My OperaVon Karman Vortices - Revisited
Tuesday, April 17, 2012 7:12:56 PM
A couple of years ago I wrote a post on Von Karman Vortices. I am now returning to this subject because NASA a few days ago released an extremely beautiful satellite image of such vortices taken on 5 April 2012.
Von Kármán vortices, which can appear as long linear chains of spiral eddies, form nearly anywhere that fluid flow is disturbed by an object. The atmosphere behaves like a fluid, so the wing of an airplane, a bridge, and even an island can cause the vortices to form. Von Karman vortices are named after aeronautical engineer Theodore von Kármán
Strong northerly winds frequently blow across Greenland, carrying cold, relatively dry polar air out across the Greenland Sea. As the air passes over the moist, warmer waters, conditions are right for cloud formation. As a result, the Greenland Sea is often cloud-filled.
In the image above the Jan Mayen Island in the Greenland Sea is responsible for the spiraling cloud pattern. Jan Mayen Island is situated about 500 km east of central Greenland, and about 600 km west of North Cape, Norway, positioning it an area prone to both clouds and wind. The island is dominated by the Beerenberg volcano, which rises 2,277 m on the northeastern end. This tall, ice-capped mountain forms a formidable barrier to wind flow. When a strong wind slams against the tall, immovable volcano, the air becomes quite turbulent, and forms a swirling pattern as it passes by. As winds pass around the volcano, the disturbance in the flow propagates downstream in the form of a double row of vortices that alternate their direction of rotation.
The image below covers a larger area. In the left lower corner of this image, a pattern of feathery white can be seen. This pale veil appears to float above the lower clouds, because the von Karman vortex patterns can be seen beneath the veil in some areas. Viewing the 5 minute swath data used to create this image, the veil can be seen to extend well over Greenland. The color, texture and altitude are suggestive of a large plume of wind-borne snow, possibly mixed with high clouds. It is difficult, however, to differentiate snow from clouds using true-color imagery.
Various Views of von Karman Vortices can be found at this NASA page, including the animation below (courtesy of Cesareo de la Rosa Siqueira at the University of Sao Paulo, Brazil), that shows how a von Karman vortex street develops behind a cylinder moving through a fluid.
Image: NASA/Wikipedia
Here is the technical explanation:
"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."
Von Kármán vortices, which can appear as long linear chains of spiral eddies, form nearly anywhere that fluid flow is disturbed by an object. The atmosphere behaves like a fluid, so the wing of an airplane, a bridge, and even an island can cause the vortices to form. Von Karman vortices are named after aeronautical engineer Theodore von Kármán
Strong northerly winds frequently blow across Greenland, carrying cold, relatively dry polar air out across the Greenland Sea. As the air passes over the moist, warmer waters, conditions are right for cloud formation. As a result, the Greenland Sea is often cloud-filled.
In the image above the Jan Mayen Island in the Greenland Sea is responsible for the spiraling cloud pattern. Jan Mayen Island is situated about 500 km east of central Greenland, and about 600 km west of North Cape, Norway, positioning it an area prone to both clouds and wind. The island is dominated by the Beerenberg volcano, which rises 2,277 m on the northeastern end. This tall, ice-capped mountain forms a formidable barrier to wind flow. When a strong wind slams against the tall, immovable volcano, the air becomes quite turbulent, and forms a swirling pattern as it passes by. As winds pass around the volcano, the disturbance in the flow propagates downstream in the form of a double row of vortices that alternate their direction of rotation.
The image below covers a larger area. In the left lower corner of this image, a pattern of feathery white can be seen. This pale veil appears to float above the lower clouds, because the von Karman vortex patterns can be seen beneath the veil in some areas. Viewing the 5 minute swath data used to create this image, the veil can be seen to extend well over Greenland. The color, texture and altitude are suggestive of a large plume of wind-borne snow, possibly mixed with high clouds. It is difficult, however, to differentiate snow from clouds using true-color imagery.
Various Views of von Karman Vortices can be found at this NASA page, including the animation below (courtesy of Cesareo de la Rosa Siqueira at the University of Sao Paulo, Brazil), that shows how a von Karman vortex street develops behind a cylinder moving through a fluid.
Image: NASA/Wikipedia
Here is the technical explanation:
"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."
- http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=77654&src=eorss-nh
- http://modis.gsfc.nasa.gov/gallery/individual.php?db_date=2012-04-11
- http://www.redorbit.com/images/pic/61026/cloud-vortices-2/
- http://my.opera.com/nielsol/blog/karman-vortex-canarie-islands
