Tunable covert-inspired flow control: An experimental and model-based investigation

Bird upperwing coverts are contour feathers observed to passively deploy during high angle of attack maneuvers. This study investigates the dynamics of passive covert-inspired torsionally hinged flaps at Re=200,000, assessing their impact on the surrounding flow field and the resulting aerodynamic performance. First, the study shows that the torsionally hinged flaps effectively mitigate flow separation at a post-stall angle of attack α=24^∘ with up to 22% lift improvements and 7% drag reduction. Second, results from a newly developed analytical model reveal that the flap's oscillation frequency depends on the local flow velocity ahead of it and varies significantly as a function of the flap location. Trailing-edge flaps that experience low local velocities oscillate at their structural natural frequency. Meanwhile, leading-edge flaps with higher local velocity oscillate at a higher frequency than their natural frequency due to the aerodynamically induced added stiffness. The flap dynamics also depend on the energy spectrum of the flow, where flaps with lower effective oscillation frequencies exhibit higher fluctuations because they coincide with the flow's high-energy frequency band. Finally, varying the flap's inertia and hinge stiffness tunes its mean position and fluctuations to enhance the flow control mechanism employed by the flap. For instance, trailing-edge flaps enhance lift by acting as a pressure dam, with maximum effectiveness when the flap is perpendicular to the airfoil's surface. Leading-edge flaps, on the other hand, improve lift by reducing adverse geometric pressure gradients, with maximum aerodynamic benefits occurring when the flap interacts with the shear layer with minimal fluctuations. This article provides a physics-based framework to enhance our understanding of covert-inspired flaps. Such knowledge can inform the design of passive and adaptive flow control strategies for various engineering applications.