Generation, Propagation, and Reduction of Aerodynamic Broadband Noise from Wind Turbines

Abstract

Aerodynamic broadband noise generated from horizontal axis wind turbines is investigated in this research. First, a numerical method to predict wind turbine aerodynamic noise is proposed by using an analytic trailing edge noise model and a wall-pressure spectral model for flow with adverse pressure gradient. Reynolds-averaged Navier-Stokes simulation and XFOIL code are used to obtain the wall-point pressure spectra at blade sections. This method is fast and reliable numerical model which is applicable for use in industrial applications. For the validation of the numerical method, noise measurement is also carried out for a 10kW wind turbine, and a comparison is made between the predicted results and the measured data. Using the proposed method, the aerodynamic noise from a 3MW wind turbine is predicted at various inflow wind speeds and distances. Moreover, time domain simulation for the wind turbine aerodynamic noise is performed to examine the main cause of the amplitude modulation of wind turbine noise, as well as to compare the acoustical characteristics depending on the observer locations. Analytic trailing edge noise and turbulence ingestion noise models are used to determine the unsteady pressure on the blade surface. The far-field acoustic pressure due to the unsteady pressure is calculated using the acoustic analogy theory. By using a strip theory approach, the two-dimensional noise model is applied to rotating wind turbine blades. The numerical results indicate that, although the operating and atmospheric conditions are identical, the acoustical characteristics of wind turbine noise can be quite different with respect to the distance and direction from the wind turbine. Using these prediction results, it is investigated why the amplitude modulation of wind turbine noise is heard even at long distances from a wind turbine. Furthermore, to reduce trailing edge noise from a 10kW wind turbine, the airfoil shape and the planform of the wind turbine blade are modified using optimization techniques based on genetic algorithms. The optimized airfoil is first determined based on a section of the rotor blade, and then the optimized blade is designed with this airfoil. A wind tunnel experiment is also performed to validate the design optimization. In addition, wind turbine blades with serrated trailing edge are also examined for the noise reduction. To examine the effect of serrated trailing edge on the trailing edge noise reduction, a field experiment is carried out based on a 10kW wind turbine.