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

Ziyu Ren,Kainan Hu,Li Wen,Tianmiao Wang

doi:10.5772/63571

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.

Highlights

As one of the most successful taxonomical groupings in the world, the fish occupies most of the water areas on earth
The fish tail has always been modelled as a simple foil, capable of conducting only twodimensional (2D) flapping movements as an extension of the undulatory wave of the body [21], and hydrodynamic evaluation has been limited to thrust performance in the horizontal plane
Under the coordi‐ nation system shown in Figure 3a, the undulatory wave can be expressed as: yu (z,t) = au sin(2p + f) where yu is the horizontal displacement of the fin ray tip; z denotes the vertical coordinate of the fin ray tip; t denotes the time instant; au indicates the amplitude of the fin ray tip discursion; l is the chord length of the caudal fin; f is the motion frequency; λ is the undulation wavelength and Φ is the phase angle between the fin ray motion and the caudal peduncle motion

Summary

Introduction

As one of the most successful taxonomical groupings in the world, the fish occupies most of the water areas on earth. As many recent studies have reported that fish can actively deform their fins to achieve different types of locomotion, more and more researchers have realized the importance of these flexible propulsion surfaces on enchancing ability of swimming. Such propulsion surfaces include dorsal fins [2,3,4] and pectoral fins [5,6,7] in body and caudal fin (BCF) propulsion and ribbon fins [8,9,10] in median and paired fin (MPF) propulsion. The significant impact of heave and pitch motions on hydrodynamic propulsion has gained great attention [18, 20]

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Discussion

Conclusion