Efficiency of Fish Propulsion

TL;DR

This paper introduces the concept of quasi-propulsive efficiency as a meaningful metric for evaluating the propulsion efficiency of fish and similar mechanisms, emphasizing its importance over traditional measures.

Contribution

It defines quasi-propulsive efficiency and demonstrates its effectiveness through simulations and examples, highlighting its role in assessing fish propulsion performance.

Findings

Quasi-propulsive efficiency is the key metric for fish propulsion.

Hydrodynamic interactions can cause low efficiencies not explained by drag.

Simulations show the importance of considering body-propulsor interactions.

Abstract

It is shown that the system efficiency of a self-propelled flexible body is ill-defined unless one considers the concept of quasi-propulsive efficiency, defined as the ratio of the power needed to tow a body in rigid-straight condition over the power it needs for self-propulsion, both measured for the same speed. Through examples we show that the quasi-propulsive efficiency is the only rational non-dimensional metric of the propulsive fitness of fish and fish-like mechanisms. Using two-dimensional viscous simulations and the concept of quasi-propulsive efficiency, we discuss the efficiency two-dimensional undulating foils. We show that low efficiencies, due to adverse body-propulsor hydrodynamic interactions, cannot be accounted for by the increase in friction drag.