The flow downstream of a body immersed in a stream or the flow behind a body propagating through a fluid. Wakes are narrow elongated regions, filled with large and small eddies. The wake eddies of a bridge pier immersed in a river stream, or of a ship propelled through the water, are often visible on the surface. On windy days, similar wakes form downstream of smoke stacks or other structures, but the eddies in the air are not visible unless some smoke or dust is entrained in them.
Turbulence in the wake of bluff bodies consists of all sizes of eddies, which interact with each other in their unruly motion. Yet, out of this chaos emerges some organization, whereby large groups of eddies form a well-ordered sequence of vortices. The sense of rotation of these vortices alternates, and their spacing is quite regular. As a result, they can drive a structure that they encounter, or they can exert on the body that created them a force alternating in sign with the same frequency as that of the formation of the vortices. Such forces can impose on structures unwanted vibrations which often lead to serious damage. Flow-induced forces can be catastrophic if they are in tune with the frequency of vibration of the structure. See Fluid flow, Turbulent flow
Wakes are sustained for very large distances downstream of a body. Ship wakes retain their turbulent character for miles behind a vessel and can be detected by special satellites hours after their generation. Similarly, condensation in the wake of aircraft sometimes makes it look like a narrow braided cloud, traversing the sky.