Flapping dynamics of a flexible membrane attached to the leading edge of a forward-facing step

Abstract

The flapping dynamics of a flexible membrane (FM) and its effect on the flow fields over and pressure fluctuations on a forward-facing step (FFS) have been investigated experimentally. The two-dimensional FM with 40 mm in length and 0.15 mm in thickness was vertically attached to the leading edge of a FFS with 40 mm in height. The deformation of the flapping FM was recorded by a high-speed camera. Velocity data in the vertical central plane and the pressure fluctuations on the step surface were measured by planar particle image velocimetry and pressure sensors, respectively. The results demonstrate that as the dimensionless bending rigidity (γ) of the FM decreases, the FM displayed two distinct modes, i.e., the bending mode and the flapping mode. In the bending mode, the bent FM is similar to a curved barrier, which elevates the shear layer and delays the reattachment of separation flow. In the flapping mode, the amplitude of the FM increases with the decrease in γ, which in turn effects the scale of flapping-induced vortices (FIVs). In proper orthogonal decomposition analysis, the results reveal a transition in the dominant flow structure from large-scale separation to FIVs with reducing γ. The FIVs significantly affect the pressure distribution on the step surface of the FFS, and the range where the coherent contribution dominates expands with the decreasing γ.