Locomotor forces on a swimming fish: Three-dimensional vortex wake dynamics quantified using digital particle image velocimetry

Quantifying the locomotor forces experienced by swimming fishes represents a significant challenge because direct measurements of force applied to the aquatic medium are not feasible, However, using the technique of digital p article image velocimetry (DPIV), it is possible to quantify the effect of fish fins on water movement and hence to estimate momentum transfer from th e animal to the fluid. We used DPIV to visualize water how in the wake of t he pectoral fins of bluegill sunfish (Lepomis macrochirus) swimming at spee ds of 0.5-1.5 L s(-1), where L is total body length. Velocity fields quanti fied in three perpendicular planes in the wake of the fins allowed three-di mensional reconstruction of downstream vortex structures, At low swimming s peed (0.5 L s(-1)), vorticity is shed by each fin during the downstroke and stroke reversal to generate discrete, roughly symmetrical, vortex rings of near-uniform circulation with a central jet of high-velocity flow. At and above the maximum sustainable labriform swimming speed of 1.0 L s(-1), addi tional vorticity appears on the upstroke, indicating the production of link ed pairs of rings by each fin. Fluid velocity measured in the vicinity of t he fin indicates that substantial spanwise flow during the downstroke may o ccur as vortex rings are formed. The forces excited by the fins on the wate r in three dimensions were calculated from vortex ring orientation and mome ntum, Mean wake-derived thrust (11.1 mN) and lift (3.2 mN) forces produced by both fins per stride at 0.5 L s(-1) were found to match closely empirica lly determined counter-forces of body drag and weight. Medially directed re action forces were unexpectedly large, averaging 125 % of the thrust force for each fin. Such large inward forces and a deep body that isolates left- and right-side vortex rings are predicted to aid maneuverability. The obser ved force balance indicates that DPIV can be used to measure accurately lar ge-scale vorticity in the wake of swimming fishes and is therefore a valuab le means of studying unsteady flows produced by animals moving through flui ds.