The evolution and scaling of controlled flow separation at the inlet of a diverging channel

The structure and scaling of flow separation in the adverse pressure gradient at the inlet of a diverging channel are investigated experimentally. The channel flow is diverted from an adjacent uniform flow over a surface through a surface opening, and the separation forms a fluidic constriction across the inlet that severely limits the fraction of the diverted flow. The cross-stream and streamwise scales of the separation domain are progressively diminished by forced streamwise attachment that is effected using a spanwise array of fluidically oscillating control jets placed across the inlet from the main flow. Variable momentum coefficient enables efficient regulation of the diverted fraction of the flow through the diverging channel. The evolution of the flow at separation and within the separation domain is measured using planar PIV and characterized using conditional averaging, spectral analysis, and decomposition methods in the absence and presence of fluidic actuation. Although the streamwise migration of the separation point in the presence of actuation results in changes in the characteristic cross-stream scale of the wall-tangential velocity distributions at separation, the time-averaged velocity distributions in the absence and presence of actuation collapse when scaled by the local vorticity thickness and velocity deficit of the separating shear layer. Proper orthogonal decomposition analysis indicates that despite the energy shift across the flow scales in the presence of actuation, local vorticity modes in the base and controlled flows about separation are remarkably similar, as the uncontrolled flow modes primarily undergo tilting and stretching as separation migrates downstream in the presence of actuation. The global effectiveness of the actuation is assessed by the increased fraction of the diverted flow from the main stream and the accompanying reduction in total pressure losses.