Vortex rings in air

Vortex ring is a remarkable hydrodynamic or aerodynamic object - a vortex whose vortex line forms a closed loop. The vortex emerges as a result of the relative motion of air ejected from a hole and the surrounding air in rest. The velocity of the air on the inner/outer side of the ring has the same/opposite direction as the ring motion. Vortex rings can move over large distances and can have surprising effects - for example we managed to blow out a candle from the distance of 15 meters. We used a self-made gun with a rubber membrane in the back and a hole in the front (see the first picture) to create the vortex rings. However, even a usual PET bottle will do. We injected some smoke into the gun and the smoke was then carried by the rings and visualized them. We also used the laser beam method (similar to that from Turbulences section) to visualize a cut through the ring, as can be seen in the last nine pictures.