Will Vortices Cause Alternating Force on a Bridge in Wind Tunnel Testing?

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Discussion Overview

The discussion revolves around the effects of vortices on a bridge structure during wind tunnel testing, specifically focusing on whether these vortices will generate alternating forces on the bridge. Participants explore the relationship between vortex shedding, force measurements, and the structural response of a suspension bridge cross-section in a controlled wind environment.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if vortices will apply alternating forces to a bridge structure in a wind tunnel, suggesting that they expect to see distinct peaks in force corresponding to the Strouhal frequency.
  • Another participant suggests that the response may be unpredictable for complex shapes like a bridge, noting that inherent damping in the structure could affect the response.
  • A participant mentions that the bridge will deform under the influence of shedding vortices, which may complicate the measurement of force and introduce additional natural frequencies.
  • There is a proposal to use a smoke wand and high-speed camera to observe vortex shedding and measure its frequency, with a question about whether the forces would be strong enough to correlate with observed peaks in force.
  • One participant expresses a belief that it is possible to match force peaks to observed frequencies based on literature, while also noting the presence of both shedding and natural frequencies.
  • Concerns are raised about the utility of measuring shedding frequencies directly from the structure, with skepticism about obtaining useful data without a frequency analyzer.
  • A participant reflects on the proximity of bending and shedding frequencies observed in a related study, indicating variability in results across different bridge designs.

Areas of Agreement / Disagreement

Participants express a mix of viewpoints, with some agreeing that vortex shedding could produce measurable forces while others remain uncertain about the predictability and clarity of the results. The discussion does not reach a consensus on the effectiveness of the proposed measurement techniques or the expected outcomes.

Contextual Notes

Participants note the complexity of the bridge shape and the potential influence of structural damping on the results. There are references to specific studies that highlight variability in natural frequencies, indicating that results may depend on the specific design and conditions of the test bridge.

Gordy
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So, let's say I have a bluff body in a moderate-speed wind environment, as in wind across the deck of a bridge. As the vortices form across its surface, do they apply alternating force to the structure?

(Maybe I'm completely misunderstanding the nature of vortices)

http://img9.imageshack.us/img9/2107/bridgevortices.jpg

Specifically, I'm putting a suspension bridge cross-section in a wind tunnel, and I'm hoping that if I attach a force gauge to the tail end, the graph will show distinct positive-negative peaks in force, in accordance with the Strouhal frequency. Is this what will happen?

For reference, the force gauge I own graphs at 5000 samples/second, measured in nano-Newtons.
 
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Or will I just end up with a squiggly-wiggly mess? (To speak in scientific terms)
 
You'll probably get a little of column A a little of column B. For a simple shape such as a cylinder, the response is predictable. One can examine the Reynolds number to find a Strouhal number, and from there find the shedding frequency.

However, for a complex shape such as a bridge, I imagine it would be difficult to identify one dominant frequency. In addition to that, inherent damping in the structure will tend to damp the response.

I just looked a paper up and found another good point. As you apply force in the form of shedding vortices on the bridge, the bridge will deform and in addition to the vortices, you will start to measure the actual bending natural frequencies on the bridge.

p.s. Paper was:
Journal of Wind Engineering and Industrial Aerodynamics 74-76 (1998) 829-838
Reynolds number effects in the flow around a blunt bridge deck cross section
Gunter Schewe, Allan Larsen
 
Right, but say that I also used a smoke wand + high-speed camera to observe the vortex street and measure the frequency. What I am asking is, would the forces be strong enough that I could match certain peaks in force to the observed frequency?
 
After reading some of that paper I would be inclined to say yes. Look some papers on the subject, people seem to get some decent results. Do note though that you will get both shedding and natural frequencies.
 
Shedding frequencies are usually pretty low compared to the static structure modes. I would doubt you'll see anything useful if your chart is attached to the structure itself. If you were looking for structural modes you may see something. Without a frequency analyzer you're just going to get garbage.
 
I'm not sure what you mean by structural modes, but take in mind that this is a relatively short span (just decking, no trusses/etc.) so flexing/torsion along the length won't really be an issue. By frequency analyzer, do you mean a piece of hardware or software?
 
I would have thought so too, but if you look at this chart which I copied from the above-mentioned paper, the bending and shedding frequencies for this particular test bridge were pretty close.

This particular bridge was meant to simulate the Great Belt East Bridge in Denmark, which is a 1624m main span, with 193 m between piers. The shape is a trapezodial girder, which the test section emulates.

I did however see that in a foot bridge study from japan that they had a natural frequency at 2 Hz. So...not sure.
 

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