Global vorticity shedding is a phenomenon where the boundary layer vorticity on an object is released into the fluid as a free vortex when the object is vanished. Retracting a wing can be a good approximation to ‘vanishing’ if the retraction is fast enough.
A towed foil can be ‘vanished’ by rapidly retracting the foil in the span-wise direction. As illustrated above, the boundary layer vorticity on a towed foil reforms into two vortices in the fluid that produce strong forces to be used for maneuverability.
We see global vorticity shedding in action above. The left column shows experimental Particle Imaging Velocimetry vorticity data. The right column shows a 3D simulation; curtains of vorticity shed and collapse into two vortex tubes in the wake. (Simulation by Dr. Gabe Weymouth)