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frequency domain using reduced basis (modal)

Posted 10.09.2013, 04:15 GMT-4 Acoustics & Vibrations, Studies & Solvers Version 4.3a 4 Replies

Sami Karkar
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Hi all,

I'm trying to make a frequency sweep on a (big) model of an acoustic cavity, with specified surface impedance as boundary conditions.
If I use the "Frequency domain" study, everything goes well, except that it is really, really slow.
Considering that 1) few eigenmodes participate to the vibrations at these frequencies (20-200Hz), that 2) I would like to have a good frequency resolution in my sweep, and 3) I need to test a lot of combinations of parameters for the boundary conditions, I figured that using the "Modal frequency domain" study type is perfect for me.

However, it does not seem to work : I always get a null solution, for all frequencies, when I use this type of study.
The predefined study contains two step : one for finding the eigenfrequencies and eigenmodes, and the second to use these modes as a reduced basis for the frequency sweep. The first step seems to work well (I can visualize the modes, they seem ok to me), but the second gives mysteriously a null result everytime (without any error message).

If anyone knows that problem and how to overcome it, I'd be really glad to know!

Regards,
Sami

PS : I'm using Comsol 4.3a.
PS2 : I use a point volume flow source, which works as expected with the traditional "frequency domain" study.
4 Replies Last Post 22.11.2013, 09:11 GMT-5
Edgar J. Kaiser Certified Consultant
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Posted: 1 decade ago 10.09.2013, 06:28 GMT-4
Sami,

in the frequency sweep step you have to excite the model some way. Did you include that?

Cheers
Edgar

--
Edgar J. Kaiser
emPhys Physical Technology
Sami, in the frequency sweep step you have to excite the model some way. Did you include that? Cheers Edgar -- Edgar J. Kaiser emPhys Physical Technology
Sami Karkar
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Posted: 1 decade ago 10.09.2013, 09:02 GMT-4
Dear Edgar,

Thanks for your answer.

As I mentioned at the end of my message, yes I excited my model with a point source (volume flow). I think the excitation is not the problem here as the "complete" model does work perfectly.

I attach here a file with a simpler model, where the modal frequency sweep does not work either.

Thanks

Regards,
Sami
Dear Edgar, Thanks for your answer. As I mentioned at the end of my message, yes I excited my model with a point source (volume flow). I think the excitation is not the problem here as the "complete" model does work perfectly. I attach here a file with a simpler model, where the modal frequency sweep does not work either. Thanks Regards, Sami

Attachments:

Sami Karkar
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Posted: 1 decade ago 20.11.2013, 15:21 GMT-5
Just in case someone reads this someday : I finally found how to overcome this.

One have to set "Linearity" to "Linear" in the options of the Modal Solver (you have to generate the default solver config first, and then modify it).

It's weird, because it should work as well in the mode "linear perturbation", but somehow it doesn't...

Sami
Just in case someone reads this someday : I finally found how to overcome this. One have to set "Linearity" to "Linear" in the options of the Modal Solver (you have to generate the default solver config first, and then modify it). It's weird, because it should work as well in the mode "linear perturbation", but somehow it doesn't... Sami
Sami Karkar
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Posted: 1 decade ago 22.11.2013, 09:11 GMT-5
See this thread for the detailed answers of Comsol :
www.comsol.com/community/forums/general/thread/40810
See this thread for the detailed answers of Comsol : http://www.comsol.com/community/forums/general/thread/40810

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