Effects of Surface Roughness on a Pitching Foil in Quiescent Flow Conditions

A pitching foil in quiescent flow conditions produces an unstable wake characterized by jet meandering and strong interactions between consecutively shed vortices. This can result in a chaotic and deflected wake, leading to low-performance scenarios. In this study, we explore the impact of surface dimples on the wake characteristics of a pitching foil using Particle Image Velocimetry (PIV). The dimple depth to chord ratio (k/c) is fixed at 0.003, while the Reynolds number, based on the trailing edge velocity, is varied from 3000 to 15000. Our findings reveal that surface roughness can have an influence on the jet characteristics past the pitching foil by increasing the jet velocity at locations far from the trailing edge. This enhancement boosts the longevity of the jet. Notably, the effect of roughness is dependent on the Reynolds number, becoming less pronounced at lower Reynolds numbers. In this presentation, we will discuss PIV results, focusing on the averaged wake shape and profile, as well as the instantaneous characteristics of the vortices shed by the foil. We will also examine the implications of these findings on the thrust production of the foil.