Estimating the unsteady angle of attack from blade pressure measurements on the NREL phase VI rotor in yaw using a free-wake vortex model

The unsteady aerodynamic phenomena associated with yawed wind turbines are still poorly understood and are therefore challenging to predict accurately. The main issues concern the geometry of the skewed vortical wake formed behind the turbine, the unsteady flow field at the rotorplane induced by the vortical wake, as well as the aerodynamic effects of unsteady flow over the blade sections. Blade pressure measurements on a rotating blade of a yawed wind turbine can be very useful in obtaining further insight of yaw aerodynamics. However, in doing so, knowledge of the time-dependent angle of attack and induced velocity distributions at the rotoplane is an indispensable requirement. This paper presents a method to derive such distributions for the NREL phase VI turbine using a free-wake vortex model. The study considered different operating conditions in yaw that yielded both attached and separated flows over the blades. The derived free-wake geometry solutions are plotted together with the corresponding wake trailing and shed circulation distributions. These plots help investigate how the unsteady bound circulation formed at the blades is eventually convected into the wake. The derived results are helpful to develop more reliable aerodynamic models for wind turbine design codes.