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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.

Due to the unsteady nature of the flow around horizontal-axis wind turbines, the blades are subjected to severe unsteady and fatigue loads. This necessitates an in-depth aerodynamic analysis of flow control techniques to enhance the performance of a wind turbine as well as the lifetime of its components. Using OpenFOAM package in this study, a series of two-dimensional incompressible simulations are performed to present a deeper insight into the aerodynamic characteristics of an oscillating deformable trailing-edge flap, as a promising flow control device, in a sinusoidal pitching motion of an S809 airfoil. Herein, it is of particular interest to investigate the effects of deformable trailing-edge flap size, oscillation frequency, and the phase shift with reference to airfoil motion on lift and drag hysteresis loops. For this purpose, a pure-pitching motion of an S809 airfoil without flap deflection is considered as the benchmark problem in which the airfoil oscillates in the near-stall region at [Formula: see text]. After validation and verification of our simulations through comparison against the corresponding experimental and numerical work, a comprehensive investigation is conducted to study the effects of the aforementioned parameters on the aerodynamic loads. Our results reveal the fact that an out-of-phase deflection of the deformable trailing-edge flap with a frequency equal to the airfoil frequency can significantly mitigate the fatigue load. Under these circumstances, an increase in the deformable trailing-edge flap size can also help the airfoil experience less-severe loads in a cycle of motion. Furthermore, higher values of deformable trailing-edge flap frequency or other values of phase shift except the out-of-phase oscillation cannot alleviate fatigue loads. An airfoil under these conditions can, however, enhance the resultant load required for a blade rotation in the case of low wind periods.

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