Effects of the observation angle on wind turbine noise levels are today only partly understood. It is clear that positions close to the rotor plane fall within generally low levels area, with a ~-4 dB offset compared to downstream positions. However asymmetries and complex lobes patterns have been observed but not fully explained. To provide additional experimental material, a novel measurement campaign is conducted within the framework of the TWAIN European research project. A commercial wind turbine is fully surrounded by 24 microphones to analyse the change in both sound magnitude and directivity. OASPL patterns are observed and analysed in general agreement with the literature, together with variations of the spectral content. An assessment of the validity of the omnidirectionality assumption used in the IEC61400-11 standard is proposed using the microphone circle measurements, indicating that this hypothesis may not lead to very large deviations in the sound power estimates. Another part of the experiment consists in testing the turbine under yaw offset with the perspective of wind farm control. Observed effects of yaw angle on the directivity boils down to a rotation of the directivity pattern with the turbine orientation, but also to a general variation in noise levels. In yawed configurations, the microphone circle allows to partially counteracts the pattern rotation to estimate variations of sound power levels. In the case of a positive yaw of +20°, we observe a slight increase of 1 to 2 dB.
Link to paper: https://zenodo.org/records/22030648