Aquatic animals perform a wide array of complex locomotor behaviors to meet the daily demands of prey acquisition, predator avoidance and reproduction. Investigating how such locomotor tasks are accomplished requires biomechanical analysis of propulsor anatomy and kinematics, and study of neuromuscular control. My current research focuses on a further level of analysis critical to an understanding of how animals swim: investigation of the hydrodynamic interface between an organism and its fluid surroundings.

		Digital Particle Image Velocimetry (DPIV) is a method I have employed for visualizing water flow in the wake of freely swimming fishes. DPIV was developed for examining man-made flows in engineering applications, but is now seeing increasing use in studies of biological fluid flow. The technique involves seeding the water in a flow tank with reflective neutrally buoyant particles and illuminating the particles with a laser which is focused into a thin sheet of light. In Figure 1 (at top) the laser is shown in a vertical light sheet defining the region of flow to be examined behind a fish's pectoral fin. High speed video images of the laser plane are recorded by camera 1 so that movement of particles in the illuminated slice of flow can be visualized. Camera 2 and a mirror within the flow downstream are used to film the posterior perspective of the fish, which is helpful for showing the position of the laser plane relative to the fish's fin. Figure 1 (at bottom) gives an example of the synchronized video signals provided by cameras 1 and 2.
Figure 1. Illustration of digital particle image velocimetry system.

By measuring changes in the pattern of particle images from one video field to the next using the statistical method of cross-correlation (see: "How DPIV works"), one can calculate a water velocity vector field that describes flow patterns within the laser plane. In collaboration with Dr. George Lauder, I have examined flow patterns in three perpendicular planar transections of the wake of the pectoral fin of bluegill sunfish (Figures 2, 3).
 

Figure 2. Water velocity vector fields calculated for three perpendicular planar sections of the pectoral fin wake of sunfish. Flow patterns are shown near the end of fin upstroke during labriform swimming at 0.5 body length per second. Note paired vortices in each plane. Scales: arrow, 20 cm/s; bar, 1 cm.

Calculated velocity vectors are used to compute vorticity components which reflect the local fluid rotation within each flow field (Figure 3). Flow patterns in orthogonal planar transections of the wake allow reconstruction of three-dimensional wake morphology (Figure 4).

Figure 3. Fluid vorticity components in three perpendicular wake planes during swimming at 0.5 body length per second. Paired vortices are represented by regions of red and blue. Curved arrows show direction of fluid jet (cf. reference planes in Figure 2).		Figure 4. Schematic three-dimensional reconstruction of the vortex ring wake produced by the pectoral fin of sunfish during swimming at low speed. Red arrows indicate direction of fluid flow measured in planar wake transections.

Characterizing the strength and three-dimensional structure of the wake shed by moving fins has allowed calculation of the hydrodynamic forces exerted by fishes while they swim (Figure 5). Analysis of the fluid forces generated by swimming animals during the course of everyday locomotor activities is the continuing focus of my current research.
 

Figure 5. Empirically determined equilibrium of stroke-averaged forces acting on sunfish during labriform swimming at 0.5 body length per second.