Vortex

In fluid dynamics, a vortex is a region within a fluid where the flow is mostly a spinning motion about an imaginary axis, straight or curved. That motion pattern is called a vortical flow. (The original and most common plural of "vortex" is vortices, although vortexes is often used too.)

Vortices form spontaneously in stirred fluids, including liquids, gases, and plasmas. Some common examples are smoke rings, the whirlpools often seen in the wake of boats and paddles, and the winds surrounding hurricanes, tornadoes and dust devils. Vortices form in the wake of airplanes and are prominent features of Jupiter's atmosphere.

Vortices are a major component of turbulent flow. In the absence of external forces, viscous friction within the fluid tends to organize the flow into a collection of so-called irrotational vortices. Within such a vortex, the fluid's velocity is greatest next to the imaginary axis, and decreases in inverse proportion distance from it. The vorticity (the curl of the fluid's velocity) is very high in a core region surrounding the axis, and nearly zero in the rest of the vortex; while the pressure drops sharply as one approaches that region.

Once formed, vortices can move, stretch, twist, and interact in complex ways. A moving vortex carries with it some angular and linear momentum, energy, and mass. In a stationary vortex, the streamlines and pathlines are closed. In a moving or evolving vortex the streamlines and pathlines are usually spirals.