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2D vs 3D Frequency Response
Posted 24 avr. 2012, 01:12 UTC−4 Structural Mechanics Version 4.2a 13 Replies
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I am trying to compare the frequency response (displacement amplitude [y-axis] vs frequency [x-axis]) of a simple beam both in 2D and 3D to compare the differences in the responses.
The resonant frequency peaks align between the 2D and 3D model; however, the amplitudes of the peaks for the 3D model are 10 times higher than the 2D model.
How can I scale the 2D model to have not only the peaks align in frequency but in amplitude as well; in other words, how can I make the 2D model response match the 3D model response?
Thank you!
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how do you treat the damping ? and are you sure your frequency stepping are the same between 2D and 3D ?
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Good luck
Ivar
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I have made sure they have the same starting frequency, step, and ending frequency during the frequency sweep.
Thanks, Ivar!
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the resonance peak height to Full Width at Half Maximum (FWHM) is the Q factor or half the damping value (check it out on Wiki)
en.wikipedia.org/wiki/Q_factor
If you have no damping (apprt from some numerical effects that might come from the solving stabilisation process you will not get any finite peak values
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Good luck
Ivar
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if the model is too big, clear the solutiomn, mesh and reset the model then save and upload, it should pass ;)
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Good luck
Ivar
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They simply have a point load in the center of each. Whenever I plot the frequency response (just to be clear--frequency, x-axis, and amplitude, y-axis) the resonant frequency peaks match up fairly well in terms of frequency but again not in amplitude.
I've added damping to the 3D model, but not the 2D model--and i've experimented with several variations (adding damping to both, just the 2D and no damping on either model) and still am unable to get the amplitudes to match.
Thanks again for your help!
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My comments are the following:
1) do not use point loads as these are singularities and give large errors around the point of application hence tend to perturbate the solver sequence and convergence estimator. It's easier to cut the beam in two in the length and apply a boundary load in both 2D and 3D along this cut line/surface
2) if you have only 1 element in the thickness, you could as well use a line/surface and beam or shell elements, so if you want to stay in "solid" ensure you have enough elements across the the beam thickness (I would say > 3). I mostly cut my biam geoemtry into 2 along the neutral line and then impose 2 mesh elements as minimum resolution per domain
I aso try to avoid free free models, as you can get other errors due to the free motion and rotation you allow. It's better to constrain your device some way to limits its free inertial motion
finally a material damping of 0.004 is rather low (realistic still) but it will hardly be observed, try 2-5%, then you should se your Bode plots as second order roll-off elements
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Good luck
Ivar
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check the layer feature under the advanced tab note that you cannot have different layer thicknesses in different directions, all directions selected get the same layer thickness. to cut your beam in 1 along the length , append 2 1/2 rectangles, or duplicate your first one by a copy
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Good luck
Ivar
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Nagi Elabbasi
Veryst Engineering
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Good point, I'm getting so used to COMSOL now that I start to automatically think "per meter" in 2D, (or per m^2 in 1D) but to get absolute values one should not forget to multiply by the TRUE depth ;)
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Good luck
Ivar
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