The flow transients associated with controlled reattachment and separation of the flow over a stalled airfoil are investigated in wind tunnel experiments. Control is effected using surface-mounted synthetic jet actuators that are typically operated at frequencies, which are at least an order of magnitude higher than the characteristic shedding frequency of the airfoil. While at these actuation frequencies the circulation (and hence the lift) of the attached flow is nominally time invariant, actuation at lower frequencies that are commensurate with the shedding frequency results in a Coanda-like attachment of the separated shear layer, organized vortex shedding and substantial oscillation of the circulation. The transients associated with flow reattachment and separation are investigated using amplitude modulation of the actuation waveform. Phase-locked measurements of the velocity field in the near wake of the airfoil and corresponding flow visualizations show that the transients that are associated with the onset of reattachment and separation are accompanied by the shedding of large-scale vortical structures and oscillations of the circulation. Pulsed modulated actuation of the actuation waveform is used to capture these transient effects and augment the increase in lift that is obtained by conventional time-harmonic actuation.
International Journal of Heat and Fluid Flow, Volume 23, Issue 5, p. 690-299, 2002.