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.

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Another possible 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.

See also

• Whirlpool
• Whirlwind
• River Eddies in Whitewater