Dynamic Stall Control by Periodic Excitation, Part 2: Mechanisms

Dynamic e ow separation and its control over a stationary dee ected surface are used to demonstrate the timescale disparity between the process of dynamic stall, which is dominated by the dynamic stall vortex (DSV), and the excitation-induced large coherent structures that effect its control. Appreciation of this disparity provided a framework for analyzing dynamic stall control on a NACA 0015 airfoil, where leading-edge excitation had effectively eliminated the DSV and signie cantly attenuated trailing-edge separation. Within this framework, a comparisonofstaticandairfoilphase-lockeddynamicpressuredataacquiredin thevicinity ofmaximum incidence (® o 25 deg) revealed that chordwise pressure distributions were independent of the airfoil pitching frequency and that the generation and advection of LCSs were not signie cantly affected by the dynamic airfoil pitching motion.Furthermore,disparitiesbetweenstaticanddynamicdatadiminishedastheexcitationfrequencyincreased relative to the airfoil pitching frequency. Oscillations of the aerodynamic coefe cients induced by the excitations were negligibly small but served to regulateairfoil cycle-to-cycle disparities typical of the baselinepoststall regime.

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