Vortex shedding 1

[OAP]

Hi all,
Do anybody know simply calculation of the effect of vortex shedding on structure? If I have a circular plate 10.0m dia. and wind blows right to it.
Thanks
OAP

 

[lutein]

You could check on ASME STS-1-2000 code, you would be able to get the vortex shedding response on circular shell and be able to calculate the stresses based on the formulas shown in the code.

 

[ishvaaag]

I can send you a worksheet for Mathcad 2000 (I think posted to the Collaboratory) where you see what to do for the building case. Only variation should be how the natural frequency for your setup is established.

If you want so please post an e-mail address.

It is based in

Structural Dynamics in Practice
Arthur Bolton
Mc Graw Hill International
UK 1994

 

[Bannu]

I am not aware what is vortex shedding, can anybody possibly elborate on it?

 

[boo1]

Flows over bluff (non-streamlined) bodies tend to be characterised by a cyclical shedding of vortices in the wake of the body. This periodic forcing frequency can have disastrous consequences.

Often very large structures conduct additional wind studies to determine the flow characteristics of air flow from down draft and vortex sheading .

Cheers

 

[butelja]

3 WORDS: Tacoma Narrows Bridge. See

http://www.eng.vt.edu/fluids/msc/gallery/vortex/tac.htm

 

[TheBlacksmith]

I knew "Galloping Gertie" would come up soon in this thread.

Blacksmith

 

[IJR]

OAP

You might also want to check out this thread507-6585
Pals here did a good job there.

respects
IJR

 

[meca]

There is some information on my website concerning vortex shedding on stacks/chimneys at

http://www.mecaconsulting.com/vortex_shedding.htm

.

I also have software available for design of stack/chimneys, which includes checks for vortex shedding per ASME STS-1-2000 and CICIND 1999.

OAP - I will be happy to run your situation on my software if you want to give me the specifics (Ht, Dia, Thk, Mat'l, etc...)

Regards,

Chris
chris@mecaconsulting.com

http://www.mecaconsulting.com

 

[OAP]

Thanks all,
Actually, my structure is not a stack. It's a logo sign dia. of 10.0 m. which stand on the roof top of building. The level of the sign is about 210.0m from ground level. The thickness of sign is 0.30m and when included the supporting frame is to be 1.50m. There is a steel louvre wall behind of the sign 3.00m. away. This sign is fixed to a column at the mid of the sign. At top of a sign, I will provide the space truss which fixed to the toop of column to take the vertical & horizontal load fron the sign. At bottom of sign , I can provide the several plane trusses which fixed to preimeter beams to support the sign frame. The wind speed (returns periods 100 years) is about 29 m/s and I calculated the wind pressure at right angle to sign is 3 kN/sq.m.
By the way, due the behind of sign is empty, so I'm worried about the effect of Vortex shedding. If you guys have any further suggestion, it will be appreciate.
Thank again
OAP

 

[IJR]

To my knowledge, vortex shedding is a consideration only if the element subjected to it is so slender and natural frequency of vibration coincides with that of vortex oscilation frequency. If that is the case you are going to have galloping(amplified oscilations)

Your case? I guess the chance is that vortex shedding will be less a problem than unbalanced wind loads and suctions and gusts.

I suggest you design your support system for wind pressures and suctions given in codes(I know DIN 1055 part 4 for examp
le give you pressure coefficients accordingly). The empty(leeward) side will be subjected to suction and you may consider different combinations of pressure/suctions etc a bit conservatively

We carried out a similar design before for a roof structure using code coefficients, and worried about dynamics, we submitted our design for wind tunnel testing. Results did not differ much from those given in the codes.

good luck

IJR

 

[RHK]

With a flat plate or disc, the critical case is sometimes not when the disc is facing directly into the wind, but when the wind is from the side. Then, the wind creates lift, not drag. The major force is still at right angles to the plane of the disc, but now the centre of pressure is offset from teh centre. Theoretically, it is at the 1/4 point for a 2-D aerofoil, but for a circular disc it will be somewhere else. If your supports are strong enough to carry the eccentric load then it should behave and not vibrate too much. I think the vibration will come from rotation of the disc under eccentric wind. Check the angle of rotation that occurs for the "strong enough" support. Classical torsion-bending flutter might occur if this increment of angle of attack causes a significant increase in the lift force. Russell Keays

 

[OAP]

Thanks again you guys,
Your suggestion is very helpful for me. Furthermore, after I did the dynamic analysis in 3D, I got the first 3 mode frequency as follow:
1st mode = 16.05 Hz
2nd mode = 19.01 Hz
3rd mode = 19.7 Hz.
I think the dynamic problem will not be significant, right? Am I correct?

OAP