Dynamical Characteristics of the Body-Caudal Fin Joint of a Carangiform Swimmer and its Influence on Hydrodynamics

TL;DR

This study presents a computational model of a fish-inspired swimmer with a passive caudal fin that interacts with body undulation, revealing how passive fin mechanics influence swimming efficiency and wake structures at Re=3000.

Contribution

It introduces a novel computational framework for a Jackfish-inspired swimmer with a passively pitching caudal fin, highlighting the role of nonlinear peduncle mechanics in hydrodynamics.

Findings

Passive fin synchronization enhances thrust via coherent vortex formation.

Phase differences affect wake patterns and drag.

Nonlinear peduncle mechanics regulate fin motion naturally.

Abstract

The hydrodynamics of fish swimming depend on the interaction between the undulation of the body and the flapping of the caudal fin. This study develops a computational framework of a Jackfish-inspired swimmer with an independently mounted caudal fin that pitches passively under fluid forces and a nonlinear torsional spring. The fin synchronizes with the body when damping and stiffness parameters are tuned correctly, producing passive pitching that closely resembles to the displacement of the actively pitching tail. At Re = 3000, synchronized passive pitching generates coherent hairpin and ring vortices that reinforce streamwise momentum and contribute to thrust, whereas larger phase differences lead to wake spread in lateral direction and drag-dominated behavior. These results reveal that nonlinear peduncle mechanics naturally regulate amplitude, phase, and recoil, offering a biologically inspired pathway toward underwater robotic design using passive kinematics.