Frequency clarification

[suviuuno]

I get peculiar results from turbine rotor. Adding constraints to bearing locations lowers natural frequencies. Higher frequencies with no constraints comparing to same mode shapes as in constrained model. No damping included at bearing locations. Beam and shell elements are used. Adding any constraints should add onto natural frequencies, right?

Thanks a bunch

 

[GregLocock]

Normally adding constraints to a system would increase the frequencies seen, crudely because you are suppressing motion, which is a lot like adding stiffness. More accurately you are coupling an infintely stiff system with no mass to your original system.

But, this does not always increase the frequencies, although I doubt the modes are truly identical. I am struggling to think of an example, but I think I've seen this in multibearing shafts before.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

I notice someone in your thread in the FEA forum has correctly identified a system where adding constraints reduces the frequency of the first flexural mode. That's always interested me. I think I understand why, but only in a hand waving sense.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.