Half Vortex Rings

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Vortices are one of the most common structures in fluid dynamics. In this video, Dianna from Physics Girl explores an unusual variety of vortex you can create in a pool. Dragging a plate through the water at the surface creates a half vortex ring, which can be tracked either by the surface depressions created or by using food dye for visualization. Vortex rings are quite common, but a half vortex ring is not. The reason is that, ignoring viscous effects, a vortex filament cannot end in a fluid. The vortex must close back on itself in a loop, or, like the half vortex ring, the ends of the vortex must lie on the fluid boundary. It is possible to break vortex lines like those in smoke rings, but the lines will reattach, creating new vortex rings–just as they do in these vortex knots. (Video credit: Physics Girl; submitted by Tom)

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