Discussion Forum

Hi

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

I havent incorporated any damping into either model (2D or 3D), would I need to in order to match the amplitude of the peaks?

I have made sure they have the same starting frequency, step, and ending frequency during the frequency sweep.

Thanks, Ivar!

Hi

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

I have tried modifying the damping, but regardless of what I enter, the amplitudes still dont match up; so I've attached the models for clarification. Any help is appreciated!

Hi

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

Ivar, you've been a life-saver! I was going to re-make the models without the solutions but that made it much easier! Here they are.

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!

Hi

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

Thanks for all your help, Ivar. I will try it out and see how it goes!

I am not quite sure how to make the cuts in the models. Is there an easy way to do this? I am trying to essentially estimate a vibrational load that sweeps through the various frequencies, so I was using a point load previously

Hi

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

The main reason for the difference in results is the difference between the beam widths in 2D and 3D. For the 2D the out-of-plane width is 1.0 by default. If you change it to 0.0254 as in the 3D model the results will be closer. You should also change the 2D beam to plane stress instead of plane strain which is a closer approximation of your 3D beam. The damping, mesh density and singularity of point load are other reasons for the difference in results.

Nagi Elabbasi
Veryst Engineering

Hi Nagi

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

Thank you for your help guys! Nagi, that definitely helped to change the out-of plane width; peaks are very close--working on matching the models now using the previous suggestions. You guys are life-savers!