Comparing unsteady and steady jets issued into a flat plate laminar boundary layer

Jets have been widely used for flow control applications, due to their ability to enhance mixing and control separation, but it is unclear the role jet steadiness plays in flow control effectiveness. Here we compare unsteady (synthetic) and steady rectangular jets issued into a laminar boundary layer with special focus on the flow features and statistics. The compared jets had matched velocities, geometry, and orientation. The effects of varying pitch (the angle between the jet exit centerline and the wind tunnel floor) and skew (rotation about the jet centerline) of the jet orifice are highlighted. We used stereoscopic particle image velocimetry to capture spanwise planes upstream and downstream of the orifice. The vortices produced by synthetic jets were shown to be much stronger than those produced by steady jets, despite producing similar flow patterns. Exploring the time-- and phase--averaged vorticity transport equation reveals that the time varying vorticity term is the reason for the enhanced vortex structure. In addition, we compare and contrast steady and unsteady jets through measures of flow control effectiveness---including mixing, boundary layer momentum addition, and jet penetration.