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# Vortex Shedding

/Vortex Shedding
Vortex Shedding2012-11-30T19:56:00+00:00
In vortices on the downstream side of the object. The object will tend to move toward the low-pressure zone.

If the cylindrical structure is not mounted rigidly and the frequency of vortex shedding matches the chimneys constructed of thin-walled steel tube can be sufficiently flexible that, in air flow with a speed in the critical range, vortex shedding can drive the chimney into violent oscillations that can damage or destroy the chimney. These chimneys can be protected from this phenomenon by installing a series of fences (sometimes called strakes or spoilers) at the top and running down the exterior of the chimney for approximately 20% of its length. The fences are usually located in a helical pattern. The fences prevent strong vortex shedding with low separation frequencies. The optimal pitch for vortex shedding is a 5D pitch (5 x the diameter of the stack).

Vortex shedding was one of the causes proposed for the failure of the original [1]

A thrill ride, “VertiGo” at Cedar Point in Sandusky, Ohio suffered vortex shedding during the winter of 2001, causing one of the three towers to collapse. The ride was closed for the winter at the time.[2]

## Governing equation

The frequency at which vortex shedding takes place for a cylinder is related to the Strouhal number by the following equation:

$St = frac{fD}{V}$

Where $St$ is the Strouhal number, $f$ is the vortex shedding frequency, $D$ is the diameter of the cylinder, and $V$ is the flow velocity.

The Strouhal number depends on the body shape and on the Reynolds number.

## Mitigation of vortex shedding effects

Modern tall smokestacks usually have a corkscrew fin (a strake) to deliberately introduce turbulence, so that the load is less variable and resonant load frequencies have negligible amplitudes.[3]