How vortex rings form
A toroidal, or doughnut-shaped, vortex is formed when fluid flows back on itself, making a spinning ring around an invisible core. The best-known examples of this phenomenon are smoke rings, produced by volcanoes and artillery, but perhaps the most intriguing are the bubble rings generated by marine creatures like whales and dolphins.
These sea mammals,known as cetaceans, generate vortex rings by flicking the tip of their dorsal fin, or quickly moving their head. This causes the rapid acceleration of a small mass of water. Drag at the outer edges of the fast-flowing packet of water slows the flow relative to the centre, causing the edges to wrap back on themselves, which results in a doughnutshaped vortex of rotating fluid.
These water-based vortices are invisible to the eye, but cetaceans blow air into the vortex, which gets caught up in the core of spinning water as bubbles and forms a visible ring. The rings are surprisingly stable and persist in the water for a long time. When a fluid flows quickly the pressure drops, so as the vortex spins its pressure relative to the water that surrounds it is lowered. This leads to slight compression, which helps to stabilise the vortex. Additionally, as the ring rises to the surface, the fluid at its edges drags against the water, maintaining its spin, unless disturbed.
Interestingly, dolphins and whales seem to create these bubble rings for fun more than anything else. They move them with their fins, manipulate their shape and can even pull smaller rings out of larger ones. When they have finished playing they often bite the ring, destroying it in a flurry of tiny bubbles.