Mechanisms of Controlled Flow Attachment of 3-D Separation Cells in Internal Flows

Reattachment within a spanwise separation cell that forms in an adverse pressure gradient over an inner surface of a diffuser duct is investigated in wind tunnel experiments using a spanwise array of fluidically-oscillating wall jets. The separation cell is characterized by two spanwise-outboard counter-rotating surface-normal vortices that bound a central, nominally 2-D reverse flow domain. It is shown that the time-averaged flow attachment across the span of the cell is effected by segmentation of its base flow into spanwise-periodic narrower cells. Each segmented cell is bounded by two streamwise vorticity regions of opposing sense that are induced by an upstream actuation jet and lead to simultaneous downwash and upwash flows along the jet centerlines and between the jets, respectively. PIV measurements in several streamwise-normal planes show that these time-averaged vorticity pairs are apparently formed by advection and accumulations of streamwise vorticity strands of opposing sense that are shed during each oscillation cycle of and are organized by the actuation wall jets in the cross flow. The progressive structuring of the vorticity concentrations in the separated flow is captured by the PIV measurements as flow attachment progresses with increasing actuation strength, and is further evident in near-surface spanwise distributions of the turbulent kinetic energy that form arch-like concentrations over the interaction domains of the vorticity pairs.