Investigation of the noise sources on a vertical axis wind turbine using an acoustic array

Small scale vertical axis wind turbines (VAWTs) are capable of capturing a significant proportion of the energy in a gusty wind. This makes them ideal for deployment in built environments but in such locations they are inherently close to people meaning that the noise is a significant problem. In order to better understand the mechanisms of noise generation on a VAWT a series of experiments was conducted using an acoustic array to locate the dominant sources of noise on a model scale VAWT over a range of tip speed ratios. Conventional beamforming methods, using a monopole or dipole source model, provided some information about the noise sources but are of limited utility in the case of such a complex, moving source. A detailed examination of the microphone cross spectra was therefore undertaken to identify the nature of the sound sources. A strong harmonic component was observed in the spectra, and was found to be the dominant source for frequencies up to the 30th harmonic of the rotor angular velocity. The harmonic source was found to be strongest at low tip speed ratios and is understood to be radiated by the unsteady loads on the blades, which are exacerbated when they experience dynamic stall. At higher frequencies spectral scaling methods were applied to attempt to identify the noise generating mechanisms. Below 1kHz it was found that the noise scaled with the fifth power of velocity implying that the sound was being radiated from the trailing edge. Above 1kHz a broadband peak was observed and the spectral scaling methods identified this as laminar boundary layer tonal noise. A comparison of the spectra from the wind tunnel model with those from a full-scale VAWT confirmed that the two are qualitatively similar meaning that results from the wind tunnel measurements are transferable to a real VAWT