The influence of oscillating trailing-edge flap on the dynamic stall control of a pitching wind turbine airfoil

Abstract

Unsteady operating environment of a horizontal axis wind turbine can induce excessive loads on the blades, originating from rapid variations in angle of attack and consequently dynamic stall (DS) occurrence. Therefore, it is of utmost importance to control the flow around a blade by which fatigue damage is likely to happen. Using two-dimensional incompressible unsteady Reynolds-averaged Navier–Stokes equations in OpenFOAM package, a series of simulations are carried out to assess the viability of an oscillating deformable trailing-edge flap (DTEF) in load and DS control on a pitching wind turbine airfoil which experiences deep DS at Re = 420,000. Results reveal whether or not the airfoil is equipped with an oscillating DTEF, DS vortex forms at high angles of attack. The size, strength and traveling of the DS vortex, however, can be influenced by out-of-phase deflection of the DTEF. More effectively, the change in the airfoil camber line during flap oscillation can remarkably affect the pressure distribution around the airfoil, and hence, significant load alleviation and mean lift enhancement are achievable, all of which help the wind turbine performance and enhance the life span of the components. Moreover, a parametric study on flap size and amplitude of deflection together with a comparison between a discrete flap and a DTEF suggests an out-of-phase oscillation of a large gently curved DTEF, up to 30% of the total chord, with similar amplitude and frequency with respect to the airfoil is the best condition under which fatigue load control as well as enhancement in resultant load for a blade rotation can take place.