation such as renewable energy sources (RESs) into charging infrastructure. Because of the intermittent and indispatchable nature of RESs, it becomes very challenging to coordinate EVs charging with other grid load and renewable generation. In this paper, EVs charging problem in the presence of smart grid technologies is investigated, and the interaction with renewable energy is reviewed. An overview about EVs and RESs is first presented, which mainly introduces major types of EVs and renewable energy estimation methods. Then, according to the objectives, the existing research works are divided into three categories: cost-aware, efficiency-aware, and emissions-aware interactions between EVs and RESs. Each category׳s discussion includes the description of core ideas, summarization of solutions, and comparison between different works. Finally, some key open issues about EVs interacting with RESs are given and some possible solutions are also discussed. <s> BIB009 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> This research uses multi-objective optimisation to determine the optimal mixture of energy and transportation technologies, while optimising economic and environmental impacts. We demonstrate the added value of using multi-objective mixed integer linear programming (MOMILP) considering economies of scale versus using continuous multi-objective linear programming assuming average cost intervals. This paper uses an improved version to solve MOMILPs exactly. To differentiate optimal solutions with and without subsidies, the impact of policy on the Pareto frontier is assessed. We distinguish between minimising economic life cycle costs (complete rationality) and required investments (bounded rationality). The approach is illustrated using a Belgian company with demands for electricity and transport. Electricity technologies are solar photovoltaics and the grid; transportation includes internal combustion engine vehicles, grid powered battery electric vehicles (BEVs), and solar-powered BEVs. The impact of grid powered BEVs to reduce GHG emissions is limited, yet they are less costly than solar panels to decrease emissions. Current policy measures are found to be properly targeting rational investors who consider life cycle costs, while private (potentially bounded rational) investors often focus on required investments only. <s> BIB010 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> The next-generation electricity grid (“smart grid”) is expected to integrate interoperable technologies – particularly in the energy, transport, information and communication fields – with the aim to increase reliability, affordability and sustainability considering at the same time, distribution systems and market operation. In order to gauge the implications of the anticipated paradigm shift for the electricity system, new reference architectures and assessment methodologies shall be developed to properly capture the interactions between the different actors (especially utilities, operators, energy aggregators, end-users, etc.) and technologies to value and allocate the costs and benefits of such transformation. Against this background, this paper proposes a conceptual architecture and an assessment framework to explore how high penetration scenarios of electric vehicles and intermittent renewable generation can complement each other in emerging distribution networks. We start from the identification of the smart grid functionalities to be implemented in a system with distributed power injections under the need to supervise and coordinate myriads of decentralized and interoperable energy sources and actors. Relying upon the proposed smart grid conceptual architecture, we develop an assessment framework to maximize the renewable electricity and electric vehicle penetration for given electricity and transport systems. The application of the proposed assessment framework to a realistic case study, representing the distribution and mobility systems of a typical mid-size Italian city, illustrates how some of the limitations and constraints of the current electricity network operation and design approaches can be addressed and overcome. We show how integration of substantial amounts of energy production and electric-based transport technologies can be achieved while improving the reliability and sustainability performances of the emerging power systems. <s> BIB011 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> SUMMARY ::: ::: In recent years, there has been a growing interest in ecofriendly technologies such as residential photovoltaic (PV) systems and electric vehicles (EVs). PV systems and EVs will contribute to reducing CO2 emissions in the residential sector and the transportation sector, respectively. In spite of that, high penetration of PV systems into the power grid can cause grid voltage and frequency stability problems. Also, the growth of the EV market will create an extra electricity load (for charging the EV fleet), leading to an increase in power utility fuel costs. In this research, we proposed the usage of the PV-EV system as a method of mitigating the impact the spread of residential PV systems and EV on the power grid. We built an PV-EV system simulation model and investigated the PV-EV system contribution to the balance of power supply and demand and to reducing the total cost of the household under different electricity pricing scenarios. We also evaluated the effect of uncertainty in the forecasting of load and PV output on the performance of the PV-EV system. <s> BIB012 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> This study assesses the impact of electric vehicle (EV) uptake and large-scale photovoltaic (PV) investment on the economics of future electricity-generation portfolios. A Monte-Carlo-based portfolio modeling tool was used to assess the expected overall industry cost, associated cost uncertainty, and CO2 emissions of future generation portfolios, where both EVs and PV generation have achieved major deployment. The Australian National Electricity Market (NEM) was used as a case study under uncertain future fuel and carbon prices, electricity demand, and plant capital costs. Two EV charging scenarios were considered: 1)unmanaged charging which commences immediately as the EVs arrive at suitable charging infrastructure and 2)managed charging where EV charging loads are managed so that they better align with PV output. Results show that there are potentially valuable synergies between PV generation and EV charging demand in minimizing future electricity industry costs, cost uncertainties, and emissions, particularly when EV charging loads can be managed. The value of PV generation and managed EV charging is greater for higher EV fleet size and moderate carbon prices. <s> BIB013 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Territory <s> This paper investigates the impact of the grid integration of roof-top Photovoltaic (PV) generation and Electric Vehicles (EVs) energy storage on the demand response. The risk indices are introduced and risk map is created to predict the potential weather impact on the PV and EV owners. Based on the predictions, the aggregator bidding strategies to enable the EVs' stored energy and PV generation to participate in the ancillary service market considering the stochastic behavior are proposed. The role of programs for demand-side management (DSM) (in daily operation) and outage management (OM) (when fault happens) to mitigate the negative weather impacts on the customers is explored. Numerical experiments are implemented to validate the proposed approach and illustrate the impact of PV generation and EVs' ability to charge/discharge the stored energy on the flexibility of the electricity customer demand. <s> BIB014

In this section, we review articles dealing with EV/PV synergy at territory scale. This scale is much larger than the previous ones. It can be cities, regions, countries. Territories can host a large number of EV and high PV capacities. The power grids considered are large distribution grids and transmission grids. The issue of coupling EV and PV matters to companies with substantial transport and electricity needs. BIB008 BIB010 BIB004 look at joint investments possibilities in EV and PV. For a Chinese taxi company, BIB004 uses an energy optimization model to assess the net present cost and carbon footprint of combinations of technologies (including grid and PV power, EV and battery storage). The authors find that grid-powered EV are the most economical option compared to PV powered EV but emit twice as much carbon. Nevertheless, high carbon taxes, higher feed-in tariffs and lower interest rates are as many parameters that can make PV powered EV more profitable. In every case, PV-grid power is always less costly than PV-battery power as the one power source, although it emits more than twice less CO 2 than PV-grid power. BIB008 develops a detailed decision model to calculate the economic payoffs of investments in EV or/and PV technologies. Synergy between EV and PV is defined by the difference between net present value of optimal combination in PV and EV, with the sum of net present value of optimal investment in separated technologies. In the scope of managing a company fleet and electricity provision, the authors find a synergy of investment in EV or PV. The key parameter appears to be the grid electricity price. Indeed, EV/PV synergy only exists in a specific range of this price. Below the lower bound of grid price of this range, investing in EV only is more beneficial, while PV technology dominates above the upper bound. In a case study focused on Singapour, BIB001 models an aggregator controlling a large EV fleet (up to 600 000 vehicles) recharge. The aggregator formulates a unit commitment problem in order to organize power generation of conventional plants with renewable energy sources and different ancillary services. EV charging is incorporated with 8 different levels of complexity. If the authors conclude that smart charging only has marginal effects on overall mix CO 2 emissions, they show that it is crucial to cover regulation power needed by strong PV capacities penetration. An increased EV share in the automobile fleet permits a larger penetration of PV energy. For EV owners, smart charging leads to a reduction of individual charging cost. Moreover, CO 2 emissions decrease by 30%. In the US context, BIB002 exhibits prospective load curves of a power system with a large penetration of plug-in hybrid EV (for now preferred to battery EV in the US) and PV capacities (up to 10%). Smart charging, synchronized with maximum PV generation times is evaluated by the peak load reduction and PV energy curtailment (due to over-generation). A 50% penetration of plug-in hybrid EV in the automobile fleet allows to reduce curtailments by 5-6% and a smart charging to extend this reduction by 1-2%. BIB005 determines a collaborative strategy between PV energy producers and EV fleets. PV generation is scheduled in day-ahead with the system operator. The producer sends its generation prevision and is penalized depending on its error of predictions. Vehicle-to-grid support of EV allows producers to diminish their penalties. A particle-swarm optimization algorithm uses vehicle-to-grid capacity of an EV fleet to fill the gap between the scheduled PV power and the actual one. Over a year, the authors find a reduction of penalties by a factor from 2 to 10 for PV producers. Nevertheless, this strategy does not seem to bring any benefits to vehicle-to-grid participants, while they support for their own battery degradation. BIB011 seeks to maximize EV and PV penetration in a mid-sized city. Maximal penetration is defined by the distribution grid technical limits. The authors develop a control strategy in case study simulating a smart grid of midsize city. Compared to an uncontrolled charge, the authors find an increase of the maximal EV and PV penetration of 64% and 18%. In this case, joint CO 2 emissions from transport and power sector lower by 28%. BIB012 investigates the large scale effects of EV/PV couplings in households by extrapolating the results of a study of 51 Japanese households equipped with PV systems and EV. Based on a rule-based decision making algorithm including optimization tools the authors demonstrate a global cost decrease of 17% in the power system. BIB006 determines the effects of a large combined deployment of EV and PV (respectively up to 2M and 620MWp ) in a province of northern Italy. The authors consider three charging strategies (uncontrolled, controlled with and without vehicle-to-grid), and formulate a unit commitment problem in the new electric mix. The authors demonstrate beneficial effects in term of peak load and ramping reductions. BIB013 runs Monte-Carlo simulations to estimate the different determining characteristics of joint EV/PV development in Australia. Simulations use as inputs technology costs, fossil fuels prices and carbon price. The authors demonstrate that beyond a certain level of carbon tax (typically 50$/tC) and with high EV fleet size, PV become economically viable in the electricity mix. BIB014 studies the behavior of an aggregator in two ancillary markets (demand-side management and outage management). 15000 EVs and 140MWp PV capacity are included into a distribution system composed of variable weather-sensible zones. Demand-side management strategy is found to reduce 94% of customer costs increase induced by weather changes. At large scales, EV fleet can also interact with other renewable energy sources such as wind power. As EVs and PV capacities are aggregated at high level, aggregators or virtual power plant managers could include these supplementary capacities. Multiple studies have investigated the possible synergies of wind power and electric vehicles BIB003 BIB007 BIB009 . Hence EV/PV synergy seems then less specific at these scales.

Interactions between electric mobility and photovoltaic generation: A review <s> Electric storage system <s> Integration of PV system and EV charging station is a typical way of spot utilization of renewable resources. Research on multi-objective optimal scheduling of a DC micro-grid consisted of PV system and EV charging station is proposed in this paper. Based on the structure of the DC micro-grid and function of each component, mathematical model of the multi-objective optimization for scheduling, which takes the cost of electricity purchasing and energy circulation of storage batteries as objective functions, is built and then solved with NSGA-II. Constraints including EVs' charging time, the range of the charging/discharging power and SOC (state of charge) of the storage batteries, power supply rate of the distribution network and system power balance are considered in this model. A certain case is studied and verifies the model's rationality. Compared with an instant charging plan, the optimal scheme is superior in reducing cost and energy circulation. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Electric storage system <s> Depending on charging locations and business interest, EV charging facilities can be classified into Commercial Charging (CC), Business Charging (BC), and Home Charging (HC). Diurnal load distribution in these EV charging facilities can be dissimilar on the account of different customers' behavior. Therefore, these charging facilities will put forth different degrees of impacts on the grid. A portion of the impacts can be reduced through onsite deployment of PV and battery energy storage system (BESS). However, the extent of reduction depends on wide range of factors such as costs, correlation between EV load profile and PV generation profile etc. Thus, the objective of this work is to find out which one of these three charging facilities is technically and economically feasible for PV and BESS based EV charging station. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Electric storage system <s> This paper investigates the possibility of charging battery electric vehicles at workplace in Netherlands using solar energy. Data from the Dutch Meteorological Institute is used to determine the optimal orientation of PV panels for maximum energy yield in the Netherlands. The seasonal and diurnal variation in solar insolation is analyzed to determine the energy availability for EV charging and the necessity for grid connection. Due to relatively low solar insolation in Netherlands, it has been determined that the power rating of the PV array can be oversized by 30% with respect to power rating of the converter. Various dynamic EV charging profiles are compared with an aim to minimize the grid dependency and to maximize the usage of solar power to directly charge the EV. Two scenarios are considered – one where the EVs have to be charged only on weekdays and the second case where EV have to be charged all 7days/week. A priority mechanism is proposed to facilitate the charging of multiple EV from a single EV–PV charger. The feasibility of integrating a local storage to the EV–PV charger to make it grid independent is evaluated. The optimal storage size that reduces the grid dependency by 25% is evaluated. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Electric storage system <s> This paper presents a novel framework for designing an electric vehicle charging facility (EVCF) as a smart energy microhub from the perspectives of both an investor and a local distribution company. The proposed framework includes a vehicle decision tree, a queuing model, a distribution margin assessment model, a distributed generation (DG) penetration assessment model, an economic assessment model, and a distribution operations model. Three design options for the EVCF are examined; battery energy storage systems (BESS), renewables based DG, and a microhub that incorporates both BESS and renewables-based DG with the option of exchanging power with the main grid. Test results considering a 33 bus distribution system and realistic vehicle statistics extracted from the 2009 (US) National Household Travel Survey are presented and discussed. The findings demonstrate the effectiveness of the proposed smart energy microhub design framework. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Electric storage system <s> Abstract The direct coupling of solar electricity with electric vehicles is very topical due to their many co-benefits, such as reducing the electricity flow into and from the grid. This work illustrates this solution using a park-and-ride lot outside Lisbon to study its solar energy potential to charge electric vehicles. The solar resource, its exploitation and coordination with the vehicles under different charging approaches are discussed in detail, using methods extendible to other cases. A genetic-algorithm optimisation was performed to maximise the solar energy generation. To ascertain the techno-economic feasibility of the concept, several sets of assumptions are analysed, including the use of energy storage and smart charging. For current market conditions, the payback time is found to be 14 years; a modest financial public incentive would significantly improve the project economics – making the payback drop to 7 years – enabling conditions to put this inescapable and green approach at the forefront. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Electric storage system <s> Electric vehicle (EV) charging stations will play an important role in the smart city. Uncoordinated and statistical EV charging loads would further stress the distribution system. Photovoltaic (PV) systems, which can reduce this stress, also show variation due to weather conditions. In this paper, a hybrid optimization algorithm for energy storage management is proposed, which shifts its mode of operation between the deterministic and rule-based approaches depending on the electricity price band allocation. The cost degradation model of the energy storage system (ESS) along with the levelized cost of PV power is used in the case of EV charging stations. The algorithm comprises of three parts: categorization of real-time electricity price in different price bands, real-time calculation of PV power from solar irradiation data, and optimization for minimizing the operating cost of EV charging station integrated with PV and ESS. An extensive simulation study is carried out with an uncoordinated and statistical EV charging model in the context of Singapore to check effectiveness of this algorithm. Furthermore, detailed analysis of subsidy and incentive to be given by the government agencies for higher penetration of renewable energy is also presented. This work would aid in planning of adoption of PV-integrated EV charging stations, which would expectedly replace traditional gas stations in future. <s> BIB006

Stand-alone battery electric storage system (ESS) is an obvious technology when it comes to variable distributed renewable sources. It allows a system equipped with variable energy capacities to increase its self-consumption. In the selected literature, EV/PV systems often incorporate a ESS as an additional flexibility source. We already pointed out that ESS can be beneficial in EV/PV systems especially when EV charging demand and PV generation profile were uncoordinated, or when EV demand were not flexible BIB006 . A few papers give us some insights on the actual added value of ESS in those systems. [60] studies the influence of adding an ESS in a PVCS. The authors maximize PV self-consumption using an optimization algorithm. Among their results, they show that the ESS could increase charging completeness and decrease overall grid electricity exchange by a factor two. BIB004 builds a PVCS architecture from several modules estimating EV arrival and charging need and maximal PV capacity and chargers power respecting grid constraints. Then, the authors minimize the facility net investment costs. Operational functioning are designed to minimize power losses of the PVCS. One main result in the paper shows that an optimal charging station with battery storage reaches twice a better return on investment than a station only equipped with PV. In comparison with EV charging demand profiles of BIB002 for PVCS, the authors analyze the sensibility of their strategy performance to a 50% decrease ESS and PV costs. Results show that when EV charging demand is more desynchronized with the PV generation profile, ESS reduction costs appears to be more advantageous to the EV/PV system than PV costs reductions. BIB003 studies the influence of an ESS capacity on the performance of the smart control strategy of a building EV/PV system. The authors show that ESS efficiency is exponentially decreasing. 13% of the power grid reduction is achieved with ESS capacity of 10kWh, while maximal reduction is about 25%, reached with a 30kW ESS. For a DC PVCS with stand-alone ESS, BIB001 co-optimizes charging costs and ESS use (i.e. number of discharging of ESS). The authors enlighten the trade-off between both objectives. Disregarding ESS use leads to a cost reduction of half while disregarding cost minimization lowers energy use in ESS up to 90%. Already discussed earlier, the authors of BIB005 show in their study of a PVCS from an investor point of view, that due to its investment high costs, stationary storage led to profit loss of 58%. Nevertheless, as the battery price should decline in the future, the authors find that delaying the ESS installation make it profitable.

Interactions between electric mobility and photovoltaic generation: A review <s> Heat-ventilation-air-conditioning systems <s> Renewable energy – mainly solar power – is expected to be increasingly rapidly integrated into future electricity systems in Japan especially after the Fukushima Nuclear Accident. On the other hand, one of the most crucial elements of future electricity systems will be the capability for “smart” controls on both supply and demand sides to perform under real-time dynamics. Therefore, the purpose of the study is to evaluate the impacts of integrating PV power into future electricity system with electric vehicles (EVs) and heat pumps (HPs) under smart control strategies in Kansai Area, Japan through scenario analysis. The analysis was conducted using a model that is organized into an input–output framework, and actualized using an hour-by-hour computer simulation to derive a real-time supply-demand balance. Electricity system scenarios with different penetration levels of PV power, EV and HP were obtained. The scenario results show that EV and HP are helpful for integrating more PV power by absorbing excess electricity in future smart electricity systems, and the high level penetration of PV power can reduce CO2 emission greatly and effectively. One million EVs and one million HPs can reduce excess electricity by 3TWh which is 2% of total electricity, and 30GWp PV can reduce a maximum of 11.6 million tonnes CO2 emissions which are 43% of total CO2 emissions. Finally, the annual and monthly operation patterns of EV and HP are also uncovered in different solar irradiation and temperature situations. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Heat-ventilation-air-conditioning systems <s> Abstract 95 percent of American households own at least one car, and with the help of newly introduced Vehicle to Home technology, it is now possible for buildings, vehicles, and renewable energy sources to work together as a single techno-ecological system to meet the requirements of a net zero energy building. Vehicle to Home technologies use idle electric vehicle battery power as a grid storage tool to mitigate fluctuations from renewable electric power sources and to help supply backup power in the event of an emergency. This study aims to investigate the role of Vehicle to Home technology in satisfying the energy requirements for a net zero energy building. For this purpose, an optimization analysis is performed first to select the best design alternatives for an energy-efficient building under the relevant economic and environmental constraints. Next, solar photovoltaic sources are used to supply the building’s remaining energy demand and thereby minimize the building’s grid reliance. Finally, Vehicle to Home technology is coupled with the renewable energy source as a substitute for power from the grid. The results indicate that, with the help of this system, it is possible to not only lower the monetary value of the required grid electricity for such a building to zero in certain months of the year but also to earn money to compensate for the installation costs of solar panels and other technologies necessary for a net zero energy building, while grid electricity consumption for the rest of the year can still be effectively reduced by up to 68% compared to that of a conventional building design. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Heat-ventilation-air-conditioning systems <s> Abstract The paper analyzes the introduction of a photovoltaic system satisfying electric, space heating and cooling demand of an office building located in southern Italy. The electric load is due to an electric heat pump, used to satisfy space heating and cooling load, a pure electric demand (personal computers, printers, lighting, etc.) and an electric vehicle charged during working hours. Dynamic simulations to evaluate the energy and environmental performance of the analyzed system considering different photovoltaic peak powers (4.5–9.0 kW), electric vehicle distance per day (40–120 km) and charging mode is carried out. The solar based system shows primary energy saving and equivalent carbon dioxide emission reduction higher than 40% in comparison to the reference conventional system based on a natural gas fired boiler, an electric chiller and a diesel car. The results highlight that the solar energy system is more competitive when DC charging system is provided. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Heat-ventilation-air-conditioning systems <s> Heating electrification powered by distributed renewable energy generation is considered among potential solutions towards mitigation of greenhouse gas emissions. Roadmaps propose a wide deployment of heat pumps and photovoltaics in the residential sector. Since current distribution grids are not designed to accommodate these loads, potential benefits of such policies might be compromised. However, in large-scale analyses, often grid constraints are neglected. On the other hand, grid impact of heat pumps and photovoltaics has been investigated without considering the influence of building characteristics. This paper aims to assess and quantify in a probabilistic way the impact of these technologies on the low-voltage distribution grid, as a function of building and district properties. The Monte Carlo approach is used to simulate an assortment of Belgian residential feeders, with varying size, cable type, heat pump and PV penetration rates, and buildings of different geometry and insulation quality. Modelica-based models simulate the dynamic behavior of both buildings and heating systems, as well as three-phase unbalanced loading of the network. Additionally, stochastic occupant behavior is taken into account. Analysis of neighborhood load profiles puts into perspective the importance of demand diversity in terms of building characteristics and load simultaneity, highlighting the crucial role of back-up electrical loads. It is shown that air-source heat pumps have a greater impact on the studied feeders than PV, in terms of loading and voltage magnitude. Furthermore, rural feeders are more prone to overloading and under-voltage problems than urban ones. For large rural feeders, cable overloading can be expected already from 30% heat pump penetration, depending on the cable, while voltage problems start usually at slightly higher percentages. Additionally, building characteristics show high correlations with the examined grid performance indicators, revealing promising potential for statistical modeling of the studied indicators. Further work will be directed to the assessment of meta-modeling techniques for this purpose. The presented models and methodology can easily incorporate other technologies or scenarios and could be used in support of policy making or network design. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Heat-ventilation-air-conditioning systems <s> This paper investigates heat pump systems in smart grids, focussing on fields of application and control approaches that have emerged in academic literature. Based on a review of published literature technical aspects of heat pump flexibility, fields of application and control approaches are structured and discussed. Three main categories of applications using heat pumps in a smart grid context have been identified: First stable and economic operation of power grids, second the integration of renewable energy sources and third operation under variable electricity prices. In all fields heat pumps - when controlled in an appropriate manner - can help easing the transition to a decentralized energy system accompanied by a higher share of prosumers and renewable energy sources. Predictive controls are successfully used in the majority of studies, often assuming idealized conditions. Topics for future research have been identified including: a transfer of control approaches from simulation to the field, a detailed techno-economic analysis of heat pump systems under smart grid operation, and the design of heat pump systems in order to increase flexibility are among the future research topics suggested. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Heat-ventilation-air-conditioning systems <s> Abstract With the increasing penetration of distributed renewable energy generation and dynamic electricity pricing schemes, applications for residential demand side management are becoming more appealing. In this work, we present an optimal control model for studying the economic and grid interaction benefits of smart charging of electric vehicles (EV), vehicle-to-grid, and space heating load control for residential houses with on-site photovoltaics (PV). A case study is conducted on 1–10 net zero energy houses with detailed empirical data, resulting in 8–33% yearly electricity cost savings per household with various electric vehicle and space heating system combinations. The self-consumption of PV is also significantly increased. Additional benefits through increasing the number of cooperating households are minor and saturate already at around 3–5 households. Permitting electricity transfer between the houses and EV charging stations at workplaces increases self-sufficiency significantly, but it provides limited economic benefit. The additional cost savings from vehicle-to-grid compared to smart charging are minor due to increased battery degradation, despite a significant self-sufficiency increase. If the optimization is conducted without taking the battery degradation cost into account, the added monetary value of vehicle-to-grid can even be negative due to the unmanaged degradation. Neglecting battery degradation completely leads to overestimation of the vehicle-to-grid cost benefit. <s> BIB006

Heating, ventilation and air-conditioning (HVAC) represent more than half of buildings energy consumption [65] . As a distributed energy resource, heat pumps are meant to play a role in smart grids BIB004 . Among other, their interaction with PV energy has been noticed because of the heat pump peak shaving potential BIB005 . While studying EV/PV systems BIB006 BIB002 BIB003 BIB001 considered additional residential heat pumps as part of the smart energy management. In BIB002 , the building energy performance is at first optimized, so the influence of its thermal system on the EV/PV smart strategy is not visible. Along with its smart control of households EVs, BIB006 utilizes an HVAC optimized management. By comparing to an uncontrolled case, the authors are reviewing the performance of their EV charging and HVAC management separately. HVAC optimal strategy only brings a reduction in yearly energy costs of 8-16%, while smart EV charging only brings a 12-20% reduction. Then, joined HVAC and EV strategies reduce energy bill by 19-33%. In this case, HVAC and EV charging have comparable benefits when controlled smartly. Moreover, their performance seems almost additive, showing that one management has little effect on the other. In BIB001 , the authors have shown that a large penetration of EV could alleviate a high portion of PV curtailment. They simulate a similar scenario with as many heat pumps as EVs. Compared to the EV-only case, this new scenario shows that the heat pumps allow an additional reduction of curtailed PV energy. If the heat pumps impact is lower than PV, HP seasonal patterns are opposite to PV profiles with a higher use in winter for HP. This is why there is an interesting complementarity to be benefited for EV.

Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> As environmental concerns have focussed attention on the generation of electricity from clean and renewable sources, wind energy has become the world's fastest growing energy source. The authors dr ... <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> Electric vehicles (EVs) and renewable energy sources offer the potential to substantially decrease carbon emissions from both the transportation and power generation sectors of the economy. Mass adoption of EVs will have a number of impacts and benefits, including the ability to assist in the integration of renewable energy into existing electric grids. This paper reviews the current literature on EVs, the electric grid, and renewable energy integration. Key methods and assumptions of the literature are discussed. The economic, environmental and grid impacts of EVs are reviewed. Numerous studies assessing the ability of EVs to integrate renewable energy sources are assessed; the literature indicates that EVs can significantly reduce the amount of excess renewable energy produced in an electric system. Studies on wind–EV interaction are much more detailed than those on solar photovoltaics (PV) and EVs. The paper concludes with recommendations for future research. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> This paper discusses the charging of plug-in hybrid electric vehicles (PHEVs) in an existing office building microgrid equipped with a photovoltaic (PV) system and a combined heat and power (CHP) unit. Different charging strategies and charging power ratings for workplace charging are examined for their grid impact and their impact on the self-consumption of the locally generated electricity. The grid impact can be significantly reduced by using strategies that require limited future knowledge of the EV mobility behavior and limited communication infrastructure. These strategies allow a high number of EVs to be charged at an office building, even with a limited number of charging spots, due to the large standstill times. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> This paper presents a comprehensive review and assessment of the latest research and advancement of electric vehicles (EVs) interaction with smart grid portraying the future electric power system model. The concept goal of the smart grid along with the future deployment of the EVs puts forward various challenges in terms of electric grid infrastructure, communication and control. Following an intensive review on advanced smart metering and communication infrastructures, the strategy for integrating the EVs into the electric grid is presented. Various EV smart charging technologies are also extensively examined with the perspective of their potential, impacts and limitations under the vehicle-to-grid (V2G) phenomenon. Moreover, the high penetration of renewable energy sources (wind and photovoltaic solar) is soaring up into the power system. However, their intermittent power output poses different challenges on the planning, operation and control of the power system networks. On the other hand, the deployment of EVs in the energy market can compensate for the fluctuations of the electric grid. In this context, a literature review on the integration of the renewable energy and the latest feasible solution using EVs with the insight of the promising research gap to be covered up are investigated. Furthermore, the feasibility of the smart V2G system is thoroughly discussed. In this paper, the EVs interactions with the smart grid as the future energy system model are extensively discussed and research gap is revealed for the possible solutions. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> Abstract Electric vehicles charging and discharging management as well as large scale intermittent renewable power generation management are known as the two most important challenges in the future distribution system operation and control. Proper integration of these energy sources may introduce a solution for overcoming to challenges. In this paper, a stochastic charging and discharging scheduling method is proposed for large number of electric vehicles parked in an intelligent parking lot where intelligent parking lots are potentially introduced as aggregators allowing electric vehicles interact with the utilities. A self-scheduling model for an intelligent parking lot equipped with photovoltaic system and distributed generators is presented in this paper in which practical constraints, solar radiation uncertainty, spinning reserve requirements and electric vehicles owner satisfaction are considered. The results show that the proposed parking lot energy management system satisfies both financial and technical goals. Moreover, electric vehicle owners could earn profit by discharging their vehicles as well as having desired state of charge in the departure time. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> Electric vehicles (EVs) represent one of the most promising technologies to green the transportation systems. An important issue is that high penetration of EVs brings heavy electricity demand to the power grid. One effective way to alleviate the impact is to integrate local power generation such as renewable energy sources (RESs) into charging infrastructure. Because of the intermittent and indispatchable nature of RESs, it becomes very challenging to coordinate EVs charging with other grid load and renewable generation. In this paper, EVs charging problem in the presence of smart grid technologies is investigated, and the interaction with renewable energy is reviewed. An overview about EVs and RESs is first presented, which mainly introduces major types of EVs and renewable energy estimation methods. Then, according to the objectives, the existing research works are divided into three categories: cost-aware, efficiency-aware, and emissions-aware interactions between EVs and RESs. Each category׳s discussion includes the description of core ideas, summarization of solutions, and comparison between different works. Finally, some key open issues about EVs interacting with RESs are given and some possible solutions are also discussed. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Other energy sources <s> In order to reach significant CO2 emissions reductions, future energy systems will require a large share of renewable energies, such as wind and solar photovoltaic power. However, relying on such renewable energy sources is expected to generate considerable excess power during certain periods of the day, in particular during night time for wind and daytime for solar power. This excess power may be conveniently used to power electric mobility. This paper explores the possible complementarities between wind and solar power and electric vehicles charging, based on 2050 scenarios for the case study of Portugal. Model results show that CO2 emissions targets can only be achieved with high levels of photovoltaics penetration and electric vehicles, reinforcing the need for daytime charging infrastructures, presumably at or near work facilities. <s> BIB007

We mentioned earlier that many efforts have been done in investigating the combination between EVs and RES in general. Especially, wind power interaction with EV has been widely investigated. Besides this, aspects of EV/PV systems studies also included micro-CHP (combined heat and power) generators or small fossil fuels generators. Micro-CHP units could show an interesting seasonal complementarity with PV energy. As CHP power depends on heat generation, its power is the highest in winter, contrary to PV power. BIB003 studies the energy management of a workplace equipped with PV and CHP units and an EV parking. It was not possible at first to discern respective effects of both technologies on the global system performance, because of the magnitude of the CHP power (one tenth of PV peak power) and its synchronization with PV power. Small dispatchable electric generators are included in BIB005 in their PVCS management study. They can provide the additional energy source, if the grid power price is high enough or in an islanded mode. Wind power, as the other major renewable source for electricity generation, is often considered with PV energy BIB002 BIB004 BIB006 . Major differences with PV energy, are that the wind power peak is generally at night BIB001 , and that it comes in large energy systems. Those systems have large flexibility needs and therefore only concerns large coordinated EV fleets. In those situations, EV fleet could charge with both PV (in day charging) and wind energy (night charging). BIB007 presents a comparison of charging strategy based on wind at night, and PV at mid-day. The authors develop an energy scenario for Portugal with among others, a large deployment of PV capacities, wind power and EV fleets. They point out that with an EV smart charging strategy, PV energy reduces curtailed renewable energy and CO 2 emissions better than wind.

Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> This paper presents the latest comprehensive literature review of AC and DC microgrid (MG) systems in connection with distributed generation (DG) units using renewable energy sources (RESs), energy storage systems (ESS) and loads. A survey on the alternative DG units' configurations in the low voltage AC (LVAC) and DC (LVDC) distribution networks with several applications of microgrid systems in the viewpoint of the current and the future consumer equipments energy market is extensively discussed. Based on the economical, technical and environmental benefits of the renewable energy related DG units, a thorough comparison between the two types of microgrid systems is provided. The paper also investigates the feasibility, control and energy management strategies of the two microgrid systems relying on the most current research works. Finally, the generalized relay tripping currents are derived and the protection strategies in microgrid systems are addressed in detail. From this literature survey, it can be revealed that the AC and DC microgrid systems with multiconverter devices are intrinsically potential for the future energy systems to achieve reliability, efficiency and quality power supply. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> In this paper, a new energy management strategy (EMS) for a dc distribution system in buildings is being proposed. The dc distribution system is considered as a prospective system according to the increase of dc loads and dc output type distribution energy resources (DERs) such as photovoltaic (PV) systems and fuel cells. Since the dc distribution system has many advantages such as feasible connection of DERs and electric vehicles (EVs), reduction of conversion losses between dc output sources and loads, no reactive power issues, it is very suitable for industrial and commercial buildings interfaced with DERs and EVs. The establishment of an appropriate EMS based on the economic point of view can reduce energy costs of buildings and provide benefits to participants in energy management. Applicable elements for the dc distribution system are identified and the real-time decision-making-based algorithm for minimizing operating costs is proposed in this paper. The EV service model for the EMS to offer incentive to EV owners who participate in battery discharging is described. To verify the performances of the proposed algorithm, computer simulation and economic analysis are being performed where the results show that the proposed EMS reduces energy costs, motivates EV owners, and can be applied to the dc distribution buildings. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> Integration of PV system and EV charging station is a typical way of spot utilization of renewable resources. Research on multi-objective optimal scheduling of a DC micro-grid consisted of PV system and EV charging station is proposed in this paper. Based on the structure of the DC micro-grid and function of each component, mathematical model of the multi-objective optimization for scheduling, which takes the cost of electricity purchasing and energy circulation of storage batteries as objective functions, is built and then solved with NSGA-II. Constraints including EVs' charging time, the range of the charging/discharging power and SOC (state of charge) of the storage batteries, power supply rate of the distribution network and system power balance are considered in this model. A certain case is studied and verifies the model's rationality. Compared with an instant charging plan, the optimal scheme is superior in reducing cost and energy circulation. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> The predicted operation and performance of a 6.65 kW PV system assumed to be located in Galway, Ireland to charge 4 electric vehicles (EVs) is modeled. The performance of AC and DC distribution systems are compared. Models account for the variable efficiencies of power converter blocks as power levels vary. A control strategy is developed to choose when/how many EVs charge based on available PV power thereby maximizing efficiency and solar energy usage in the EVs. It is shown that for an EV user with a home charge point and a 100 km daily return commute, a ~90% SOC can be supported by the system during the summer months. It is also shown that the DC system is 4.67% more efficient than the AC system over the course of a year. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> This paper investigates the opportunity for electricity loss minimization in commercial facilities where a photovoltaic system and electric vehicle charging stations coexist, through combined operation. A new metering architecture is proposed that can be used for coupling the two independent circuits and consequently reduce energy losses in grid side converters. The metering topology is valid for every kind of pricing (flat rates, feed-in tariffs, feed-in premiums). The problem is modeled as a non-linear optimization problem using BARON solver, taking into account the non-linear efficiency curve of the grid side converters. A case study with a sensitivity analysis on both photovoltaic system and electric vehicle charging fleet is carried out and useful results regarding losses minimization and profit increments are concluded. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> Recent years have seen increased interest in green technologies such as photovoltaic (PV) electricity and plug-in hybrid electric vehicles (PHEVs). Such technologies, however, have been found to be detrimental to distribution networks. This paper introduces a novel distribution system architecture that can better accommodate the expected growth in PV electricity and PHEVs. In the proposed architecture, the distribution system becomes a bilayer system composed of the traditional ac layer that serves existing system loads, plus an embedded dc layer that interfaces with PV arrays and PHEVs. A bidirectional converter interconnects the two layers and controls the power flows between them. This paper presents the key design and operational aspects of the proposed architecture, with consideration of different uncertainties inherent in the system. To this end, a probabilistic benchmark has been developed for modeling these uncertainties and for use with the sizing and scheduling of different system components. Monte Carlo (MC) simulations confirmed the technical and economic merit of the proposed design methodology. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Innovative network technologies <s> Abstract When charging or discharging electric vehicles, power losses occur in the vehicle and the building systems supplying the vehicle. A new use case for electric vehicles, grid services, has recently begun commercial operation. Vehicles capable of such application, called Grid-Integrated Vehicles, may have use cases with charging and discharging summing up to much more energy transfer than the charging only use case, so measuring and reducing electrical losses is even more important. In this study, the authors experimentally measure and analyze the power losses of a Grid-Integrated Vehicle system, via detailed measurement of the building circuits, power feed components, and of sample electric vehicle components. Under the conditions studied, measured total one-way losses vary from 12% to 36%, so understanding loss factors is important to efficient design and use. Predominant losses occur in the power electronics used for AC-DC conversion. The electronics efficiency is lowest at low power transfer and low state-of-charge, and is lower during discharging than charging. Based on these findings, two engineering design approaches are proposed. First, optimal sizing of charging stations is analyzed. Second, a dispatch algorithm for grid services operating at highest efficiency is developed, showing 7.0% to 9.7% less losses than the simple equal dispatch algorithm. <s> BIB007

Some innovative features regarding electrical network components can be incorporated in EV/PV systems. Those are: DC networks, solid-state transformers and reactive power exchange. While current power grids use almost exclusively AC current, low-voltage DC distribution grids are regarded as the appropriate framework for microgrids BIB001 . Such proposition seems justified because those systems will host mainly DC distributed energy resources (as PV panels, fuel cells, batteries...). EV batteries run on DC current and are therefore equipped with an AC/DC inverter that causes main charging/discharging losses BIB007 . DC systems should therefore be beneficial to EV/PV systems. BIB003 BIB006 BIB002 also studied EV/PV systems in DC networks, but no comparison were made with AC networks. BIB004 shows that for a small PVCS using a smart charging strategy, a DC system increases energy efficiency of EV charging from PV panels by 5%. One could think that DC networks would particularly be useful in larger EV/PV systems, using vehicle-to-grid technology. BIB005 show that within a DC EV/PV system, revenues are increased by 0.5-1%.

Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> This article establishes the relationship between the static axiomatic theory of bargaining and the sequential strategic approach to bargaining. We consider two strategic models of alternating offers. The models differ in the source of the incentive of the bargaining parties to reach agreement: the bargainers' time preference and the risk of breakdown of negotiations. Each of the models has a unique perfect equilibrium. When the motivation to reach agreement is made negligible, in each model the unique perfect equilibrium outcome approaches the Nash bargaining solution with utilities that reflect the incentive to settle and with the proper disagreement point chosen. The results provide a guide for the application of the Nash bargaining solution in economic modelling. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> The electric energy distribution infrastructure is undergoing a startling technological evolution with the development of the smart grid concept, which allows more interaction between the supply- and the demand-side of the network and results in a great optimization potential. In this paper, we focus on a smart grid in which the demand-side comprises traditional users as well as users owning some kind of distributed energy source and/or energy storage device. By means of a day-ahead demand-side management mechanism regulated through an independent central unit, the latter users are interested in reducing their monetary expense by producing or storing energy rather than just purchasing their energy needs from the grid. Using a general energy pricing model, we tackle the grid optimization design from two different perspectives: a user-oriented optimization and an holistic-based design. In the former case, we optimize each user individually by formulating the grid optimization problem as a noncooperative game, whose solution analysis is addressed building on the theory of variational inequalities. In the latter case, we focus instead on the joint optimization of the whole system, allowing some cooperation among the users. For both formulations, we devise distributed and iterative algorithms providing the optimal production/storage strategies of the users, along with their convergence properties. Among all, the proposed algorithms preserve the users' privacy and require very limited signaling with the central unit. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> When preparing for the widespread adoption of Electric Vehicles (EVs), an important issue is to use a proper EVs’ charging/discharging scheduling model that is able to simultaneously consider economic and environmental goals as well as technical constraints of distribution networks. This paper proposes a multi-objective operational scheduling method for charging/discharging of EVs in a smart distribution system. The proposed multi-objective framework, based on augmented e-constraint method, aims at minimizing the total operational costs and emissions. The Vehicle to Grid (V2G) capability as well as the actual patterns of drivers are considered in order to generate the Pareto-optimal solutions. The Benders decomposition technique is used in order to solve the proposed optimization model and to convert the large scale mixed integer nonlinear problem into mixed-integer linear programming and nonlinear programming problems. The effectiveness of the proposed resources scheduling approach is tested on a 33-bus distribution test system over a 24-h period. The results show that the proposed EVs’ charging/discharging method can reduce both of operation cost and air pollutant emissions. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> In the context of smart grid, the building can freely connect with other buildings to form clusters which are termed as building clusters to share energy. However, less study is conducted to develop optimal operation strategy for building clusters and evaluate the performance of building clusters in terms of different measures under different operation modes. Therefore, this research proposes a collaborative decision model to study the energy exchange among building clusters where the buildings share a combined cooling, heating and power system, thermal storage, and battery, and each building aims to minimize its energy cost, carbon emission or primary energy consumption. A collaborative decision framework is proposed to obtain Pareto operation decisions for the building clusters. We compare the performance of the collaborative strategy with the non-cooperative strategy where no energy sharing among the buildings. It is demonstrated that the collaborative strategy can significantly reduce energy cost, carbon emission and primary energy consumption under both grid connected and disconnected operation modes. The collaborative strategy under dynamic pricing plan is more cost effective than the strategy under flat pricing plan, which indicates that the collaborative strategy can motive buildings to more efficiently utilize the shared energy under dynamic pricing plan. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> The owners of PV systems and EV charging station are probably different. Combining PV systems with EV charging station can contribute to more benefit, according to previous research, but also lead to some problem. In order to solve the problem of energy management and profit distribution, a multi-party energy management for EV charging station cooperated with PV systems in smart grid is proposed. Firstly, the feasible model of EV charging energy demand is established based on starting charging time, departure time and SOC, which is varied with the PV power. The actual charging power can be then decided according to the relationship between charging demand and PV power. Finally, the operation profit is distributed to each project using Shapley value method, which considers the contribution of each project. A comprehensive result obtained from simulation tests has shown that the proposed energy management can obviously increase the profit of each party in cooperation mode compared to independent mode, which can be utilized to practical application. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> Smart grids are the result of a dynamic co-evolution process that leverages the integration of new technological advances in the energy systems and information and communication technologies. This process is accompanied by changes in business models, organizational structures, roles, and operating practices. In this context, collaboration among multiple entities becomes a crucial element, justifying the term Collaborative Smart Grid. The purpose of this article is to systematically review recent literature with a view to identifying trends, opportunities, and challenges regarding the application of models, approaches, and tools from collaborative networks to the energy domain. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation in smart strategies <s> The realisation of the smart grids and markets vision constitutes a substantial transition of electricity systems affecting multiple stakeholders and creating various technical, social, economic, political, and environmental challenges. These need to be considered adequately in decision support tools for agents in electricity systems. Agent-based modelling and simulation as a flexible and rich modelling framework can serve as a testbed for analysing new paradigms in the field of smart grids, such as demand response, distributed generation, distribution grid modelling, and efficient market integration. While so far wholesale electricity markets have been the focus of agent-based modelling and simulation, this paper provides a detailed review of literature using such techniques for analysing smart grids from a systems perspective. For that purpose, a general classification of applying agent-based modelling and simulation techniques to electricity systems is provided. The literature review of specifically using agent-based modelling and simulation for analysing smart grids shows that, although being still a limited field of research, quite different applications are identified with the number of contributions having increased in recent years. Agent-based modelling and simulation can deliver specific insights in how different agents in a smart grid would interact and which effects would occur on a global level. Thereby, the approach can deliver valuable input for decision processes of stakeholders and policy making. Future research could feature more focused analyses of storage systems, local market concepts, interactions with centralised markets, and the role of intermediaries. <s> BIB007

To enable a smart control strategy, cooperation between participants in the EV/PV system is often required. We present the implicit methods introduced in smart control strategies that take such behaviors into account. First, we note that the issue of control mode has been widely studied in new energy systems with cooperative and non-cooperative game theory approaches BIB006 and agent-based models BIB007 . Mainly studies involving distributed generation and storage have been studied in this literature, as in BIB002 , and could be easily applied on EV/PV systems. When an EV/PV system control strategy uses such methods, it typically seeks to optimize its own objective function, along with the one of a another agent. Most common multi-objective method is the weighted-sum method. For instance, let's consider two agents in EV/PV systems with objective functions X 1 and X 2 . Without cooperation, each agent define its strategy by maximizing (or minimizing) its own objective function independently from the other agent. With cooperation, agents agree to define their strategy by maximizing (or minimizing) w 1 X 1 + w 2 X 2 . As pointed out in BIB004 , w 1 and w 2 can be interpreted as the preference of the decision maker towards the related objective. Alternatively, this method can be interpreted with game theoretical concepts. By posing W i = αi Xi , the latter problem can be considered as the linearized Nash bargaining solution in cooperative games theory. The following Nash bargaining problem would be to maximize (X 1 −X 1 ) α1 (X 2 −X 2 ) α2 under both constraints of agents 1 and 2. In this framework, α i and X i can be interpreted as the agents' bargaining power and non-cooperative payoff BIB001 . Of course, weighted-sum method can be extended to multiple objectives without loss of generality. A Pareto frontier is the optimal solutions set defined by all the combination of w i . An alternative method is the −constraint multi-objective optimization, as used in BIB003 . As its interpretation is less obvious in term of cooperation, and its use much less frequent than that of the weighted-sum method, we do not expand on this method. Cooperation between agents requires a value sharing scheme. This issue has been practically unaddressed in the selected literature. Only in BIB005 , a Shapley value method is used in a cooperative game theory model to distribute the total benefits from the EV/PV synergy. More generally, the literature lacks analysis of business models.

Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> Plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs) have high potential for reducing fuel consumption and emissions, and for providing a way to utilize renewable energy sources for the transportation sector. On the other hand, charging millions of PEVs could overload the power grid, increase emissions and significantly alter economic characteristics. A day-time photovoltaic (PV) based, plug-in electric vehicle charging station located in a workplace parking garage is considered in this research. The results show the impact of PV based workplace charging on the economics and emissions from the power grid. An optimal charge scheduling strategy is compared with an uncontrolled charging case to perform the economics and emissions analysis. Two locations (Columbus, OH and Los Angeles, CA) are selected such that the analysis includes different scenarios of yearly variation of solar radiation and finance structure. A high fidelity hourly simulation model for energy economic analysis is developed considering different types of vehicles, statistical data for driving distances, parking time, installation cost, tax rebates and incentives. An incremental parking rate for accessing the charging facility is considered for economic analysis for the garage owner and the vehicle owner. The analysis is extended to consider the impact of carbon tax implementation on the driver economics and shows the feasibility of such PV based charging stations. Parametric analysis for different parking rates and installed capacities show (i) the feasibility of a PV based workplace charging facility, (ii) benefits to the vehicle owner and the garage owner, and (iii) the need for an optimal charging controller. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> The owners of PV systems and EV charging station are probably different. Combining PV systems with EV charging station can contribute to more benefit, according to previous research, but also lead to some problem. In order to solve the problem of energy management and profit distribution, a multi-party energy management for EV charging station cooperated with PV systems in smart grid is proposed. Firstly, the feasible model of EV charging energy demand is established based on starting charging time, departure time and SOC, which is varied with the PV power. The actual charging power can be then decided according to the relationship between charging demand and PV power. Finally, the operation profit is distributed to each project using Shapley value method, which considers the contribution of each project. A comprehensive result obtained from simulation tests has shown that the proposed energy management can obviously increase the profit of each party in cooperation mode compared to independent mode, which can be utilized to practical application. <s> BIB002 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> This paper proposes a novel electric vehicle (EV) classification scheme for a photovoltaic (PV) powered EV charging station (CS) that reduces the effect of intermittency of electricity supply as well as reducing the cost of energy trading of the CS. Since not all EV drivers would like to be environmentally friendly, all vehicles in the CS are divided into three categories: 1) premium, 2) conservative, and 3) green, according to their charging behavior. Premium and conservative EVs are considered to be interested only in charging their batteries, with noticeably higher rate of charging for premium EVs. Green vehicles are more environmentally friendly, and thus assist the CS to reduce its cost of energy trading by allowing the CS to use their batteries as distributed storage. A different charging scheme is proposed for each type of EV, which is adopted by the CS to encourage more EVs to be green. A basic mixed integer programming (MIP) technique is used to facilitate the proposed classification scheme. It is shown that the uncertainty in PV generation can be effectively compensated, along with minimization of total cost of energy trading to the CS, by consolidating more green EVs. Real solar and pricing data are used for performance analysis of the system. It is demonstrated that the total cost to the CS reduces considerably as the percentage of green vehicles increases, and also that the contributions of green EVs in winter are greater than those in summer. <s> BIB003 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> Abstract In order to maximize the operation profit while maintaining the service quality, an effective energy management scheme is highly needed for the Photovoltaic-assisted Charging Station (PVCS). Considering the uncertainty of Electric Vehicle (EV) charging demand and PV power output, it will be challenging to determine the charging power for EVs to make informed real-time decisions. In this study, an online energy management method leveraging both offline optimization and online learning is proposed. In order to maximize the self-consumption of Photovoltaic (PV) energy and decide the power supplied from the power grid with Time-of-Use (TOU) pricing, here online learning is coupled with the rule-based decision-making to obtain a real-time online algorithm. The knowledge base for online learning is derived and updated from the results of offline optimization after every operation day. The PVCS located at workplace parking lots is used as an example to test the proposed method. The simulation results show that the method can be implemented without the information on future PV power and charging demand. The obtained results are close to the optimal results from offline optimization. <s> BIB004 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> Integration of Electric Vehicles (EV) and Renewable Energy Sources (RES) with Energy Management System (EMS) in microgrid has been widely discussed. In this paper, we present a two-tier EMS system currently in operation on campus of University of California, Los Angeles. The upper level system, called Super Control Center (SCC), processes grid-wide energy coordination while the lower level systems manage local EV charging and other micro-scale services. We use the concept of Solar-to-Vehicle (S2V) to demonstrate the combined function of the EMS system. For lower level system, we present two queuing algorithms with user priority calculated from user's Solar-Friendliness Index (SFI), i.e., solar composition of user's energy consumption profile. The algorithms have shown capabilities to increase RES utilization, while actively encouraging users to change consumption behavior and promoting higher RES utilization. Simulation results have shown that SCC can reduce up to 73% of the EV load. The lower level EV coordination algorithms are able to increase the RES utilization from 0.504 to 1. <s> BIB005 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> Abstract The direct coupling of solar electricity with electric vehicles is very topical due to their many co-benefits, such as reducing the electricity flow into and from the grid. This work illustrates this solution using a park-and-ride lot outside Lisbon to study its solar energy potential to charge electric vehicles. The solar resource, its exploitation and coordination with the vehicles under different charging approaches are discussed in detail, using methods extendible to other cases. A genetic-algorithm optimisation was performed to maximise the solar energy generation. To ascertain the techno-economic feasibility of the concept, several sets of assumptions are analysed, including the use of energy storage and smart charging. For current market conditions, the payback time is found to be 14 years; a modest financial public incentive would significantly improve the project economics – making the payback drop to 7 years – enabling conditions to put this inescapable and green approach at the forefront. <s> BIB006 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between EV drivers and a photovoltaic charging station <s> Abstract The electric vehicle (EV) can be utilized as a dynamically configurable dispersed energy storage in the vehicle-to-grid (V2G) and vehicle-to-building (V2B) operation mode to balance the energy demand between buildings and EVs. This paper proposes a mixed integer linear programming based collaborative decision model to study the energy sharing between a building and an EV charging station (CS). The building has its distributed generator, and electric and thermal energy storage, and the CS has its own renewable energy source. To model the V2G/V2B integration, we introduce three sets of decision variables to represent the energy exchange among building, CS and power grid. A set of parameters are introduced to model the driver behaviors, such as initial and desired state of charge level of EV battery, and available hours of EV, and sixteen different building categories (e.g., office, restaurant, hotel, warehouse, etc.) are studied. The impacts of driver behaviors and building categories on the economic performance of V2G/V2B integration are characterized and analyzed. The results from this research can recommend best V2G/V2B integration considering various driver behaviors and building categories which can provide valuable insight for smart community design. <s> BIB007

An issue raised by studies in the framework of PVCS is the cooperation between PVCS managers and EV drivers. Indeed, if uncontrolled strategies at least satisfy drivers' charging needs, efficient smart charging requires basic involvement from EV drivers, who should renounce some of their charging needs. A driver can be more or less flexible on his mobility needs, environmentally friendly or willing to participate in the power system stabilization 3 . Hence, charging station managers should give incentives to drivers to make a trade-off between their own needs and the PVCS needs. In the literature, no behavioral assumptions were made on EV drivers in term of reactions to incentives to cooperate in EV/PV systems. Beyond the techno-economic nature of the selected literature, this point is yet largely unknown since no typical behavior of EV drivers has risen yet. [87] points out the importance of EV drivers preferences for their charging needs. From a PVCS investor's perspective, the authors simulate a smart control strategy in two scenarios where EV drivers are expecting from the PVCS to handle the totality or only half of their charging needs. The PVCS operates with a rulebased decision making algorithm which prioritizes vehicle recharge according to their parking duration and aim at maximizing their PV self-consumption. Results show that full charging service leads to operator's profits almost fifteen times higher than the case when drivers' demand is only half of their charging needs. References BIB003 BIB004 BIB002 define several charging modes at different tariffs. For all of them, users who require charging should indicate a departure time to the PVCS. In BIB004 BIB002 , there are two types of mode: premium and general. Premium mode offers maximum charging power at high tariff while general mode follows the PVCS smart control strategy to define power delivered to the EV. BIB003 defines a more detailed business model. Three modes are proposed to users: premium, conservative and green. Premium mode offers maximal power at the highest price. Conservative mode offers charging at less cost, but not very different than premium. Green mode only ensures that the state of charge selected by the driver is reached at departure time. In the meantime, PVCS can freely organize the charging schedule of the green EV. In return, the offered tariff is 15% lower than the premium tariff. Green mode also allows PVCS to use vehicle-to-grid capacity of the vehicle as desired, in exchange for a remuneration for the EV owner (here at 85% of the green tariff). Green mode can be interpreted as a cooperative behavior from EV drivers. The only inconvenient for the Green driver is that if he decides to leave before his indicated departure time, his state of charge may even be lower than when he first plugged. Committing to the indicated departure time is therefore a form of additional cooperation with the PVCS. In the end, the authors' simulations show a cost reduction of around 48% for green users compared to premium and conservative users. For the PVCS, a 100% proportion of green drivers leads to total operational cost reduction of 80%. BIB005 proposes an alternative way to promote cooperation drivers and the PVCS to maximize PV energy utilization. It defines a "solar-friendliness index" for each driver, that measures the correlation between the power consumed by the EV driver for charging and the PV generation profile. A queuing algorithm is then used to organize the charging of a set of EVs based on their solar-friendliness index. As we discussed earlier, BIB001 addresses the issue of trade-offs between PVCS owner's revenues and EV drivers' costs of charging in the PVCS. Indeed, a PVCS owner has the dilemma of raising its parking fees to improve its profitability. On the other hand, EV owners would prefer charging at home or somewhere else if the parking fees are too high. The authors show by using a parametric analysis, that payback time appears to be concave with parking fees, meaning that marginal benefits of increasing the fees is decreasing. Compared to the charging costs of uncontrolled home charge at night, the authors are able to determine a maximal parking fee. BIB006 tests different customer tariffs for a PVCS. They show that a flat fee (e.g. monthly fee with unlimited free power for EV after) is much less profitable than a power-based fee. As already discussed in BIB007 , benefits of the defined collaborative strategy depended on building types. Moreover, the authors show that the driver's behavior has a major effect on the optimization of the building energy costs. Typically it is shown that long and diurnal availability of EV is not always a necessary condition for reaching best strategies. For instance a short duration availability is best fitted for a small hotel. In this case, total costs are reduced by 18% because of the smart control strategy. Moreover optimal energy needs for charging vary across building types from 20 to 70% of the state of charge of the vehicle. Along with these papers, the literature on EV/PV interactions misses a proper realistic study on PVCS tariffs from both station manager and user's perspective.

Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between an EV aggregator and photovoltaic energy producer <s> Simultaneous upcoming of photovoltaic generation and electric vehicles increases constraints on electric power system. This paper explores the possible synergy between these players so as to jointly improve the production predictability while ensuring a low carbon mobility. First a context is defined for this collaboration. It consists in the association of a photovoltaic producer and some electric vehicles owners so as to both manage the EV recharge and meet a day-ahead production commitment. Several currently studied questions such as commitment strategies or optimal charging can be transposed into the proposed context called collaborative system. Here, we mainly focus on its sizing in terms of PV rated power and number of vehicles. A simplified model of the system is thus realised, including a day ahead commitment and an optimal vehicle charging planning, based on deterministic vehicle characteristics.First results show a strong influence of the sizing on the potential added value of vehicles in this association. Then, we assess the impact of day-ahead production forecast quality by comparing persistence forecast with some meteorological data. It appears that other things remaining equal, an imprecise forecast will increase the optimal number of vehicles that are supposed to get into the collaborative system. Finally, the robustness of the charging planning is investigated. <s> BIB001 </s> Interactions between electric mobility and photovoltaic generation: A review <s> Cooperation between an EV aggregator and photovoltaic energy producer <s> The owners of PV systems and EV charging station are probably different. Combining PV systems with EV charging station can contribute to more benefit, according to previous research, but also lead to some problem. In order to solve the problem of energy management and profit distribution, a multi-party energy management for EV charging station cooperated with PV systems in smart grid is proposed. Firstly, the feasible model of EV charging energy demand is established based on starting charging time, departure time and SOC, which is varied with the PV power. The actual charging power can be then decided according to the relationship between charging demand and PV power. Finally, the operation profit is distributed to each project using Shapley value method, which considers the contribution of each project. A comprehensive result obtained from simulation tests has shown that the proposed energy management can obviously increase the profit of each party in cooperation mode compared to independent mode, which can be utilized to practical application. <s> BIB002

An EV aggregator manages the charging of an EV fleet. It can typically be a charging station manager, or a company fleet manager. It can contract with a PV energy producer to form a EV/PV system. When it comes to design a coalition of cooperating agents, it is important to assess the optimal dimension of this coalition. BIB001 seeks to determine the best sizing possible of a EV/PV collaborative system. The system is defined by a coalition of an EV aggregator and a PV energy producer. They found that a EV/PV system added value depends strongly on the ratio of the average aggregated vehicle capacity over the peak power of the PV energy producer. The authors determine a clear space of this ratio that makes the collaborative system globally profitable. Nevertheless, the length of this space is strongly dependent on the PV generation forecast. BIB002 studies cooperative game theory model where owners of charging station and PV capacities are different agents. In the non-cooperative case, PV producer sells its power to the grid, and the charging station buys power also from the grid. The authors then propose a cooperative scheme that brings the two entities in one coalition that maximizes the overall profits. Total profit is then shared according to the Shapley value method. Finally, cooperation raises total profits of 20%. [89] tackles the cooperation issues between a workplace building and a charging station. Building energy system includes solar panels, a fixed battery, workplace load and a heating system. Charging station is equipped with PV panels. The authors compare the cases without cooperation (separated optimization) and with cooperation. Without cooperation, both parties optimize their cost independently. With cooperation, the authors formulate a multi-optimization approach with a weighted-sum on each agent costs (as described in section 4.1.1) and exhibit the resulting Pareto frontier. The optimal weighting for the global system leads to a cost decrease of 22%. It is interesting to note that global optimum is not the optimum for either agents. For the building owner, global optimum leads to 23% higher costs compared to its own optimum. At the same time, costs are 16% lower for the charging station at its own optimum compared to the global optimum. One will even note that building optimum leads the charging station to a worse outcome than in non-cooperative strategy. As stated in 4.1.1, weights can be linked to bargaining power in the negotiation, these results show the necessity of a fair sharing of cooperation benefits.