Research on the rotor speed and aerodynamic characteristics of a dynamic yawing wind turbine with a short-time uniform wind direction variation

Dynamic yawing caused by wind direction variation is an important operating case for wind turbines. Because experimental data are lacking, existing studies assume that the rotor speed and pitch angle are constant, which cannot reflect the actual working states of wind turbines in the short-time variation of the wind direction. In this study, the rotor speed and power characteristics of wind turbine with the condition of variable wind direction were obtained through experiments. Using the experimental time-varying rotor speed curve, the aerodynamic characteristics were further studied using computational fluid dynamics method. The results showed that the rotor speed decreased due to the short-term wind direction variation. The final rotor speed and the stable time were related to the initial tip speed ratio and the wind direction variation rate, respectively. The dynamic yawing with the assumption of invariable rotor speed overestimated the average torque and underestimated the variation of torque. The continuous decreasing rotor speed made the angle of attack and its fluctuation degree larger. This caused a wide range of the flow separation for the whole blade and large fluctuation of the axial/tangential force load, and it doubled the fluctuation frequency of the aerodynamic load in the blade root region.