English: Schematic illustration of particle trajectories in an idealized irrotational stationary vortex (where the linear velocity is constant and the angular velocity is inversely proportional to distance from the axis).

The dashed line represents the axis of the vortex. The small spheres are sample particles, and the thin circles are their streamlines (or pathlines). Each niti transparent tube represents a surface of equal angular momentum; the distance r from the axis to each surface is 1, 2, 3 and 4 times some arbitrary unit. Note that this schematic animation is not entirely accurate. In a real vortex with curved axis, as shown here, the streamlines are not perfect concentric circles. Moreover, the pattern must break down in a core region close to the axis.

This is one frame of an animation created on 2012-09-24 by J. Stolfi using POV-Ray. The code is available at http://www.ic.unicamp.br/~stolfi/EXPORT/videos/vortex-flow/2012-09-24-vortex-anim.tgz