Forced Flow Reattachment Process and the Effect of Pressure Gradient

Abstract

Stereo particle image velocimetry experiments were conducted to explore the flowfield resulting from the interaction of finite-span synthetic jet actuators and a turbulent boundary layer over an S817 airfoil at various angles of attack. Three specific scenarios were investigated: a favorable pressure gradient, an adverse pressure gradient, and a separated flow in which the synthetic jet actuators reattached the flow. Multiple coherent vortical structures were shed during the reattachment process, for which the circulation associated with these structures was composed of the “trapped” vorticity within the separated shear layer. The shedding of this vorticity was shown to be an inherent part of the reattachment mechanism. In a smaller, local measurement domain, volumetric flowfields were reconstructed near the synthetic jet orifice. It was shown that the presence of an adverse pressure gradient resulted in an increase of momentum diffusion through the boundary layer, in addition to the rate at which the jet’s axis switching occurred. At the highest angle of attack tested, where the flow was reattached through actuation, the coherent structures associated with the synthetic jet dissipated significantly quicker spatially, as much of the energy associated with the actuation was used to maintain attached flow.