You probably have seen smoke rings blown by cigarette smokers or bubble rings from playful dolphins. But do you know that they are closely related to each other and officially known as the vortex ring phenomenon?
A vortex ring is a donut-shaped vortex made of gas or fluid. Inside the vortex, the gas or fluid molecules sustain a spinning momentum around an imaginary axis line that forms a closed loop. This motion is also called a poloidal spin, meaning that the spin is directed towards the poles (like in the magnetic field ).
Using the same working principle, people build air vortex cannons for either entertainment or scientific purpose. It is a device that releases doughnut-shaped air vortices that are close to smoke rings or bubble rings, but larger, stronger and often invisible.
Source: The Royal Institution via Youtube