Vortex shedding

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Vortex shedding is an unsteady flow that takes place in special flow velocities (according to the size and shape of the cylindrical body). In this flow vortices are created at the back of the body and periodically from both sides of the body.

Vortex shedding is caused when air flows past a blunt structure. The airflow past the object creates alternating low-pressure vortexes on the downwind side of the object. The object will tend to move toward the low-pressure zone.

Eventually, if the frequency of vortex shedding matches the resonant frequency of the structure, the structure will begin to resonate and the structure's movement can become self-sustaining.

Vortex shedding was one of the causes proposed for the failure of the Tacoma Narrows Bridge in 1940, but was rejected because the frequency of the vortex shedding did not match that of the bridge.