The Karman vortex street is one of the best known vortex patterns in fluid mechanics. The vortex street is just a special type of unsteady separation over bluff bodies such as the flat plate in the image at the right. The vortex street is highly periodic having a frequency which is proportional to U/D, where D is the length of the bluff body measured transverse to the flow and U is the incoming flow speed. This periodicity is responsible for the "singing" of telephone wires. In fact, vortex streets are almost always involved when the wind generates a fairly pure tone as it blows over obstacles.
A practical consequence of the regular, period flow is that the forces on the body are also periodic. Because the flow is asymmetric fore and aft as well as in the direction transverse to the flow the body will experience both and oscillating drag and lift. If the frequency of the shedding is close to a structural frequency, resonance can occur, usually with unpleasant results.
One example in day-to-day life where one can see such fluid-structure resonances is the antenea of your automobile. You will notice dramatically different vibrations as the speed of the vehicle changes. As the speed changes, the driving frequency changes, thus giving rise to different structural resonances of the antenea.
More von Karman vortex streets can be found at the following links:
The image at the above right is a screen cap of a mpeg found on the ITSC Movie site. The folks at ITSC were kind enough to give me permission to take the capture and use it here. Their collection of simulations and movies is definitely worth a look. Either click on the highlighted text or the image itself to go to their site.
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