Vortex shedding

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 detach periodically from either side of the body. See Von Kármán vortex street.

Vortex shedding is caused when air flows past a blunt object. The airflow past the object creates alternating low-pressure vortices 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 resonance frequency of the structure, the structure will begin to resonate and the structure's movement can become self-sustaining. Tall 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 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.

Vortex shedding was one of the causes proposed for the failure of the original Tacoma Narrows Bridge (Galloping Gertie) in 1940, but was rejected because the frequency of the vortex shedding did not match that of the bridge. The bridge actually failed by aeroelastic flutter [K. Billah and R. Scanlan (1991), "Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks", American Journal of Physics, 59(2), 118--124 [http://www.ketchum.org/billah/Billah-Scanlan.pdf (PDF)] ] .

A thrill ride "Vertigo" at Cedar Point in Sandusky, Ohio Suffered the fate of vortex shedding during the winter of 2001, one of the three towers collapased. The ride was closed for the winter at the time.

Governing equation

The frequency at which vortex shedding takes place for a cylinder can be derived using the following equation::$St\; =\; frac\{fD\}\{V\}$Where $St$ is the Strouhal number, $f$ is the frequency, $D$ is the diameter of the cylinder, and $V$ is the flow velocity.

For Reynolds numbers greater than 1000, the Strouhal number is approximately equal to 0.21.

See also

* Vortex
* Vortex induced vibration
* Von Kármán vortex street

References

* [http://www.tech.plym.ac.uk/sme/Interactive_Resources/tutorials/FailureCases/sf1.html Failure of the Tacoma Narrows Bridge]
* [http://www.mecaenterprises.com/vortex_shedding.htm Another explanation of vortex shedding] (includes a helpful diagram)
* [http://www.civeng.carleton.ca/Exhibits/Tacoma_Narrows/ A movie of the Tacoma Narrows Bridge failure]

External links

* [http://www.youtube.com/watch?v=_AJgEa2dbJU Flow visualisation of the vortex shedding mechanism on circular cylinder using hydrogen bubbles illuminated by a laser sheet in a water channel. Courtesy of G.R.S. Assi.]