Dynamic stall control by periodic excitation, Part 2: Mechanisms

Dynamic flow separation and its control over a stationary deflected 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 significantly attenuated trailing-edge separation. Within this framework, a comparison of static and airfoil phase-locked dynamic pressure data acquired in the vicinity of maximum incidence (α ≈ 25 deg) revealed that chordwise pressure distributions were independent of the airfoil pitching frequency and that the generation and advection of LCSs were not significantly affected by the dynamic airfoil pitching motion. Furthermore, disparities between static and dynamic data diminished as the excitation frequency increased relative to the airfoil pitching frequency. Oscillations of the aerodynamic coefficients induced by the excitations were negligibly small but served to regulate airfoil cycle-to-cycle disparities typical of the baseline poststall regime.