Investigation of Fish Caudal Fin Locomotion Using a Bio-Inspired Robotic Model

Abstract

Due to its advantages of realizing repeatable experiments, collecting data and isolating key factors, the bio-robotic model is becoming increasingly important in the study of biomechanics. The caudal fin of fish has long been understood to be central to propulsion performance, yet its contribution to manoeuverability, especially for homocercal caudal fin, has not been studied in depth. In the research outlined in this paper, we designed and fabricated a robotic caudal fin to mimic the morphology and the three-dimensional (3D) locomotion of the tail of the Bluegill Sunfish (Lepomis macrochirus). We applied heave and pitch motions to the robot to model the movement of the caudal peduncle of its biological counterpart. Force measurements and 2D and 3D digital particle image velocimetry were then conducted under different movement patterns and flow speeds. From the force data, we found the addition of the 3D caudal fin locomotion significantly enhanced the lift force magnitude. The phase difference between the caudal fin ray and peduncle motion was a key factor in simultaneously controlling the thrust and lift. The increased flow speed had a negative impact on the generation of lift force. From the average 2D velocity field, we observed that the vortex wake directed water both axially and vertically, and formed a jet-like structure with notable wake velocity. The 3D instantaneous velocity field at 0.6 T indicated the 3D motion of the caudal fin may result in asymmetry wake flow patterns relative to the mid-sagittal plane and change the heading direction of the shedding vortexes. Based on these results, we hypothesized that live fish may actively tune the movement between the caudal fin rays and the peduncle to change the wake structure behind the tail and hence obtain different thrust and lift forces, which contributes to its high manoeuvrability.

A Fluid-Structure Interaction Study on a Passively Deformed Fish Fin

“…They have opposite rotation directions with one in counterclockwise while the other clockwise, and their vortices are approximately equal. These results match with those obtained in the experiments [11] and [23] using digital particle image velocimetry (DPIV) techniques, and to our knowledge, no similar vorticity patters are reported by previous FSI studies. In comparison, the intensity of the vortices behind the flexible fin outperforms that of a rigid counterpart and it also has farther influence zone in the wake.…”

“…In the experiment of Ren et al [23], the fin consists of rigid rays and elastic membrane. The fin membrane is glued with the rays.…”

A Fluid–Structure Interaction Study on a Bionic Fish Fin With Non-Uniform Stiffness Distribution

“…W-shape, can be achieved by imposing specific distribution of the flexibility of fish rays (fin). obtained in the experiments [16] and [33] using digital particle image velocimetry (DPIV)…”

“…In the experiment by Ren et al [33], the fin consists of rigid rays and elastic membrane. The fin membrane is glued with the rays.…”

“…The mass ratio is reduced frequency is 1 f   . These parameters are chosen to match those in the experiment by Ren et al[33].…”