AQUATIC PROPULSION IN FISHES BY VORTEX RING PRODUCTION.

E.G. Drucker* and G.V. Lauder. Brown University, Providence, RI and University of California, Irvine. Society of Integrative and Comparative Biology, 1998 Annual Meeting.

The difficulty in directly measuring properties of unsteady fluid flow surrounding freely-swimming fishes has led to qualitative analyses of hydrodynamic structures in the wake and steady-state models (e.g. "lift-based" and "drag-based") for estimating swimming thrust. To gain new insights into mechanisms of unsteady aquatic force production, we used Digital Particle Image Velocimetry to visualize quantitatively the structure of the wake behind the pectoral fins of slowly swimming bluegill sunfish (Lepomis macrochirus, 20 cm total body length, TL). Fish swam in a flow tank at 0.5, 1.0, and 1.5 TL/s against a current seeded with 12 µm silver-coated glass beads. Particle motion was illuminated by a 3 W argon laser focused into a 1-2 mm-thick light sheet in three separate perpendicular planes (horizontal, parasagittal, frontal) and imaged using high-speed video (250 fields/s). Use of two video cameras simultaneously permitted a three-dimensional reconstruction of the flow. Cross-correlation analysis of pairs of consecutive video images allowed calculation of planar flow velocity fields and changes in fluid angular velocity (vorticity). Abduction of the pectoral fin resulted in fluid flow with opposite-sign vorticity around the dorsal and ventral fin edges. This downstroke produced a vortex ring (in the shape of a torus) 2-5 cm in diameter that traveled laterally, posteriorly, and ventrally. Each pectoral fin generated one vortex ring per complete downstroke/upstroke cycle. A momentum jet directed through the center of each ring is the source of both "lift-based" and "drag-based" thrust forces for swimming. The deep body of bluegill separates left- and right-side vortices and, by limiting hydrodynamic interactions between the two fins, is expected to enhance maneuverability.