WILGA, C.D.; LAUDER, G.V.*; Univ. of Rhode Island; Harvard University, Cambridge, MA: Locomotor hydrodynamics in dogfish (Squalus acanthias): function of the heterocercal tail and pectoral fins

The hydrodynamic function of the heterocercal caudal fin and pectoral fins in the spiny dogfish Squalus acanthias was studied with high-resolution Digital Particle Image Velocimetry (DPIV). Wake flow patterns were quantified at 40 ms intervals for time series of up to several seconds, with each time interval represented by a matrix of 2200 vectors reflecting water flow at that time. This allowed a much more detailed analysis of wake flow patterns for shark fins than was possible in our previous research. Three centers of vorticity were noted in vertical slices though the wake of the tail: dorsal and ventral counterrotating tip vortices as well as a central region of vorticity located between the tip vortices. Velocity transects though these three centers confirm the presence of two vortex rings generated with each tail stroke: a small dorsally located vortex ring is nested within a larger vortex ring which has a diameter effectively equal to tail height. This contrasts sharply with the single vortex ring generated by each stroke of the homocercal tail in teleost fishes. Time series of wake velocity patterns show that an initial dorsal high velocity jet forms that is directed posteroventrally, and that this is joined later by a ventral jet that merges with the dorsal jet to form the broad flow region reflecting momentum added by the tail. The nested vortex rings appear to be a consequence of heterocercal tail kinematics in which the dorsal and ventral tail lobes rotate around the central horizontal body axis. High-resolution analyses of pectoral fin wake velocities confirm the lack of significant downwash behind the fin during steady horizontal swimming, and the generation of significant lift forces during maneuvering.