What is the bubble ring (vortex ring)?
The bubble ring is a bubble which became the shape of the ring in water. The bubble ring is a phenomenon known to scuba diver well. Expert's diver can make the bubble ring from the expiration. A part of dolphin also makes the bubble ring. The bubble ring is the much beautiful phenomenon.

If air is discharged in water, the shape of the bubble does not become like the ring usually. The reason is that the bubble is about to become globular by the surface tension. (Globular is shape with the fewest surface areas.)
Air which forms the bubble ring rotates in the direction indicated in figure. (From the inside to the outside)
Therefore, shape of ring is kept as for it. And, the ring shape stabilizes the rotation.
The bubble ring is a certain kind of whirlpool, and it is the fluid mechanics phenomenon. It can be thought that the bubble ring is a ring formed by connecting both ends of a horizontal whirlpool. In a word, the part of air at the center of the whirlpool is a bubble ring.

We can make the bubble ring by discharging fixed amount of air toward for above momentarily. However, only when some conditions become complete, the bubble ring is formed.