Various physical phenomenons related to flow are admirable. If you are enthusiastic to understand the truth below these phenomenons, observations (experiments) are the most effective way to examine problems. All in all, what about the flow go down through a cylindrical object ? As far as I know, experimental measurements show that the pressure on the reas side (lee side) of the cylinder is less than free stream. Therefore, it follows that the real fluid causes a large pressure-drag forece. Besides, the streamlines are not symmetrical with respect to the y-axis. Flow separates from the ream surface and forms a broad, pulsating wake. Wake consists of pairs of vortices shed alternately from the upper and lower part of the real surface. They called Karman Vortex Streets which is s stable configuration for vortex pair. A vortex street can be observed only over a given range of Reynolds numbers (Re), normally above a limiting Re value of about 90. As Re increases, the wake structure becomes more complicated but the alternating shedding can still be detected at Re=10000000. The shedding frequency is defined by the non-dimensionless Strouhal number. Vortex shedding is a viscous flow that evolces into unsteady motions because of flow instability. Where do you encounter with this phenomenon ? Look the sky, light the sky! Here is an example: 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. Remember the bridge that fell in the wind? Here is another example of Karman vortex regarding flow induced vibrations: To sum up, karman vortex is a street that enables you to reach you target. Keep in touch with Karman Vortex that you probably encounter a problem related to it. Source: [1]https://disc.gsfc.nasa.gov/education-and-outreach/additional/science-focus/ocean-color/vonKarman_vortices.shtml [2]https://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_vortex_street
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