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This study presents a novel spatially disrupted flow control system inspiredby the covert feathers on bird wings. The system is a passive flow controlsystem consisting of multiple feather-inspired flaps that dynamically interactwith the surrounding flow to mitigate stall. Incorporating covert-inspiredflaps on the suction side of the airfoil resulted in a substantial increase inlift (up to 50%) and a substantial reduction in drag (up to 30%) in post-stallconditions. Using wind tunnel experiments and time-resolved particle imagevelocimetry, the physical mechanisms responsible for post-stall liftimprovements and drag reduction are identified as (1) shear layer interactionand (2) pressure dam effect. In the first mechanism, flap deployment reducesthe geometric adverse pressure gradient that the flow encounters, reducing thedegree of flow separation. In the second mechanism, the deployed flap acts as abarrier, preventing the relatively high pressure downstream from propagatingupstream of the airfoil. The flow control mechanism employed was a function ofthe location of the flap. Flaps near the leading edge interacted mainly withthe shear layer, while flaps near the trailing edge induced a pressure dameffect. Increasing the number of flaps along the chord increased the gain inlift and the reduction in drag. However, additive performance enhancements weresensitive to spatial distribution and flow control mechanisms. The shear layerinteraction mechanism is found to be additive; that is, the deployment ofadditional flaps increases the lift gain, whereas the pressure dam effect isnot.

Feather-inspired flow control: The flow physics of spatially distributed covert flaps