Role of actuation frequency in controlled flow reattachment over a stalled airfoil

The effect of the actuation frequency on the manipulation of the global aerodynamic forces on lifting surfaces using surface-mounted fluidic actuators based on synthetic (zero mass flux) jet technology is demonstrated in wind-tunnel experiments. The effect of the actuation is investigated at two ranges of (dimensionless) jet formation frequencies of the order of, or well above, the natural shedding frequency. The vortical structures within the separated flow region vary substantially when the dimensionless actuation frequency F+ is varied between O(1) and O(10). When F+ is O(1), the reattachment is characterized by the formation of large vortical structures at the driving frequency that persist well beyond the trailing edge of the airfoil. The formation and shedding of these vortices leads to unsteady attachment and, consequently, to a time-periodic variation in vorticity flux and in circulation. Actuation at F+ of O(10) leads to a complete flow reattachment that is marked by the absence of organized vortical structures along the flow surface. This suggests that when the actuation frequency is high enough, the Coanda-like attachment of the separated shear layer to the top (suction) surface of the airfoil can be replaced by completely attached flow for which separation may be bypassed altogether.