Eddy (fluid dynamics)

In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid flows past an obstacle. The moving fluid creates a space devoid of downstream-flowing water on the downstream side of the object. Fluid behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle, followed by a short reverse flow of fluid behind the obstacle flowing upstream, toward the back of the obstacle. This phenomenon is most visible behind large emergent rocks in swift-flowing rivers.

Another possible type of turbulence is the vortex. This notion is rather applied to gases than to liquids. Here, no void is created, but only an area of lower pressure, but again, a backflow causes the gas to rotate.

The concept of eddy is often used in phenomenology of turbulence. An ”eddy” of size l can be loosely described as the velocity in the Fourier mode with wave-vector q=1/l.