Large Low-Frequency Oscillations Initiated by Flow Control on a Poststall Airfoil

Abstract

A large-amplitude low-frequency oscillation has been reported in the literature for some airfoils in a small range of angles of attack before stall. In the current study, the same low-frequency oscillations are shown to be triggered by two methods of active flow control at poststall angles. Active control is applied in the form of external acoustic excitation or a row of discrete jets that are operated in a pulsed or steady mode. Conditionally averaged particle image velocimetry and time-resolved surface pressure measurements are used to evaluate the spatial and temporal characteristics of the oscillation. By systematically increasing forcing amplitude, each flow control technique is shown to first excite low-frequency oscillation up to a maximum lift increase, after which subsequent increases in forcing amplitude attenuate and ultimately eliminate low-frequency oscillation altogether. At maximum oscillation amplitude, the behavior is highly periodic in the case of acoustic forcing and intermittent in the case of pulsed or steady jets. The dynamics associated with the oscillation are shown to include the shedding of a large vortex structure, with noted similarity to the dynamic stall vortex seen on pitching or plunging airfoils.