SUMMARY

Internal flows subjected to adverse pressure gradients are susceptible to three-dimensional separation on flow boundaries that can result in flow instabilities and significant losses. Active, surface-integrated flow control offers an attractive approach for mitigating these adverse effects by delaying separation or bypassing it altogether. The present investigations focus on the interactions between a separation cell that forms over a diffuser surface and a spanwise array of fluidically oscillating jets that lead to flow attachment with specific emphasis on understanding the actuation-induced changes in the structure and dynamics of the base flow. These effects are investigated in two diffuser configurations having significant differences in their inlet conditions namely, an open-end diffuser duct branching from a channel, and a curved surface insert that forms a diffuser within a channel using planar and stereo particle image velocimetry. Actuation is effected by spanwise arrays of surface-integrated fluidically oscillating jets that issue tangentially to the diffuser’s surface. It is shown that separation cells formed in the adverse pressure gradient are receptive to fluidic actuation and that increasing actuation strength incrementally delays separation by the manipulation of the flow dynamics in the vicinity of separation and creation of spanwise concentrations of streamwise vorticity that subdivide the separation cell of the base flow into smaller spanwise-periodic reattachment cells that mitigate the adverse effects of reversed flow along the surface. The demonstrated control of separation indicates that these active flow control technologies have the potential for improving system performance in multiple internal flow applications including diffusers, flow diverters, and engine inlets.