The response of a NACA 0012 airfoil in the poststall region was examined when the unsteady freestream velocity varied sinusoidally as a function of time. Several operational parameters, including the reduced frequency, the angle of attack, and the amplitude of the freestream oscillation were systematically varied over a large range of values. Lift measurements showed that under the optimum condition, the phase-averaged lift force becomes maximum. For the optimal case, flow visualization shows that the high aerodynamic loads are due to a large coherent vortex which sheds from the leading-edge and stays on the wing for an appreciable portion of the cycle.
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