Editing Vortex (section)

====Rotational vortices====
[[File:Saturn north polar vortex 2012-11-27.jpg|thumb|[[Saturn|Saturn's]] north polar vortex]]
A rotational vortex – a vortex that rotates in the same way as a rigid body – cannot exist indefinitely in that state except through the application of some extra force, that is not generated by the fluid motion itself. It has non-zero vorticity everywhere outside the core. Rotational vortices are also called rigid-body vortices or forced vortices.

For example, if a water bucket is spun at constant angular speed {{mvar|w}} about its vertical axis, the water will eventually rotate in rigid-body fashion. The particles will then move along circles, with velocity {{mvar|u}} equal to {{mvar|wr}}.<ref name=LJC7.5/> In that case, the free surface of the water will assume a [[paraboloid|parabolic]] shape.

In this situation, the rigid rotating enclosure provides an extra force, namely an extra pressure [[gradient]] in the water, directed inwards, that prevents transition of the rigid-body flow to the irrotational state.