tubes - vortexes - second mode

[Tmoose]

One of our groups routinely analyzes vortex shedding frequencies of boiler tubes. I hear we are real concerned that they not match standing wave frequencies within the enclosure. Recently I was asked if I'd expect vortices could significantly excite the tubes "second mode" if frequencies are close. I'm checking to see if the "second mode" is really an S shape, and not another lateral mode )). I am not sure about the proximity of other tubes

My guess is things probably could get locked together, but I have no experience or references that offer anything useful.

Any suggestions?

thanks

Dan T

 

[hacksaw]

The modeshapes and even the form of excitation are a function of your tube layout and spacing, the over all response is dominated by locking effects. Blevin's is a good starting point, but your equipment manufacturer problably is the best reference.

 

[FeX32]

you would need significantly higher energy to excite the 2nd mode. ...although if the energy exerted by the vortex shedding can provide this then you would systematically expect the mode to be excited


Fe

 

[GregLocock]

Or you could do a modal analysis - loudspeaker, microphone, time.

I see no reason why vortices couldn't excite higher modes, though I don't think an S shape is the second mode of an open tube.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[FeX32]

oh... the energy i am referring to is 'in' the flow

and the 2nd mode is likely to be 'more damped' so the amplitude of oscillation would be less noticeable.
Of course, this is system specific, some systems are not like this.
I would worry more about the 1st mode


Fe

 

[GregLocock]

So would I but the second mode may be preferentially excited for structural reasons or because you have excitation by a vortex street.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[FeX32]

That's true Greg. I agree.
I guess it is all dependent on the OP's system.



Fe