POST-STALL FLOW-CONTROL ON AN AIRFOIL BY LOCAL UNSTEADY FORCING

By using a Reynolds-averaged two-dimensional computation of a turbulen t flow over an airfoil at post-stall angles of attack, we show that th e massively separated and disordered unsteady flow can be effectively controlled by periodic blowing-suction near the leading edge with low- level power input. This unsteady forcing can modulate the evolution of the separated shear layer to promote the formation of concentrated li fting vortices, which in turn interact with trailing-edge vortices in a favourable manner and thereby alter the global deep-stall flow field . In a certain range of post-stall angles of attack and forcing freque ncies, the unforced random separated how can become periodic or quasi- periodic, associated with a significant lift enhancement. This opens a promising possibility for flight beyond the static stall to a much hi gher angle of attack. The same local control also leads, in some situa tions, to a reduction of drag. On a part of the airfoil the pressure f luctuation is suppressed as well, which would be beneficial for high-a lpha buffet control. The computations are in qualitative agreement wit h several recent post-stall flow control experiments. The physical mec hanisms responsible for post-stall flow control, as observed from the numerical data, are explored in terms of nonlinear mode competition an d resonance, as well as vortex dynamics. The leading-edge shear layer and vortex shedding from the trailing edge are two basic constituents of unsteady post-stall flow and its control. Since the former has a ri ch spectrum of response to various disturbances, in a quite wide range the natural frequency of both constituents can shift and lock-in to t he forcing frequency or its harmonics. Thus, most of the separated flo w becomes resonant: associated with much more organized flow patterns. During this nonlinear process the coalescence of small vortices from the disturbed leading-edge shear layer is enhanced, causing a stronger entrainment and hence a stronger lifting vortex. Meanwhile, the unfav ourable trailing-edge vortex is pushed downstream. The wake pattern al so has a corresponding change: the shed vortices of alternate signs te nd to be aligned, forming a train of close vortex couples with stronge r downwash, rather than a Karman street.