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CCX modal mass with density / with lumped masses
Hi all,
I am trying to compute the effective modal mass of the modes of a 3D model with only rectangular bars, using Mecway 12 and CCX. I do not get satisfactory results using both density and lumped masses with my model. So I tried a simple example derivated from "2.5 Free Vibration of a Cantilever Beam" of the manual in 3D to investigate what could be wrong.
In this example, the manual show how to compute the first mode of a cantilever bar, using shell elements, with Modal Vibration 2D activated. I use the same geometric and mecanical properties, but with 32 bar elements and Modal Vibration 3D activated. The expected results are : the mass of the cantilever beam is 94kg and its first eigen mode is 28Hz (see mecway manual).
If I compute the modes using a material density (7860kg/m3) for the bar and no lumped mass :
- Mecway gives a total mass of 94 kg with menu Tools/Mass → OK
- CCX returns the good frequency (28Hz)
- CCX outputs a dat file showing a total effective mass of 841 (kg ?) → it seems to be false. Unit problem ?
- see file Modal3D_Cantilever_ccx_density.liml
If I compute the modes using lumped mass (total lumped mass of 94kg) instead of density (1kg/m3 instead of 7860kg/m3 to neutralize density)
- Mecway gives a total mass of 94 kg with menu Tools/Mass → OK
- CCX returns the good frequency (28Hz)
- CCX outputs a dat file showing a total effective mass of 91 (kg ?). Since there is 1 fixed node and 33 nodes in total in this example, 32/33*94=91kg → OK
- see file Modal3D_Cantilever_ccx_lumped_mass.liml
I do not understand why CCX does not give the same (or almost the same) total effective mass when I use only density and when I use lumped masses.
My final aim is not to compute the total effective mass mtot (I can get it directly from Mecway without CCX), but to compute the effective modal mass mi of each node. But since I cannot get CCX computing mtot for now, I am not sure if the values mi are correct or not.
Does anybody know why using density or lumped mass give so different results regarding modal mass ?
Thanks for your help
I am trying to compute the effective modal mass of the modes of a 3D model with only rectangular bars, using Mecway 12 and CCX. I do not get satisfactory results using both density and lumped masses with my model. So I tried a simple example derivated from "2.5 Free Vibration of a Cantilever Beam" of the manual in 3D to investigate what could be wrong.
In this example, the manual show how to compute the first mode of a cantilever bar, using shell elements, with Modal Vibration 2D activated. I use the same geometric and mecanical properties, but with 32 bar elements and Modal Vibration 3D activated. The expected results are : the mass of the cantilever beam is 94kg and its first eigen mode is 28Hz (see mecway manual).
If I compute the modes using a material density (7860kg/m3) for the bar and no lumped mass :
- Mecway gives a total mass of 94 kg with menu Tools/Mass → OK
- CCX returns the good frequency (28Hz)
- CCX outputs a dat file showing a total effective mass of 841 (kg ?) → it seems to be false. Unit problem ?
- see file Modal3D_Cantilever_ccx_density.liml
If I compute the modes using lumped mass (total lumped mass of 94kg) instead of density (1kg/m3 instead of 7860kg/m3 to neutralize density)
- Mecway gives a total mass of 94 kg with menu Tools/Mass → OK
- CCX returns the good frequency (28Hz)
- CCX outputs a dat file showing a total effective mass of 91 (kg ?). Since there is 1 fixed node and 33 nodes in total in this example, 32/33*94=91kg → OK
- see file Modal3D_Cantilever_ccx_lumped_mass.liml
I do not understand why CCX does not give the same (or almost the same) total effective mass when I use only density and when I use lumped masses.
My final aim is not to compute the total effective mass mtot (I can get it directly from Mecway without CCX), but to compute the effective modal mass mi of each node. But since I cannot get CCX computing mtot for now, I am not sure if the values mi are correct or not.
Does anybody know why using density or lumped mass give so different results regarding modal mass ?
Thanks for your help
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Comments
The lumped mass file doesn't seem to have any lumped masses. Maybe the wrong file? I'm curious to see the 91 result.
The files in my previous message were identical (my bad). The file with lumped masses in attached to this new message.
I got an answer from the ccx forum (https://calculix.discourse.group/t/total-effective-mass-differs-if-using-density-or-lumped-masses/532)
Long story short : do not use line2 elements if you need to compute modal mass with ccx, instead use line3 elements. Then using density or lumped masses will give very similar results. Here the trouble lies with density + line2.
@Victor : do you think it might be possible to compute the modal mass of each mode with the intern solver in future version ? Mecway is much more permissive in terms of element types for vibration analysis 3D so it would be great not to have to simplify models (converting general section, hollow sections and so on in "analogous" rectangular sections) in order to compute modal mass with ccx.
Modal mass/etc is on my list as a few people have asked for it before but it probably won't be done for the next version, sorry.