Vortex
A coherent region of swirling fluid — the basic building block of turbulent and near-turbulent flow.
Definition
A vortex is a region of fluid in which the flow rotates around an axis, either straight or curved. The simplest vortex is a rigid-body rotor, with velocity linearly proportional to distance from the axis. More realistic is the Rankine vortex, with solid-body rotation inside a core radius and potential (1/r) flow outside.
Vortices shed from moving objects at intermediate Reynolds number produce the Kármán vortex street behind a cylinder, the wingtip vortices of aircraft, and the cyclones and anticyclones of the atmosphere. At high Reynolds number, vortices nest at every scale — Richardson's cascade — and their statistics obey Kolmogorov's −5/3 law.