Separation Control by External Acoustic Excitation at Low Reynolds Numbers

AtReynolds numbers approaching those ofmicro air vehicles (both engineered and natural), the Eppler 387 airfoil (in common with many other smooth profiles) can have multiple lift and drag states at a single wing-incidence angle. Prestall hysteresis and abrupt switching between stable states result from sudden flow reattachment and the appearance of a large separation bubble. It is shown that control of the dynamics can be achieved using external acoustic forcing. Separation control, hysteresis elimination, and more than 70% increase in lift–drag ratio are obtained at certain excitation frequencies and sound pressure levels. The global flow around the wing is effectively modified, and large, stable vortical structures appear in the separated shear layer. Correlation between the effects of acoustic excitation and wind-tunnel resonance shows that the antiresonances in an enclosed chamber correspond to the largest improvement in wing performance. Implications for control and stabilization of small aircraft inside and out of enclosed boxes are considered.