Vortex shedding

Vortex shedding behind a circular cylinder. In this animation, the flows on the two sides of the cylinder are shown in different colors, to show that the vortices from the two sides alternate. Courtesy, Cesareo de La Rosa Siqueira.
Vortex shedding as winds pass Heard Island (bottom left) in the southern Indian Ocean resulted in this Kármán vortex street in the clouds

In fluid dynamics, vortex shedding is an oscillating flow that takes place when a fluid such as air or water flows past a bluff (as opposed to streamlined) body at certain velocities, depending on the size and shape of the body. In this flow, vortices are created at the back of the body and detach periodically from either side of the body forming a Kármán vortex street. The fluid flow past the object creates alternating low-pressure vortices on the downstream side of the object. The object will tend to move toward the low-pressure zone.

If the bluff structure is not mounted rigidly and the frequency of vortex shedding matches the resonance frequency of the structure, then the structure can begin to resonate, vibrating with harmonic oscillations driven by the energy of the flow. This vibration is the cause for overhead power line wires humming in the wind,[1] and for the fluttering of automobile whip radio antennas at some speeds. Tall chimneys constructed of thin-walled steel tubes 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.

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.[2]

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.[3] In northeastern Iran, the Hashemi-Nejad natural gas refinery's flare stacks suffered vortex shedding seven times from 1975 to 2003. Some simulation and analyses were done, which revealed that the main cause was the interaction of the pilot flame and flare stack. The problem was solved by removing the pilot.[4]

  1. ^ The Mechanical Universe: Mechanics and Heat, Advanced Edition, p. 326
  2. ^ K. Billah and R. Scanlan (1991), Resonance, Tacoma Narrows Bridge Failure, and Undergraduate Physics Textbooks, American Journal of Physics, 59(2), 118--124 (PDF)
  3. ^ Maureen Byko (May 2002). "Materials Give Roller Coaster Enthusiasts a Reason to Scream". The Minerals, Metals & Materials Society. Archived from the original on 8 February 2007. Retrieved 2009-02-22.
  4. ^ "Engineering Service". Archived from the original on December 31, 2015. Retrieved 2016-06-22.{{cite web}}: CS1 maint: unfit URL (link)