Vortex Shedding 1

[sandman21]

In ASCE 7-05, you cannot use the analytical procedure if your building has crosswind loading, vortex shedding, etc. Can anyone provide an explanation as to what vortex shedding is? Thank you

 

[sms]

As air flows around a structure, one result may be vortices are generated in the flow. Go to an online encyclopedia for the more info on a vortex, but it is a circular flow of fluid (in your case air). Vortex generation can cause alternating air pressure on the structure, that if you are unlucky might be resonant with the structure causing it to collapse.

The tacoma narrows bridge is a good example of this.

Check here for a video...

http://www.youtube.com/watch?v=3mclp9QmCGs

"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" Oddball, "Kelly's Heros" 1970

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[msquared48]

You get the same effect off the wingtips of jet aircraft. That's the reason for the vertical spoilers on the tips of the wings - to minimize the vortices and hence, decrease drag and increase fuel efficiency. Just look at the wingtips of aircraft as they go thru a smoke plume. The rotation is toward the aircraft at the top of the vortice.

Structurally, the problem occurs when you incite harmonic effect via the vortex shedding. The presence of these does constitute a force to be dealt with, and can be very destructive if not designed for.

Mike McCann
McCann Engineering

 

[UcfSE]

ASCE 7 deals with along-wind response. Cross-wind effects occur when the building/structure has response components normal to the wind direction. Some structures may be especially sensitive to these effects.

Wiki has some good introductory material and pictures the last I looked.

 

[jike]

A practical illustration of what vortex shedding is: A vertical radio antenna on a automobile is subject to wind pressure as the air rushes by it. Because the antenna is interupting the air flow, tiny vortecies are formed on each side in an alternating pattern. The wind and the vortex formation creates alternating forces at right angles to the wind flow. At wind speed nearing the natural frequency of the antenna, you will see "flutter" when the antenna moves back and forth at right angles to the wind.

Another place vortex shedding can occur is on cantilevered steel stacks. If it is a problem, strakes are usually added to change the frequency and formation of the vortecies. Strakes are either individual plates or a continuous plate applied in a helical pattern along the length of the stack.

 

[UcfSE]

This wiki page about the Von Karman vortex street has a good aerial photo of this effect in the cloud cover. Lower on the page is a nice animated photo of the effect.