Horseshoe vortex capping a cylindrical obstacle at the wall. This sink vortex characterizes the three-dimensionla
separation of the flow along a flat wall with a cylindrical obstacle. The colored streak emitted at the wall reveals the
boundary of the separated zone and the counterflow in front of the obstacle (in white), the sense of rotation of the main
vortex whose two branches (in yellow) curve downstream, and lastly the separated zone behind the obstacle (in green).
This type of vortex occurs, in particular, around bridge piles, for which it presents a risk of undermining.
Image created by Henri Werlé. Reproduced with permission from his spectacular
collection Courants et Couleurs published in 1974 by ONERA, the French Aerospace Lab.
For further information, see
On the Flow of Fluids Made Visible,
Leonardo, Vol. 8, No. 4. (Autumn, 1975), pp. 329-331.