A classical example of axisymmetric unsteady flow, this phenomenon precedes the establishment of a permanent regime, and characterizes a starting movement.

The red streak that covers the front of the disk is emitted exactly at the point from which all streamlines are initiated (that is, the stagnation point); along the side of the disk it turns green through dilution, and reveals the vortex structure of the toroidal wake.

Image created by Henri Werlé. Reproduced with permission from his spectacular collection Courants et Couleurs published in 1974 by ONERA, the French Aerospace Lab.