Comparative Investigations on Active and Passive Tails of Undulating Swimmers

TL;DR

This study compares active and passive tail pitching in fish-like swimmers using fluid-structure interaction simulations, revealing how tail actuation affects thrust, power, and stability at different Reynolds numbers, informing bio-inspired vehicle design.

Contribution

It provides a detailed analysis of passive versus active tail pitching effects on hydrodynamics, highlighting design principles for bio-inspired underwater robots based on swimmer size.

Findings

Active tails generate more thrust at higher Reynolds numbers.

Passive tails reduce power demand at lower Reynolds numbers.

Performance varies with joint stiffness, damping, and inertia.

Abstract

Fish display remarkable swimming capabilities through the coordinated interaction of the body and caudal fin, yet the potential role of a passively pitching tail in enhancing hydrodynamic performance remains unresolved. In this work, we evaluate the performance of a carangiform swimmer equipped with either an actively pitching tail or a passively pitching tail. Fluid-structure interactions-based simulations are employed to asses how variations in joint stiffness, damping, and inertia influence thrust generation, power demand, and overall stability at two representative Reynolds numbers of 500 and 5000. The results reveal that actively pitching tails tend to generate greater thrust, while passively pitching tails deliver improved outcomes in terms of the power demand at the lower Reynolds number. Larger pitching amplitudes contribute positively only when associated with higher swimming frequency, when produced by reduced inertia for more flexible joints, they lead to unfavorable effects. At the higher Reynolds number, active tails consistently outperform passive ones, although a small subset of passive cases still achieve favorable performance. Across all cases, a recurring balance emerges, with thrust production and power expenditure varying inversely. These findings clarify the hydrodynamic consequences of passive versus active tail motion and establish design principles for bio-inspired underwater vehicles, where smaller swimmers may benefit from passive tail pitching, while larger swimmers are better served by active control