This paper deals with large onshore wind turbine aeroacoustics. Noise from the NREL 5-MW device is predicted by the permeable-surface Ffowcs Williams-Hawkings equation (FWH-P), starting from the post-processing of LES data on different acoustic surfaces S. Their size and placement is aimed at embedding most of the aerodynamic sources of sound surrounding rotor and nacelle. Due to the presence of eddies that inevitably cross S, the paper compares results from open and closed acoustic surfaces, and the outflow disk averaging technique. The issues related to the interpolation process of LES data on S are discussed, too. In order to assess the LES/FWH-P aeroacoustic platform, LES and FWH-P pressures are compared in the very-near field. It is shown that, within the limits of the discretization settings imposed by the interpolation procedure, and for the Reynolds number working condition herein investigated, the lack of quadrupole sources outside the permeable surface(s) deeply affect the quality of FWH-P acoustic pressures with respect to direct LES signals.
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