Three-dimensional effects of the wake on wind turbine sound propagation using parabolic equation

The influence of three-dimensional (3D) wind turbine wake effects on sound propagation is investigated. To study this, numerical simulations are conducted using a 3D parabolic equation model at low frequencies, with comparisons made to a two-dimensional (2D) approach that neglects transverse horizontal propagation. Three atmospheric stability conditions are investigated using analytical wind profiles that incorporate the wake effects. The wind turbine noise source is specified using an aeroacoustic extended source model. 3D effects due to the wake are shown to be significant, especially for the stable atmosphere. Indeed, horizontal refraction induces focusing that a 2D approach fails at predicting. As the wind turbine blades are rotating, the focal zones are moving accordingly, yielding large variations of the sound levels. Downstream the turbine, amplitude modulation can locally reach values as high as 16.5 dB over long distances. In addition, higher average SPL are predicted by 3D simulations compared to 2D ones, with deviations up to 4.5 dB. For neutral and unstable conditions differences in 2D and 3D sound propagation approaches are smaller, as velocity gradients in the wind turbine wake are smaller.