The formation and evolution of flow structures due to the interaction of a finite-span synthetic jet with a zero-pressure gradient laminar boundary layer were experimentally investigated using stereoscopic particle image velocimetry. A synthetic jet with three orifice aspect ratios of AR = 6, 12, and 18 was issued into a free-stream velocity of U∞ = 10 m/s (Reδ = 2000) at blowing ratios of Cb = 0.5–1.5. The interaction was found to be associated with two sets of flow structures: (1) a recirculation region downstream of the orifice due to virtual blockage, and (2) a steady streamwise vortex pair farther downstream. These two flow structures were characterized in detail. Tube-like velocity deficits in the free-stream were evident, as well as regions of increased velocity within the boundary layer. Reducing the aspect ratio of the orifice decreased the spacing of the edgewise vortices (generated due to the finite span of the orifice) as well as reducing the virtual blockage of the jet. A control volume analysis of the fluid streamwise momentum indicates that there is a momentum deficit just downstream of the jet orifice and the change in streamwise momentum is proportionally similar for all cases.
A method for constructing well-posed one-way equations for calculating disturbances of slowly-varying flows was recently introduced (Towne & Colonius, JCP, Vol. 300, 2015). The linearized Navier-Stokes equations are modified such that all upstream propagating modes are removed from the operator. The resulting equations, termed one-way Navier-Stokes equations, are stable and can be solved efficiectly in the frequency domain as a spatial initial value problem in which initial perturbations are specified at the domain inlet and propagated downstream by spatial integration. To date, the method has been used to predict large-scale wavepacket structures and their acoustic radiation in turbulent jets. In this paper, the method is extended and applied to wall-bounded flows. Specifically, we examine the spatial stability of two-and three-dimensional boundary layers, corresponding to the Blasius and the Falkner-Skan-Cooke flows, and predict the evolution of unstable Tollmien-Schlichting waves and crossflow vortices, respectively. The method is validated against well-known results from the literature.
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