yade-dev team mailing list archive

[Robert Caulk <rcaulk@xxxxxxxxxxxx>]

Hello Jérôme,

I support clarifying the meaning of particle young in the class reference
as you suggest. There is no doubt that newcomers to DEM and Yade confuse
micro young's modulus with macro young's modulus all the time.

I would vote for clarifying the general doc instead of full removal of
stiffness and normal stiffness section in general doc, I think they provide
good examples of how Yade treats micro stiffness. If we decide to keep the
sections I can look into improving their clarity. But if others think
removal is best, I am not against it :-)

Best,

Robert

On Wed, Feb 21, 2018 at 8:59 AM, Jerome Duriez <jerome.duriez@xxxxxxxxx>
wrote:

> Hi,
>
> After seeing Yet Another Doubts on "young" meaning / Ip2_FrictMat_FrictMat_FrictPhys
> contact model (a PhD student in my institute), I'm proposing to explicitly
> mention the underlying equations in the doc, once for all.
> The corresponding would-be diff appears at the end of the email, you may
> also compare the attached screenshot with https://yade-dem.org/doc/yade.
> wrapper.html#yade.wrapper.Ip2_FrictMat_FrictMat_FrictPhys.
> From the diff, you will see I'm also proposing to remove the "harmonic
> average stiffness" comment, for instance because there actually is a "2"
> factor missing/extra between the comment and the actual code.
>
>
> In the case of the above-mentioned PhD student, confusion also arose
> because of https://yade-dem.org/doc/formulation.html#stiffnesses and
> https://yade-dem.org/doc/formulation.html#normal-stiffness.
> As such, I would also propose to simply remove these two sections from the
> doc.
> From my point of view, they actually are too Ip2-specific to appear in the
> general doc.
>
>
> If necessary, a reference to the (equivalent) 2.2.1 and 2.2.1.1. sections
> from Vaclav's thesis ([Smilauer2010b] reference) could be introduced e.g.
> in Ip2_CpmMat_CpmMat_CpmPhys and Ip2_FrictMat_FrictMat_FrictPhys
> docstrings, to keep track of these comments that could be made concerning
> the Ip2 equations.
>
>
> This email probably is Yet Another non-critical one, but since it deals
> about (the doc of) central pieces of code, and lines probably written by
> Vaclav, I'm opening a possible discussion here before performing the
> changes.
>
>
> Jérôme
>
> PS: next step would be to move https://yade-dem.org/doc/
> formulation.html#strain-evaluation to Ig2 docstrings but I'm not planning
> on it...
>
>
> ****************** Diff herein proposed for commit **************
>
> diff --git a/pkg/dem/FrictPhys.cpp b/pkg/dem/FrictPhys.cpp
> index cb904da..4f8ffe8 100644
> --- a/pkg/dem/FrictPhys.cpp
> +++ b/pkg/dem/FrictPhys.cpp
> @@ -26,9 +26,7 @@ void Ip2_FrictMat_FrictMat_FrictPhys::go( const
> shared_ptr<Material>& b1
>         Real Va         = mat1->poisson;
>         Real Vb         = mat2->poisson;
>
> -       //harmonic average of the two stiffnesses when (2*Ri*Ei) is the
> stiffness of a contact point on sphere "i"
>         Real Kn = 2*Ea*Ra*Eb*Rb/(Ea*Ra+Eb*Rb);
> -       //same for shear stiffness
>         Real Ks = 2*Ea*Ra*Va*Eb*Rb*Vb/(Ea*Ra*Va+Eb*Rb*Vb);
>
>         Real frictionAngle = (!frictAngle) ? std::min(mat1->frictionAngle,mat2->frictionAngle)
> : (*frictAngle)(mat1->id,mat2->id,mat1->frictionAngle,mat2->
> frictionAngle);
>
> diff --git a/pkg/dem/FrictPhys.hpp b/pkg/dem/FrictPhys.hpp
> index eeb2ef4..12c85ee 100644
> --- a/pkg/dem/FrictPhys.hpp
> +++ b/pkg/dem/FrictPhys.hpp
> @@ -44,7 +44,7 @@ class Ip2_FrictMat_FrictMat_FrictPhys: public
> IPhysFunctor{
>                         const shared_ptr<Material>& b2,
>                         const shared_ptr<Interaction>& interaction);
>         FUNCTOR2D(FrictMat,FrictMat);
> -       YADE_CLASS_BASE_DOC_ATTRS(Ip2_FrictMat_FrictMat_FrictPhys,IPhysFunctor,"Create
> a :yref:`FrictPhys` from two :yref:`FrictMats<FrictMat>`. The compliance of
> one sphere under point load is defined here as $1/(E.D)$, with $E$ the
> stiffness of the sphere and $D$ its diameter. The compliance of the contact
> itself will be the sum of compliances from each sphere, i.e.
> $1/(E_1.D_1)+1/(E_2.D_2)$ in the general case, or $2/(E.D)$ in the special
> case of equal sizes and equal stiffness. Note that summing compliances
> corresponds to an harmonic average of stiffnesss (as in e.g.
> [Scholtes2009a]_), which is how kn is actually computed in the
> :yref:`Ip2_FrictMat_FrictMat_FrictPhys` functor:\n\n $k_n =
> \\frac{E_1D_1*E_2D_2}{E_1D_1+E_2D_2}=\\frac{k_1*k_2}{k_1+k_2}$, with
> $k_i=E_iD_i$.\n\n The shear stiffness ks of one sphere is defined via the
> material parameter :yref:`ElastMat::poisson`, as ks=poisson*kn, and the
> resulting shear stiffness of the interaction will be also an harmonic
> average. In the case of a contact between a :yref:`ViscElMat` and a
> :yref:`FrictMat`, be sure to set :yref:`FrictMat::young` and
> :yref:`FrictMat::poisson`, otherwise the default value will be used.",
> +       YADE_CLASS_BASE_DOC_ATTRS(Ip2_FrictMat_FrictMat_FrictPhys,IPhysFunctor,"Create
> a :yref:`FrictPhys` from two :yref:`FrictMats<FrictMat>` as per the
> following expressions: \n\n$k_n = \\frac{E_1D_1*E_2D_2}{E_1D_1+E_2D_2}$
> where $k_n=$ :yref:`FrictPhys.kn`, the $E_i$ are :yref:`FrictMat.young` for
> each contacting body $i=1;2$, and the $D_i$ are the bodies' (equivalent)
> diameters\n\n$k_s = \\frac{E_1D_1P_1*E_2D_2P_2}{E_1D_1P_1+E_2D_2P_2}$
> where $k_s=$ :yref:`FrictPhys.ks` and the $P_i$ are the bodies'
> :yref:`FrictMat.poisson`\n\n$\\mu = \\tan(min(\\phi_1,\\phi_2))$ by
> default (see :yref:`frictAngle attribute<Ip2_FrictMat_
> FrictMat_FrictPhys.frictAngle>`), where $\\mu=$ :yref:`FrictPhys.tangensOfFrictionAngle`
> and the $\\phi_i$ are the bodies' :yref:`FrictMat.frictionAngle`.",
>
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