Local Acoustic Forcing of a Wing at Low Reynolds Numbers

At transitional Reynolds numbers (104-105), many smooth airfoils experience laminar flow separation and possible turbulent reattachment, where the occurrence of either state is strongly influenced by small changes in the surrounding environment. The Eppler 387 airfoil is one of many airfoils that can have multiple lift and drag states at a single wing incidence angle. Prestall hysteresis and abrupt switching between stable states occur due to sudden flow reattachment and the appearance of a separation bubble close to the leading edge. Here, we demonstrate control of the flow dynamics by localized acoustic excitation through small speakers embedded beneath the suction surface. The flow can be controlled not only through variations in acoustic power and frequency, but also through spatial variations in forcing location. Implications for control and stabilization of small aircraft are considered.