Interaction between vortex shedding and flap oscillation in separation control of a stalled aerofoil

Flow control for lifting surfaces at high angle of attack is essential to increase the lift coefficient. Inspired by covert feathers of birds, a passive rotatable flap is added to the suction surface of an aerofoil to analyze its effectiveness and flow control mechanism from the perspective of fluid structure interaction. The lift coefficients of the aerofoil are measured by a balance, increasing near the stall angle and reaching maximum augmentation of 25% with the flap. Particle image velocimetry measurements show that the transition of trailing edge shear layer to vortex shedding occurs later due to the flap, and the separation zone becomes smaller, which work together to enhance the lift-to-drag ratio. Flap movement is influenced by unsteady trailing edge vortex shedding, with the flap pop-up angle increasing together with the aerofoil angle of attack and reaching a maximum value around ${84}^{\circ }$, beyond which it decreases again. Moreover, the experimental data observes the low frequency flow pulsation and flap oscillation in addition to the high frequency trailing edge vortex shedding, which has a modulation effect on the flow fluctuation and flap oscillation magnitude.