Vortex Ring - Formation

Formation

One way a vortex ring may be formed is by injecting a compact mass of fast moving fluid (A) into a mass of stationary fluid (B) (which may be the same fluid). Viscous friction at the interface between the two fluids slows down the outer layers of A relative to its core. Those outer layers then slip around the mass A and collect at the rear, where they re-enter the mass in the wake of the faster-moving inner part. The net result is a poloidal flow in A that evolves into a vortex ring.

This mechanism is commonly seen, for example, when a drop of colored liquid falls into a cup of water. It is also often seen at the leading edge of a plume or jet of fluid as it enters a stationary mass; the mushroom-like head ("starting plume") that develops at the tip of the jet has a vortex-ring structure.

A variant of this process may occur when a jet within a fluid hits a flat surface, as in a microburst. In this case the poloidal spinning of the vortex ring is due to viscous friction between the layer of fast outward flow near the surface and the slower-moving fluid above it.

A vortex ring is also formed when a mass of fluid is impulsively pushed from an enclosed space through a narrow opening. In this case the poloidal flow is set in motion, at least in part, by interaction between the outer parts of the fluid mass and the edges of the opening. This is how a smoker expels smoke rings from the mouth, and how most vortex ring toys work.

Vortex rings may also be formed in the wake of a solid object that falls or moves through a fluid at sufficient speed. They may form also ahead of an object that abruptly reverses its motion with the fluid, as when producing smoke rings by shaking an incense stick. A vortex ring can also be created by a spinning propeller, as in a blender.