Effect of a Flexible Wall on a Reattaching Turbulent Shear Layer

Digital particle image velocimetry was applied to investigate turbulent flow of air between a flexible wall and a rigid surface containing a backward-facing step (BFS). The inflow condition corresponded to a Coanda jet issuing from a nozzle that was located upstream of the BFS. The flexible wall was represented by a sheet of paper under tension that was positioned above the BFS. Two additional configurations, which involved the BFS without the flexible wall and the BFS in proximity to an inclined rigid upper wall, were considered in this study. In all three cases, the flow fields were characterized in terms of patterns of time-averaged velocity, out-of-plane vorticity, streamline topology, and turbulence statistics. High-speed photography and unsteady pressure measurements were employed to characterize the flow-induced deformation of the flexible wall and the flow oscillations. The profile of the paper sheet could be approximated by linear segments, which, in conjunction with the rigid surface that contained the BFS, formed a diverging channel configuration. Confinement of the incoming flow by the flexible wall delayed flow reattachment to the rigid bottom surface downstream of the BFS. Patterns of turbulence statistics in the presence of the flexible wall shared qualitative similarity with the corresponding parameters of diverging channel flows as well as classical Couette–Poiseuille flows.