Turbulent-boundary-layer trailing-edge noise reduction technologies including porous materials

Noise reduction is one of the major challenges to the societal acceptance of wind turbines. Aerodynamic noise sources are the dominant ones, assuming that mechanical noise can be reduced by correct insulation and periodic maintenance of the gearbox and drive train and by adding insulating material in the wind-turbine nacelle. The dominant aerodynamic noise source is turbulent boundary layer trailing edge noise, caused by the scattering of the pressure fluctuations beneath the turbulent boundary layer at the trailing edge of an airfoil. In order to reduce this source of noise, several strategies have been proposed such as: optimized airfoil design for noise performances; installation of add-ons such as brushes or trailing-edge serrations; and recently, permeable materials at the trailing edge. In this chapter, after a brief introduction to the aerodynamic noise sources, the noise reduction physical mechanisms of trailing-edge serrations, as an industrial state-of-the-art solution, and of porous materials, as an innovative noise reduction strategy, will be discussed.