Wind tunnel studies of wind turbine yaw and speed control effects on the wake trajectory and thrust stabilization

Abstract

Yaw-based wake steering is pursued as a means to avoid upstream turbine wake influence on downstream turbines. However, the method's effectiveness is likely restricted by rapid wind direction changes that cause wake trajectory oscillations. To address this issue, a thrust-based wake trajectory stabilization method is proposed here. The changes to the cross-stream thrust, resulting from unintentional yaw misalignments, caused due changes in inflow wind direction could be counteracted by changing rotor speed and blade pitch, thus stabilizing wake trajectory oscillations. This paper presents a wind tunnel case study to explore the effect of combining yaw-based wake steering and fast-changing rotor speed control on the cross-stream thrust component and hence on the wake trajectory. Although a feedback controller was not implemented, it is demonstrated that wake trajectory oscillation could be contained using faster actuating mechanisms, such as rotor speed or pitch. Here the wake oscillations due to wind direction changes emulated by sinusoidally yawing the turbine, were restricted by varying turbine rotation speed. Direct thrust measurements as well as detailed Particle Image Velocimetry (PIV) and hotwire wake data show how this combination can successfully suppress the dynamic cross-stream thrust component perturbations and subsequently the wake trajectory oscillation around a desired position.