Control of Flow Circulations in a Programmable Offset Diffuser

A flow control approach to address total pressure distortions at the AIP of a reconfigurable diffuser testbed with variable bend section (20 <  < 60o) was developed and tested in a joint experimental/numerical study. The present investigation showed that distortion at the AIP is caused in large part by the presence of a pair of time-averaged counter-rotating streamwise vortices that evolve as a result of Dean circulations during flow turning in the bend. The circulation transports streamwise vorticity concentrations of opposite sense along the inner-radius surface about the bend’s plane of symmetry that roll and are advected through a short diffuser to the AIP. The present flow control approach is designed to weaken or suppress the wall-bound formation of streamwise vorticity on the inner-radius surface of the bend using actuation by arrays of surface wall jets. Preliminary investigations of this fluidic control approach using fluidically-oscillating jets showed that it can successfully suppress and delay vorticity transport to the AIP by destructive interactions of jets-induced small-scale vorticity concentrations of opposite sense relative to the circulations-induced concentrations that pool along the bend surfaces. The effectiveness of the present configuration of fluidic actuation depends strongly on the Mach number and the bend angle, while the bend radius has only a secondary effect. Time-resolved analysis of flow demonstrated the impact of the actuation on the structure of the instantaneous flow and underscores the differences between the instantaneous and time-averaged flow fields.