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von karman vortices

Vortices of Von Karman!

The Canary Islands and the vortices of Von Kármán.

The spectacular images are usually given in satellite views of the Canary Islands with the right conditions

One of the most striking structures observed in satellite images of subtropical areas, in this case a photograph of last Friday, June 16 over the Canary Islands, are the whirlpools or vortices of Von Karman. These structures are generated when the air flow affects an orographic obstacle, such as an island.

Under certain atmospheric conditions, a family of cyclonic and anticyclonic vortices form downwind of the obstacle, extending up to several hundred km downstream of the obstacle. These vortices do not resemble the cloud structure in which they are embedded, usually stratocumulus.

As we move away from the obstacle, the vortices grow in spatial dimension while dissipating or weakening, until they disappear. On certain occasions the eddies located in the two lines or opposite streets can interact with each other. Other times, vortices do not become apparent to our eye, due to the absence of clouds or a very dry environment, but they exist as disturbances of the wind field.

The presence of Von Karman eddies is seen in many fluids where the stationary obstacle passively opposes the flow impinging on it. Vortices of this type have been detected in the marine areas themselves downwind of islands, as is the case of the islands, and the photograph of yesterday Friday that illustrates this article.

How they are formed

Von Karman, who was not a meteorologist, physically explained why these eddies form aerodynamically. In his time, there was no evidence of cloud structures that remembered the shape of such disturbances. It was with the arrival of meteorological satellites when their existence was proven. We will focus on the case of vortices generated by islands in the lower troposphere, for obvious reasons.

When satellite images of these structures are analyzed, they have to be formed, predominantly, in areas where there are stratocumulus clouds or clusters of little vertical development, areas dominated by subtropical anticyclones (case of the Canary Islands, Madeira,..). This leads to great stability and the presence of a strong thermal inversion on the top of the stratocumulus. Let's see and explain these facts, qualitatively.

A layer of air where temperature increases with height is called a thermal inversion layer. The effect is like that of an atmospheric "cover". This layer is a symptom of stability. In areas subject to the influence of subtropical anticyclones, subsidence or generalized declines are very common and generate these layers or thermal inversions. The airflow, below this level or layer, is forced to move horizontally and the vertical movements below it are very weak allowing the development of cumuliform clouds but organized in strata.

If an island or mountain is situated in this flow with the tops above said layer, the air flow, which is below the inversion, is forced to surround it without passing over it. Under certain conditions, these vortices can be generated, whose dimensions are relatively small compared to those of storms or hurricanes of synoptic type. We are talking about mesoscale vortices, with a size of the order of tens of kms.

The pair of meso vortices rotate in the opposite direction and are generated on each of the flanks of the island. For the northern hemisphere, and placing ourselves behind the obstacle and facing the wind, the one on the left does it cyclonically and the one on the right, anticyclonically. If the layer where they develop has cloudiness, for example stratocumulus, the vortices become visible from satellite.

Who was Von Karman

Todor (Theodore) von Kármán was born on May 11, 1881 in Budapest, Hungary. He was an aeronautical engineer and professor at different schools and universities. It was in 1911 that he made a detailed analysis of the alternation of vortices aligned in two rows or streets behind a body immersed in a fluid.

In 1912 he became professor and director of the Aeronautical Institute of Aachen in Germany at the age of 31. Later, he became Director of Caltech's Guggenheim Aeronautical Laboratory from 1930 to 1949. In the USA, he was one of the founders of the prestigious Jet Propulsion Laboratory. The formulation of the vortices that we have described here led to them being baptized with his name. Many consider him a genius in the world of modern aeronautics.

link to article in case you can't view the pic on here

Attached Files
.docx   Von Karman.docx (Size: 2.02 MB / Downloads: 80)
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