# Vortex Force on a Bluff Body?

1. Jan 26, 2010

### Gordy

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 [Broken]

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.

Last edited by a moderator: May 4, 2017
2. Jan 26, 2010

### Gordy

Or will I just end up with a squiggly-wiggly mess? (To speak in scientific terms)

3. Jan 27, 2010

### minger

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

4. Jan 27, 2010

### Gordy

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?

5. Jan 27, 2010

### minger

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.

6. Jan 27, 2010

### FredGarvin

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.

7. Jan 27, 2010

### Gordy

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?

8. Jan 27, 2010

### minger

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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