Modal Vibration of Struture

keithmcarthur

Hello,

I have modeled a fan base structure and am looking for the modal vibration frequencies of the structure. The structure is fabricated from c-channel and plate. I am running into an issue where the majority of the modes found are localized resonances in the structural shapes. e.g. localized twisting of the c-channel. Is there a method to only look for significant mode shapes? More interesting in looking for bending/twisting modes for the entire structure, etc. Not sure if there is a way to change a setting that would influence the results or not.

Thanks.

mmartin

Dear KEITHMCARTHUR:

If you read the file *.dat you will find extra information about the mass participation for each degree of freedom and vibrational mode.
The higher value (related to the dof) for each mode shapes will tell you about the bias of the structural shape (twist/bending...).

mmartin

Victor

It's a bit strange because typically the lowest frequencies are the global modes. So make sure what's happening is reasonable for your structure and not a bug.

Then to do it the "dumb" way, increase the number of modes until you can find the ones you want.

If you write the *FREQUENCY card yourself, you can specify a minimum frequency. That still finds all the modes below that but then discards them, so it probably isn't any faster to solve but might keep the solution size down if that's a problem.

prop_design

one thought i have is if you're contact is setup correctly. also make sure the model is fixed appropriately.

Victor

prop_design has a good point. It sounds like either the whole structure might be constrained too much or some members might be underconstrained causing 0 frequency rigid body modes.

mmartin

Regarding the *.dat file. This file is generated along *.frd and *.eng ... after calculix ccx solve the analysis.
Configure MW to place the ccx temp files into the same MW working directory.
Doing that , you will see the *.dat file along your MW project.
Just open it with your favourite ASCII editor,

rwhirley

Often it's helpful to run some simple static analyses first, just to be sure the model constraints are doing what you want, before running the modal analysis. It's usually very quick as a model debugging tool.