Yade

Cell.homoDeform & PeriTriaxController.dynCell

Asked by Jérôme Duriez

Hi,

I'm discovering periodic boundaries and I have some questions...

- concerning Cell.homoDeform, the doc does not in fact explain what's going on when Cell.homoDeform=0..
      From the code it seems to me that particles are moved according to cell deformation only when homoDeform. Thus, if !homoDeform, the sample deformation will be caused "indirectly" by boundaries particles being closer from the center of the cell ? Leading to a non-uniform strain in the cell ?
May someone confirm this, or not ?

- and for PeriTriaxController.dynCell. The "laws of dynamic" of the doc, and the corresponding line in code ([1] ?) are not very clear to me. Is this line only a numerical trick to deform the cell until the wanted stress is reached ? Or something that can be physically proven ?
[1] : https://github.com/yade/trunk/blob/master/pkg/dem/PeriIsoCompressor.cpp#L176

- and what if !dynCell ? I confess having difficulties to understand lines 118 and 171 of PeriIsoCompressor.cpp (a strain_rate in m/s ?)
Does someone use dynCell = 0 by the way ?

Any reference (different than this one : http://<email address hidden>/msg04996.html ;-) ) is welcome.
I found http://rsta.royalsocietypublishing.org/content/356/1747/2763.abstract but it is for me not enough at the moment. Do for ex homoDeform = 1 and dynCell =1 correspond to equations (1.1) and (1.2) of this paper ?

Thanks !!

Jerome

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Bruno Chareyre (bruno-chareyre) said : 		#1

>if !homoDeform, the sample deformation will be caused "indirectly" by boundaries particles being closer from the center of the cell ? Leading to a non-uniform strain in the cell ?

Yes.
Of course, it will not remain non-uniform. It will equilibrate in the long run, just like a stress increment applied by a rigid boundary is propagating over time.
It is not documented because it is really useless.

> Or something that can be physically proven ?

I think it is impossible to define a dynamic situation consistently in PBC. Of course I would be glad if someone could find a logic in it, but at the moment I typed the equation without a proof. Some time ago Chiara even mentionned that the equation was dimensionally wrong.
So, take it as a trick. And apparently the trick works. :)

>if !dynCell

It becomes strain=stress/stiffness instead of rate+=stress/mass. The complex lines are stiffness evaluation (Jan will maybe confirm or not).

>Any reference

I guess I should write a paper about it... I don't think it is explained anywhere in details.
There is a little bit of explanation here:
https://yade-dem.org/doc/formulation.html#deformations-handling
https://github.com/yade/trunk/blob/master/pkg/dem/NewtonIntegrator.cpp#L181

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Jérôme Duriez (jduriez) said : 		#2

Ok, it is already helpful, thanks !

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Václav Šmilauer (eudoxos) said : 		#3

Hi, there are some equations to the deforming space in http://woodem.eu/doc/theory/leapfrog.html#motion-in-uniformly-deforming-space - they are enhanced from what is homoDeform in Yade, but perhaps you could find some use of that. Cheers, Vaclav

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Jérôme Duriez (jduriez) said : 		#4

Thanks also Vaclav !

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