Fig. 7. Progressive visualisation of a discrete ring vortex shed from the tail fin at the end of a half-stroke. (A) The leading edge of the vortex is being outlined by the white streamer; the arrow indicates the direction of propagation of the vortex. (B) The vortex has passed through the streamer and is drawing dye into the trailing edge. (C) Dye has progressed beyond the ring plane, and the locations of the opposite cores of the vortex can be seen. Below, drawings of the main features shown in the videographic sequence.
