Combining unsteady blade pressure measurements and a free-wake vortex model to investigate the cycle-to-cycle variations in wind turbine aerodynamic blade loads in yaw

摘要

Prediction of the unsteady aerodynamic flow phenomenon on wind turbines is challenging\ud\udand still subject to considerable uncertainty. Under yawed rotor conditions, the wind turbine blades\ud\udare subjected to unsteady flow conditions as a result of the blade advancing and retreating effect and\ud\udthe development of a skewed vortical wake created downstream of the rotor plane. Blade surface\ud\udpressure measurements conducted on the NREL Phase VI rotor in yawed conditions have shown that\ud\uddynamic stall causes the wind turbine blades to experience significant cycle-to-cycle variations in\ud\udaerodynamic loading. These effects were observed even though the rotor was subjected to a fixed\ud\udspeed and a uniform and steady wind flow. This phenomenon is not normally predicted by existing\ud\uddynamic stall models integrated in wind turbine design codes. This paper couples blade pressure\ud\udmeasurements from the NREL Phase VI rotor to a free-wake vortex model to derive the angle of\ud\udattack time series at the different blade sections over multiple rotor rotations and three different yaw\ud\udangles. Through the adopted approach it was possible to investigate how the rotor self-induced\ud\udaerodynamic load fluctuations influence the unsteady variations in the blade angles of attack and\ud\udinduced velocities. The hysteresis loops for the normal and tangential load coefficients plotted against\ud\udthe angle of attack were plotted over multiple rotor revolutions. Although cycle-to-cycle variations\ud\udin the angles of attack at the different blade radial locations and azimuth positions are found to be\ud\udrelatively small, the corresponding variations in the normal and tangential load coefficients may be\ud\udsignificant. Following a statistical analysis, it was concluded that the load coefficients follow a normal\ud\uddistribution at the majority of blade azimuth angles and radial locations. The results of this study\ud\udprovide further insight on how existing engineering models for dynamic stall may be improved\ud\udthrough the integration of stochastic models to be able to account for the cycle-to-cycle variability in\ud\udthe unsteady wind turbine blade loads under yawed conditions.