19.8 Brief comms AM

within a larger vortex ring, which includes centres (1) and (3). Counter-rotating centres in the dorsal vortex ring produce jet A; a second jet B develops later as a result of flow induced by the ventral tip vortex. Jets A and B are directed on average at 35° to the freestream flow, before combining to produce a single broad jet C. Initially, little downstream flow is seen between same-sign vorticity centres (2) and (3) (Fig. 1a). Velocity transects though the centres of vorticity confirm the presence of a small dorsal vortex ring linked to a larger vortex ring that has a diameter equivalent to the tail height (Fig. 1b,c). Circulation calculations show that mean vortexcentre (1) circulation (0.0160 m s ) is not significantly different from the sum of negative circulation in vortex centres (2) and (3) ( 0.0019 and 0.0165, which is 0.0184 m s 1 in total; paired t-test, P 0.3), as required by Kelvin’s law. These findings contrast with the single vortex rings generated by the symmetrical tail of the bony fishes. A strong posteroventrally directed jet flow is visible as a result of sharktail motion (Fig. 1a,c), corroborating one classical model of shark-tail function. We propose that the mechanism for generating the ring-within-a-ring vortex structure in swimming dogfish is similar Biomechanics