This photo shows a cylinder in a cross-flow exhibiting the beginning of a von Kármán vortex street: a pattern of repeating, alternating vortices that often appear in wakes. This was visualized by flowing a rheoscopic fluid over a cylinder.

The cylinder in the photo has a 1″ diameter, and the fluid is water which contains steric acid crystals. The depth of the tank is 1.25″ to approximate a 2D flow.