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

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 is 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 are monitored using both a nacelle-mounted and a ground-based lidar. A test protocol closely aligned with IEC 61400-11-1 is followed to characterize the turbine's noise emissions under various yaw misalignment settings, ranging from −20 to +20° across a broad spectrum of flow conditions. A dedicated data cleaning and analysis procedure is developed to derive ground-level turbine noise directivity patterns. In the absence of yaw misalignment, the directivity patterns exhibit a typical two-lobe structure. However, slight but statistically significant asymmetries are also observed: the downstroke side is on average 0.6 dB(A) louder than the upstroke side, and the downwind side is 0.4 dB(A) louder than the upwind side. These specificities are not captured by most of the analytical models used in the field. When yaw misalignment is applied, a modest increase of approximately 0.6 dB(A) is observed in the estimated overall sound power level. The results from this innovative experiment confirm the fact that operators must consider metrics other than just power production when implementing WFFC in their projects and that more advanced noise models are required for the development of a framework allowing multi-objective WFFC.