yade-dev team mailing list archive

[Václav Šmilauer <eudoxos@xxxxxxxx>]

I re-read my first post over to make sure; no reference to macroscopic
(packing's) modulus was done. I was merely expressing my surprise over
the weird definition of contact area, since I thought it was generally
accepted that it is cross-section of cylinder between particles (Hentz,
but also some other papers by Wenjie, Frederic etc -- I vaguely
remember, but can be wrong here). You can still see at
Ip2_FrictMat_FrictMat_FrictPhys.cpp:61:

	//Real Sinit 	= Mathr::PI * std::pow( std::min(Da,Db) , 2);

I am wondering at which point it was commented -- it must have lead to
packing stiffness change by that factor 1.57.

> I could put, a factor 2,3,7,PI,sqrt(2) in front of Kn = ..., it would 
> not change the fact that A is unknown.

Just because we don't define that, it means that anybody is free to
change the logic in Ip2_FM_FM_FP; running the same simulation few weeks
later will give you different results. That is not right. 

>  And currently, _there is no exact 
> theory giving A_. If there was a theory for that, well, we could just 
> quit DEM and go derive equations.
(FYI there is, for some special arrangements
http://www.fisica.ufc.br/hans/p/256.pdf, but that is not our subject
really)

To summarize in the most general way (since you seem to miss the point
that I was not objecting to the E!=E* at all) -- let me know over which
points we disagree:

1. Contact stiffness is something like a[N]/lengthOfContact[m]

2. a[N] is some quantity proportional to some particle-defined modulus
(not saying it is E of continuous medium)

3. a is something like Young's modulus [Pa] * b[m²] (by dimensionality)

4. let us take (2r) as lengthOfContact (same radii, for simplicity) (the
most obvious choice)

5. there is a thing Material::young [Pa] (note E below), defined in
Material class; we take is as the value of Young's modulus [Pa] (the
most obvious choice again)

6. let's call this b[m²] "contact area", since it is area related to the
contact (pure terminology thing)

7. the current equation in yade is: kn=Er=E(2r²)/(2r), so "contact area"
b[m²]=2r². Contrary to other obvious choices (4,5), it is very much
non-obvious what is the geometrical meaning of 2r².

What I was saying was merely that it would be nicer to use πr² instead
of 2r², which is cross-section area of cylinder between the particles.

vaclav