Near-wake structures of a finite square cylinder with a flapping film at its free end

As a follow-up study of Wang et al. [“Control of the flow around a finite square cylinder with a flexible plate attached at the free end,” Phys. Fluids 34(2), 027109 (2022)], this paper presents an experimental study of flow around a wall-mounted finite square cylinder with a vertically clamped flapping film at its free end. The width (d) of the square cylinder was 40 mm, and the aspect ratio (H/d) was 5, where the height H was 200 mm. The flexible film was made of low-density polyethylene, with a thickness of 0.04 mm and the width and length (l) each of d. Flow visualization and particle image velocimetry were conducted in the central lateral plane and several horizontal planes to reveal the 3D structure of the flapping induced vortex (FIV) and its effects on the cylinder near wake. All measurements were done in a low-speed wind tunnel at a flow speed of U∞ = 5 m/s with a Reynolds number of 13 700 based on U∞ and d. Previous study suggests that the flapping film reduces aerodynamic forces of the cylinder significantly and that the fluctuating lateral force is reduced by 60% [Wang et al., “Control of the flow around a finite square cylinder with a flexible plate attached at the free end,” Phys. Fluids 34(2), 027109 (2022)]. Vortices that shed from the trailing edge of the flapping film connect those from the side edges, forming n-shape FIVs downstream. FIVs induce more high-speed flow downwards into the wake, which suppresses the mean recirculation zone near the free end but enlarges it in the lower part of the wake. The two legs of n-shape FIVs are symmetrically arranged near the cylinder free end, whose effects diminish gradually as approaching the bottom wall, where alternating Karman vortex shedding still prevails.