Computational Fluid Dynamics Based Optimal Design of Vertical Axis Marine Current Turbines
Abstract Marine turbines are being increasingly used to harness kinetic energy of water and convert it into other useful forms of energy. Widespread commercial acceptability of these machines depends upon their efficiency. This largely depends upon the geometric features of the marine turbines such as number of blades, shape of blades etc. Researchers have been using experimental facilities to optimise these machines for maximum power generation. With the advent of advanced computational techniques, it has now become possible to numerically simulate the flow of water in the vicinity of marine turbines and monitor their performance output. In this work Computational Fluid Dynamics (CFD) based techniques have been used to analyse the effects of number of blades within the stator and rotor, of an in-house built Vertical Axis Marine Current Turbine (VAMCT), on the performance output of the turbine. Furthermore, an effort has been put forward towards better understanding of the flow structure in the vicinity of the blades during transient interaction between rotor and stator blades. This study provides vital information with regards to the flow sensors’ requirements and placements in order to monitor various blade configurations of a VAMCT in real world. The results of this study show that the torque output from a VAMCT is a strong function of blade configurations and there is a significant degradation in the performance output of marine turbines as the inequality between the number of rotor and stator blades increases. Hence, CFD has the potential to optimise the design of marine turbines and can be used as a potential modelling tool in the near future for on-line health monitoring of such systems.
Computational flow field analysis of a Vertical Axis Wind Turbine
The present work has used Computational Fluid Dynamics (CFD) to obtain the flow field characteristics of a novel Vertical Axis Wind Turbine (VAWT). The turbine used in this study features a multi blade design where both a stator and rotor blade array is used. The computational model is three-dimensional and contains full-scale turbine geometry measuring 2.0m in diameter and 1m in height. The pressure field across the wind turbine has been computed for operating conditions of λ=0 and λ=0.4 where it is shown to be non-uniform. The velocity field across the turbine has been obtained at λ=0 and λ=0.4 and has highlighted the presence of a jet flow passing through the central core of the turbine as it exits the rotor assembly. It is evident that under dynamic conditions and due to the direction of this jet flow a strong flow interaction occurs with the downstream rotor blades. This interaction results in rotor blades 5, 6 and 7 generating an opposing torque that acts against the direction of motion. Furthermore, it is noticed that rotor blades 10, 11, 12 and 1 contribute to the majority of overall rotor torque, which is a characteristic of this machines design. 
 
Key words 
 
Vertical axis wind turbine, CFD, flow field.
Effect of rotor blade position on Vertical Axis Wind Turbine performance
The need for sustainable energy sources becomes greater each year due to the continued depletion of fossil fuels and the resulting energy crisis. For the past decade there has been a call for immediate and concrete solutions to the climatic challenges the world is currently facing. An answer to the need for a sustainable, low cost energy solution is potentially in the form of wind turbines, which are receiving increased political support. In this paper a numerical study is presented with the aim of evaluating the performance output of three Vertical Axis Wind Turbine (VAWT) configurations. Here using Computational Fluid Dynamics (CFD) a two-dimensional Multi Reference Frame (MRF) approach has been used to perform steady state simulations. For this purpose an inlet velocity of 4m/s has been used along with rotor blade tip speed ratios (λ) in the range of 0 to 0.6. The effects of varying rotor blade position on the global and local flow fields have been quantified as shown in Figure.1. Figure.1 ’Pressure field (Pa) with rotor at 0°, λ=0.4’ From this figure it can be seen that the pressure distribution across the wind turbine is non uniform and features large pockets of high and low pressure. The regions of the turbine that are in direct contact with the air stream (windward) feature the highest pressure. It is also evident that each blade has both a pressure and a suction side. The torque contribution made by each rotor blade for each rotor position is presented in Figure.2. This contribution has been non-dimensionalised relative to overall rotor torque output (T/TAVG). Figure.2 ’Variation of rotor blade torque (T/TAVG) contribution at R=0,10 and 20 ’ Here, the rotor blade passage between blades 11 and 12 generates maximum torque when the rotor is at 20 . Furthermore, it has been found that within a typical rotor blade passage, maximum torque is obtained at a unique rotor angle. The power curves for all three configurations are depicted in Figure.3 and show the variation of power coefficient with respect to TSR. From the data obtained it can be concluded that the VAWT power curve characteristics vary with relative rotor blade position. For positions 10 and 20 peak power is generated at a tip speed ratio of 0.4 whereas position 0 generates maximum power at 0.6. A maximum power coefficient of 0.26 is present when the rotor is positioned at 0 and operating at a tip speed ratio of 0.6. Figure.3 ‘Variation of power output with respect to λ’