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 counter rotating 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 with a diameter effectively equal to tail height. This contrasts
sharply with the single vortex rings generated by each stroke of the tail in
teleost fishes with homocercal tails. 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
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.