I understand that Karman Vortex street are generated in presence of blunt obstacles. But why Karman Vortex street show alternating rotating structures?
If the upper side sheds a vortex, the lower one should also shed it. And finally to balance the overall flow structure somewhere downstream, if one is clockwise the other has to be anticlockwise
Hi Hassan,
Karman vortex street is formed by the vortices shed ALTERNATELY from the top and bottom surfaces of a blunt body. We have boundary layers on both surfaces. Within the boundary layers we have zero velocity on the surface (due to no-slip condition) and close to free stream velocity on the edge. This velocity gradient within the boundary layer, in a direction normal to surface has the tendency to rotate the fluid elements. This happens on both surfaces (upper and lower surfaces) and the sense of rotation is given by the Right Hand Rule (i.e. make your right hand as if you are giving thumbs up sign with the velocity at the boundary layer edge pointing in the direction of your fingers, then the vorticity or rotation vector has the direction of your thumb). When the boundary layer flow reaches the end of the blunt body, it separates from the body. It is called ‘vortex shedding’ as the separating boundary layer due to the 'rotational tendency' stated above doesn’t just leave the surface as a straight stream BUT leaves as a rotating chunk of fluid. These rotating chunks rotate ‘clockwise’ on the top side and ‘counter clockwise’ on the bottom side (you can check that using RH rule). The chain of these continuously shed vortices downstream of the body is the Karman vortex street.
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