As shown in Figs.7(a)-7(c) for f = 0.6 Hz , the
leading-edge vortex first moves along the upper surface
of the hydrofoil to the trailing edge and falls off
while a new leading-edge vortex emerges on the leading
edge of the hydrofoil. The shedding leading-edge
vortex is then connected with the tip vortices, lying in
the two sides of the flapping hydrofoil, and is eventually
closed with the trailing edge leading to vortex loops
in the tail flow field. This phenomenon is consistent
with the experimental observations of Von Ellenrieder[
5,6].