Missing nodal diameter
Missing nodal diameter
(OP)
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.
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.





RE: Missing nodal diameter
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Missing nodal diameter
RE: Missing nodal diameter
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Missing nodal diameter
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.
RE: Missing nodal diameter
RE: Missing nodal diameter
RE: Missing nodal diameter
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 ?
RE: Missing nodal diameter
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
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.
RE: Missing nodal diameter
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...