Comparative Investigations on Active and Passive Tails of Undulating Swimmers

TL;DR

This study compares active and passive tail strategies for a carangiform-like swimmer using two-dimensional fluid–structure interaction simulations at $Re=500$ and $Re=5000$. A biomimetic two-foil model with a torsional spring–damper joint captures tail pitching dynamics, and OpenFOAM-based DNS resolves the coupled flow with rigorous grid and time-step validation. The results reveal a consistent thrust–power trade-off: actively pitching tails produce higher thrust, while passively pitching tails can offer higher energy efficiency at low Reynolds numbers, with the balance shifting toward active tails at high Reynolds numbers. The work identifies how joint stiffness, damping, inertia, and Strouhal frequency govern performance and stability, providing design guidelines for bio-inspired underwater vehicles where tail actuation strategy should scale with swimmer size. Overall, the findings highlight when passive versus active tail control is advantageous and illuminate vortex dynamics underlying the observed trends.

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

Figures (15)

• Figure 1: Medial view of a Bluegill sunfish lauder2000function and the superior view for its $2D$ Kinematics.
• Figure 2: Two-dimensional model of the fish at (a) static position (${t}/{\tau} =0$), and (b) mid-oscillation instant (${t}/{\tau} =0.55$).
• Figure 3: Flow domain and boundary conditions.
• Figure 4: Results for convergence of Grid size.
• Figure 5: Results for convergence of Time step.