14th International Symposium on Particle Image Velocimetry – ISPIV 2021
August 1–5, 2021

Bubble PIV technique to measure the velocity field of a
free-swimming California sea lion

Gino Perrotta1, Frank E. Fish2, Megan C. Leftwich1∗

1 George Washington University, Department of Mechanical and Aerospace Engineering, Washington,
USA

2 West Chester University, 2Department of Biology, West Chester, USA
∗mleftwich@gwu.edu

1 Introduction
Fish et al. (2014) adapted laboratory PIV for safe use on larger animals. As opposed to seeding the entire
flow with reflective particles and illuminating a plane of the flow with a laser, they produced a sheet of
small bubbles and used sunlight for global illumination. Underwater cameras imaged the flow in a method
similar to traditional PIV. This technique was used to measure the flow around a swimming dolphin and
estimate the thrust produced during a tail stand maneuver (Fish et al. (2014, 2018)). In the current work,
we will extend the modification of PIV of Fish et al. to measure the flow produced by a swimming sea lion
also using bubbles as seeding particles and sunlight as illumination. This is the first time that the flowfield
of a swimming sea lion has been directly measured. We will present an extensive extension to the image
processing required to measure flow under field conditions. Finally, we will present the flow generated by
propulsive strokes of an adult female (Cali) sea lion freely swimming through a pool of stationary water.

2 Methods
Data collection: Measurements were taken at Science Learning and Exploration With The Help of Sea lions
(SLEWTHS) in Moss Landing, California. Three adult California Sea Lions trained to follow an instructor?s
target were guided to swim through a sheet of bubbles in front of a submerged camera. Small bubbles were
generated in a thin flat sheet by releasing compressed air through a porous irrigation hose placed on the floor
of the pool. The motion of the bubbles was recorded by a submerged GoPro Hero 5 at 1920x1080 pixels
and 120 fps. Image processing: The raw images need extensive processing to yield clear velocity fileds. The
process, and results are shown in Fig. 1. The process involves xxx steps: image data reduction (Fig. 1(b)),
a sliding minimum subtraction (Fig. 1(c)), a spatial median subtraction (Fig. 1(d)), a heuristic refinement
(Fig. 1(e)), a two-dimensional PIV calculation (Fig. 1(f)), and a three-dimensional median subtraction (Fig.
1(g)). This lengthy process is necessary largely due to the use of global illumination via sunlight and the
resulting reflections throughout the large field of view.

3 Results
The flowfield measured in the bubble-plane was measured for 173 runs of three separate individuals. Fig.
1(g) shows the velocity magnitude in a representative run. Because the animals swim freely in the pool,
ensuring in-plane motion and propulsive stroke timing necessitate a large number of runs, a feat made more
challenging by the difficulties of working with marine mammals. From these data we are able to examine
the full-scale flowfield produced by the swimming sea lion, and estimate the strength of the resulting fluid
jet. Overall, XXX flowfields were measured showing distinct flow features created by the sea lion at various
points in the propulsive stroke. The method was repeated in a water channel at West Chester University at
known steady velocity. This allowed for validation of the technique by measuring a known flowfield in a
controlled setting (something this is not possible in the field experiments).



Figure 1: Each panel shows a step in the data processing from the raw data (a) to the measured flowfield (h).

4 Conclusions
The flowfield of swimming California sea lions was measured by adapting and advancing bubble PIV tech-
niques. These results not only illuminate this particular flow phenomenon, they also provide additional
options for measuring flowfields in field settings where typical PIV techniques are not feasible.

Acknowledgements
Finding for this study was provided by the Office of Naval Research Award N000141712312 (PO Tomas
McKenna) to MCL and FEF and Award N000141712448 (PO Robbert Brizzolera) to MCL. As well as the
National Science Foundation Award CBET 1604876 to MCL.

References
Fish FE, Legac P, Williams TM, and Wei T (2014) Measurement of hydrodynamic force generation by

swimming dolphins using bubble dpiv. Journal of Experimental Biology 217:252–260

Fish FE, Williams TM, Sherman E, Moon YE, Wu V, and Wei T (2018) Experimental measurement of
dolphin thrust generated during a tail stand using dpiv. Fluids 3:33


	Introduction
	Methods
	Results
	Conclusions