Application of multiple scattering model towards investigating the reflection of wind turbine generated sound from rough ground

• Wind turbine sound levels are predicted using semi-analytical scattering model. • The scattering model allows faster computation of sound compared to boundary element methods. • Orientation of rough ground surface has significant influence on scattering effects. • Distributions with axes in the direction of the incident wave give smallest scattering effect. This work investigates the reflection of wind turbine generated sound from a rough ground represented by a distribution of semicylinders. Formulations for a multiple scattering model are developed by making some modifications to an existing multiple scattering theory. The modified multiple scattering model accounts for the angle of incidence outside the plane perpendicular to the semicylinder axes as well as the impedance of the ground plane. To account for the impedance of the semicylinders themselves, an impedance boundary condition is applied at their surfaces. Furthermore, in the context of the sound generated by wind turbines, where the source is often far from the receiver’s location, the incident wave is approximated as a plane wave. A study is carried out to compare predictions from the modified multiple scattering model to results from a boundary element method. The results between the modified model and the boundary element method agree satisfactorily. The results from the multiple scattering model alone have highlighted the significance of the orientation of a ground roughness on propagation. When the wind turbine sound propagates in the direction of the semicylinder axes, the rough ground effect is most like the flat ground equivalent. That is, the rough ground effect is at its smallest. The purpose of validating the modified multiple scattering model is to establish an efficient method for investigating the effect of a rough ground surface on the propagation of wind turbine noise.