  A good understanding of wind turbine noise propagation is relevant to better
measure the impact of turbines on the environment. In this study, we developed
numerical simulations to study the impact of a 2D hill on the sound propagation
for a turbine in non flat terrain. The simulations employed a propagation model
derived from the linearized Euler equations, solved in a moving frame following
the wavefront. The wind turbine noise was modeled using a point source
approach. The flow structure in the atmospheric boundary layer and the wind
turbine wake were obtained from large-eddy simulations. We find that the wind
turbine wake acts as a waveguide that focuses sound propagation towards the
ground. This effect is most pronounced for the flat terrain case and when the
turbine is placed on the hill. The sound attenuation is more localized and
closer to the turbine when the wind turbine is placed on the hilltop than in
the flat case. As the wind speed increases, the sound focusing is observed
closer to the turbine. For a turbine placed in front of the hill, the sound
propagation is mainly determined by the flow over the hill. This demonstrates
that terrain topography can have surprising effects on wind turbine noise
propagation.
