WAKE DYNAMICS AND LOCOMOTOR FORCE IN SLOW AND FAST SWIMMING FISHES.

E.G. Drucker* and G.V. Lauder. University of California, Irvine. Society of Integrative and Comparative Biology, 1999 Annual Meeting.

Hydrodynamic forces for fish locomotion were quantified by characterizing the wake behind freely swimming bluegill sunfish (Lepomis macrochirus) and black surfperch (Embiotoca jacksoni). Digital Particle Image Velocimetry (DPIV) was used to visualize unsteady water flow near the oscillating pectoral fins and to measure the structure and strength of the wake shed up to and above Up-c, the speed of transition from labriform locomotion to axial undulation. Fish 20 cm in length swam at 50, 100 and 150 % Up-c against a current seeded with 12 µm glass beads. Particle motion was illuminated by a 3 W argon laser focused into three separate perpendicular light sheets and imaged using high-speed video (250 fields/s). The pectoral fin stroke produces paired vortices of opposite rotation in each orthogonal plane, indicating that the wake is comprised of three-dimensional vortex rings. Sunfish produce single rings with each fin at low speeds and multiple, linked rings at higher speeds. Surfperch, capable of swimming twice as fast as sunfish with the labriform gait, generate a high-thrust, multiple-ring wake at all speeds studied. For both species, the upstroke is the primary force generator, producing up to 70 % of the total forward thrust per stride. The downstroke generates low-magnitude lift, and large laterally directed forces whose reaction provides stability. A balance between forces calculated from the wake and empirically determined body drag and weight indicates that DPIV successfully detects the major vortical structures shed by swimming animals and validates the technique for studies of unsteady aquatic force production.