Flow over a forward-facing step with a flexible membrane at its leading edge

Abstract

The effects of a flapping membrane at the leading edge of a forward-facing step (FFS) on the flow over and pressure fluctuation on it were experimentally investigated. The FFS was immersed in the oncoming boundary layer (δ/h = 0.4, where δ and h are the oncoming boundary layer thickness and step height, respectively) and the Reynolds number, based on the step height and oncoming flow velocity (U∞) was 3.4×104. Both the length and width of the membrane were identical to h, the height of the FFS. Two-dimensional particle image velocimetry (PIV) and smoke-wire flow visualization techniques were used to investigate the flow over the FFS. Pressure scanners have also been used to study the pressure distribution on an FFS. It was found that the membrane flapped periodically under the effects of the separated shear flow from the leading edge of the FFS, which significantly suppressed the separation bubble on the FFS and prompts an early recovery of the time-averaged pressure. The phase-averaged results revealed that the flapping membrane generated alternating vortices over the FFS. These flapping-induced vortices (FIVs) enhance the Reynolds stresses over the FFS, increasing momentum transport in the wall-normal direction. Moreover, these FIVs synchronize with the pressure fluctuation on the FFS.