Mechanisms of anguilliform locomotion in fishes studied using simple three-dimensional physical models

Abstract

Physical models enable researchers to systematically examine complex and dynamic mechanisms of underwater locomotion in ways that would be challenging with freely swimming animals. Previous research on undulatory locomotion, for example, has used rectangular flexible panels that are effectively two-dimensional as proxies for the propulsive surfaces of swimming fishes, but these bear little resemblance to the bodies of elongate eel-like swimming animals. In this paper we use a polyurethane rod (round cross-section) and bar (square cross-section) to represent the body of a swimming Pacific hagfish (Eptatretus stoutii). We actuated the rod and bar in both heave and pitch using a mechanical controller to generate a propulsive wave at frequencies between 0.5 and 2.5 Hz. We present data on (1) how kinematic swimming patterns change with driving frequency in these elongate fish-like models, (2) the thrust-generating capability of these simple models, (3) how forces and work done during propulsion compare between cross-sectional shapes, (4) the wake flow patterns in these swimming models using particle image velocimetry. We also contrast kinematic and hydrodynamic patterns produced by bar and rod models to comparable new experimental data on kinematics and wake flow patterns from freely swimming hagfish. Increasing the driving frequency of bar and rod models reduced trailing edge amplitude and wavelength, and above 2 Hz a nodal point appeared in the kinematic wave. Above 1 Hz, both the rod and bar generated net thrust, with the work per cycle reaching a minimum at 1.5 Hz, and the bar always requiring more work per cycle than the rod. Wake flow patterns generated by the swimming rod and bar included clearly visible lateral jets, but not the caudolaterally directed flows seen in the wakes from freely swimming hagfish.

Propulsive performance of an under-actuated robotic ribbon fin

“…In black ghost knifefish swimming, it has been reported numerous complex fin bending patterns including the curling of the rays into the flow, smooth curvatures and sharp peaks (Youngerman et al 2014). In addition, previous studies (Lauder et al 2007, Wen and Lauder 2013, Feilich and Lauder 2015, Lim and Lauder 2016 have observed similar bending and deforming behaviors in other types of fins. In our study, the interaction between the elastodynamic property of the flexible rays, the tensile behavior of the elastic membrane and the surrounding fluid leads to the deflection amplitude for a fullyactuated fin starting from around 6 cm at 0.5 Hz to greater than 8 cm after 4 Hz.…”

Robotics-inspired biology

“…Examples of this phenomenon are commonly seen in recent studies of flexible swimming models of aquatic locomotion (e.g. Akanyeti et al, 2017;Alben et al, 2012;Lauder et al, 2012;Lim and Lauder, 2016;Lucas et al, 2015;McHenry et al, 1995) where input motions to the 'head' of a swimming simple model fish result in complex patterns of body bending and fluid flow around and behind the model due to interactions between the fluid and the structure. As another example, a simple model of terrestrial locomotion, the bristlebot (Becker et al, 2014), is designed with flexible toothbrushlike bristles serving as 'legs' that interact with the ground.…”

Intermittent locomotion of a fish-like swimmer driven by passive elastic mechanism

“…In an experimental demonstration by Ramananarivo et al [3], an elastic swimmer actuated at the head by the magnetic field was shown to locomote on the free surface with an emergent anguilliform kinematics. In a series of studies by Lauder and coauthors, the effects of frequency and amplitude, body length and stiffness, planform and cross-section shape on the propulsive performance were assessed [4][5][6][7][8][9][10][11][12][13]. This type of artificial swimmer provides us a useful tool for analyzing flow structure and energy efficiency across a wide range of parameter values.…”