Effects of surface roughness on the propulsive performance of pitching foils

TL;DR

This study investigates how surface roughness affects the unsteady hydrodynamic performance of pitching foils, revealing that roughness has minimal impact on performance and wake flow, especially at higher Reynolds numbers.

Contribution

It provides experimental evidence that surface roughness has limited effect on the performance of flapping foils, contrasting with static cases, and highlights the robustness of unsteady propulsion systems.

Findings

Roughness does not improve performance over smooth surfaces in unsteady conditions.

Flow wake patterns are nearly identical regardless of surface roughness.

Drag penalty from roughness decreases significantly at higher Reynolds numbers.

Abstract

The hydrodynamic influence of surface texture on static surfaces ranges from large drag penalties (roughness) to potential performance benefits (shark-like skin). Although, it is of wide-ranging research interest, the impact of roughness on flapping systems has received limited attention. In this work, we explore the effect of roughness on unsteady performance of a harmonically pitching foil through experiments using foils with different surface roughness, at a fixed Strouhal number and within the Reynolds number (Re) range of 15k-30k. The foils' surface roughness is altered by changing the distribution of spherical-cap shaped elements over the propulsor area. We find that the addition of surface roughness does not improve the performance compared to a smooth surface over the Re range considered. The analysis of the flow fields shows near identical wakes regardless of the foil's surface roughness. The performance reduction mainly occurs due to an increase in profile drag. However, we find that the drag penalty due to roughness is reduced from 76% for a static foil to 16% for a flapping foil at the same mean angle of attack, with the strongest decrease measured at the highest Re. Our findings highlight that the effect of roughness on dynamic systems is very different than that on static systems, thereby, cannot be accounted for by only using information obtained from static cases. This also indicates that the performance of unsteady, flapping systems is more robust to the changes in surface roughness.