Sound propagation from a wind turbine in a hilly environment

Sound propagation in the atmospheric boundary layer can be strongly affected by vertical gradients in the wind speed and air temperature, and by atmospheric turbulence. In addition, undulating terrain will either partly shield or focus sound waves, highly impacting sound exposure levels. These aspects become especially important in case of wind turbines present on a ridge, such placement often being efficient to harvest wind energy due to the wind speeding up. In this study, sound propagating from a wind turbine, positioned at a ridge, towards receivers along a valley, is numerically simulated with the Parabolic Equation (PE) method. A combination of an analytical starting field and the conformal mapping method shows to be a useful and numerically efficient approach, overcoming some issues as identified and discussed in this work. Example calculations show the importance of the valley ground impedance for wind turbine noise exposure.