Advanced numerical techniques for the acoustic modelling of materials and noise control devices in the exhaust system of internal combustion engines
This Thesis is focused on the development and implementation of general and efficient numerical methods for the acoustic modelling and design of noise control devices in the exhaust system of combustion engines. Special attention is paid to silencers, that can be divided into reactive and dissipative configurations. For the latter, significant differences are likely to appear in their acoustic behaviour, depending on the temperature variations within the absorbent material. Also, material heterogeneities associated with uneven filling processes and time degradation due to the exhaust gases, can alter the silencer attenuation performance. Therefore, numerical techniques considering all these features are required to guarantee an accurate prediction of the acoustic behaviour. First, a literature review is carried out. This is mainly related to one dimensional models and their associated matrix representation, as well as to acoustic models for absorbent materials and perforated surfaces. It is worth noting that plane wave model limitations make indispensable using alternative multidimensional methods, since the latter are valid for a wide frequency range and for silencers with complex geometries. In addition to developing acoustic models to simulate exhaust silencers in general terms, the possibility of using new acoustic elements is also explored in this Thesis. These elements have as an objective being an interesting potential alternative to the fibrous absorbent materials commonly used in practice, whose negative impact on health is a current cause for concern. In this sense, and following earlier studies, the Thesis considers the use of microperforated surfaces and goes into detail about the study of surfaces manufactured by sintering. The latter present, in some cases, the particularity of having a nearly constant acoustic impedance, whose value depends, among others, on the thickness and porosity of the plates. Both kind of surfaces have been proved to be an interesting alternative to dissipative silencers; the microperforated surfaces in the low frequency range and the sintered screens in the mid and high frequency range. General acoustic models, in which the wave equation permits to consider variable properties and mean flow, are considered. Thus, a finite element approach is first