  Citation: Perrotta, G.; Fish, F.E.; Adams, D.S.; Leahy, A.M.; Downs, A.M.; Leftwich, M.C. Velocity Field Measurements of the California Sea Lion Propulsive Stroke Using Bubble PIV. Fluids 2022, 7, 3. https:// doi.org/10.3390/fluids7010003 Academic Editors: Sean P. Colin and John H. Costello Received: 2 November 2021 Accepted: 10 December 2021 Published: 22 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fluids Article Velocity Field Measurements of the California Sea Lion Propulsive Stroke Using Bubble PIV Gino Perrotta 1 , Frank E. Fish 2 , Danielle S. Adams 2 , Ariel M. Leahy 2 , Abigal M. Downs 2 and Megan C. Leftwich 3, * 1 The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20703, USA; gino.perrotta@jhuapl.edu 2 Department of Biology, West Chester University, West Chester, PA 19383, USA; ffish@wcupa.edu (F.E.F.); DA762671@wcupa.edu (D.S.A.); AL916349@wcupa.edu (A.M.L.); ad846650@wcupa.edu (A.M.D.) 3 Department of Mechanical and Aerospace Engineering, George Washington University, Washington, DC 20052, USA * Correspondence: mleftwich@gwu.edu 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. Keywords: marine mammal; California sea lion; velocity fields; bubble PIV; thrust production 1. Introduction In situ flow measurements of swimming animals present significant technical and logistical challenges. Despite this, a rich body of literature exists for measuring both the locomotion and kinematics of swimming fish and the flow that they generate. Early suc- cesses 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 [1–4] or freely swim- ming in stationary tanks [5,6]. Although 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]. Notably, this is in no way an exhaustive overview of laboratory- based 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]. The studies briefly mentioned above provide extensive insight into the mechanisms of unsteady propulsion, particularly for body/caudal fin-type swimming. However, they are laboratory studies; the animals’ environments have been altered. Additionally, some animals cannot be safely studied in a laboratory setting. To combat this, a handful of in situ techniques have been developed to measure the flow field of animals swimming in their natural environments. Notably, the Self-Contained Underwater Velocimetry Apparatus (SCUVA) device [13], as well as its precursor studies [14], tracked particles naturally occurring in the water with a camera and laser setup operated by a freely swimming human Fluids 2022, 7, 3. https://doi.org/10.3390/fluids7010003 https://www.mdpi.com/journal/fluids