Experimental investigation of the effect of wake steering on the noise emission of a commercial wind turbine

Abstract. Wake steering is a wind farm flow control (WFFC) strategy that involves intentionally misaligning the most upstream wind turbines to deviate their wakes away from the downstream wind turbines. This study investigates the acoustic implications of such a strategy. A novel acoustic setup was implemented, involving 24 ground-based sound level meters arranged on a circle around an industrial 2.2 MW wind turbine, positioned at tip-height distance. This configuration enables a fine angular resolution of 15°. Incoming wind conditions were monitored using both a nacelle-mounted plus a ground-based lidar. A test protocol closely aligned with IEC 61400-11-1 was followed to characterize the turbine's noise emissions under various yaw offset settings, ranging from −20° to +20° across a broad spectrum of flow conditions. A dedicated data cleaning and analysis procedure was developed to derive ground-level turbine noise directivity patterns. In the absence of yaw misalignment, the directivity patterns exhibit the typical two-lobe structure. However, slight but statistically significant asymmetries were also observed: the downstroke side was in average 0.6 dB(A) louder than the upstroke side, and the downwind side was 0.4 dB(A) louder than the upwind side. These specificities are not captured by most of the analytical models used by the profession. When yaw control was applied, a modest increase of approximately 0.6 dB(A) was observed in the estimated overall sound power level. The results from this innovative experiment confirm the fact that operators must consider other metrics than just power production when implementing WFFC on their projects, and that more advanced noise models are required for the development of a framework allowing multi-objective WFFC.