Feather-inspired Passive Flaps for Flow Control on a Finite Rectangular Wing

Bird wings are equipped with multiple flow control devices, one of which is the covert feathers.The covert feathers act as high-lift aeroelastic flow control devices for stall mitigation. Thisstudy investigates the performance characteristics of covert-inspired passive torsionally hingedflaps mounted on the suction side of a finite rectangular wing with AR= 4.67 at Re= 200, 000.Wind tunnel experiments including force and flow field measurements are used to analyze theeffect of flap location, hinge stiffness, and flap inertia on the wing aerodynamic forces (i.e., liftand drag) and the flow physics of the wing-flap system. Two post-stall α ranges are defined inthe study: the partial stall regime (19° ≤ 26°) and the global stall regime (28°≤ α≤ 36°).Results show that the flaps can reduce drag by up to 15% and improve lift by up to 5% inthe partial stall regime. The leading edge flaps are effective in reducing drag in this regimesince they can lessen the effect of induced drag at the wing tip, where the downwash effect isprominent, and decrease pressure drag at the wing areas with separated flow by bringing theshear layer closer to the wing surface, leading to a substantial overall drag reduction. In theglobal stall regime, the flaps reduce drag by up to 5% and improve lift by up to 20%. Theleading edge flap mitigates flow separation in this regime by interacting with the shear layer.However, the lift increase for the flap cases leads to an induced drag penalty, counteracting thereduction in pressure drag, resulting in a minimal overall drag reduction. Furthermore, weshow that the flap inertia and hinge stiffness control the flap deflection angle which in turncontrols to the change in the lift and drag forces relative to the baseline case.