Flow speed has little impact on propulsive characteristics of oscillating foils

TL;DR

This study shows that the propulsive performance of oscillating foils is largely unaffected by changes in flow speed, emphasizing the importance of mean trailing edge velocity over flow velocity.

Contribution

It demonstrates that thrust, power, and wake characteristics are independent of flow speed, simplifying experimental analysis of swimming propulsion.

Findings

Thrust and power are independent of flow velocity over a two-fold change.

Wake vortex organization remains unchanged with flow speed variations.

Mean trailing edge velocity is the key velocity scale for propulsion performance.

Abstract

Experiments are reported on the performance of a pitching and heaving two-dimensional foil in a water channel in either continuous or intermittent motion. We find that the thrust and power are independent of the mean freestream velocity for two-fold changes in the mean velocity (four-fold in the dynamic pressure), and for oscillations in the velocity up to 38\% of the mean, where the oscillations are intended to mimic those of freely swimming motions where the thrust varies during the flapping cycle. We demonstrate that the correct velocity scale is not the flow velocity but the mean velocity of the trailing edge. We also find little or no impact of streamwise velocity change on the wake characteristics such as vortex organization, vortex strength, and time-averaged velocity profile development---the wake is both qualitatively and quantitatively unchanged. Our results suggest that constant velocity studies can be used to make robust conclusions about swimming performance without a need to explore the free-swimming condition.