Undulating fins produce off-axis thrust and flow structures

TLDR

The wake of freely swimming undulating fins remains uncharacterized, yet these elongated fins enable knifefish agility and inspire maneuverable underwater vehicles, with vortex patterns resembling distorted linked vortex rings seen in carangiform swimmers. The study presents the flow structure of an undulating robotic fin model by measuring wake velocity fields with particle image velocimetry. Experimental data from particle image velocimetry were complemented by high‑fidelity CFD simulations of both a virtual fish—whose fin kinematics and morphology were derived from a freely swimming knifefish—and a virtual rendering of the robot. The wake consists of linked vortex tubes shed along the fin’s long edge, producing an oblique jet that propels the fish forward.

Abstract

While wake structures of many forms of swimming and flying are well characterized, the wake generated by a freely swimming undulating fin has not yet been analyzed. These elongated fins allow fish to achieve enhanced agility exemplified by the forward, backward and vertical swimming capabilities of knifefish, and also have potential applications in the design of more maneuverable underwater vehicles. We present the flow structure of an undulating robotic fin model using particle image velocimetry to measure fluid velocity fields in the wake. We supplement the experimental robotic work with high-fidelity computational fluid dynamics, simulating the hydrodynamics of both a virtual fish, whose fin kinematics and fin plus body morphology are measured from a freely swimming knifefish, and a virtual rendering of our robot. Our results indicate that a series of linked vortex tubes is shed off the long edge of the fin as the undulatory wave travels lengthwise along the fin. A jet at an oblique angle to the fin is associated with the successive vortex tubes, propelling the fish forward. The vortex structure bears similarity to the linked vortex ring structure trailing the oscillating caudal fin of a carangiform swimmer, though the vortex rings are distorted because of the undulatory kinematics of the elongated fin.