Large Low-Frequency Oscillations Initiated by Sub-Optimal Flow Control on a Post-Stall Airfoil

Abstract

A large amplitude low frequency oscillation in the lift force is detected for an airfoil experiencing leading edge stall. This behavior has been documented experimentally and numerically in the literature, but with an incomplete understanding as to its origin. In the current study this behavior is shown to interact with standard methods of active flow control. 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. Time-resolved PIV and surface pressure measurements are used to evaluate the spatial and temporal characteristics of the oscillation. The use of forcing allows for further investigation into the physics of this oscillation, specifically in the dynamic behavior and primary time scale. The paper is concluded with a discussion of implications for active flow control. Each flow control technique is shown to excite the oscillation at moderate forcing amplitudes and remove the oscillation with higher forcing amplitude. 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. Unsteady behavior of this amplitude could have adverse effects for structural mechanics and should be considered in the design of a robust active flow control system.