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Maximum number of eigenfrequencies?

Posted Jul 2, 2012, 7:58 a.m. EDT Version 4.1 10 Replies

Silviu Zilberman
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Hi,

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.
10 Replies Last Post Jul 10, 2012, 3:34 a.m. EDT
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jul 2, 2012, 8:34 a.m. EDT
Hi

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.

--
Good luck
Ivar
Hi 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. -- Good luck Ivar
Silviu Zilberman
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Posted: 1 decade ago Jul 3, 2012, 3:06 a.m. EDT
Thanks Ivar,

I tried refined meshes and nothing changed. I'll continue dwelling on this issue and update the thread if I gain new insight .

Silviu.
Thanks Ivar, I tried refined meshes and nothing changed. I'll continue dwelling on this issue and update the thread if I gain new insight . Silviu.
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jul 3, 2012, 3:14 a.m. EDT
Hi
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 ;)

--
Good luck
Ivar
Hi 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 ;) -- Good luck Ivar
Magnus Ringh COMSOL Employee

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Posted: 1 decade ago Jul 3, 2012, 5:37 a.m. EDT
Hi,

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
Hi, 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
Silviu Zilberman
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Posted: 1 decade ago Jul 3, 2012, 6:33 a.m. EDT
Hi Magnus,

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.
Hi Magnus, 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.
Magnus Ringh COMSOL Employee

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Posted: 1 decade ago Jul 3, 2012, 7:19 a.m. EDT
Hi,

In the attached screen capture, the vertical scrollbar is on the right in the list of eigenfrequencies.

Best regards,
Magnus Ringh, COMSOL
Hi, In the attached screen capture, the vertical scrollbar is on the right in the list of eigenfrequencies. Best regards, Magnus Ringh, COMSOL
Silviu Zilberman
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Posted: 1 decade ago Jul 3, 2012, 7:25 a.m. EDT
Hi Magnus,

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.
Hi Magnus, 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.
Silviu Zilberman
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Posted: 1 decade ago Jul 3, 2012, 7:30 a.m. EDT
Hi Ivar,

When writing 'from frequency' I assume you meant 'Search for frequencies around:' ? If correct, I tried changing that value, but no success...

Silviu.
Hi Ivar, When writing 'from frequency' I assume you meant 'Search for frequencies around:' ? If correct, I tried changing that value, but no success... Silviu.
Nagi Elabbasi Facebook Reality Labs
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Posted: 1 decade ago Jul 3, 2012, 10:18 a.m. EDT
Hi Silviu,
Look at the log output. It could be that only 25 frequencies have converged.
Hi Silviu, Look at the log output. It could be that only 25 frequencies have converged.
Silviu Zilberman
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Posted: 1 decade ago Jul 10, 2012, 3:34 a.m. EDT
Hi,

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.
Hi, 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.

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