Resonance frequency query

[reno001]

Dear all,

I am working on a micro cylindrical structure. Exactly the structure looks as a dish upside down onto a disc and the disc is on a small stand. Finally the stand is attached to the ground.

I am looking for the resonance frequencies of this structure. Because of the axial symmetry I thought I could use 1/4th of the structure and apply customized boundary conditions. This idea could help me to get quicker results. But getting the first results I realize that is not the case. The results are different between the whole structure and the 1/4th .

Is there anyone who experiment some thing similar?
Could you confirm my results?
Could you explain me why?
Is there any published work (references)?

Any help would be appreciated.

Thanks in advance

Renaud

 

[meshparts]

Hello Renaud,

I can confirm your results. By using any kind of symmetry conditions, you leave out those eigenmodes, who do not respect the symmetry condition. Is that what you meant?

Regards,
Alex

 

[cbrn]

Hi,
good point Mihaiupb! In fact, it is possible to take advantage of cyclic symmetry in eigenmodal extraction, but it MUST be done with "cyclic-type" constraint equations; these can automaticaly be set up by ANSYS if you define a "cyclic sector". This doesn't ensure that you get by default ALL the posible non-symmetric eigenmodes, though: you have to tell ANSYS the range of "harmonic indexes" you want him to compute. For the meaning of "harmonic index", see the help manual; simply said, you can think the wave of your eigenform being expressable as a Fourier series of harmonic functions: the harmonic index is the order of the expansion.
Hope this helps,

regards

 

[reno001]

Hi Cbrn and Alex

Thanks for you answers.
I will add one information for the others Ansys users.
The following web site has some good staff:

http://www.dermotmonaghan.com/feInformation/analysisTypes/modal.php

These could help anybody doing the same job than I.

Best regards

Renaud