A whirling mass of fluid such as seen in a
tornado, a whirlpool, a smoke ring, or water
running out of a bath. The term is used in hydrodynamics
for a portion of fluid in which the individual particles have circular motions.
A vortex may be produced when two adjacent fluid streams have different
velocities or when a solid body moves through a fluid and vortex lines cannot
begin or end inside the fluid.
PHENOMENA AND STRUCTURES