Turbulence

Turbulence is the formation of eddies in a fluid (liquid or gas). It is produced whenever a fluid (under certain conditions) is in contact with a solid and there is relative motion between them; for example: when wind flows past a building or past a mountain; when the ocean flows past an island; when a baseball flies by; when a jet plane moves in the stratosphere; or when a river flows past a bridge pier. In all these cases, eddies form behind the obstacle (i.e., "downstream"), and eventually are carried away by the main body of fluid.

Turbulence has long been observed, but its scientific study began with the work of William John Macquorn Rankine (1820-1872); later, Osborne Reynolds (1842-1912) defined the number bearing his name, and Ludwig Prandtl (1875-1953) put forth the limiting-layer hypothesis.

Today, the study of turbulence, experimental and theoretical, continues; but an agreement between both approaches is still in the future.