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Precision of the eigenfrequency analysis (structural mechanics)
Posted 07.09.2010, 08:35 GMT-4 3 Replies
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Hi,
This problem is not very realistic, but I would like to use Comsol for some demonstration purposes.
I would like to visualize the first, second, third etc. mode of vibrations of a few, i.e. n (identical) cantilever beams hanging out from a common piece of solid.
When I start the eigenfrequency analysis in Comsol it calculates n nearly the same frequencies. The difference is at the second or third place after a comma, depending on the solver type and its settings.
How can I make the solver to "merge" these n eigenmodes into one, as these four beams were connected using an invisible and infinitely stiff connectors? I have thought about limiting the resolution of the solver that it does not distinguish between these closely spaced eigenmodes, but I suppose it is rather not feasible without changing the Comsol solver code.
Have any of you had a similar problem? Any clues for solving it?
Thank you in advance!
Kind regards
Stanislaw Kalicinski
This problem is not very realistic, but I would like to use Comsol for some demonstration purposes.
I would like to visualize the first, second, third etc. mode of vibrations of a few, i.e. n (identical) cantilever beams hanging out from a common piece of solid.
When I start the eigenfrequency analysis in Comsol it calculates n nearly the same frequencies. The difference is at the second or third place after a comma, depending on the solver type and its settings.
How can I make the solver to "merge" these n eigenmodes into one, as these four beams were connected using an invisible and infinitely stiff connectors? I have thought about limiting the resolution of the solver that it does not distinguish between these closely spaced eigenmodes, but I suppose it is rather not feasible without changing the Comsol solver code.
Have any of you had a similar problem? Any clues for solving it?
Thank you in advance!
Kind regards
Stanislaw Kalicinski
3 Replies Last Post 08.09.2010, 05:05 GMT-4
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Posted:
1 decade ago
Hi
if you haev 4 identical beams, meshed more or less the same way, you will see 4 identical modes (to the mesh-precision, eigenmode calcualtion precision). This is normal. Just as if you do a calculaton of free-free modes without fixing your object you get 6x0Hz (or close to0Hz) for the first 6 free rigid-body modes.
I'm afraid you cannot do much about that
--
Good luck
Ivar
if you haev 4 identical beams, meshed more or less the same way, you will see 4 identical modes (to the mesh-precision, eigenmode calcualtion precision). This is normal. Just as if you do a calculaton of free-free modes without fixing your object you get 6x0Hz (or close to0Hz) for the first 6 free rigid-body modes.
I'm afraid you cannot do much about that
--
Good luck
Ivar
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Posted:
1 decade ago
Hi Ivar,
Thank you for your answer.
Is then currently any way to display more than one eigenmode at a time in Comsol (preferably combined using logical OR)?
At first sight this may seem a useless feature, but it may be interesting in some cases when two or more eigenmodes are located very close to each other (e.g. in quartz resonators). Such visualization would help in understanding the influence of the spurous modes in this type of devices.
Regards
Stanislaw
Thank you for your answer.
Is then currently any way to display more than one eigenmode at a time in Comsol (preferably combined using logical OR)?
At first sight this may seem a useless feature, but it may be interesting in some cases when two or more eigenmodes are located very close to each other (e.g. in quartz resonators). Such visualization would help in understanding the influence of the spurous modes in this type of devices.
Regards
Stanislaw
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Posted:
1 decade ago
Hi
True , I see your point, but I do not think of a way within Comsol to overlap two modes on the same image.
Via Matalb/script it should be rather straightforward (basically its just a "keep plot" function to activate),
now it's worth to check carefully all the plot options.
--
Good luck
Ivar
True , I see your point, but I do not think of a way within Comsol to overlap two modes on the same image.
Via Matalb/script it should be rather straightforward (basically its just a "keep plot" function to activate),
now it's worth to check carefully all the plot options.
--
Good luck
Ivar