Numerical Simulation Concept for Low-Noise Wind Turbine Rotors

Abstract

This thesis deals with the acoustic simulation of wind turbine noise. Turbulent boundary layer trailing-edge interaction noise is the main noise source for modern wind turbines. A hybrid approach was used to simulate this phenomenon on a two-demensional airfoil level. In a first step a numerical flow simulation (CFD) is conducted. It yields aerodynamic coefficients as well as the flow field around the airfoil and turbulence statistics from the used turbulence model. In a second step these turbulence statistics are used to reconstruct a time and space resolved turbulence field by utilizing the stochastic reconstruction method FRPM. Together with the underlying mean flow, the turbulent sound sources are used in a subsequent computational aeroacoustic (CAA) simulation. The result is the sound pressure field around the airfoil. The method was validated using experimental results from wind tunnel tests. It shows better accuracy than semi-emperical approaches and is much faster than scale resolving methods. Throughout geometry variations it was shown that airfoils with the same aerodynamic performance could have totally different overall sound emissions in a range of up to 4dB. Relations between parameters from the suction side boundary layer and the noise spectrum were shown. Guidelines for the design of low-noise airfoils were derived. To evaluate the noise levels for a whole wind turbine, a strategy was developed to simulate the overall noise by a summation of independent airfoil sections. Time and space resolved noise maps were calculated for arbitrary observer positions around the wind turbine. Noise reductions up to 2 dB were possible without drawbacks on turbine performance or loads. The presented method can be used in the early stages of future wind turbine design.