yade-users team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

Chiara,

If you have your noze in papers now, I'll let you think about the code 
below (uncommited). The first line is what is in the code already.
Checking with paper and pen, I found that  current equation is 
equivalent to Cundall/Thornton expression only in the limit case of slow 
deformations. Out of this limit, they can differ a little. I wouldn't 
bet on which one is better, for now I can only tell they are slightly 
different. What do you think (if you read on molecular dynamics, you 
probably know more than me already)?

The interesting thing is we can still write something _exactly_  
equivalent to Cundall equation by updating velocity, this is what is 
done below. I'll give again the choice between two types of scaling 
probably.

Cheers.

Bruno



/Vector3r 
dVel((scene->cell->velGrad-prevVelGrad)*/*scene->cell->wrapShearedPt(*/state->pos/*)*/);
       state->vel+=dVel;
//This is the convective term, appearing in the time derivation of 
Cundall/Thornton expression (dx/dt=velGrad*pos -> 
//d²x/dt²=dvelGrad/dt+velGrad*vel), negligible in many cases but not for 
high speed large deformations (gaz, turbulent flow,...). Emulating 
Cundall //in an option, I don't especially recommend it. I know 
homothetic 1 and 2 expressions tend to identical values in the limit of 
dense quasi-static //situations. They can give slitghly different 
results in other cases, and I'm still not sure which one should be 
considered better, if any.
if (homotheticCellResize>1) state->vel+=scene->cell->velGrad*state->vel*dt;
/

chiara modenese a écrit :
>     Hello Bruno,
>     I am a little bit confused about the periodic. In
>     NewtownIntegrator we update the velocities and _then_ we integrate
>     the positions. This is exactly the way of Cundall (I am checking
>     the equations right now) except that he chooses to update
>     positions and not velocities (defining a strain rate instead of a
>     gradient of velocity which we said is fine but not in the general
>     case). I understand that the reason why Cundall updates the
>     relative velocities at contact (btw, according to his paper this
>     is not meant to be only for the shear direction) before going for
>     the computation of contact forces is that he was not updating the
>     velocities at the level of the integration. So my guess is: why do
>     we need to update the relative velocities if that has already been
>     done in NewtonIntegrator? That would have made sense if updating
>     positions and not velocities. Bruno, you said there was a problem
>     using periodic ScGeom before the last changes. Where you testing
>     with homothetic=1? Because I think that case you would have needed
>     to update velocities (which is not a problem in Dem3dofGeom).
>
>
> I have been looking at molecular dynamics (sorry for my lack of 
> knowledge in this field) and I see that "shift2" as well as the 
> "shift" introduced recently by Bruno for the velocities, are 
> independent by the way we update displacements (or velocities) in 
> NewtonIntegrator. If I have got it right, this shift has to deal with 
> the periodicity (though I will reread carefully the part of the code 
> where this is computed). This shift is not described in Cundall paper, 
> that is why I got confused.
> Btw, why do you choose to update the cell in Scene?
>
> cheers.
> Chiara
>
>
>
>
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--
_______________
Bruno Chareyre
Associate Professor
Grenoble INP
Lab. 3SR
BP 53 - 38041, Grenoble cedex 9 - France
Tél : 33 4 56 52 86 21
Fax : 33 4 76 82 70 43
________________