  The evolution of semicircular quantum vortex loops in oscillating potential
flow emerging from an aperture is simulated in some highly symmetrical cases.
As the frequency of potential flow oscillation increases, vortex loops that are
evolving so as eventually to cross all of the streamlines of potential flow are
drawn back toward the aperture when the flow reverses. As a result, the escape
size of the vortex loops, and hence the net energy transferred from potential
flow to vortex flow in such 2 Pi phase-slip events, decreases as the
oscillation frequency increases. Above some aperture-dependent and
flow-dependent threshold frequency, vortex loops are drawn back into the
aperture. Simulations are preformed using both radial potential flow and
oblate-spheroidal potential flow.
