Control of flow distortion in offset diffusers using trapped vorticity

Controlled concentrations of trapped vorticity within an offset, subsonic (M_AIP ≤ 0.7) diffuser are explored for active suppression of flow distortion in joint experimental and numerical investigations. The coupling between trapped vorticity, used to model boundary-layer separation, and secondary-flow vortices is manipulated using an array of fluidic oscillating jets, which are spanwise distributed just upstream of the trapped vortex. Actuation energizes the separated shear layer, reducing the size of separation and effecting an earlier reattachment of the boundary layer, which favorably effects the flow field downstream of reattachment. It is shown that optimal interactions between actuation and the trapped vortex fully suppress the central vortex pair, and redistributes the residual vorticity around the diffuser's circumference. This results in a 68% reduction in circumferential distortion at the Aerodynamic Interface Plane (AIP), using an actuation mass flow rate that is only 0.25% of the diffuser mass flow rate.