Various Views of von Karman Vortices
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.
SeaWiFS has the unique ability to observe evidence of fluid motion in both the ocean and the atmosphere from space. Many other meteorological satellites can observe cloud patterns that show the fluid dynamics of the atmosphere, but SeaWiFS (under the right conditions) can also view plankton blooms that display fluid motion in the marine environment.
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 (courtesy of Cesareo de la Rosa Siqueira at the University of Sao Paulo, Brazil) shows how a von Karman vortex street develops behind a cylinder moving through a fluid.
"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."
The "Reynolds number" is the ratio of inertial forces to viscous forces in a fluid. The Reynolds number indicates the likelihood of turbulent (rather than laminar) flow in a fluid. As an example, two paddles moving at the same speeds—one through a bucket of water and one through a bucket of paint—will have different Reynolds numbers associated with the fluid flowing around them. The Reynolds number in the tub of paint will usually be lower.
Von Karman vortices form at all scales of fluid motion. The picture below shows a complex vortex street formed in a flowing film of soap with two cylinders in the fluid flow.
The picture below shows what happens when the fluid flow rate is increased, and a comb (rather than a single cylinder) is placed in the film. (These soap film vortex images are courtesy of Dr. Maarten Rutgers, in the "Science" section, "Vortex Street" subsection of his Web site. To see how the apparatus works, click "Soap Intro". The picture below is cropped from a larger version in the "Vertical Combs" subsection)
Compare the image above to these SeaWiFS images of phytoplankton blooms near the Shetland Islands in the North Sea (top) and the Falkland Islands east of Argentina (bottom):
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|Shetland Islands ||Falkland Islands |
It is clear to see that vortex patterns form on both the large and small scale. These patterns are particularly impressive in the atmosphere, the cloud spirals that form in the wind-wake of islands. Below are several more SeaWiFS images of von Karman vortex streets in clouds; a unique SeaWiFS image of swirls in sea ice (see note) on the coast of Hudson Bay; links to other images of vortex streets taken from Skylab, the Space Shuttle, and a geostationary meteorological satellite (GOES); and a GOES movie of a vortex street forming above Guadalupe Island.
Vortex street, Guadalupe Island
Guadalupe Island, March 10, 2000
Vortex streets, Cape Verde Islands:
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|January 1, 2000 ||January 19, 2000 |
|May 8, 2000 |
Vortex streets, Canary Islands:
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|April 24, 2000 ||June 4, 2000 |
Swirls in Hudson Bay, Canada: (Note: It's not absolutely certain that this is ice. Instead, it could be fog. The sharpness of "cracks" in the center and right of the image make ice a more likely candidate.)
July 22, 2000
- von Karman Vortices over the Pacific Ocean, from Skylab, August 1, 1973
- von Karman Vortices over Socorro Island, Mexico, from the Space Shuttle, May 16, 1992
- von Karman Vortices over Heard Island, Antarctica, from the Space Shuttle, November 14, 1994
A final note: Recent studies of insect flight have revealed that one facet of their flight dynamics is their ability to borrow energy from the vortices that form around their wings during flight. Normally the vortices are simply lost energy, also called "drag". Yet insects can recapture some of the energy in the vortices and use it to aid their flight speed and maneuverability.
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