Wind Turbine Trailing Edge Noise: Mitigation of Normal Amplitude Modulation by Individual Blade Pitch Control

Abstract

This work deals with the elimination or mitigation of amplitude modulation (AM) at any single listener position in the near proximity of a state-of-the-art three-bladed horizontal axis wind turbine by applying individual azimuthal pitch control to the blades. The investigation is restricted to normal AM (NAM) in the close vicinity of the turbine. The effects of the listener position, shear rate of the inflow, and flexibility of the blades are studied. The methodology is model-based: Among others, the fundamental sub-models are the Brooks, Pope and Marcolini airfoil self-noise model, Howe's directivity model for trailing edge sound sources, Rozenberg's convective amplification model for sound sources in motion and an atmospheric damping model. The aeroelastic behaviour of the complete wind turbine is modelled via a multibody approach as encoded in the Horizontal Axis Wind turbine simulation Code 2nd generation (HAWC2) from the Danish Technical University (DTU). The complete model is integrated into an optimisation scheme that seeks an azimuthal pitch control of each blade for minimal modulation depth of the overall sound pressure level at an arbitrarily chosen listener position. The sound prediction model is validated by experimental data from 160 commercial 3.2 MW turbines and the University of Siegen small-scale research wind turbine. As a result, individual blade pitch control as obtained by the optimisation scheme eliminates NAM at any targeted listener position, at the expense of an increase of NAM at other ground locations. Cancelling NAM for a lateral listener position requires the largest amplitude and gradients in the azimuthal pitch control. In case of elastic blades, the required amplitude of the optimised azimuthal pitch control is substantially larger as compared to rigid blades. It is also shown that azimuthal pitch control causes small fluctuations of the axial thrust and aerodynamic power output of the turbine.