Vortex rings are toroidal flows that self-propel away from their source

Evidence

What happens when fast-moving air is injected into still air through a circular hole? If the air is injected as a short burst, a single ring vortex may form. Videos of vortex rings produced by homemade air cannons, whales, dolphins, and volcanoes can be found on YouTube. Vortex rings are toroidal (doughnut-shaped) vortices, rotating around the ring. The sense of rotation is such that the outer part of the torus travels back toward the source, while the inner part propagates in the direction of the initial stream of air, even as the whole ring moves away from the source, traveling perpendicular to the plane of the ring axis. — Eric J. Heller, Why You Hear What You Hear- An…, 1321

Categorization

• DEFINE: Defines the toroidal vortex ring and its sense of rotation, the basic unit behind edgetones and whistles.