Hydrodynamics of linear acceleration in bluegill sunfishLepomis macrochirus

Abstract

In their natural habitat, fish rarely swim steadily. Instead they frequently accelerate and decelerate. Relatively little is known about how fish produce extra force for acceleration in routine swimming behavior. In this study, we examined the flow around bluegill sunfish Lepomis macrochirus during steady swimming and during forward acceleration, starting at a range of initial swimming speeds. We found that bluegill produce vortices with higher circulation during acceleration, indicating a higher force per tail beat, but they do not substantially redirect the force. We quantified the flow patterns using high speed video and particle image velocimetry and measured acceleration with small inertial measurement units attached to each fish. Even in steady tail beats, the fish accelerates slightly during each tail beat, and the magnitude of the acceleration varies. In steady tail beats, however, a high acceleration is followed by a lower acceleration or a deceleration, so that the swimming speed is maintained; in unsteady tail beats, the fish maintains the acceleration over several tail beats, so that the swimming speed increases. We can thus compare the wake and kinematics during single steady and unsteady tail beats that have the same peak acceleration. During unsteady tail beats when the fish accelerates forward for several tail beats, the wake vortex forces are much higher than those at the same acceleration during single tail beats in steady swimming. The fish also undulates its body at higher amplitude and frequency during unsteady tail beats. These kinematic changes likely increase the fluid dynamic added mass of the body, increasing the forces required to sustain acceleration over several tail beats. The high amplitude and high frequency movements are also likely required to generate the higher forces needed for acceleration. Thus, it appears that bluegill sunfish face a trade-off during acceleration: the body movements required for acceleration also make it harder to accelerate.

Airfoil-like mechanics generate thrust on the anterior body of swimming fishes

“…Trout produced a mean thrust of 1.5 ± 0.4 mN. All values were on the same order of magnitude as previous estimates from wake analyses (39,40). Fig.…”

Tuna robotics: hydrodynamics of rapid linear accelerations

“…This finding was corroborated by detailed analyses of bluegill sunfish showing increased head and tail amplitudes [7] and lamprey showing higher amplitudes along the entire body [6] during linear acceleration. Wise et al [7] also found indications of larger force production rather than the reorientation of axial forces during acceleration compared to steady swimming patterns, which differs slightly from the conclusions of [5] who documented vortex wake reorientation during acceleration. Whereas fish show increased tail beat amplitudes during linear accelerations, Akanyeti et al [5] also showed that different amounts of accelerations are largely modulated by the tail beat frequency.…”

Red muscle activity in bluegill sunfish Lepomis macrochirus during forward accelerations

“…Wise et al . 52 showed that the acceleration reaction in bluegill sunfish is increased by the very same kinematics bluegill sunfish use during accelerations. It may be that fishes can produce these additional forces economically by using eccentric activity.…”

“…Even the red muscle studied here probably still generates net positive power, because it remains active for the same amount of time during shortening. To accelerate in a fluid, bluegill sunfish require a force both to accelerate their own mass, but also to overcome the acceleration reaction 52 , an additional force required to accelerate the fluid around them 53 . Wise et al .…”