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#1 2014-02-01 10:05:56

keeswouters
Member
From: kuringen
Registered: 2007-12-01
Posts: 144
Website

RBE3 behaviour

Hoi,

after reading this post http://www.code-aster.org/forum2/viewtopic.php?id=18439 I tried the RBE3 command myself.
Onnomobile is satisfied with the behaviour, but I run into problems when using an asymmetric mesh.
I used the RBE3 command to apply a torque on the axial face of the cylinder. For a symmetric mesh it really works fine, for an asymmetric mesh the behaviour is less satisfactory. Maybe it is because of my adjustments of the RBE3 command:

nodeRBE3=AFFE_CHAR_MECA(MODELE=CylModel,
                      LIAISON_RBE3=_F(GROUP_NO_MAIT='Nmaster',
                                      DDL_MAIT=('DX','DY','DZ','DRX','DRY','DRZ',),
                                      #DDL_MAIT=('DRZ',),
                                      GROUP_NO_ESCL='Nbot',
                                      DDL_ESCL='DX-DY-DZ',
                                      COEF_ESCL=1,),);

I apply the torque to the master node Nmaster. The nodes of the axial face 'Abot' of the cylinder are contained in 'Nbot'.
The complete command file is attached (it will not run because of lack of additional files, but see next line).
Further details can be found here: http://www.caelinux.org/wiki/index.php/ … ue_by_RBE3.

Did I use the wrong settings for this command or is it inherent to the RBE3 connection with asymmetric mesh?
The nice part is that using the transition of the poutre-3D-solid just works fine for both cases:

replace LIAISON_RBE3=.... by:
LIAISON_ELEM=_F(GROUP_MA_1='Abot',
                                      GROUP_NO_2='Nmaster',
                                      OPTION='3D_POU'),);

Last edited by keeswouters (2014-02-01 10:31:12)


Attachments:
cylinder_nodepoutre_v24.comm, Size: 7.47 KiB, Downloads: 237

kind regards - kees
--
I a parallel univers the laws of mechanics may be different.

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#2 2014-02-01 10:13:16

keeswouters
Member
From: kuringen
Registered: 2007-12-01
Posts: 144
Website

Re: RBE3 behaviour

A picture of the local deformation.

Last edited by keeswouters (2014-02-01 10:14:03)


Attachments:
kw_rbe3_conrbe3_2.png, Size: 117.38 KiB, Downloads: 356

kind regards - kees
--
I a parallel univers the laws of mechanics may be different.

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#3 2014-02-06 15:22:55

onnomobile
Member
From: Switzerland
Registered: 2010-05-26
Posts: 93
Website

Re: RBE3 behaviour

Hello,

Scary !
I just reviewed my latest use of the RBE3 command and indeed, I always use tetrahedral mesh, which has only little surface difference between all the elements, hence little error in the behavior I suppose. I'm eager to hear the end of this story.

Thx for your professional cross-checking.

Cheers,

Last edited by onnomobile (2014-02-06 15:23:09)


Running Code Aster v11.5 on Scientific Linux 6

There are 10 types of people in this world, those who understand binary and those who don't.

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#4 2014-02-06 16:08:15

RichardS
Member
From: Munich, Germany
Registered: 2010-09-28
Posts: 549
Website

Re: RBE3 behaviour

Hello Kees,
I think the behaviour you observed is not caused by a wrong definition on your side,
it is a result of the mechanism how the forces are distributed from the master to the slave nodes.

The moment you apply on the Master node is applied as nodal forces to the slave nodes with weighting factors depending on the distance from the center of gravity of the slave nodes.
In your case this is the same for all nodes (R).
In the areas with a dense mesh the force distribution is higher and thus leads to the observed deformation of the cylinder.

I would expect exactly the same behaviour with commercial codes.

Best regards,
Richard


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www.simscale.com
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#5 2014-02-06 17:15:49

onnomobile
Member
From: Switzerland
Registered: 2010-05-26
Posts: 93
Website

Re: RBE3 behaviour

Hello,

I tried this out of course :-) With ANSYS and a "remote point" loaded with a torque just like Kees did, the radial displacement is only 1e-4 times the tangential displacement and this is only on the nodes where the element size changes. Sorry no Nastran at hand to test the genuine RBE3.

I think it would be an improvement to also distribute the forces to the surrounding element's area and not only to the distance. LIAISON_ELEM is rigid I believe, so not an equivalent replacement even if the result is better in this case.

All in all, these complex boundary conditions are always tricky; I was not careful enough myself. Thx for this lesson.

Cheers,

Last edited by onnomobile (2014-02-06 17:20:12)


Attachments:
9453_maillage_1.png, Size: 174.3 KiB, Downloads: 219

Running Code Aster v11.5 on Scientific Linux 6

There are 10 types of people in this world, those who understand binary and those who don't.

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#6 2014-02-08 17:41:13

keeswouters
Member
From: kuringen
Registered: 2007-12-01
Posts: 144
Website

Re: RBE3 behaviour

Hoi Richard, Onno,

thanks for the comments.

The moment you apply on the Master node is applied as nodal forces to the slave nodes with weighting factors depending on the distance from the center of gravity of the slave nodes.
In your case this is the same for all nodes (R).
In the areas with a dense mesh the force distribution is higher and thus leads to the observed deformation of the cylinder.

I was hoping that the RBE3 would not depend on mesh density. And like the Ansys example shows, may be it should not.
The case using the solid-mesh connection indeed shows that this implementation does not depend on the mesh density.
I will have a look at the Code-Aster implementation, although I recall that the last time I read it, I cannot judge whether it can be made independent of the mesh density easily ( http://www.code-aster.org/V2/doc/defaul … .03.08.pdf ).

Anyway, if used carefully, RBE3 can be / might be an effetively tool.

Last edited by keeswouters (2014-02-09 15:28:03)


kind regards - kees
--
I a parallel univers the laws of mechanics may be different.

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