yade-users team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxx>]

Ok, I received this e-mail.
I double-checked but couldn't find your previous one.
Regarding a possible collaboration, yes, it would be great. It seems 
that your experience could help us a lot, and I hope we can help you 
too. If you can come in grenoble to discuss that, its perfect. Prof. 
Frederic Donze would participate in the discussions probably, as he is 
the one who initiated the development of Yade (and he still supervizes 
it). He does not participate directly to the project on erosion however.

> I can see what you are trying to do. The upscaling from Stokes
> to Darcy was the topic of my D.E.A., and of part of my Ph.D.
> thesis. Your problem seems, however, a bit more complicated
> than just that, as the internal erosion might create
> wormhole-like fluidized-bed regions where Darcy's law is not
> necessarily valid anymore. Have you already developed a
> conceptual macroscopic model for this kind of flow?

1/ Assume that the packing is always "dense", so that
2/ the velocity of grains is always low compared to the velocity of the 
fluid, and
3/ the size of pores is of the same order of magnitude as the size of 
grains, making low-Reynolds assumption valid.
3/ You can triangulate the packing, and define the geometry for the flow 
as a series of adjacent "voids" (where each void is a tetrahedron cut by 
four spheres).
4/ Here comes comsol and the most complex part : let us compute the 
Stokes flow and bring out the hydraulic properties of a generic void. 
Given the size of the terahedron, the size and velocity of the four 
spheres, and the averaged pressures in the four adjacent voids, we need 
approximated values of the flow rates (a sort of local Darcy law) and 
hydraulic forces on the four grains.
5/ Mass balance between adjacent voids and the transmitivity equations 
from 4) will give a big system, which (i hope) will have one solution...
6/ Take this solution, compute for one grain the sum of hydraulic forces 
from all tetrahedral voids which have this grain as vertex. Do the same 
for all grains, and go back to the classical DEM scheme taking into 
account this set of hydraulic forces.

I agree that wormhole formation will be out of the scope of such a 
model. The concept of small voids between grains is not relevant in this 
case, neither is the low-Reynolds condition. This model will apply for 
bulk erosion, clogging effects,... but it will not handle the cases 
where there is a localization of the erosion process and formation of 
big holes. But isn't that exactly what you are working on now?

> (1) a packing of (rigid) sand grains (of spherical shape) is
> hold together by friction and confining stress;
> (2) a jet of water is directed against the internal wall of
> this borehole to apply normal and tangential stresses to the
> individual grains in the packing;
> (3) hydrodynamic forces and torques acting on the individual
> grains are then passed to a DEM code to verify the breaking of
> the grain bonds, remove the grains with a broken link,
> recalculate the equilibrium position for the packing,


Some physical issues comes to mind :
1/ the internal wall of the borehole will not be stable if you simulate 
a dry sand with only friction at contact, even before you apply 
hydraulic forces (except if you have strong arching effects, but I don't 
think you will have them). You will need a little bit of cohesion at 
contact. In a real situation, the stability of a borehole in sand can be 
due to a little bit of clay or to capillary forces if the sand is not 
saturated (capillary forces are now implemented in Yade).
2/ You can't consider a one-step procedure for removing the grains. 
Frictional contacts slip, they don't break. Even if the hydraulic forces 
cause slip at some contacts when you apply them, you can't be sure that 
the packing will not stabilize after some small movements. It would 
sound more reasonable to me to keep the hydraulic forces constant - 
which is correct as long as there is only small displacements of the 
grains - for a number of DEM iterations (100 iterations should be 
enough). Then see which grains are removed from the packing. Perhaps it 
is what you want to do already...

> (4) pass the new geometric configuration to the fluid flow code
> to recalculate stresses -> goto (2)
>
> I have modeled the hydrodynamic stresses by means of COMSOL
> Multiphysics (see attached), and I now need to pass this information 
> to a DEM
> code.
>
> Any help on how to start the coupling with yade will be highly
> appreciated.


Look at the class ForceEngine. It takes a series of grains and add 
external forces on them. You could create a class HydroForceEngine 
similar to this one. HydroForceEngine would read a file containing grain 
id's and hydraulic forces on them, then it would add force(i) on grain 
(i), when ForceEngine add the same force on all grains.

Ciao :)

Bruno





--
_______________
Chareyre Bruno
Maître de conférence

Institut National Polytechnique de Grenoble
Laboratoire 3S (Soils Solids Structures) - bureau I08
BP 53 - 38041, Grenoble cedex 9 - France     
Tél : 04.56.52.86.21
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