An eddy is a fluid current whose flow direction differs from that of the general flow; the motion of the whole fluid is the net result of the movements of the eddies that compose it. Eddies can transfer much more energy and dissolved matter within the fluid than can molecular diffusion in nonturbulent flow because eddies actually mix together large masses of fluid. Flow composed largely of eddies is called turbulent; eddies generally become more numerous as the fluid flow velocity increases. Energy is constantly transferred from large to small eddies until it is dissipated.
In the lee of an obstacle, eddies form only when the flow around the obstacle reaches a critical velocity; they represent a flow of fluid into the space behind the obstacle, and this inflow begins only when the general flow is fast enough to produce a lowered pressure there. Eddies or vortices (whirlpools) so produced can also cause sound. Many sounds, both natural and manmade, occur in this way.
Excerpt from the Encyclopedia Britannica without permission.