Meeting Abstract

18.4  Thursday, Jan. 3  What�s the point? Undulatory swimming with the tapering tail of the alligator FISH, Frank E*; LAUDER, George V; West Chester Univ.; Harvard Univ. ffish@wcupa.edu

Many hydrodynamic models, often used to calculate thrust production and efficiency during undulatory swimming by aquatic animals, assume that the end of the tail has a broad trailing edge. However, many swimming animals have a tapering tail that ends at a point. The hydrodynamic models predict that no thrust could be developed with a tapering tail, because the end of the tail would be unable to act on any significant mass of water. To address how an undulatory swimmer with a tapering tail develops thrust, high-speed videography (250 Hz) and digital particle image velocimetry (DPIV) were used to examine the kinematics of juvenile American alligators (Alligator mississippiensis) and the associated flow around the tail and in the wake. The alligators swam steadily against a constant water current (1.0 L/s) in a flow tank. Alligators were observed to swim in an anguilliform mode. The posterior quarter of the tail moved in a whip-like manner such that this segment was oriented nearly perpendicular to the axis of progression. Lateral movements of the tail generated vorticity along the sides of the tail anterior of the tail tip. The fluid entrained along the side of the tail was shed into the wake as elongate vortex rings. The direction of the vortex jet was 148 deg from the axis of progression of the animal. Thrust force magnitude was estimated to be approximately twice the lateral force produced by the movements of the tail. Despite being classified as anguilliform swimmers, both the eel and the alligator produced wakes that were found to be different in structure. The tapering tail of the alligator modifies the structure of the vortex ring but does not appear to be the position along the tail where vorticity is shed, unlike animals with broad tailing edges.