Surface roughness influences vortex interactions and jet stability in pitching foils in quiescent flow

Abstract

This study investigates the impact of surface indentations, shaped as dimples, on the flow dynamics of a pitching foil under zero-freestream conditions. A series of systematic experiments were conducted employing flow field measurements using Particle Image Velocimetry. The dimple depth ratio ($d/D$, where $d$ is the dimple depth and $D$ is the dimple diameter) was varied from 0.022 to 0.088 across Reynolds numbers ($Re=V_{T{E}_{max}}c/\nu$, where $V_{T{E}_{max}}$ is the maximum trailing edge velocity, $c$ is the foil chord, and $\nu$ is the fluid kinematic viscosity) of 3700, 10000 and 20000. The impact of dimples on the wake characteristics was evaluated by analyzing the time-averaged jet behavior and vortex dynamics. The results reveal that the deepest dimpled case modified the far wake of the pitching foil, particularly at higher Reynolds numbers. Under these conditions, the vortices shed from the trailing edge persisted longer, and the jet exhibited greater coherence. The dimples appear to influence dipole interactions in the wake, reducing the jet deflection. These findings suggest that surface roughness can be strategically employed to modulate wake dynamics and improve the stability of the jet, potentially enhancing the propulsion efficiency of bio-inspired flapping foil systems.