AN UNCOMMON THOUGHT

"The real trick to life is not to be in the know, but to be in the mystery."
-Fred Alan Wolf

01 August 2016

Eddies.


The phenomenon that is shown in the image of Guadalupe Island at the top of this page (acquired on August 20, 1999) features a ubiquitous occurrence in the motion of fluids—a vortex street, which is a linear chain of spiral eddies called von Karman vortices. Von Karman vortices are named after Theodore von Karman, who first described the phenomenon in the atmosphere. Dr. von Karman was a co-founder of NASA's Jet Propulsion Laboratory.

von Karman vortices form nearly everywhere that fluid flow is disturbed by an object. In the cloud images shown on this page, the "object" that is disturbing the fluid flow is an island or group of islands. As a prevailing wind encounters the island, the disturbance in the flow propagates downstream of the island in the form of a double row of vortices which alternate their direction of rotation. The animation below shows how a von Karman vortex street develops behind a cylinder moving through a fluid.


Both the ocean and atmosphere are fluids, in constant motion. On our limited “human”-scale, we are aware of this motion when we feel the wind blow,  or when we encounter a current running along the beach while swimming. Yet our eyes alone can rarely observe the larger scale of fluid motion in the  ocean and atmosphere.

Technical description: As a fluid particle flows toward the leading edge of a  cylinder, the pressure on the particle rises from the free stream pressure  to the stagnation pressure. The high fluid pressure near the leading edge   impels flow about the cylinder as boundary layers develop about both sides. The high pressure is not sufficient to force the flow about the back of the  cylinder at high Reynolds numbers. Near the widest section of the cylinder,  the boundary layers separate from each side of the cylinder surface and form  two shear layers that trail aft in the flow and bound the wake. Since the  innermost portion of the shear layers, which is in contact with the cylinder,  moves much more slowly than the outermost portion of the shear layers, which  is in contact with the free flow, the shear layers roll into the near wake,  where they fold on each other and coalesce into discrete swirling vortices.  A regular pattern of vortices, called a vortex street, trails aft in the wake.

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