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Maximum number of eigenfrequencies?
Posted 02.07.2012, 07:58 GMT-4 Version 4.1 10 Replies
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I am experiencing a funny problem in calculating eigen frequencies of a mechanical system. One in principle has to specify the number of desired eigen modes. However, when specifying a number greater than 25, I keep getting only 25 modes.
Is there a maximum on the number of modes?
Thanks, Silviu.
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I do not know about any limitation, but on the other hand, depending on your mesh you will not be able to resolve more than N, where N is such that this last mode shape must be resolved by at least 3-5 elements per periode.
But this is very geometry shape dependent, this might be your limitation.
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Good luck
Ivar
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I tried refined meshes and nothing changed. I'll continue dwelling on this issue and update the thread if I gain new insight .
Silviu.
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Have you tried to set "from frequency", and see if you can get more modes (and are you still blocked 25 at the time ?
Else I would say try support ;)
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Good luck
Ivar
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Perhaps it's a coincidence, but I get exactly 25 eigenfrequencies in the drop-down list that appears when selecting the eigenfrequency solution to plot. When there are more than 25 eigenfrequencies, a vertical scrollbar appears so that you can reach all computed eigenfrequencies even though you can only see a range of 25 at the time. This might be a reason why you think there is a limit of 25 eigenfrequencies.
Best regards,
Magnus Ringh, COMSOL
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Thank you for your reply. I was looking for the vertical scroll bar, but I can't see one. Could you send a screen capture?
Many thanks!
Silviu.
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In the attached screen capture, the vertical scrollbar is on the right in the list of eigenfrequencies.
Best regards,
Magnus Ringh, COMSOL
Attachments:
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Thanks. Of course I saw the vertical scroll bar and the 25 eigen modes were counted after scrolling down all the way. For some reason I read the word 'vertical' in your reply and though about 'horizontal'. That's why I asked for a screen capture.
The problem of 25 eigen values is still in place... :-(
Many thanks,
Silviu.
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When writing 'from frequency' I assume you meant 'Search for frequencies around:' ? If correct, I tried changing that value, but no success...
Silviu.
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Look at the log output. It could be that only 25 frequencies have converged.
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It seems that the problem is that COMSOL somehow remembers the original number of eigen values in the 1st calculation and would not update it even though the user changed the number of requested solutions. In my case the solution was to rebuilt the model from scratch and make sure that in the 1st calculation I specify a large enough number of required eigen values.
Silviu.