The formation, in the wake of a plate, of alternate vortices carried along with the flow makes up a classical example of a flow with accentuated unsteady character: it is the well-known phenomenon of Bénard-Kármán vortices, the subject of many theoretical and experimental studies. The liquid tracers emphasize the vortex cores and the emission lines rolling up around these cores. In the image shown here, the red streak remains concentrated in the cores, thus revealing their locations; in the next image the emission has turned green during its dilution in water.

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