Menu

Discussion Forum

New Discussion
Filter

Discussion Closed This discussion was created more than 6 months ago and has been closed. To start a new discussion with a link back to this one, click here.

Eigenfreguency

Posted Jun 23, 2010, 8:08 a.m. EDT 3 Replies

Tewodros Tedo
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Dear Comsolers,

I was using Eigenfrequency analysis in the structural mechanics. when I am looking at the results of the eigenfrequencies some of the values are repted. what does it mean when two eigenfrequencies have the same value?

best regards
3 Replies Last Post Aug 12, 2010, 3:18 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Jun 23, 2010, 12:53 p.m. EDT
Hi

Usually if you do not attach your structure (at all) you get 6 frequencies at 0Hz (when in 3D) corresponding to the 6 free DOF.
Now if you model has a symmetry, such as an axial cylindrical symmetry (let say along Z), you might have a frequency bending along X, and one along Y, but as you model is symmetric you should see the same (or almost) frequency value for both so there is free variable along Rx and Ry. It's the degeneracy of the mode.

If you have no symmetry and your model is well fixed they should all be different. This is eneral physics and has nothing to do with COMSOL or A, or F or N or whatever the FEM software name is ?

Have fun Comsoling
Ivar
Hi Usually if you do not attach your structure (at all) you get 6 frequencies at 0Hz (when in 3D) corresponding to the 6 free DOF. Now if you model has a symmetry, such as an axial cylindrical symmetry (let say along Z), you might have a frequency bending along X, and one along Y, but as you model is symmetric you should see the same (or almost) frequency value for both so there is free variable along Rx and Ry. It's the degeneracy of the mode. If you have no symmetry and your model is well fixed they should all be different. This is eneral physics and has nothing to do with COMSOL or A, or F or N or whatever the FEM software name is ? Have fun Comsoling Ivar
Dan Costley
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Aug 11, 2010, 6:32 p.m. EDT
I am doing eigenfrequency analysis of a Pratt truss bridge, similar to the example in the COMSOL documentation. However, the solution outputs 30 eigenfrequencies between 0 and 4 Hz, or 20 between 6 and 7 Hz. In many of these, just one or two structural elements are distorted when plotted. In other words, over half the eigen-functions appear to be garbage. Is this natural? Is their some way to filter these out, like putting a quality factor on the mode, or something like that?
Thanks.
I am doing eigenfrequency analysis of a Pratt truss bridge, similar to the example in the COMSOL documentation. However, the solution outputs 30 eigenfrequencies between 0 and 4 Hz, or 20 between 6 and 7 Hz. In many of these, just one or two structural elements are distorted when plotted. In other words, over half the eigen-functions appear to be garbage. Is this natural? Is their some way to filter these out, like putting a quality factor on the mode, or something like that? Thanks.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
Send Private Message Flag post as spam

Please login with a confirmed email address before reporting spam

Posted: 1 decade ago Aug 12, 2010, 3:18 a.m. EDT
Hi

It does not sound "normal" for me, I would say check your BC's again, you might have missed a power of ten somewhere in the materials, or something in the truss dimensions

sometimes a reset model and solve helps if you have edited a lot the file

--
Good luck
Ivar
Hi It does not sound "normal" for me, I would say check your BC's again, you might have missed a power of ten somewhere in the materials, or something in the truss dimensions sometimes a reset model and solve helps if you have edited a lot the file -- Good luck Ivar

Note that while COMSOL employees may participate in the discussion forum, COMSOL® software users who are on-subscription should submit their questions via the Support Center for a more comprehensive response from the Technical Support team.