A large plastic garbage can with a hole cut in the bottom and a rubber membrane covering its mouth is filled with vapor from a fog machine. When the rubber membrane is hit large smoke rings are produced which travel the length of the lecture hall with little dissipation.

Neat demo: attach a glass rod to the front of a laser pointer, making the light diverge in one direction and thus getting a plane of light. Now shine the plane to a vortex ring to get a cross-sectional view of the vortices.

K top, P1

PIRA DCS 2C50.10