POST-STALL FLOW CONTROL ON AN AIRFOIL BY LOCAL UNSTEADY FORCING Part I. Lift, Drag, and Pressure Characteristics

By using a Reynolds-averaged two-dimensional computation of a turbulent flow over an airfoil at poststall angles of attack, we show that the massively separated and disordered unsteady flow can be effectively controlled by a local unsteady excitation with low-level power input. In a certain range of post-stall angles of attack and forcing frequency, the unforced random separated flow can become periodic or quasi-periodi c, associated with a strong lift enhancement, which opens a promising possibility for one to fly beyond the static stall till to a much higher angle of attack. The same local control also leads to, in some situations, a reduction of the drag. On a part of the airfoil the pressure fluctuation is suppressed as well, which would be beneficial for higha buffet control. The computations confirm the physical principles for controlling a massively separated unsteady flow proposed by Wu, Vakili, and Wu (Prog. Aerospace Sci. 28 (1991), 73-131) and are in qualitative agreement with several recent post-stall flow control experiments. In Part I of this two-part paper, we examine the characteristics of lift, drag, and surface pressure without and with control, focusing mainly on the effect of forcing frequency and angles of attack. The necessary conditions for unsteady control to be successful are identified.