S1-2.1 Jan. 4 Escaping fishes: experimental hydrodynamics of wake flow patterns TYTELL, E. D.; LAUDER, G. V.*; Harvard University; Harvard University firstname.lastname@example.org
Despite a vast literature on the kinematics, mechanics, and neural control of c-start escape responses in fishes, no study has yet examined the hydrodynamic effect of fish body motion on the fluid environment. We thus have little idea of how power produced by body musculature is transmitted to the water, and what type of hydrodynamic wake is left by escaping fishes. We investigated the wake generated by bluegill sunfish, Lepomis macrochirus, executing c-start escape responses. We used two laser light sheets in orthogonal horizontal orientation to eliminate shadows and allow full field resolution of flow structures, and imaged the light sheet and fish with two synchronized high-speed cameras in ventral and lateral view respectively at 1000 Hz. C-starts were obtained from five fish at three distinct horizontal planes: mid-body, mid-dorsal fin, and mid-anal fin positions. Body mass distribution and body and fin areas were also measured. Three distinct vortex rings with fluid jets, nearly orthogonal to each other, are formed during the c-start response. Jet #1 is the first ring formed by the tail during Stage 1, and moves in the same direction as Stage 2 fish movement, reducing final escape velocity but also rotating the fish. Jet #2, in contrast, moves approximately opposite to the final direction of fish motion and contains the bulk of the total fluid momentum. It forms during Stage 1 primarily as a result of suction on the inside of the c-bend. Jet #3 forms during Stage 2 in the mid-body region and moves in a direction approximately perpendicular to both jets 1 and 2, across the direction of body movement. The c-start escape response thus involves a complex pattern of fluid movement, with effectively all momentum contributing to the escape generated during Stage 1 by suction on the concave body surface.