| In order to further increase the installed capacity of wind power in the future within limited land resources,onshore wind farms will inevitably be much more closer to residential areas,leading to a more significant problem of annoying noise generated by wind turbines.Wind farm noise will not only have a negative impact on the daily life of nearby residents as well as their working and production orders,but also change the living habits and growth cycle of surrounding animals and plants,thus having a certain influence on the local ecosystem.Therefore,in order to pursue the harmonious development of economy and ecology,the noise reduction of wind farm is imperative.At present,the noise reduction in the scale of wind farm is mainly to optimize the layout design scheme of wind turbines,so as to meet the requirements of low noise in sensitive areas.While for noise reduction of a single wind turbine,the first method is to add in the noise constraint based on considering the power and loads of the airfoil and blade aerodynamic shapes;Secondly,starting from the operation control strategy of wind turbine,the low-noise operation mode is selected by integrating power output and noise level;The third is to carry out design innovations on the basis of existing wind turbines,mainly including the research on noise reduction accessories of wind turbines.The research of this paper belongs to the third category.Since the noise source of blade is mainly the trailing edge turbulent boundary layer noise,this paper considers applying porous media to the trailing edge of airfoil section to fundamentally reduce the sound generation and realize noise reduction.Calculations and analysis are mainly carried out in this study to investigate the flow field and sound field of NACA0018 airfoil and its several types of porous airfoils filled with different porous trailing edges.By using Computational Fluid Dynamics（CFD）and Computational Acoustics（CAA）methods,the internal mechanism of the application of porous media in the noise reduction technology of wind turbine blades is explored.The work also evaluates the noise reduction effectiveness of porous medium,compares and analyzes the aerodynamic performances and sound characteristics of airfoils with different porous filling forms in the trailing edge,and finally their advantages and disadvantages are discussed.Therefore,the main work of this paper is divided into the following parts:1.Firstly,the paper expounds the current development status of wind turbine noise reduction,especially the related suppression technology on airfoil trailing edge noise,which accounts for a large proportion of noise generated by blades.Then it mainly introduces the current researches on the application of porous media in wind turbine noise reduction,and finally the research direction,objectives,main work and innovation of this paper are presented.2.The basic theories of sound generation and Computational Aeroacoustics（CAA）are introduced,where basic concepts,definitions and general methods in aeroacoustics are briefly described.Then an emphasis is given to the detailed derivation of FW-H model in the research.In addition,the typical fluid flow governing equations and the modified governing equations in porous media are described.Besides,the geometric characteristics and mathematical modeling of porous media are introduced.3.The geometric model and structural parameters of the research object in this study are determined.The introduction of the turbulence model,acoustic model,initial conditions,boundary conditions and other parameter settings selected in the simulation are depicted in detail.At the same time,in order to ensure the reliability of the model,the grid independence is verified and the results are compared with the experiment data.4.The influence of porous media on the flow field is mainly studied.The changes of velocity field,boundary layer（boundary layer thickness,boundary layer momentum thickness,boundary layer displacement thickness）,pressure distribution along the airfoil and the effects of porous trailing edge on vortex development and shedding in near-wall area and wake as well as wake velocity are illustrated.Thus the noise reduction mechanism can be explored through the comparison and analysis of flow field parameters.5.Taking the lift coefficient,drag coefficient and lift drag ratio as the main measurement parameters,the aerodynamic performance of the reference airfoil and several types of porous airfoils are analyzed,through which period their respective aerodynamic advantages and disadvantages are evaluated.At the same time,in order to further explore the influence of porous materials on lift and drag force,two types of porous media with different parameters are selected for simulation and discussion.On the other hand,comparison of the noise characteristics of the reference and porous airfoils,including their sound pressure fluctuation,power spectral density distribution,sound pressure level spectrum and their respective sound Directivity are approached.This paper also intuitively discusses the noise reduction effect of several types of airfoils compared with the reference airfoil,including the frequency range where the noise reduction is realized and the spatial angle range that can achieve the noise reduction effect.What’s more,the sound pressure level distribution and sound directivity characteristics of several types of airfoils at different angles of attack is studied.In addition,the noise reduction performances of two kinds of porous media with different structural parameters are stated.6.Finally,the research contents are summarized,the aerodynamic performance and noise reduction characteristics of the airfoils are combined to give a comprehensive consideration.Several types of porous airfoils with different filling forms are compared,analyzed and evaluated,after which a relatively better porous media filling scheme is proposed.On the basis of the whole work,this paper puts forward prospects for the main direction and contents of future feasibility study and expansion. |
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