yade-dev team mailing list archive

[Václav Šmilauer <eudoxos@xxxxxxxx>]

> >
> > 	//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.
> >
> >   
> My bad habit of taking one file, changing what I like, and keeping some 
> old commented parts in case I want to know what was there before (I 
> obviously coded the "basic" version starting from the law inherited from 
> SDEC, and actually I think it was my first coding in Yade!).
> For sure I'll remove that next time I commit a change in this file.
   
> That would be a problem, but I see no solution. The factor is 1 
> currently, couldn't be simpler, and _should not_ be modified. Everybody 
> should know (i.e. I'll have to document that) that in this law kn 
> between spheres will be E.d.
The solution is to define things even if the definition is arbitrary.

To me it seems as if you were saying: Oh yes, at one point I just
removed Sinit from Ip2_FM_FM_FP, because it is arbitrary anyway. But in
any case I don't want people to do the same.

> > 4. let us take (2r) as lengthOfContact (same radii, for simplicity) (the
> > most obvious choice)   
> Ok, we converge here. The thing is, in theorems of dimensional analysis 
> (Buckingam), you really put the basic physical quantities, and size of 
> particles is really the fundamental size in the system, not just an 
> approximation of contact "length".
My point was stronger. If you have particles of different radius, you
will probably define contact-level "reference distance" by summing their
radii. That can be different for each contact and is not just property
of the system. I am reasoning for 2 particles in contact here.
> > 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)   
> No. E is not "Young" modulus, it is the stiffness of contacts. It is 
> called young because again, I adapted existing code the lazy way, and 
> also because we have same data class for different laws, where the 
> meaning of the data is not the same in each law.
> For a similar reason, I renamed Poisson -> KsDivKs some time ago in 
> preprocessors. I can't (or can I?) change the name in the data class though.
Whatever you call it, it has dimension of stiffness (even you don't
imagine stiffness of balls, which I tend to do somehow).
> 
> > 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.
> Irrelevant for me : no area and no length.
As soon as you define length of a particular contact (not some average
length dimension characterizing the packing -- see above), you might
define the area as well...?
> However, adopting your philosophy (which is as correct as mine I think), 
> I could say that contact area is obviously more than the  projection of 
> the grain, since there is void around grains which should be associated 
> to each interaction (the sum of interaction "volumes" should be the 
> total volume of the packing right?).
I wanted to avoid this volume discussion, because this pulls in packing
configuration and leads to nowhere. For that reason I would not want
having sum of interaction volumes equal to packing volume, since that is
in general impossible. Reasoning about 2 spheres is simpler. Length and
area.

I see it is useless to continue. Howgh.