Flow Reattachment and Vortex Re-formation on Oscillating Low-Aspect-Ratio Wings

Effects of small-amplitude wing oscillations on flow reattachment and vortex re-formation in the poststall region were studied experimentally. Low-aspect-ratio delta and cropped delta wings with sweep angles in the range of A = 20-50 deg were tested. Effects of forcing frequency, amplitude, wing sweep angle, and mode of the wing oscillations were investigated. It was shown that wing oscillations at an appropriate frequency promotes the reattachment of the otherwise separated flow at poststall incidences. Moreover, with increasing frequency, leading-edge vortices develop with axial flow in the cores, followed by their breakdown. For an optimum range of Strouhal numbers, Sr = 1-2, vortex breakdown is delayed to a maximum distance from the apex. This range of Strouhal numbers compares well with the dominant frequencies of the shear layer instabilities. For sweep angles larger than 20 deg, the reattachment process is generic for all nonslender wings and there is an optimum range of Strouhal numbers.