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JAYESH S February 20, 2014 03:38

Natural frequency Disappearing. need help
Hi sir,

I was doing modal analysis of PCB board in ANSYS. I have done for full model, half model and quarter model. But some of the natural frequencies in the full model is disappearing in the half model and some more in quarter model.

Is there any article regarding this subject?

Any reasons available?
Will this affect for my analysis?


Jayesh S

Florin February 21, 2014 06:48

Hello Jayesh,
Your problem is explained in most of the modal analysis documentation.
The reason your frequencies are disappearing is the symmetry conditions.
When you use a half model, with symmetry, only the symmetric mode shapes (and their frequencies) will be picked up by the program.
To get the other half of the frequencies you need to specify anti-symmetric BC on the same half model.
The same observations apply for the quarter model.

Actually it is not recommended to use symmetry BC for modal analysis; the analysis is linear and you don't need a very refined mesh; the analysis is not very computationally expensive. Therefore it is recommended to make the analysis on the full model.

I hope this solves your problem.

JAYESH S February 22, 2014 06:47

Thank you very much for the information.

I have done modal analysis in full model only.
If I use the half model or quarter model for the further analysis, will there be any issues or errors?

Jayesh S

Florin February 28, 2014 03:32

Hello Jayesh,
No, if you use the symmetry the frequencies and modes will not be erroneous, but you will not get all of them; only the symmetric modes are computed; that is if there will be a mode shape that is not symmetric, it will not be shown.
Therefore, the results will be correct, but they will not be complete.

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