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Re: π=2 ?! (Ip2_FrictMat_FrictMat_FrictPhys)

 



chiara modenese a écrit :


2010/3/25 Václav Šmilauer <eudoxos@xxxxxxxx <mailto:eudoxos@xxxxxxxx>>


     >   Just to say to be carefull with formula in Hentz's thesis: the idea
     > was  to use as inputs for contact properties the Young modulus and
     > Poisson ratio that we want to obtain macroscopically for the granular
     > assembly. The "funny term" are use to compute kn and ks with
    respect to
     > these macroscopic Young modulus and Poisson ratio
    Yes, I know. I disregarded that funny term, it is close to 1.0 and
    doesn't play any role.

    What struct me, though, was that we deviate from any obvious definition
    of contact stiffness. Its dimensionality is correct, but it is scaled by
    some dimensionless constant (π/2 in our case) away from the "intuitive"
    definition (which is the base of what Hentz uses): stiffness of cylinder
    with radius min(r₁,r₂) between spheres' centers, with some average
    Young's modulus.

    I say nothing if such constant is properly documented and supported by
    some reasoning, but for me now, even though it works, it is just garbage
    code.

     > In my opinion, the only important point is to compute kn and ks
    such as
     > scale effects are avoided, and I think it is well done in the current
     > formula (*) in Yade.

    I disagree with the premise that the only important point is to avoid
    scale effects. I like quantities to have physical meaning, as much as it
    is meaningful with discrete models; the argument (Bruno ;-) ) that
    discrete solution doesn't converge (and doesn't approach continuous
    solution) if you refine "mesh" (packing) does not justify, in my eyes,
    gratuitously introducing random constants to the code.

    Besides that, imagine Chiara reading that code (in a few days)... guess
    what happens? ;-)


Actually if you have a look at one of my old questions (that is your plan, right? ;-) ) I asked about the way Yade computes stiffnesses since to me it was not making sense to call a variable young modulus when physically it is not (in this sense I totally agree with you). But I remember Bruno said that it was related with an estimate of the stiffness of the packing. Maybe Bruno can explain better this concept or we could look at the users mailing list to recall his answer.

cheers, Chiara

I agree: "Young modulus" is not a good name for these variables (Ea or Eb). However, independently of the geometrical configuration of the granular assembly (characterized for instance, among other, by the coordination number), the macroscopic Young modulus depends only (or at least essentially) from this variable. That could justify the name "Young Modulus". But we can give another name if you want to.

Best,

   Luc


    v



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--
Luc Sibille

Université de Nantes - Laboratoire GeM UMR CNRS

IUT de Saint Nazaire
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44606 Saint-Nazaire Cedex, France

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