Hydrodynamics of C-Start Escape Responses of Fish as Studied with Simple Physical Models

Abstract

One of the most-studied unsteady locomotor behaviors exhibited by fishes is the c-start escape response. Although the kinematics of these responses have been studied extensively and two well-defined kinematic stages have been documented, only a few studies have focused on hydrodynamic patterns generated by fishes executing escape behaviors. Previous work has shown that escape responses by bluegill sunfish generate three distinct vortex rings, each with central orthogonal jet flows, and here we extend this conclusion to two other species: stickleback and mosquitofish. Jet #1 is formed by the tail during Stage 1, and moves in the same direction as Stage-2 movement of the fish, thereby reducing final escape-velocity but also rotating the fish. Jet #2, in contrast, moves approximately opposite to the final direction of the fish's motion and contains the bulk of the total fluid-momentum powering the escape response. Jet #3 forms during Stage 2 in the mid-body region and moves in a direction approximately perpendicular to jets 1 and 2, across the direction of movement of the body. In this study, we used a mechanical controller to impulsively move passively flexible plastic panels of three different stiffnesses in heave, pitch, and heave + pitch motions to study the effects of stiffness on unsteady hydrodynamics of escape. We were able to produce kinematics very similar to those of fish c-starts and also to reproduce the 3-jet hydrodynamic pattern of the c-start using a panel of medium flexural stiffness and the combined heave + pitch motion. This medium-stiffness panel matched the measured stiffness of the near-tail region of fish bodies. This motion also produced positive power when the panel straightened during stage 2 of the escape response. More flexible and stiffer panels resulted in non-biological kinematics and patterns of flow for all motions. The use of simple flexible models with a mechanical controller and program of fish-like motion is a promising approach for studying unsteady behaviors of fish which can be difficult to manipulate experimentally in live animals.

Ear-body lift and a novel thrust generating mechanism revealed by the complex wake of brown long-eared bats (Plecotus auritus)

“…During a c-start the fish’s tail goes through a strong pitching motion that results in a high thrust force (e.g. 42 ), which is reflected in the wake by a distinct vortex loop 43 44 . Similar wakes have also been observed in plates pitched through large angles (e.g.…”

Tuna robotics: hydrodynamics of rapid linear accelerations

“…Obtaining consistent acceleration magnitudes while controlling for variables such as heading, the power used to generate the acceleration and kinematics ranges from difficult to impossible when studying live animals. As a result, a few investigators have turned to mechanical and robotic systems to provide a highly controlled experimental platform in which acceleration dynamics can be quantified [2,[12][13][14][15]. These robotic systems have shed new light on the effects of the dorsal and anal fin on linear accelerations and have helped to design new mechanical systems to emulate and understand the impressive acceleration performance of fast-starting fish.…”

Dynamics and locomotion of flexible foils in a frictional environment

“…The importance of resonances on propulsion was also highlighted by Quinn et al [28] in their study of heaving rectangular panels in a water channel. Similar systems have been used to study effects of foil shape and stiffness, centre-of-mass oscillations, wall effects, and vortex dynamics for oscillating foils in high-Reynolds-number flows [29][30][31][32][33][34][35][36][37][38][39][40][41][42]. Flexible foils have also been used to investigate propulsion in low-Reynolds-number flows [20,21].…”