Missing nodal diameter

[Domainpower]

Hi all ,

I am doing modal analysis of a rotating machine.In my first run i used a sector and applied cyclic symmetry analysis in ansys code.
i tried to extract first 10 nodal diameters of bending modes.
I was able to identify bending modes for nodal diameter = 0,3,4,5,6,7,8,9,10. However nodal diameter = 1 and 2 seemed missing.

I rerun my modal analysis this time with full model but again
I could not find bending mode for nodal diameter = 1 and 2.

Is it normal?

I am attaching some pictures of the machine below if it could help.

In case any more information is required please ask.

 

[GregLocock]

Quick hint. Many engineers don't have proprietary management aid software on their computers.

Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[Domainpower]

Sorry Greg but i couldn't pickup your hint.

 

[GregLocock]

pptx is a proprietary microsoft format.


Cheers

Greg Locock


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

http://eng-tips.com/market.cfm?

 

[Domainpower]

Thanks Greg for clarification.

I have attached the picture below in word format , i hope this should help.

Also is it likely that as i have displacement constraint not symmetrical (that is i don't have them on both sides) , i am missing nodal diameter 1 and 2?

Please suggest.

 

[Domainpower]

Can someone provide a bit insight into the problem.

 

[Domainpower]

Is any additional information required GregLocock?

 

[amanuensis]

It's difficult for me to understand your problem.
But generally, when you work with symmetrical structures, it can appear the phenomenum of degenerescence.
It means that several modes have the same resonance frequency but not the same mode shape.

Could you look at the values of the first resonance frequencies and check/verify whether or not you observe a degenerescence ?

 

[Domainpower]

Thanks amanuensis for showing interest.

May be i did not made my problem clear.

I built 1/3rd sector of model and applied cyclic symmetry.Hence results should follow ideally:


Harmonic Index

Nodal diameter set​

0 :

0,3,3,6,6,9,9,12,12......​

1 :

1,2,4,5,7,8,10,11,13.....​

However when i plot the modes under harmonic index 1 i don't find bending modes with nodal diameter 1 and 2.

I just wanted is it a normal behavior to miss certain mode shapes in case the displacement constraint is only on one side as shown in the picture.

 

[amanuensis]

Well, it seems to be an advanced problem...

Do you mean that you have a spherical harmonics decomposition (n=0, n=1, etc...)?
For each harmonic, you look at in the orthogonal direction to see the bending modes ?

So sorry, but I don't understand what are this series of numbers {0,1,2,3, etc...}

Maybe, you should save your image in pdf format so that everyone can look at it.

I don't see a clear relationship between the boundary conditions and the missing modes...