During my usual study of scientific material on the internet, I came across this so very interesting phenomenon of ‘Von Karman Vortices’, and was compelled to share it with everyone, though many of you would already be familiar with the concept.
A Kármán vortex street is a repeating pattern of swirling vortices caused by the unsteady separation of flow over bluff bodies. (A vortex (pl. vortices) is a spinning, often turbulent, flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex) - Wikipedia.com
Relative motion between an object and a fluid is common occurrence. Simple examples are the motion of a plane in flight or the wind blowing on an obstacle. Obstacles disturb the flow and create particular shapes in their wakes. This phenomenon can be easily observed behind piers of a bridge, where eddies appear and are blown by the stream.
This wake might be complex depending on the shape of the obstacle. In order to understand this phenomenon, studies are done on a simple case: a two-dimensional flow past a circular cylinder. This case illustrates Strouhal instability and the particular wake known as Von Karman Vortex Street. It is a succession of eddies created close to the cylinder that break away alternatively from both sides of the cylinder. Vortices are emitted regularly and rotate in opposite senses.
A von Karman vortex street can be seen in the wake of different shaped objects, especially cylindrical objects. The vortex street is a visualization of interesting fluid flow events, including vortices, and also provides an aesthetically appealing image. Von Karman vortex streets are so common, in fact, that satellites have captured images of naturally occurring vortex streets in the wake large objects on Earth.
Clouds make the vortices visible. The center of each vortex is clear, researchers say, because the rotating motions cause some wind to blow vertically, breaking up the otherwise solid deck of clouds.
It is also believed that insects take advantage of the energy from the vortices of the von karman vortex streets that form around their wings during flight. Insects rotate their wings before beginning the return stroke so that their wings are at an angle from 45 degrees to 90 degrees to the vortex and are lifted by the eddies of air. Vortices usually just create drag; however, this method used by insects can recapture some of the energy and use it to improve speed and maneuverability.
We often come across different formations of clouds during cyclones and other weather conditions, but I have never seen something so beautiful before. Once again, it leaves man in wonder of the magnificence and beauty of God’s creation. No doubt nature holds in it beauty that man cannot beat with his creations, and karman vortices are nature’s beauty.
Phenomena that we explain in labs, conducting experiments within little tanks and glass sheets, nature exhibits with such magnificence that makes us submit to the nature’s powers, and admit that we cannot compete with it.
Sources:
http://hmf.enseeiht.fr/travaux/CD0102/travaux/optmfn/gpfmho/01-02/grp1/presenta.htm
http://www.colorado.edu/MCEN/flowvis/galleries/2006/assignment4/WilkesandersSkallerud.pdf
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The full summary of the report can be find on the website of State Bank of Pakistan by 
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