Velocity Field Measurements of the California Sea Lion Propulsive Stroke Using Bubble PIV

Gino Perrotta,Abigal M Downs,Frank E Fish,Megan C Leftwich,Ariel M Leahy,Danielle S Adams

doi:10.3390/fluids7010003

Abstract

California sea lions are among the most agile of swimming mammals. Most marine mammals swim with their hind appendages—flippers or flukes, depending on the species—whereas sea lions use their foreflippers for propulsion and maneuvering. The sea lion’s propulsive stroke generates thrust by forming a jet between the flippers and the body and by dragging a starting vortex along the suction side of the flipper. Prior experiments using robotic flippers have shown these mechanisms to be possible, but no flow measurements around live sea lions previously existed with which to compare. In this study, the flow structures around swimming sea lions were observed using an adaptation of particle imaging velocimetry. To accommodate the animals, it was necessary to use bubbles as seed particles and sunlight for illumination. Three trained adult California sea lions were guided to swim through an approximately planar sheet of bubbles in a total of 173 repetitions. The captured videos were used to calculate bubble velocities, which were processed to isolate and inspect the flow velocities caused by the swimming sea lion. The methodology will be discussed, and measured flow velocities will be presented.

Highlights

In situ flow measurements of swimming animals present significant technical and logistical challenges
This enabled the wakes of a variety of steadily swimming animals to be measured while swimming in water channels [1,2,3,4] or freely swimming in stationary tanks [5,6]. Most of these studies present two-dimensional data, a few have recently extended these techniques to collect volumetric flow fields around swimming fish [7,8]. This is in no way an exhaustive overview of laboratorybased flow measurements of swimming fish; we refer the interested reader to Triantafyllou
The results presented above were compared to velocity and vorticity fields measured on a robotic sea lion foreflipper, as presented by Kashi et al [23]

Summary

Introduction

In situ flow measurements of swimming animals present significant technical and logistical challenges. Successes often adapted digital particle image velocimetry (PIV) techniques to laboratory environments studying swimming animals This enabled the wakes of a variety of steadily swimming animals to be measured while swimming in water channels [1,2,3,4] or freely swimming in stationary tanks [5,6]. Most of these studies present two-dimensional data, a few have recently extended these techniques to collect volumetric flow fields around swimming fish [7,8]. This is in no way an exhaustive overview of laboratorybased flow measurements of swimming fish; we refer the interested reader to Triantafyllou (2000) [9] and, more recently, Wu (2011) [10], Lauder (2015) [11], and Costello 2020 [12]

Methods

Results

Discussion

Conclusion