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Symmetry in modal analysis

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stefanos9891

Mechanical
Nov 16, 2012
3
Hello everyone, this is my first thread

I had this project recently where I had to perform modal analysis in Ansys. The geometry and the loading was symmetric in one plane. Initially I performed a full model analysis and I extracted the natural frequencies and mode shapes. I was only interested in the first mode because the project specified that the componend was moving axially (in Y axis)and that was represented in that mode. Also the result of the natural frequency lied in the frequency range specified in the project.
Then I tried to run the same analysis but only this time i used half model. What i observed is that the first mode gave almost the exact same result with the full model.
When i looked about this I found out that symmetry is not recommended for moadal analysis. But in this case could I use half model if I am only interested in that mode?

Thank you in advance
 
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"symmetry is not recommended for moadal analysis"

This is because the use of symmetery excludes non-symmetric modes. You can use symmetric models, but have to be careful about the conclusions you draw.

Take a simply-supported beam, being analysed for bending modes. You could model half of the beam (from one support to the midpoint), and put a symmetry condition at the midpoint - this would have the effect of restraining the midplane of the beam to not rotate. This symmetric model would give correct results for the 1st bending mode (where the beam essentially just forms a U-shape) because there is no rotation of the midpoint in that mode. But the 2nd mode, where the beam forms a ~ shape, does involve rotation of the midplane, is precluded by the symmetry boundary condition, and would not be extractable from the symmetric model. For the symmetric model, the reported 2nd mode would look like the 3rd mode (W shape). So, if you were not careful, you might miss some things from the symmetric model.
 
Agree, it is dangerous. I'm not even convinced that the first mode has to be symmetric in a symmetrical structure -it could be antisymmetric, which would probably need different bcs.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
Thank you for your posts!
These have been really helpful!
 
I not recommend to use simetry in modal analysis, when you run a simulation like this, and solve for, for saying, 10 normal modes, the non-symmetric modes are wrong at all, so if you gonna run a modal analysis using symmetry be careful with your results, use the entire model
 
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