Solid particles floating on a liquid
Many basic aerodynamic experiments were conducted by Prandtl and co-workers by moving the models in tanks containing water. Very fine particles of aluminum or coffee or lycopodium may be used to visualize streamlines, separation of boundary layer and eddies that are generated on the model partly submerged in the liquid. The vortices that are generated downstream of a cylinder moving in a liquid, photographed with a long exposure, are shown in Figure 6.3.
Vortices downstream of a cylinder visualized with aluminum powder