Comparative analysis of the wake structures surrounding rigid and flexible hemispheres immersed within a turbulent boundary layer

Abstract: This paper experimentally investigates the unsteady wake behaviour around a rigid 3D printed hemisphere model and two flexible polyurethane hemispheres, immersed within a turbulent boundary layer. The primary aim of the investigation is to analyse the effect of fluid-structure interaction on unsteady wake formation and propagation around a hemisphere. Tests have been conducted at three distinct Reynolds numbers, ReD = 0.97×10^5, ReD = 1.62×10^5 and ReD = 2.27×10^5. For data acquisition, a stereoscopic particle image velocimetry (SPIV) system has been used, with two low-speed, high-resolution cameras positioned to capture flow behaviour in the streamwise direction along the centreline of the models. Time-averaged data indicates that as the Reynolds number increases, the vertical deformation of the flexible model becomes greater, causing an increase in flow speed at the apex of the hemisphere. This, in turn, induces a change in the downstream unsteady wake structure, specifically the size of the recirculation region itself. The results indicate that as the Reynolds number increases, the recirculation region size decreases, which is to be expected. However, with an increase in model flexibility, there is a further decrease in wake size. In this case, for the most flexible model, increasing the Reynolds number from ReD = 0.97×10^5, ReD = 1.62×10^5 shows a 26% reduction in the size of the region of recirculation compared to roughly a 20% reduction for the rigid model. The overall wake formation of the post-deformation flexible model mimics the behaviour of flow around the rigid hemisphere at higher Reynolds numbers, without the requirement for increased flow speed. This reduction in the streamwise size of the unsteady wake has been observed across the entire Reynolds number range. The results for the rigid model are consistent with existing literature. Keywords: Turbulent Flow, Boundary Layer Flow, Vorticity, Vortex Shedding, Stereoscopic particle image velocimetry, Fluid-Structure Interaction