Dynamic stall of the wind turbine airfoil and blade undergoing pitch oscillations: A comparative study

Abstract

Dynamic stall significantly causes the unsteady aerodynamic loads on horizontal axis wind turbines. However, many studies are about dynamic stall of 2D airfoils rather than 3D rotating blades. The 3D dynamic stall is therefore still poorly understood and also challenging to predict accurately. This paper presents comparative analyses of dynamic stall among the 2D airfoil, 3D non-rotating blade and 3D rotating blade undergoing sinusoidal pitch oscillations. The parameters of 2 radial locations and 11 pitch conditions are also studied. All 3D aerodynamic responses come from the NREL Phase VI experiment. URANS simulations are used to predict the 2D aerodynamic responses of NREL S809 airfoil. Rotational augmentation is found to make the key difference between 2D airfoil flow and 3D blade flow. Rotational augmentation effectively suppresses the extension of separated flow during the upstroke process, and significantly accelerates the flow reattachment during the downstroke process. The onset of dynamic stall is therefore delayed with the maximum lift coefficient increased by 46%. The aerodynamic hysteresis intensity is also greatly reduced by 61%. Increasing the mean angle of attack (AOA), AOA amplitude and reduced frequency can further delay the onset of dynamic stall. On the other hand, rotational augmentation may unexpectedly produce a negative aerodynamic pitch damping and cause stall flutter on the inboard blade. Increasing AOA amplitude could reduce this negative damping and improve the torsional aeroelastic stability. This work might deepen the understanding of 3D dynamic stall with rotational augmentation on wind turbines.