Resistive thrust production can be as crucial as added mass mechanisms for inertial undulatory swimmers

TL;DR

This paper demonstrates that resistive forces are as important as added mass effects in the thrust production of inertial aquatic swimmers, challenging traditional classifications based solely on Reynolds number.

Contribution

It introduces a comprehensive hydrodynamic model that combines resistive and reactive forces, highlighting the significance of resistive contributions at high Reynolds numbers.

Findings

Resistive forces are crucial in high Reynolds number swimming.

Elongated swimmers rely heavily on resistive mechanisms.

Both resistive and reactive forces should be considered in swimmer modeling.

Abstract

In this paper, we address a crucial point regarding the description of moderate to high Reynolds numbers aquatic swimmers. For decades, swimming animals have been classified in two different families of propulsive mechanisms based on the Reynolds number: the "resistive" swimmers, using local friction to produce the necessary thrust force for locomotion at low Reynolds number and the "reactive" swimmers, lying in the high Reynolds range, and using added mass acceleration (described by perfect fluid theory). However, inertial swimmers are also systems that dissipate energy, due to their finite size, therefore involving strong resistive contributions, even for high Reynolds numbers. Using a complete model for the hydrodynamic forces, involving both reactive and resistive contributions, we revisit here the physical mechanisms responsible for the thrust production of such swimmers. We show, for instance, that the resistive part of the force balance is as crucial as added mass effects in the modeling of the thrust force, especially for elongated species. The conclusions brought by this work may have significant contributions to the understanding of complex swimming mechanisms, especially for the future design of artificial swimmers.