Formation of hairpin vortices has been observed in experiments and numerical simulations of single hairpins, however observational evidence for them in nature is still limited. Theodorsen has been producing sketches that indicate the presence of hairpin vortices in his flow visualization experiments. These smaller elementary structures can be seen overlaying the main vortex in the sketch to the right (image of sketch to Theodorsen's steam experiment that exposes the presence of structures). The sketch was well advanced for the time, but with the advent of computers came better depictions. Robinson in 1952 isolated two types of flow structures that he named the "horseshoe", or arch, vortex and the "quasi-streamwise" vortex (classic figure shown to the right).