yade-dev team mailing list archive

[Bruno Chareyre <bruno.chareyre@xxxxxxxxxxxxxxx>]

Hi there,
Since the capillary model of Luc Scholtès, the implementation of
capillary models in Yade has always been using sorts of hacks.

What capillary models imply is that we can have two simultaneous
interactions between the same two objects: one corresponds to solid
contact force, the other corresponds to pendular bridge forces. The
forces are additive, and both interactions can be computed using the
same IGeom, fortunately.

For Luc, the problem was (and it still is) that the dispatching will
never pull two types of interactions for two objects (only one Law2 is
called). Hence we cannot have two ordinary interactions acting between
two bodies.

The hack of Luc (blame me, I think I suggested this): make capillary law
a global engine that uses IGeom data defined in the interaction loop but
which is executed after the interaction loop. It has its own loop on
interactions independently of dispatching.
Problems:
- one more loop
- parallelization has to be reimplemented
- handling capillary data needs to add variables to IPhys and then it
needs a special IP2 functor to generate a special IPhys (FrictPhys ->
CapillaryPhys), a lot of code duplicated just for adding 1 or 2
variables to a class.

A more recent strategy (by Anton) is to implement solid contact +
capillary force in a unique Law2. Advantage: no additional loop.

The worst problem in both cases is: the capillary model is restricted to
a particular contact law, although they are independent physics model.
So, if we want capillary forces combined with e.g. HertzMindlin contacts
we need to 1/ duplicate the full capillary model in the HM context
("Anton"'s way) or 2/make the capillary model handle two different IPhys
by reimplementing a dispatching mechanism.

The second option leads to the following in
Law2_ScGeom_CapillaryPhys_Capillarity.cpp:
if (!hertzOn) cundallContactPhysics =
static_cast<CapillaryPhys*>(interaction->phys.get());//use CapillaryPhys
for linear model
            else mindlinContactPhysics =
static_cast<MindlinCapillaryPhys*>(interaction->phys.get());

A series of awkward "if (!hertzOn)" is seldom found in the rest of the
code to.
Obviously this approach cannot be generalized to more than two different
IPhys, the code would be a big mess.

What we want:
A- having N possible contact laws, and M possible capillary models...
and why not L remote electrostatic attraction models (for instance),
B- being able to combine all them in simulations with no additional
implementation, i.e. only N+M+L implementations in total

What we have, be it with Luc's way or Anton's way:
A framework in which handling all possible combinations needs N*M*L
implementations...

Can we do something about it? Any idea?

Bruno



-- 
_______________
Bruno Chareyre
Associate Professor
ENSE³ - Grenoble INP
Lab. 3SR
BP 53
38041 Grenoble cedex 9
Tél : +33 4 56 52 86 21
Fax : +33 4 76 82 70 43
________________