Re: [Yade-users] [Question #665350]: The parameter (en, es )of the DMT model

[Chiara Modenese]

Question #665350 on Yade changed:
https://answers.launchpad.net/yade/+question/665350

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi,

Glad to know the work has been useful.

Surface energy (gamma) is a measurable material parameter, and also an
input of the DMT model. You need to find out what the typical value(s)
is for your material based on the relevant literature (sorry but I can't
help you directly with this).

For the damping coefficients, en and es, these are there to replicate
dissipation forces, which you will need to calibrate in your model
against experimental values, depending on the purpose of your DEM
simulations.

I hope this is of help,
Chiara

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Re: [Yade-users] [Question #665350]: The parameter (en, es )of the DMT model

[Chiara Modenese]

Question #665350 on Yade changed:
https://answers.launchpad.net/yade/+question/665350

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi,

If you are looking to use the DMT model for adhesive particles, this is
a simple extension of the Hertz Mindlin formulation. The Hertzian
contact relationship is shifted down by the adhesive force. You can
check this on the literature, or at page 39, 40 of my PhD thesis [*].

The adhesive force should already be implemented in the current version
of the HertzMindlin.cpp contact law and it is a function of gamma.
Apologies if this has not been documented properly but it should work
fine if you are familiar with Yade. If you need further assistance, let
me know and I can provide you with a sample script.

HTH,
Chiara
[*] http://www.eng.ox.ac.uk/civil/publications/theses/modenese_pdf

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Re: [Yade-users] [Question #256283]: Problem with Damping, Facets and Contact Model

[Chiara Modenese]

Question #256283 on Yade changed:
https://answers.launchpad.net/yade/+question/256283

Chiara Modenese posted a new comment:
Great, thanks! Yes, you are right about the computation of the mass, it
shows how important it is to read & understand the code! It should be a
detail written in the documentation though - please feel free to add it
(thanks in advance!).

Glad to have confirmation that everything works fine,
Chiara

On 30 October 2014 15:06, Chareyre 
wrote:

> Question #256283 on Yade changed:
> https://answers.launchpad.net/yade/+question/256283
>
> Chareyre posted a new comment:
> Thank you for your feedback.
> PFC is excellent for benchmarks.
>
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Re: [Yade-users] [Question #256283]: Problem with Damping, Facets and Contact Model

[Chiara Modenese]

Question #256283 on Yade changed:
https://answers.launchpad.net/yade/+question/256283

Chiara Modenese proposed the following answer:
Just to say that the HM contact model has been widely tested for
sphere-sphere contacts but not with other elements. I am not familiar with
the formulation of facets but maybe somebody who is might be able to help
you?

Cheers,
Chiara

On 28 October 2014 16:46, Chareyre 
wrote:

> Question #256283 on Yade changed:
> https://answers.launchpad.net/yade/+question/256283
>
> Status: Open => Answered
>
> Chareyre proposed the following answer:
> Hi,
>
> On 28/10/14 08:16, Alexander Smith wrote:
> > I tested the Hertz Mindlin contact model and noticed that shear and
> normal viscous damping is not accounted for in a sphere-facet contact. It
> behaves correctly with a sphere-sphere contact. I have attached a simple
> script below to illustrate my point. I just wanted to check before I report
> a bug.
> To me it sounds like a bug, but maybe it is intentional. Devs of the
> viscous model will tell.
>
> > Also, I noticed that a facet is made of two "triangles" and at the
> junction of the two it appears the sphere is in contact with both surfaces.
> This increases the normal force and effects shear force and contact moments
> (shown by setting a=0 in the script below). Is this correct?
> No, it is not correct. It is a known issue of facet-sphere contacts, and
> it is more or less solutionless in the context of facets as they are now.
> Thus, I would not recommend to try and fix this by yourself.
> Some solutions are being implemented and should be released soon. They
> replace facets by more advanced shapes.
>
> > I am developing a contact model and  I need some guidance on how to fix
> it to ensure that the contact model behaves correctly on flat surfaces.
> If the surface is flat, then it can be a single facet, or a a box.
> Problem solved (or?).
>
> Bruno
>
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Re: [Yade-users] [Question #250903]: theory of hertz-mindlin

[Chiara Modenese]

Question #250903 on Yade changed:
https://answers.launchpad.net/yade/+question/250903

Chiara Modenese posted a new comment:
Hi,
Sure, I am always happy to help. Please post any Yade-related question on
launchpad though.
Thanks,
Chiarra


On 9 July 2014 16:26, Luis Barbosa 
wrote:

> Question #250903 on Yade changed:
> https://answers.launchpad.net/yade/+question/250903
>
> Luis Barbosa posted a new comment:
> Hi Chiara,
> Grazie Mille!
> I'm also working with soil, but for agriculture in the earth. I'll face
> many difficulties in a short future, can I contact you to help me in some
> cases?
>
> Thanks!
> Luis
>
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Re: [Yade-users] [Question #250903]: theory of hertz-mindlin

[Chiara Modenese]

Question #250903 on Yade changed:
https://answers.launchpad.net/yade/+question/250903

Chiara Modenese proposed the following answer:
Hi Alexander,
I hope you are having fun and please say hello to everyone on my behalf! I
do say Yade the "italian way" and have managed to convince others around
the UK that that is what it should sound like! :-)
Keep me posted,
Chiara


On 9 July 2014 01:07, Alexander Eulitz [Eugen] <
question250...@answers.launchpad.net> wrote:

> Question #250903 on Yade changed:
> https://answers.launchpad.net/yade/+question/250903
>
> Alexander Eulitz [Eugen] posted a new comment:
> Hi Chiara,
> thanks for the link to your PhD! Just was talking about you here at the
> Yade Workshop. Btw. how do you pronounce the "Yade" in Oxford as home of
> the proper English (just another discussion in Grenoble ;))?
> Alex
>
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Re: [Yade-users] [Question #250903]: theory of hertz-mindlin

[Chiara Modenese]

Question #250903 on Yade changed:
https://answers.launchpad.net/yade/+question/250903

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Luis,

You can check out my PhD thesis and in particular the chapter on contact
models for lunar soil.
http://www.eng.ox.ac.uk/civil/publications/theses/modenese

I hope it helps!
Chiara


On 28 June 2014 15:51, Luis Barbosa 
wrote:

> New question #250903 on Yade:
> https://answers.launchpad.net/yade/+question/250903
>
> Hi everybody,
>
> I'm studying Law2_ScGeom_MindlinPhys_Mindlin. I've simulated a simple
> problem with one sphere over a face with a normal and shear force and get
> the values (the rotation of the sphere is blocked).
> I would like, in order to understand the law, solve the same problem
> analytically, just learn the theory HM. The problem is the equations on the
> script [1] are not so clear to me, for instance how to calc the shear force
> and some parameters of damping.
> Is there some easier doc with this equations of HM?
>
> Thanks!
>
> [1]
> http://bazaar.launchpad.net/~yade-pkg/yade/git-trunk/view/head:/pkg/dem/HertzMindlin.cpp
>
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Re: [Yade-users] [Question #241471]: mindlin contact law

[Chiara Modenese]

Question #241471 on Yade changed:
https://answers.launchpad.net/yade/+question/241471

Chiara Modenese proposed the following answer:
Hi Seungcheol,

First of all I would post your script and highlight any chances you have
done from the version you said was working fine. When I talk about
calibration I mean the approach you take to select the input parameters
of your simulations, be them for use in YADE or another DEM code. Surely
if you decide to use a contact law in place of another, you already have
some ideas on how to determine those parameters or where they should
come from (contact parameters, geometrical parameters etc.).

Chiara

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Re: [Yade-users] [Question #241108]: mindlin-Deresiewicz contact law

[Chiara Modenese]

Question #241108 on Yade changed:
https://answers.launchpad.net/yade/+question/241108

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi,

The contact law you are attempting to use is in fact not available. I started 
coding it during my PhD but I never completed it since I decided that the 
simplified HM law was good enough for my purposes. Please see below the link 
with the list of contact laws currently in use with Yade and which have been 
tested and accurately verified by means of elements contact tests.
https://www.yade-dem.org/wiki/ConstitutiveLaws

Please feel free to add your contact law should you need the complete
path described by the HM law (and not the simplified version which is
now in the code).

Cheers,
Chiara

On 27 Dec 2013, at 18:56, Sina Jafari wrote:

> Question #241108 on Yade changed:
> https://answers.launchpad.net/yade/+question/241108
> 
>Status: Answered => Open
> 
> Sina Jafari is still having a problem:
> for example in the following script I have modified the triaxial example 
> script a little bit to study the effect of contact law on the output, the 
> point is when I use mindlin-Deresiewicz contact law, no shear force is 
> recorded in the output which is in contradiction with the results obtained 
> by, say, cundall law. Why is this happening? do I need to define a parameter 
> or somethin'? Thanks for your answers. here is the script:
> # -*- coding: utf-8 -*-
> #*
> #  Copyright (C) 2010 by Bruno Chareyre  *
> #  bruno.chareyre_at_grenoble-inp.fr *
> #*
> #  This program is free software; it is licensed under the terms of the  *
> #  GNU General Public License v2 or later. See file LICENSE for details. *
> #*/
> 
> ## This script details the simulation of a triaxial test on sphere packings 
> using Yade
> ## See the associated pdf file for detailed exercises
> ## the algorithms presented here have been used in published papers, namely:
> ## * Chareyre et al. 2002 
> (http://www.geosyntheticssociety.org/Resources/Archive/GI/src/V9I2/GI-V9-N2-Paper1.pdf)
> ## * Chareyre and Villard 2005 
> (https://yade-dem.org/w/images/1/1b/Chareyre&Villard2005_licensed.pdf)
> ## * Scholtès et al. 2009 (http://dx.doi.org/10.1016/j.ijengsci.2008.07.002)
> ## * Tong et al.2012 (http://dx.doi.org/10.2516/ogst/2012032)
> ##
> ## Most of the ideas were actually developped during my PhD.
> ## If you want to know more on micro-macro relations evaluated by triaxial 
> simulations
> ## AND if you can read some french, it is here: 
> http://tel.archives-ouvertes.fr/docs/00/48/68/07/PDF/Thesis.pdf
> 
> from yade import pack,plot
> import matplotlib; matplotlib.rc('axes',grid=True)
> import pylab
> 
> ###   DEFINING VARIABLES AND MATERIALS   ###
> 
> key='_Kenney_'
> num_spheres=48710
> psdSizes,psdCumm=[0.262*0.89185,0.53*0.89185,0.97*0.89185,1.76*0.89185,2.49*0.89185,3.4*0.89185,4.87*0.89185,6.4*0.89185],[0.1,3.4,9.1,19.1,29.4,48.7,81.5,100]
> #targetPorosity = 0.387 #the porosity we want for the packing
> compFricDegree = 26.5 # initial contact friction during the confining phase 
> (will be decreased during the REFD compaction process)
> finalFricDegree = 26.5 # contact friction during the deviatoric loading
> rate=0.0001 # loading rate (strain rate)
> damp=0.2 # damping coefficient!!
> stabilityThreshold=0.01 # we test unbalancedForce against this value in 
> different loops (see below)
> young=540e6 # contact stiffness
> mn,mx=Vector3(0,0,0),Vector3(53.52,53.52,53.52) # corners of the initial 
> packing
> 
> 
> ## create materials for spheres and plates
> O.materials.append(FrictMat(young=young,poisson=0.35,frictionAngle=radians(compFricDegree),density=2000,label='spheres'))
> O.materials.append(FrictMat(young=young,poisson=0.5,frictionAngle=0,density=0,label='walls'))
> 
> # create walls around the packing
> walls=aabbWalls([mn,mx],material='walls',oversizeFactor=1)
> wallIds=O.bodies.append(walls)
> 
> ## use a SpherePack object to generate a random loose particles packing
> sp=pack.SpherePack()
> sp.particleSD2(radii=psdSizes,passing=psdCumm,numSph=7500,cloudPorosity=0.55)
> O.bodies.append([utils.sphere(center,rad,material='spheres') for center,rad 
> in sp])
> #or alternatively (higher level function doing exactly the same):
> #sp.toSimulation(material='spheres')
> 
>

Re: [Yade-users] [Question #241471]: mindlin contact law

[Chiara Modenese]

Question #241471 on Yade changed:
https://answers.launchpad.net/yade/+question/241471

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi,

Please post a script with the parameters you are using in your
simulations to reproduce the error. Saying "it is not working because
some particles are flying out" does not necessarily mean that the
contact law is not working but it may well be due to an improper
combination of input parameters you are using (strain rate etc.) - have
you done any calibration beforehand?

I have used the law before with boxes and periodic boundaries and never
found a problem.

Cheers,
Chiara


On 31 Dec 2013, at 00:06, Seungcheol Yeom wrote:

> New question #241471 on Yade:
> https://answers.launchpad.net/yade/+question/241471
> 
> Hello all,
> 
> I am wondering whether a mindlin contact law is only used for the sphere 
> interaction. 
> I have tried to use the contact law (mindlin) for the tri-axial consolidation 
> instead of using a cundall strack but it is not working.
> It is appeared that all paricles near the top and bottom of the plates are 
> flying out.
> So, I think I have to use two different contact laws somthing like mindlin 
> for the spheres and cundall for box, wall, facet if the mindlin contact law 
> only works for the spheres interaction. 
> Is this possible? or is there any method I can do?
> Thank you very much for your help.
> 
> Best,
> 
> Seungcheol
> 
> 
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Re: [Yade-users] [Question #232351]: Kinds of damping

[Chiara Modenese]

Question #232351 on Yade changed:
https://answers.launchpad.net/yade/+question/232351

Chiara Modenese posted a new comment:
Hi again,

Yes, they are. Linear or non-linear it depends on the definition as in the
linear case cn is in linear relationship with betan, for instance.
Improvements to Yade documentation are more than welcome so feel free to
add/modify text! In my experience is not very useful to try a relationship
other than linear for viscous damping unless you are unable to achieve what
you want with a simple model or unless your goal is to come up with a
contact relationship that is closer to reality.

Best,
Chiara


On 6 November 2013 16:11, Alexander Eulitz [Eugen] <
question232...@answers.launchpad.net> wrote:

> Question #232351 on Yade changed:
> https://answers.launchpad.net/yade/+question/232351
>
> Alexander Eulitz [Eugen] posted a new comment:
> Hi Chiara,
> thanks for your help. I really appreciate it.
> I will try to use linear damping only and calibrate beta_n during a simple
> sphere drop experiment.
> After this I'll have look for ideas of how beta_s can be determined.
> Adjusting the documentation is another bullet point on my todo list for
> christmas :)
>
> Just another short question: Am I right in assuming that linear contact
> damping as well as non-linear contact damping are both forms of viscous
> damping?
>
> Thanks again,
> Alex
>
>
> -Ursprüngliche Nachricht-
> Von: boun...@canonical.com [mailto:boun...@canonical.com] Im Auftrag von
> Chiara Modenese
> Gesendet: Mittwoch, 6. November 2013 16:46
> An: Eulitz, Alexander
> Betreff: Re: [Question #232351]: Kinds of damping
>
> Question #232351 on Yade changed:
> https://answers.launchpad.net/yade/+question/232351
>
> Chiara Modenese posted a new comment:
>  Hi Alexander,
>
> Yes, you are right beta_n should be called damping ratio rather than
> damping coefficient but you get the concept. Feel free to adjust the
> text if you want.
>
> At the time I set the code so that e_n and e_s would be the defined the
> same was for convenience but also because I believe that the equation
> for alpha was derived for normal impact tests only (please correct me if
> I am wrong as I do not have that paper with me right now).
>
> Looking back at this problem, I would not use this form of non-linear
> damping because actually the relationship for alpha that is implemented
> is not formally correct. It is a good first approximation but
> analytically is not consistent (I will see later if I can find the
> reference that proves that for you) - if you do some research on the
> topic you will find different relationships for alpha.
>
> Chiara
>
>
> On 6 November 2013 14:16, Alexander Eulitz [Eugen] <
> question232...@answers.launchpad.net> wrote:
>
> > Question #232351 on Yade changed:
> > https://answers.launchpad.net/yade/+question/232351
> >
> > Alexander Eulitz [Eugen] posted a new comment:
> > Hi, I'd like to reopen this question.
> > The Hertz Mindlin contact law allows for two diffrent kinds of
> > viscouse damping, i.e. linear and non-linear.
> >
> > Considering the linear case:
> > according to [1] beta_n is the viscous damping coefficient. But how is
> > it defined? I did not find a satisfying answer. I looked at [3] and
> > the named source from Schwager as wells as at [5], but I do not get it.
> >
> > In the source of the Hertz Mindlin Contact law [2] beta_n will be used
> > to calculate c_n:
> >  cn = Cn_crit*phys->betan; // Damping normal coefficient
> >
> > with Cn_crit being the critical damping coefficient. I recognized that
> > rearranging the equation for the damping ratio [4] gives:
> > damping_coeff=damp_ratio*crit_damp_coeff which equals the line from
> > the source code.
> > If this is right, then beta_n is the damping ratio and not a viscous
> > damping coefficient.
> >
> > May second question concerns non-linear viscous damping:
> > It is not commented in the documentation [1] but looking at the
> > sources [6] tells me that I can enable this kind of damping by
> > specifying the coefficients of restitution (en, es). This way a value
> > alpha will be computed from it.
> > The first strange thing is, that it does not matter whether I specify
> > en or es, only en will be used for alpha computation.
> > The second strange thing is that the documentation on [1] says:
> >  " If e_n is given, MindlinPhys.betan is computed using \beta_n=-(\log
> > e_n)/\sqrt{\pi^2+(\log e_n)^2}. The same applies to e_s."
> >
> > But this is not what is done in the source.
> >
> > Could you please help me

Re: [Yade-users] [Question #232351]: Kinds of damping

[Chiara Modenese]

Question #232351 on Yade changed:
https://answers.launchpad.net/yade/+question/232351

Chiara Modenese posted a new comment:
Hi Alexander,

Yes, you are right beta_n should be called damping ratio rather than
damping coefficient but you get the concept. Feel free to adjust the text
if you want.

At the time I set the code so that e_n and e_s would be the defined the
same was for convenience but also because I believe that the equation for
alpha was derived for normal impact tests only (please correct me if I am
wrong as I do not have that paper with me right now).

Looking back at this problem, I would not use this form of non-linear
damping because actually the relationship for alpha that is implemented is
not formally correct. It is a good first approximation but analytically is
not consistent (I will see later if I can find the reference that proves
that for you) - if you do some research on the topic you will find
different relationships for alpha.

Chiara


On 6 November 2013 14:16, Alexander Eulitz [Eugen] <
question232...@answers.launchpad.net> wrote:

> Question #232351 on Yade changed:
> https://answers.launchpad.net/yade/+question/232351
>
> Alexander Eulitz [Eugen] posted a new comment:
> Hi, I'd like to reopen this question.
> The Hertz Mindlin contact law allows for two diffrent kinds of viscouse
> damping, i.e. linear and non-linear.
>
> Considering the linear case:
> according to [1] beta_n is the viscous damping coefficient. But how is it
> defined? I did not find a satisfying answer. I looked at [3] and the named
> source from Schwager as wells as at [5], but I do not get it.
>
> In the source of the Hertz Mindlin Contact law [2] beta_n will be used to
> calculate c_n:
>  cn = Cn_crit*phys->betan; // Damping normal coefficient
>
> with Cn_crit being the critical damping coefficient. I recognized that
> rearranging the equation for the damping ratio [4] gives:
> damping_coeff=damp_ratio*crit_damp_coeff which equals the line from the
> source code.
> If this is right, then beta_n is the damping ratio and not a viscous
> damping coefficient.
>
> May second question concerns non-linear viscous damping:
> It is not commented in the documentation [1] but looking at the sources
> [6] tells me that I can enable this kind of damping by specifying the
> coefficients of restitution (en, es). This way a value alpha will be
> computed from it.
> The first strange thing is, that it does not matter whether I specify en
> or es, only en will be used for alpha computation.
> The second strange thing is that the documentation on [1] says:
>  " If e_n is given, MindlinPhys.betan is computed using \beta_n=-(\log
> e_n)/\sqrt{\pi^2+(\log e_n)^2}. The same applies to e_s."
>
> But this is not what is done in the source.
>
> Could you please help me?
>
> [1]
> https://yade-dem.org/doc/yade.wrapper.html?highlight=mindlin#yade.wrapper.Ip2_FrictMat_FrictMat_MindlinPhys
> [2]
> https://github.com/yade/trunk/blob/master/pkg/dem/HertzMindlin.cpp?source=cc#L316
> [3] https://answers.launchpad.net/yade/+question/235934
> [4] http://en.wikipedia.org/wiki/Damping_ratio
> [5] http://woodem.eu/doc/theory/contact/hertzian.html#viscous-damping
> [6]
> https://github.com/yade/trunk/blob/master/pkg/dem/HertzMindlin.cpp?source=cc#L86
>
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Re: [Yade-users] [Question #238696]: Time step

[Chiara Modenese]

Question #238696 on Yade changed:
https://answers.launchpad.net/yade/+question/238696

Chiara Modenese proposed the following answer:
Hi Lingran,

I have used contact damping with the HM law. Whether it is approprate in a
dynamic case like yours it is up to you to decide as it all depends on the
physics you are trying to reproduce. You may be able to calibrate the shear
and normal damping coefficients against real data, for instance. Finally,
keep in mind that the critical time step for particles with and without
damping is not the same as it should be reduced when damping is present.

Cheers,
Chiara

On 5 November 2013 14:51, lingran
wrote:

> New question #238696 on Yade:
> https://answers.launchpad.net/yade/+question/238696
>
> Hi, all,
>
> About 15 years ago, a girl did an impact experiment of a boulder impacting
> onto a soil layer, now I want to reproduce her experiment by using yade.
> The problem is that her soil layer is quite big, to respect the reality,
> the numerical sample will have millions of particles, and the time step
> will be as small as 1e-8, thus the sample might never be prepared by
> gravity deposition.
>
> So my question is: how could we increase the time step while not change
> the physics? I did some two particles impact tests, it turns out playing
> with radius and density (keep mass constant) could increase time step, but
> the contact forces will also be changed, not good. So, any other ideas?
>
> Second question: Does somebody use viscous damping of
> Law2_ScGeom_MindlinPhys_Mindlin()? This contact law is said to be mature,
> but is it proper to be used in dealing with dynamic problems?
>
> Thanks.
> Regards.
> Lingran
>
>
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Re: [Yade-users] [Question #237624]: constitutive law for inelastic (elasto-pastic) contacts

[Chiara Modenese]

Question #237624 on Yade changed:
https://answers.launchpad.net/yade/+question/237624

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Alex,

For inelastic collisions based on the Hertz-Mindlin model I suggest you
look into this paper:
http://www.sciencedirect.com/science/article/pii/S0032591012005670
 An investigation of the comparative behaviour of alternative contact force
models during inelastic collisions
Once you have identified the model that you think is most appropriate
to model your case then you can try to implement in Yade - contact models
are relatively easy to implement and test in Yade.

Best,
Chiara

On 19 October 2013 10:56, Alexander Eulitz [Eugen] <
question237...@answers.launchpad.net> wrote:

> New question #237624 on Yade:
> https://answers.launchpad.net/yade/+question/237624
>
> Hi there,
> is somebody dealing with inelastic contacts using Yade?
> I found pkg/dem/NormalInelasticityLaw.cpp which is quite good. But it
> makes use of the Cundall & Strack formulation whereas I need HertzMindlin.
>
> Some infos about the problem I try to solve:
> A steel ball is dropped from a given height  onto a horizontal plate which
> is made of steel, too.
> I simulated this scenario using Cundall and Strack and observed way higher
> contact forces than in my experiments.
> I.e. about 30N (experiment) to 250N (C&S simulation) for maximum force of
> the first contact.
> Changing to contact law of Hertz Mindlin which seems appropriate because
> Cundall & Stracks law was originally tested on disks rather then spheres,
> gives simulated maximum contact forces of 40N.
> A indentation test on the steel ball and the steel plate showed that there
> is a slight difference in the hardness of the two materials. So the softer
> one (the plate) can be plastically deformed by the harder steel ball. I
> think that this could explain the deviation of contact forces. That's the
> reason I am looking for a Hertz Mindlin based constitutive law for
> elasto-plastic contacts.
>
> Thanks for your help,
> Alex
>
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Re: [Yade-users] [Question #235934]: Reference for coeff of restitution in Hertz-Mindlin with non-linear damping

[Chiara Modenese]

Question #235934 on Yade changed:
https://answers.launchpad.net/yade/+question/235934

Chiara Modenese posted a new comment:
@Klaus
Yes, you are absolutely right and you refer to an excellent reference
(worth reading it for those interested in conctact models). However,
sometimes an analytical relationship is not possible to derive, such as in
the HM model.

@Bruno
At present I am not able to commit that part of the code but if somebody
follows my instruction it should be quick to do it.

All,
In my experience is not so critical to avoid tensile stresses due to
viscous damping as I have always found them to be negligible but it is a
good practice to neglect them entirely if possible. It is more of an issue
if we are modelling cohesive particles, as there you expect the cohesion to
match with your physical input.

Chiara


On 23 September 2013 11:16, Klaus Thoeni <
question235...@answers.launchpad.net> wrote:

> Question #235934 on Yade changed:
> https://answers.launchpad.net/yade/+question/235934
>
> Klaus Thoeni posted a new comment:
> > Yet, I wonder if people do not calibrate the restitution vs. viscosity
> > relation taking into account the tensile effects sometimes.
>
> At the end it depends what relation between the restitution coefficient
> and the
> viscous parameter is used. There is even a paper on this topic [1].
>
> Klaus
>
> [1] http://link.springer.com/article/10.1007/s10035-007-0065-z
>
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Re: [Yade-users] [Question #235934]: Reference for coeff of restitution in Hertz-Mindlin with non-linear damping

[Chiara Modenese]

Question #235934 on Yade changed:
https://answers.launchpad.net/yade/+question/235934

Chiara Modenese posted a new comment:
Hey Guys,

I am still here! Sorry for not replying earlier but I have recently been
very busy and the good news is that I have successfully completed my PhD
:-) (finally!). I take this occasion to say a Big Thanks to all of you
for your friendship and support during this long journey which I greatly
enjoyed, with all the lessons and gratifications that have come with it.

Now here is my reply to Vaclav (thanks for pointing this matter out).
You have spotted the problem since the normal force can become negative when 
viscous damping is applied. This should obviously be avoided as it is 
unphysical and below is the code that I have replaced in my local version of 
Yade that I have missed to commit (sorry about that).  When adhesion is 
included, the code takes also care of this by limiting the normal force with 
the maximum adhesion force. 

This issue can potentially affect all contact laws so ideally an option
like this should be available even for the linear contact law when
combined with some form of viscous damping.

Can somebody (Anton?) please replace the code below with what is
currently in trunk in HertzMindlin.cpp, lines 344 to 345? Many thanks
for this.

Let me know if there is any further question.

Cheers,
Chiara


/**/
/* VISCOUS DAMPING (Normal direction) */
/**/
// normal force must be updated here before we apply the 
Mohr-Coulomb criterion
if (useDamping){ // get normal viscous component 
phys->normalViscous = cn*incidentVn;
Vector3r normTemp = phys->normalForce - phys->normalViscous; // 
temporary normal force
// viscous force should not exceed the value of current normal 
force, i.e. no attraction force should be permitted if particles are 
non-adhesive
// if particles are adhesive, then fixed the viscous force at 
maximum equal to the adhesion force
// *** enforce normal force to zero if no adhesion is permitted 
***
if (phys->adhesionForce==0.0 || !includeAdhesion){ 
if (normTemp.dot(scg->normal)<0.0){
phys->normalForce = Vector3r::Zero();
phys->normalViscous = phys->normalViscous + 
normTemp; // normal viscous force is such that the total applied force is null 
- it is necessary to compute energy correctly!
}
else{phys->normalForce -= phys->normalViscous;} 

}   
else if (includeAdhesion && phys->adhesionForce!=0.0){  

// *** limit viscous component to the max adhesive 
force ***
if (normTemp.dot(scg->normal)<0.0 && 
(phys->normalViscous.norm() > phys->adhesionForce) ){ 
Real normVisc = phys->normalViscous.norm(); 
Vector3r normViscVector = phys->normalViscous/normVisc;
phys->normalViscous = 
phys->adhesionForce*normViscVector;
phys->normalForce -= phys->normalViscous;
}
// *** apply viscous component - in the presence of 
adhesion ***
else {phys->normalForce -= phys->normalViscous;}
}
if (calcEnergy) {normDampDissip += 
phys->normalViscous.dot(incidentVn*dt);} // calc dissipation of energy due to 
normal damping
}

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Re: [Yade-users] [Question #232953]: rolling resistance models in YADE

[Chiara Modenese]

Question #232953 on Yade changed:
https://answers.launchpad.net/yade/+question/232953

Chiara Modenese proposed the following answer:
Yes, I agree that the algorithms are different than what is in Yade at
present (thanks for the reference you have given above). Moreover, as you
say, equations are not generally  fully developed and so it is hard to
guess how was actually implemented. It would be good, however, to consider
a paper where PFC with rolling resistance was used and to try and compare
the results obtained with Yade.

As for the coordination number, I have actually been asked in the past why
I get very low values (in my case always around 3.5 towards the steady
state) but I have never used another code to be able to find an explanation
as to why other authors obtain much higher values - I guess it would
require a more systematic comparison of Yade with PFC.

Chiara

On 20 August 2013 16:41, Bruno Chareyre <
question232...@answers.launchpad.net> wrote:

> Question #232953 on Yade changed:
> https://answers.launchpad.net/yade/+question/232953
>
> Bruno Chareyre posted a new comment:
> @Chiara
> No, I did not imply that. There may be nothing wrong but for sure the
> models are different.
> Since it is not very possible to check the version of SDEC modified by JP
> Plassiard (version control only started with yade), nor re-run the same
> calculations again, it is not a very good candidate for benchmarking.
> The paper can give interesting general trends, but curves matching would
> be pointless.
>
> Namely, as discussed in [1], they had a coordination number around 7 for
> simple frictional contacts, which you will not have in Yade without
> tricking.
>
>  [1] https://answers.launchpad.net/yade/+question/207350
>
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Re: [Yade-users] [Question #232953]: rolling resistance models in YADE

[Chiara Modenese]

Question #232953 on Yade changed:
https://answers.launchpad.net/yade/+question/232953

Chiara Modenese proposed the following answer:
Bruno, when you say that the results with those other published algorithms
of rolling resistance cannot be reproduced with Yade, do you mean that
something is wrong with any of them? Thanks, Chiara

On 3 August 2013 11:01, Bruno Chareyre  wrote:

> Question #232953 on Yade changed:
> https://answers.launchpad.net/yade/+question/232953
>
> Bruno Chareyre proposed the following answer:
> Dear Fu Zuoguang, I don't see anything wrong in your summary.
>
> @Klaus, actually no, the code in Yade has been written independently and
> there are differences in the algorithms. Since the results in these
> papers can't be reproduced with Yade, it is not very helpful to put them
> forward for what concerns rotational effects. Plus they don't define the
> law completely (relative rotation or shear disp. are not really
> detailed, typically)
>
> The most detailed presentation of the model presently is here:
> http://dx.doi.org/10.1016/j.ecoleng.2013.05.002
> Fundamentally, it does not differ from the previous models with rotational
> effects (Cundall, Oda & Iwashita, Plassiard et al., ...) but as always we
> can find differences in some details.
>
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Re: [Yade-users] [Question #228886]: [Triaxial test] Kinetic Energy is way too big, how to reduce it?

[Chiara Modenese]

Question #228886 on Yade changed:
https://answers.launchpad.net/yade/+question/228886

Chiara Modenese proposed the following answer:
Hi,

Just a small comment on what you want to achieve rather than how. I
agree with Bruno saying that kinetic energy alone (however small in
absolute value) will not tell you if your test is quasi-static. You need
to compare the total kinetic energy with the elastic potential energy of
your system and make sure that the former is a small fraction of the
latter (that is to say that dynamic forces are always present but their
effect is negligible). How small a fraction you will need to determine
until the solution becomes insensitive to the walls' velocity (or strain
rate). Alternatively you can also look at the unbalanced ratio.

Chiara

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Re: [Yade-users] [Question #227868]: rolling resistance implementation

[Chiara Modenese]

Question #227868 on Yade changed:
https://answers.launchpad.net/yade/+question/227868

Chiara Modenese proposed the following answer:
Hi Ning

I think that you already answered your question. The relative rotation
that you are referring to pertains to sliding (which should be dealt
with in the shear direction). The rolling resistance considers only the
relative rotation between the two contacting particles, excluding the
sliding contribution.

Chiara

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Re: [Yade-users] [Question #224829]: O.save/O.load

[Chiara Modenese]

Question #224829 on Yade changed:
https://answers.launchpad.net/yade/+question/224829

Chiara Modenese proposed the following answer:
Hi Riccardo

My suggestion is that if you require to frequently save and load
simulations, you should try to use the same version of Yade. Otherwise
you are likely to encounter segmentation errors because of lack of
compatibility (although this will generally depend on the type of
changes between different versions of the code).

Chiara

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Re: [Yade-users] [Question #219884]: Time computation of biaxial/triaxial test

[Chiara Modenese]

Question #219884 on Yade changed:
https://answers.launchpad.net/yade/+question/219884

Chiara Modenese proposed the following answer:
Yes, of course as it determines the critical time step of the
simulation. So you might also think of increasing your particle size but
I would suggest to play with either stiffness or strain rate first as
also Bruno suggested. If you use cohesive bonds, depending on how you
define the cohesive law, there might be some scaling issues if you
change the particle size from the one of the real material.

Chiara

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Re: [Yade-users] [Question #219884]: Time computation of biaxial/triaxial test

[Chiara Modenese]

Question #219884 on Yade changed:
https://answers.launchpad.net/yade/+question/219884

Chiara Modenese proposed the following answer:
@Bruno

If you increase density and at the same time reduce the strain rate so
that the total time is unchanged, then clearly there is no speed-up, as
you already say. Either one increases the density OR the strain rate, it
is equivalent. Maybe increasing the strain rate, rather than the
density, sounds more reasonable but I can assure you that the result
would be the same (dimensional analysis can prove it).

Chiara

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Re: [Yade-users] [Question #219884]: Time computation of biaxial/triaxial test

[Chiara Modenese]

Question #219884 on Yade changed:
https://answers.launchpad.net/yade/+question/219884

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Jessica,

I would answer that all depends on your input parameters and what you
are interested to study (small or large strains behaviour, for
instance). Tuning parameters like particles density generally helps
speeding up the computation time, e.g. by increasing your time step. You
can also try reducing the interparticle stiffness if you do not care
about the actual value of your interparticle parameters (in other words,
if they do not match with real physical parameters defined at the
interparticle level). Alternatively, you may try increase the strain
rate, making sure your solution does not vary because of that.

Happy to help you more on this issue if you have further enquires.
Cheers,
Chiara
PS1 There is some literature on the topic, have a look at the book on DEM 
edited by Radjai and Dubois, for instance
PS2 Do not be afraid to play with your input parameters to try making the 
simulation faster, but be thoughtful on the changes you make

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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Status: Answered => Solved

Chiara Modenese changed the question status:
Comments #9 and #12 solve the problem.
Chiara

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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Chiara Modenese proposed the following answer:
Hi Jan,

If I correctly interpret the code by Ning, he is simply taking the
distance between the centroids of the two contacting bodies - whether
these are spheres or clumps the definition of branch vector is the same.
In the special case of two contacting spheres the branch vector then
becomes in line with the contact point. Why do you state that this
approach is wrong? It is general (as I understand it now, see my
original question #10) and it would be good to include this adaption for
clumps in trunk.

Chiara

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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Chiara Modenese proposed the following answer:
Hi Jan,

I agree with Ning, the contact point does not need to be located in line
with the branch vector for the derivation to be valid. See Bagis' paper
(or similar references).

Chiara

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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Chiara Modenese proposed the following answer:
@Ning
Your proposed solution looks fine to me. Are you planning to commit it?

@Jan
How do you define the branch vector in your case? Can you clarify your 
suggestion please?

Chiara

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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Chiara Modenese proposed the following answer:
Hi Jan,

Thanks for reply. The definition of average stress tensor in Bagi's
paper, for instance, is equal to the one in Yade [1]. I was not sure
about its validity for arbitrary grain shape (Bagi specifies that the
method is valid for grains of convex shape but I cannot see why his
derivation would not apply to concave shape too). I have adapted the
definition of branch vector in my local branch [2] but it only handles
the case of clump-clump interaction whereas it would be nice to have in
trunk all the possible combinations (sphere-clump, etc.).

[1] https://yade-
dem.org/doc/yade.utils.html?highlight=getstress#yade._utils.getStress

[2]  // compute branch vector
  if ( Body::byId(I->getId2(),scene)->isClumpMember() && 
Body::byId(I->getId1(),scene)->isClumpMember()) { // if both bodies are clump 
mumbers   
 int id_clump_2 = Body::byId(I->getId2(),scene)->clumpId;
 int id_clump_1 = Body::byId(I->getId1(),scene)->clumpId;
 branch = Body::byId(id_clump_2,scene)->state->pos + 
scene->cell->hSize*I->cellDist.cast() - 
Body::byId(id_clump_1,scene)->state->pos;
  } else { // only spheres
 branch= Body::byId(I->getId2(),scene)->state->pos + 
scene->cell->hSize*I->cellDist.cast() - 
Body::byId(I->getId1(),scene)->state->pos;
  }

Chiara

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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Status: Solved => Open

Chiara Modenese changed the question status:
Hi,

Concerning the computation of stress tensor and the question above I
would like to know if the general definition of average stress tensor
(e.g. Thornton, 2000; Bagi, 1996) holds true for clumps of particles
(provided that the branch vector is correctly defined as previously
discussed). In the paper by Bagi (1996) the definition is obtained for
grains of arbitrary _convex_ shape. Do you know if the theory is also
valid for grains of concave shape?

Thanks for opinion.
Chiara

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Re: [Yade-users] [Question #214773]: Question about "ScGeom"

[Chiara Modenese]

Question #214773 on Yade changed:
https://answers.launchpad.net/yade/+question/214773

Chiara Modenese proposed the following answer:
Hi Christian,

I noticed that in your script you use
Law2_ScGeom_MindlinPhys_HertzWithLinearShear. As explained in [1] that
law is no longer maintained or functioning. Please use
Law2_ScGeom_MindlinPhys_Mindlin instead.

[1] https://yade-dem.org/wiki/ConstitutiveLaws

Chiara

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Re: [Yade-users] [Question #212834]: Contact number estimation

[Chiara Modenese]

Question #212834 on Yade changed:
https://answers.launchpad.net/yade/+question/212834

Chiara Modenese proposed the following answer:
Hi Nait,

Here is a simple example.

ii=O.bodies[id].intrs() # list of interactions with body id
len(ii) # number of interactions to which body id participates

I hope this is clear now.
Best, 
Chiara

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Re: [Yade-users] [Question #212834]: Contact number estimation

[Chiara Modenese]

Question #212834 on Yade changed:
https://answers.launchpad.net/yade/+question/212834

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Nait,

If you are looking for the number of interactions of a specific body
(say body with id 0), then you can type this:

O.bodies[0].intrs

If instead you want the average number of contacts you can call this
(please see the relevant documentation for more explanations on this
function):

utils.avgNumInteractions()
(see https://yade-dem.org/doc/yade.utils.html#yade.utils.avgNumInteractions)

HTH,
Chiara

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Re: [Yade-users] [Question #210907]: on sample preparation of real granula materials

[Chiara Modenese]

Question #210907 on Yade changed:
https://answers.launchpad.net/yade/+question/210907

Chiara Modenese proposed the following answer:

On 11 Oct 2012, at 15:11, Chareyre wrote:

> Question #210907 on Yade changed:
> https://answers.launchpad.net/yade/+question/210907
> 
> Chareyre posted a new comment:
>> I generate the particles in a very loose state and then I apply
> compression
> 
> Then the microstructure is heterogeneous, with a gradient of porosity
> and fabric from the contour to the center of the sample (periodic BCs
> fix that).
Yes, that is right as I forgot to say I make use of PBCs.
> 
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Re: [Yade-users] [Question #210907]: on sample preparation of real granula materials

[Chiara Modenese]

Question #210907 on Yade changed:
https://answers.launchpad.net/yade/+question/210907

Chiara Modenese proposed the following answer:

On 11 Oct 2012, at 14:11, Christian Jakob wrote:

> Question #210907 on Yade changed:
> https://answers.launchpad.net/yade/+question/210907
> 
> Christian Jakob posted a new comment:
>> @Christian: If you use very _high_ stiffness with the non-linear model you 
>> need to be careful to choose the right parameters > for the expansion method 
>> as Bruno suggests. In these regards, remember that the _loading rate_ you 
>> apply has to be 
>> sufficiently small if you want the process to be quasi-static and if you 
>> want to avoid the use of a calming function like you do.
> 
> I know, but then it takes too long to get specified porosity. So I am
> much faster with method 3 ...
Yes, I understand and agree. Actually I do not use the expansion method but I 
generate the particles in a very loose state and then I apply compression to 
achieve my target. I found this method much easier and not too slow even when 
using high stiffness parameters. 

> 
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Re: [Yade-users] [Question #210907]: on sample preparation of real granula materials

[Chiara Modenese]

Question #210907 on Yade changed:
https://answers.launchpad.net/yade/+question/210907

Chiara Modenese proposed the following answer:

On 11 Oct 2012, at 11:46, Chareyre wrote:

> - In 2, there is no need to "calm". If it goes very dynamic it's
because the parameters of the growth are not well choosen, else it's
almost quasistatic during all the growth.

@Christian: If you use very _high_ stiffness with the non-linear model
you need to be careful to choose the right parameters for the expansion
method as Bruno suggests. In these regards, remember that the _loading
rate_ you apply has to be sufficiently small if you want the process to
be quasi-static and if you want to avoid the use of a calming function
like you do.

Chiara

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Re: [Yade-users] [Question #210907]: on sample preparation of real granula materials

[Chiara Modenese]

Question #210907 on Yade changed:
https://answers.launchpad.net/yade/+question/210907

Chiara Modenese proposed the following answer:

On 11 Oct 2012, at 08:56, wangxiaoliang wrote:

> Actually, I am talking about the sample controlling parameters, somebody
> use porosity, somebody use relative density, to be equal to that in the
> real experiment. Which one is more appropriate?

Good question. In fact, it would be better to use the relative density
as a reference parameter. However, this is not straightforward to define
with spherical particles although some papers offer an equivalent
definition for the numerical model (like in the paper you are reading)
but it is questionable I think as in some cases calibration is more
difficult depending on the contact model adopted.

One could match the real porosity of the experiments if the same
particle shape and PSD are used in the numerical model.

Chiara

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Re: [Yade-users] [Question #208965]: topics on rolling resistance modeling granular media

[Chiara Modenese]

Question #208965 on Yade changed:
https://answers.launchpad.net/yade/+question/208965

Chiara Modenese proposed the following answer:
Hi Wang,
We cannot help you further unless you provide us with an example. We used the 
law you mention successfully here and we did not find any strange results.
Chiara

On 25 Sep 2012, at 02:41, wangxiaoliang wrote:

> Question #208965 on Yade changed:
> https://answers.launchpad.net/yade/+question/208965
> 
> wangxiaoliang gave more information on the question:
> Btw, I also used the 2nd way I mentioned, say
> Ip2_FrictMat_FrictMat_MindlinPhys() + Law2_ScGeom_MindlinPhys_Mindlin() to 
> simulate triaxial compression
> 
> luckily ,there exists rolling moment . Very Strangely, model with
> rolling moment results a much smaller than model without rolling moment?
> 
> Can anybody tell me why?
> 
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Re: [Yade-users] [Question #208965]: topics on rolling resistance modeling granular media

[Chiara Modenese]

Question #208965 on Yade changed:
https://answers.launchpad.net/yade/+question/208965

Chiara Modenese proposed the following answer:

On 20 Sep 2012, at 10:20, Chareyre wrote:

> Question #208965 on Yade changed:
> https://answers.launchpad.net/yade/+question/208965
> 
> Chareyre proposed the following answer:
> Wang (is that your name?),
> I don't remember this specific law you are mentionning. If we removed it, 
> then it was probably unmaintained and unsafe to use.
> There is no need for a "cohesionless" law. As you see in the example script 
> (2nd loading), Law2_ScGeom6D_CohFrictPhys_CohesionMoment can do that.
> 
> 
> Chiara,
> always_use_moment_law=1 means that bending moment is always present with or 
> without cohesion.
> setCohesionNow=1 means that all existing contacts will have cohesive/moment 
> properties initialized at the next step.
> Newly created will not be cohesive because setCohesionOnNewContacts=0 by 
> default.

Excellent, sorry I have never used this law and I did not properly check the 
details in code/documentation. Then I think that the script is fine as details 
are (or should be) already explained in documentation.
Chiara

> 
> Feel free to add comments (attributes doc could be better to...), else
> I'll do it eventually but not today.
> 
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Re: [Yade-users] [Question #208965]: topics on rolling resistance modeling granular media

[Chiara Modenese]

Question #208965 on Yade changed:
https://answers.launchpad.net/yade/+question/208965

Chiara Modenese proposed the following answer:
Hi Bruno,
Thank you for your new script. I have not tested it but I would add a comment 
to lines 83, 88 and 90 
(O.engines[2].lawDispatcher.functors[1].always_use_moment_law = True, 
O.engines[2].physDispatcher.functors[1].setCohesionNow = True). If I am not 
wrong, these instructions will add a moment/cohesion only to newly created 
contacts whereas existing interactions will not be affected by. It is a detail 
but it would be good to add a comment in the script about it (so that users do 
not forget it).
Chiara

On 19 Sep 2012, at 17:31, Chareyre wrote:

> Question #208965 on Yade changed:
> https://answers.launchpad.net/yade/+question/208965
> 
> Chareyre proposed the following answer:
> I just commited a new example script with cohesion and rolling friction.
> 
> See:
> https://github.com/yade/trunk/blob/dbfa0cd01b7634292762d3a2177025ff1fa2134b/examples/test/triax-cohesive.py
> 
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Re: [Yade-users] [Question #207881]: Unphysical behaviour, and Verlet collision detection

[Chiara Modenese]

Question #207881 on Yade changed:
https://answers.launchpad.net/yade/+question/207881

Chiara Modenese proposed the following answer:
Hi Giulia,

I doubt this behaviour has anything to do with verletDist. Assuming your
cylinder is working all right, the problem might be related to one or
more of the following aspects:

- the input parameters of your particles/cylinder (friction, for instance)
- the scale of your domain which might be inadequate
- the rate of loading of your cylinder which might be too high (I do not know 
how much you reduce it)

I would try to plot the stresses to help diagnose the problem.

Chiara
 

On 6 Sep 2012, at 19:55, Giulia Macaro wrote:

> New question #207881 on Yade:
> https://answers.launchpad.net/yade/+question/207881
> 
> Hi,
> 
> I am running a simulation in which a constant vertical velocity is applied to 
> a non-dynamic cylindrical body laying on a packing of spheres with periodic 
> boundaries.
> The velocity is applied through 
> TranslationEngine(velocity=-0.01,translationAxis=[0,0,1]).
> I have coded myself the cylindrical body I am working with, in a way similar 
> to the chained cylinder.  It is still experimental, since I am still checking 
> if it works correctly.
> 
> At some point the contact force on the cylinder becomes very high, and the 
> spheres in contact with it start moving extremely fast (almost as if the 
> packing is exploding). See the images:
> http://www.2shared.com/photo/e7PFqYLJ/00790.html
> http://www.2shared.com/photo/wY5rowcJ/00791.html
> http://www.2shared.com/photo/0N4N71xE/00792.html
> 
> 1.
> I am wondering if anybody has faced the same issue with similar simulations.
> I am not sure if this is a problem of the interaction sphere-cylinder, but it 
> seems working fine for part of the test, and then the packing starts 
> exploding.  I have tried to reduce the loading velocity and the time step, 
> but it still happens.
> Could it be related to the verletDist and targetInterv, maybe because the 
> particles are moving too fast and the interaction are not detected? I am 
> using the default values for now.
> 
> 2.
> Assuming that the problem might be the optimization with the verlet distance, 
> I have few question about it.
> I see that targetInterv is =50 by default, so the distance is adjusted, isn't 
> it?
> It isn't that clear the difference between the sweepLength, the verletDist 
> and the sweepDist.  I understood what is the verletDist for, but then the 
> other two variables seem very similar.
> And also, in which range should I vary the verletDist and targetInterv? Are 
> there any references about it?
> 
> Thanks for any suggestions
> Giulia
> 
> 
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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 22 Aug 2012, at 09:46, ceguo wrote:

> Question #206415 on Yade changed:
> https://answers.launchpad.net/yade/+question/206415
> 
>Status: Answered => Open
> 
> ceguo is still having a problem:
> Hi,
> 
> Thanks Jan and Chiara. To handle rigid clumps rotation we need to define
> the mass center of the clumps, right? and if we have the center and the
> contact point position, the branch vector is handily defined. So my
> question is how clumps rotate in YADE?
This was already discussed in:
https://answers.launchpad.net/yade/+question/199704
In the end I did not have time to implement it but it is quite straightforward. 
All you need to know is the positions of the clumps, which you can do in Yade 
(see Bruno's hint in the thread above).

Chiara



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Re: [Yade-users] [Question #206415]: getStress() applies for clumps?

[Chiara Modenese]

Question #206415 on Yade changed:
https://answers.launchpad.net/yade/+question/206415

Chiara Modenese proposed the following answer:

On 21 Aug 2012, at 14:30, ceguo wrote:

> New question #206415 on Yade:
> https://answers.launchpad.net/yade/+question/206415
> 
> Hi,
> 
> Kozicki, et al. (2012) (published in Granular Matter) shows marvelous 
> simulations using clumps in YADE. I am wondering if the current functions 
> getStress() and fabricTensor() etc. applicable for clumps or not? If 
> applicable, is the branch vector determined internally by YADE. And is there 
> a function to get the clumps orientation tensor (e.g. elongated particle)?
Both fabricTensor() and getStress() (as Jan said) functions do not currently 
support clumps. In Kozicki et al. walls are used at the boundaries thus I 
suppose stresses have been computed without the need to define the branch 
vector but let's wait for Janek's confirmation.

Chiara


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Re: [Yade-users] [Question #205137]: impact of sphere on a granular media

[Chiara Modenese]

Question #205137 on Yade changed:
https://answers.launchpad.net/yade/+question/205137

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 6 Aug 2012, at 15:41, mer samuel wrote:

> New question #205137 on Yade:
> https://answers.launchpad.net/yade/+question/205137
> 
> Hi,
Hi Samuel,
> 
> I study the impact of sphere on a granular medium. To understand how the 
> Elastic Contact law works I am currently simulating
> impacts involving only two spheres, one is set as the substrate which is 
> fixed and the other one is the incident sphere.
> During my simulation I observe that for Vin bigger than 2 m/s the energetic 
> restitution coefficient is always 1 whatever the incident angle is (from 5 to 
> 80 degrees with respect to the horizontal). 
Is the coefficient of restitution with respect to the tangential or normal 
direction you are referring to? Do you have any friction or damping at all? You 
need to present your problem with more details and tell us where you think the 
problem lies.

> I tried to look in the literature to get more detail about the contact law 
> but I didn't find anything to explain such a phenomena.
> If you have any hints it would be really helpful.
Look for the references of Colin Thornton (Powder Technology, 2011-2012), from 
my prospective he has very recently published some excellent work on the 
comparison of contact models after impact analysis.

Chiara

> 
> Samuel
> 
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Re: [Yade-users] [Question #204632]: tesselationWrapper

[Chiara Modenese]

Question #204632 on Yade changed:
https://answers.launchpad.net/yade/+question/204632

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 31 Jul 2012, at 06:16, tranviet wrote:

> New question #204632 on Yade:
> https://answers.launchpad.net/yade/+question/204632
> 
> Hello,
> 
> I can run the file tesselationWrapper.py within the newest YADE version, but 
> when I copy two files TesselationWrapper.cpp and TesselationWrapper.hpp from 
> the package to version YADE-0.70 and compile and then run the example, it 
> doesn't work:
> 
> File "./yade-unknown", line 172, in runScript
>execfile(script,globals())
>  File "tesselationWrapper.py", line 10, in 
>TW=TesselationWrapper()
> NameError: name 'TesselationWrapper' is not defined
If I am not wrong, this simply means that the function is not wrapped with 
Python in your version. It was a relatively recent introduction by Bruno and it 
is what you should check out/add to your code.

Chiara


> 
> It's weird for me since  tesselationWrapper has been available in YADE for a 
> long time. I am writing my codes in YADE-0.70 and I would like to use this 
> feature without moving to the newer version.
> 
> Can anyone explain it to me? I really appreciate it.
> 
> Viet Tran
> 
> 
> 
> 
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Re: [Yade-users] [Question #203709]: Which physical parameters i need and how to add them

[Chiara Modenese]

Question #203709 on Yade changed:
https://answers.launchpad.net/yade/+question/203709

Chiara Modenese proposed the following answer:
Hi Christian,

Thanks for details.

You can attribute whatever material you want to whatever body. For
instance:

# create two materials
O.materials.append(..., label='mat_spheres');
O.materials.append(..., label='mat_box');

# assign materials to different bodies
O.bodies.append(..., material='mat_spheres')
O.bodies.append(..., material='mat_box')

Please see: 
https://yade-dem.org/doc/yade.utils.html?highlight=utils.sphere#yade.utils.sphere
You do something similar in your code (but using the material instance instead 
of a label like I do), it should work too.

As for the choice of your input parameters, that is never easy to do. To
start with, you could take some initial values from the literature for a
similar material. Then you can carry out a parametric study to assess
the influence of stiffness on your test, for instance.

Good luck with your test,
Chiara


--
Chiara Modenese, BSc MSc 
DPhil(PhD) Candidate in Engineering Science
Department of Engineering Science, University of Oxford
Parks Road, Oxford, OX1 3PJ, UK
email: chiara.moden...@eng.ox.ac.uk


On 20 Jul 2012, at 13:35, Christian Sommerfeld wrote:

> Question #203709 on Yade changed:
> https://answers.launchpad.net/yade/+question/203709
> 
> Christian Sommerfeld posted a new comment:
> Hi Chiara,
> 
> for the spheres we want to use a ceramic material and for the quader/box
> we want to use steel as it's material.  First of all we want to simulate
> a elastic behaviour, therefore i chose FricMat as the best Material to
> handle this problem.  Furthermore we want to use create a simulation
> with the law of cundall and strack to compare it afterwards with other
> law, which are maybe more suitable.
> 
> It is not such a difficult test. The spheres and the box are inside a
> cylinder, whereby there are so many spheres that they   enclose the box.
> And the box executes a rotational movement around the z-axes and is also
> fixed. Now we want to measure the agent force on the box.
> 
> Our programm is still working, but i am not sure if i take the correct
> physical parameters for this law, because i nly found the calculation
> for kN and kS in the PhD of Smilauer but there is no reference to a law.
> 
> And i can not add different materials to the spheres and the box.
> 
> My programm is the following:
> 
> 
> # import yade modules that we will use below
> from yade import pack, plot
> 
> # PhysicalParameters
> 
> 
> FacetMat=O.materials.append(FrictMat(young=5e4,poisson=0.5,frictionAngle=.6,density=2400))
> 
> 
> 
> oriBody = Quaternion(Vector3(0,0,1),(math.pi))
> quader=O.bodies.append(geom.facetBox((.5,0,.5),(.3,.3,.3),oriBody, 
> wallMask=63, color=(0,0,1), dynamic=True, material=FacetMat))
> 
> # show all bodies of the facet
> for l in O.bodies:
>   g=l.id
>   print g
> 
> # create Cylinder from facets
> cyl=O.bodies.append(utils.geom.facetCylinder((0,0,1),1.5,2,orientation=Quaternion((1,
>  0, 0),0),segmentsNumber=20,wallMask=6, color=(0,1,0), fixed=True, 
> material=FacetMat))
> 
> 
> SphereMat=O.materials.append(FrictMat(young=5e7,poisson=0.5,frictionAngle=.6,density=1000))
> 
> # create block of spheres 
> sphereRadius = 0.06
> nbSpheres = (10,10,30)
> #nbSpheres = (50,50,50)
> for i in xrange(nbSpheres[0]):
>for j in xrange(nbSpheres[1]):
>for k in xrange(nbSpheres[2]):
>x = (i*2 - nbSpheres[0])*sphereRadius*1.1
>y = (j*2 - nbSpheres[1])*sphereRadius*1.1
>z = (k*2 - nbSpheres[2])*sphereRadius*1.1
>sp1=utils.sphere([x,y,z+10],sphereRadius,color=
>  (1,0,0),material=SphereMat)
>O.bodies.append(sp1) 
> 
> sphereRadius = 0.06
> nbSpheres = (10,10,30)
> #nbSpheres = (50,50,50)
> for i in xrange(nbSpheres[0]):
>for j in xrange(nbSpheres[1]):
>for k in xrange(nbSpheres[2]):
>x = (i*2 - nbSpheres[0])*sphereRadius*1.1
>y = (j*2 - nbSpheres[1])*sphereRadius*1.1
>z = (k*2 - nbSpheres[2])*sphereRadius*1.1
>
> sp2=utils.sphere([x,y,z+4],sphereRadius,color=(1,0,0),material=SphereMat)
>O.bodies.append(sp2)
> 
> 
> O.engines=[
>   ForceResetter(),
>   InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]),
>   InteractionLoop(
>  # handle sphere+sphere and facet+sphere collisions
>  [Ig2_Sphere_Sphere_Dem3DofGeom(),Ig2_Facet_Sphere_Dem3DofGeom()],
>  #[Ip2_FrictMat_FrictMat_FrictPhys()],
>  [Ip2_2xFrictMat_CSPhys()],
>  [Law2_Dem3Dof_CSPhys_CundallStrack()]
>  #[Law2_L3Geom_FrictPhys_ElPerfPl()]
>   ),

Re: [Yade-users] [Question #203709]: Which physical parameters i need and how to add them

[Chiara Modenese]

Question #203709 on Yade changed:
https://answers.launchpad.net/yade/+question/203709

Chiara Modenese proposed the following answer:
Sorry, I get now the type of question. Christian, follow Jerome's
suggestions, you should be fine then. Chiara

On 20 Jul 2012, at 13:11, jduriez wrote:

> Question #203709 on Yade changed:
> https://answers.launchpad.net/yade/+question/203709
> 
> jduriez proposed the following answer:
>> Is it correct to input young's modulus, poission's ratio and the
> friction angle for the law of cundall and strack and the density for the
> calculation of the gravity force?
> 
> Yes, not only correct, but also required. The law you plan to use :
> Law2_Dem3Dof_CSPhys_CundallStrack, requires interaction data of "CSPhys"
> type (see the name). Data that are computed "FrictMat" type (see the
> previous "engine" Ip2_2xFrictMat_CSPhys() ).
> 
> Hence you need to define all data for FrictMat, otherwise default values
> will be used.
> 
> Note that, for simulations with "Cundall-Strack" law, there is also
> "Law2_ScGeom_FrictPhys_CundallStrack", for which you should probably
> find several examples e.g. in the scripts.
> 
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Re: [Yade-users] [Question #203709]: Which physical parameters i need and how to add them

[Chiara Modenese]

Question #203709 on Yade changed:
https://answers.launchpad.net/yade/+question/203709

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Christian,

Could you please (briefly) explain here what material you aim to
simulate and what kind of test you want to run with it? It would help us
to address your problem.

Chiara


On 20 Jul 2012, at 11:51, Christian Sommerfeld wrote:

> New question #203709 on Yade:
> https://answers.launchpad.net/yade/+question/203709
> 
> Hi,
> 
> I want to create a simulation with the basic DEM model from Cundall and 
> Strack. But i don find any calculation for it. 
> Therefore i have the following question:
> Is it correct to input  young's modulus, poission's ratio and the friction 
> angle for the law of cundall and strack and the density for the calculation 
> of the gravity force?
> I have done it in this way: 
> O.materials.append(FrictMat(young=5e4,poisson=0.5,frictionAngle=.6,density=2400))
> 
> with the following engine:
> O.engines=[
>   ForceResetter(),
>   InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]),
>   InteractionLoop(
>  [Ig2_Sphere_Sphere_Dem3DofGeom(),Ig2_Facet_Sphere_Dem3DofGeom()],
>  [Ip2_2xFrictMat_CSPhys()],
>  [Law2_Dem3Dof_CSPhys_CundallStrack()]
>   ),
>   GravityEngine(gravity=(0,0,-9.81)),
>   
> RotationEngine(rotateAroundZero=True,zeroPoint=(0,0,0),rotationAxis=(0,0,1),angularVelocity=50*(2*pi/60),ids=quader,label='rotor'),
>   NewtonIntegrator(damping=0.4),
> PyRunner(iterPeriod=20,command="myAddPlotData()"),
>   
> VTKRecorder(iterPeriod=100,fileName='/tmp/QuaderAufnahme-',recorders=['spheres','facets'])
> ]
> 
> Furthermore i am still wondering why the simulation only apply the last 
> convention of my material, when i write it in this way:
> 
> FacetMat=O.materials.append(FrictMat(young=5e4,poisson=0.5,frictionAngle=.6,density=2400))
> SphereMat=O.materials.append(FrictMat(young=5e10,poisson=0.5,frictionAngle=.6,density=1))
> 
> oriBody = Quaternion(Vector3(0,0,1),(math.pi))
> quader=O.bodies.append(geom.facetBox((.5,0,.5),(.3,.3,.3),oriBody, 
> wallMask=63, color=(0,0,1), dynamic=True, material=FacetMat))
> 
> sphereRadius = 0.06
> nbSpheres = (10,10,30)
> for i in xrange(nbSpheres[0]):
>for j in xrange(nbSpheres[1]):
>for k in xrange(nbSpheres[2]):
>x = (i*2 - nbSpheres[0])*sphereRadius*1.1
>y = (j*2 - nbSpheres[1])*sphereRadius*1.1
>z = (k*2 - nbSpheres[2])*sphereRadius*1.1
>sp1=utils.sphere([x,y,z+10],sphereRadius,color= 
> (1,0,0),material=SphereMat)
>O.bodies.append(sp1)
> 
> 
> 
> 
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Re: [Yade-users] [Question #191395]: visualize utils.box with paraview

[Chiara Modenese]

Question #191395 on Yade changed:
https://answers.launchpad.net/yade/+question/191395

Chiara Modenese posted a new comment:
Is this feature easy to implement? I would need now as I cannot display
boxes with Paraview ATM.

Thanks for advice, Chiara

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[Yade-users] [Question #203339]: Visualize displacements

[Chiara Modenese]

New question #203339 on Yade:
https://answers.launchpad.net/yade/+question/203339

Hi,

Do you know how to compute/visualise displacement in Yade? I see there is an 
attribute called displ to b.state. Can that be used for such a purpose? I 
suppose that one has to update the refPos in order to do that. Do you confirm 
it?

Thank you,
Chiara

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Re: [Yade-users] [Question #201807]: Error running rotating-cilinder.py

[Chiara Modenese]

Question #201807 on Yade changed:
https://answers.launchpad.net/yade/+question/201807

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Diego,

The error you mention occurs simply because you are using an older
version of Yade where gravity was not yet an attribute of
NewtonIntegrator class (exactly what the message tells you). You can
either update to the latest version or use yade-daily from packages.

If, for any reason, you prefer to stick to the version you are currently
working on, you should read something like this in the list of engines:

...
GravityEngine(gravity=(0,-1e3,0)),
NewtonIntegrator(),
...

HTH,
Chiara


On 29 Jun 2012, at 16:41, Diego Fregolente wrote:

> New question #201807 on Yade:
> https://answers.launchpad.net/yade/+question/201807
> 
> Dear all
> 
> I am very sorry if I am going to make a foolish question but, I just
> started to use YADE and I am trying to run some of the examples.
> Unfortunately , when I tried to run the program rotating-cilinder.py I
> got the following error.
> 
> ~/yade/examples/baraban$ yade -x rotating-cylinder.py
> Welcome to Yade 0.60.3
> TCP python prompt on localhost:9000, auth cookie `saskye'
> XMLRPC info provider on http://localhost:21000
> 
> Running script rotating-cylinder.py
> Generating cloud…
> Traceback (most recent call last):
>   File "/usr/bin/yade", line 152, in runScript
> execfile(script,globals())
>   File "rotating-cylinder.py", line 49, in 
> 
> NewtonIntegrator(damping=.3,gravity=(0,0,-1e3)), # gravity
> artificially high, to make it faster going ;-)
> AttributeError: No such attribute: gravity.
> 
> Please could you tell me how can I fix it?
> 
> with very best regards
> Diego
> 
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Re: [Yade-users] [Question #201769]: Hertz mindlin, rolling back sphere

[Chiara Modenese]

Question #201769 on Yade changed:
https://answers.launchpad.net/yade/+question/201769

Chiara Modenese proposed the following answer:
Hi Anna,

Are you aware that you input in the model some rolling resistance (if
you try with the bool includeMoment=False you will see what I mean)? I
think that explains the behaviour. Can you please try to explain here
the purpose of your test? It may help us to give you a better answer.

Thank you,
Chiara

On 29 Jun 2012, at 07:01, anna effeindzourou wrote:

> New question #201769 on Yade:
> https://answers.launchpad.net/yade/+question/201769
> 
> Hi all,
> I am using the Hertz Mindlin contact law with a simple example a sphere 
> rolling on a inclined facet and then on a horizontal facet. I've observed 
> that the sphere is rolling up  and down.
> Here is a small script to help you understand the phenomenon.
> 
> 
> #!/usr/bin/python
> # -*- coding: utf-8 -*-
> from yade import utils
> 
> h=0.2
> 
> O.materials.append(FrictMat(young=1.7e8,poisson=0.20588,frictionAngle=radians(17),density=2650))
> s1=O.bodies.append( 
> utils.sphere([0.001,0.5,h+0.1],.1,color=[0,1,0],fixed=False,wire=True))
> 
> 
> v1=(0,0,h)
> v6=(0,1,h)
> v2=(.5,0,0)
> v3=(2,0,0)
> 
> v4=(2,1,0)
> v5=(0.5,1,0)
> 
> f1=O.bodies.append( utils.facet([v1,v2,v5], dynamic=None, fixed=True, 
> wire=True, color=None, highlight=False, noBound=False, mask=1))
> f2=O.bodies.append( utils.facet([v1,v5,v6], dynamic=None, fixed=True, 
> wire=True, color=None, highlight=False, noBound=False,  mask=1))  
> f3=O.bodies.append( utils.facet([v2,v3,v4], dynamic=None, fixed=True, 
> wire=True, color=None, highlight=False, noBound=False, mask=1))
> f4=O.bodies.append( utils.facet([v2,v4,v5], dynamic=None, fixed=True, 
> wire=True, color=None, highlight=False, noBound=False,  mask=1))
> 
> ## time step
> O.dt=1e-5
> 
> O.engines=[
>   ForceResetter(),
>   InsertionSortCollider([Bo1_Sphere_Aabb(),Bo1_Facet_Aabb()]), 
>   InteractionLoop(
>   [Ig2_Facet_Sphere_ScGeom(),Ig2_Sphere_Sphere_ScGeom()],
>   [
>   Ip2_FrictMat_FrictMat_MindlinPhys(en=0.1, krot=1,eta=1)
>   ],
>   [Law2_ScGeom_MindlinPhys_Mindlin(includeMoment=True),
>   ]),
>   NewtonIntegrator(damping=0,gravity=(0,0,-9.81)),
> ]
> 
> Any ideas?
> 
> cheers, 
> Anna
> 
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Re: [Yade-users] [Question #199704]: getStress for ScGeom

[Chiara Modenese]

Question #199704 on Yade changed:
https://answers.launchpad.net/yade/+question/199704

Chiara Modenese proposed the following answer:

On 24 Jun 2012, at 11:15, Chareyre wrote:

> Question #199704 on Yade changed:
> https://answers.launchpad.net/yade/+question/199704
> 
> Chareyre proposed the following answer:
> Sure there is not another difference elsewhere giving the same result in
> the end (e.g. sign of force)?
Yes, in fact there is no reversed sign inside the function getStress but 
instead there is in PeriIso*.cpp. The result should be the same in the end 
(Jan, have you done any test in these regards?). I gather it was decided not to 
keep reversing the sign of contact forces depending on the convention but I am 
not sure it is the best solution (I think it was a good reminder to not 
overlook that feature).  

Chiara


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Re: [Yade-users] [Question #199704]: getStress for ScGeom

[Chiara Modenese]

Question #199704 on Yade changed:
https://answers.launchpad.net/yade/+question/199704

Chiara Modenese proposed the following answer:

On 23 Jun 2012, at 20:01, Chareyre wrote:

> Question #199704 on Yade changed:
> https://answers.launchpad.net/yade/+question/199704
> 
> Chareyre proposed the following answer:
> Something like this in getStress:
> 
> if (b1->clumpMember && b2->clumpMember){
> l = O.bodies[b2->clumpId]->pos - O.bodies[b1->clumpId]->pos
> } else {
> //like now
> }
> 
> It needs also to adapt this to clump vs. standalone.
> 
> For the periodic case, it needs to be shifted, but the current line should 
> work without any change:
> if (isPeriodic) branch-= scene->cell->hSize*I->cellDist.cast();
Bruno, why is it -=? In PeriIso*.cpp it used to be +=. Am I missing something?
Thank you, Chiara

> 
> You can use triaxial test on clumps to validate (if boxes are used for
> boundaries, it gives an alternate calculation of stress). There is an
> example script from Janek IIRC.
> 
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Re: [Yade-users] [Question #199704]: getStress for ScGeom

[Chiara Modenese]

Question #199704 on Yade changed:
https://answers.launchpad.net/yade/+question/199704

Chiara Modenese proposed the following answer:

On 23 Jun 2012, at 20:01, Chareyre wrote:

> Question #199704 on Yade changed:
> https://answers.launchpad.net/yade/+question/199704
> 
> Chareyre proposed the following answer:
> Something like this in getStress:
> 
> if (b1->clumpMember && b2->clumpMember){
> l = O.bodies[b2->clumpId]->pos - O.bodies[b1->clumpId]->pos
> } else {
> //like now
> }
OK, thank you.
> 
> It needs also to adapt this to clump vs. standalone.
Yes, right.
> 
> For the periodic case, it needs to be shifted, but the current line should 
> work without any change:
> if (isPeriodic) branch-= scene->cell->hSize*I->cellDist.cast();
> 
> You can use triaxial test on clumps to validate (if boxes are used for
> boundaries, it gives an alternate calculation of stress). There is an
> example script from Janek IIRC.
Yes, I know which script. So will try a comparison and let you know.
Chiara
> 
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Re: [Yade-users] [Question #199704]: getStress for ScGeom

[Chiara Modenese]

Question #199704 on Yade changed:
https://answers.launchpad.net/yade/+question/199704

Chiara Modenese proposed the following answer:
Hi Bruno, 
I need this feature (actually thanks for pointing this out) so I would be happy 
to code it. How do you suggest it?
Thanks, Chiara

On 23 Jun 2012, at 18:11, Chareyre wrote:

> Question #199704 on Yade changed:
> https://answers.launchpad.net/yade/+question/199704
> 
> Chareyre proposed the following answer:
> If it works for clumps, it's really chance.
> I would guess it does not work, because for clumps the "l" branch should 
> correspond to clump-clump distance not member-member (and the latest is 
> probably what happens).
> It's probbaly not very difficult to handle the clump case, anyone want to try?
> 
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Re: [Yade-users] [Question #199704]: getStress for ScGeom

[Chiara Modenese]

Question #199704 on Yade changed:
https://answers.launchpad.net/yade/+question/199704

Chiara Modenese proposed the following answer:
OK, then you mean the tangential stiffnesses. I thought you referred to some 
equivalent constant value.
Thanks for reply,
Chiara

On 23 Jun 2012, at 14:45, ceguo wrote:

> Question #199704 on Yade changed:
> https://answers.launchpad.net/yade/+question/199704
> 
> ceguo posted a new comment:
> Hi Chiara,
> 
> Thanks for the reply. The equivalent stiffness is defined as K=dF/dU,
> where dF is change of contact force and dU is change of overlap or
> sliding displacement. So e.g. in Hertzian model, equivalent K_n, K_s are
> (from PFC mannual):
> 
> K_n = (2G sqrt(2R)/(3(1-nu)))sqrt(U_n);
> 
> K_s = (2(G^2*3*(1-nu)R)^1/3/(2-nu))*F_n^1/3.
> 
> So they are not constant but depend on the current overlap and sliding
> condition.
> 
> Ning
> 
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Re: [Yade-users] [Question #199704]: getStress for ScGeom

[Chiara Modenese]

Question #199704 on Yade changed:
https://answers.launchpad.net/yade/+question/199704

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 23 Jun 2012, at 10:26, ceguo wrote:

> Question #199704 on Yade changed:
> https://answers.launchpad.net/yade/+question/199704
> 
>Status: Answered => Open
> 
> ceguo is still having a problem:
> Hi Jan,
> 
> I noticed that you deprecated stressTensorOfPeriodicCell and suggested
> to use getStress instead. I think this function works for clumps also.
> Am I right?
> 
> To make general expression of elastic moduli, I think we can use
> equivalent kn and kt in nonlinear contact models (also change L*n_i and
> L*n_k to L_i and L_k for non-spherical particles).
Hi Ning,
How would you define the equivalent stiffnesses?
Thanks, Chiara

> 
> Ning
> 
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Re: [Yade-users] [Question #200537]: deleting a body

[Chiara Modenese]

Question #200537 on Yade changed:
https://answers.launchpad.net/yade/+question/200537

Chiara Modenese posted a new comment:

On 15 Jun 2012, at 14:36, jduriez wrote:

> Question #200537 on Yade changed:
> https://answers.launchpad.net/yade/+question/200537
> 
>Status: Open => Answered
> 
> jduriez proposed the following answer:
> Hello,
> 
> See O.bodies.erase(id of the body).
Fernando,
Thanks to various fixes by Anton, the line above should work fine (even with 
saving/reloading simulations). Make sure you use the latest Yade version (or 
Yade daily).
Chiara


> 
> It has been often discussed on the various lists, but I'm not aware of
> all details and never used it personly, so I can not confirm strictly
> that you will not have to perform another manual task so that things go
> well after (about refreshing the list of interactions for ex) => give a
> look (or wait for other answers)
> 
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Re: [Yade-users] [Question #197576]: Contact time duration

[Chiara Modenese]

Question #197576 on Yade changed:
https://answers.launchpad.net/yade/+question/197576

Status: Expired => Open

Chiara Modenese is still having a problem:
Hi Bruno,

As for this question, I was thinking to something like

if (uN<0) {
phys->contact_time=(scene->iter-contact->iterMadeReal)*scene->dt; // 
contact time duration
scene->interactions->requestErase(contact->getId1(),contact->getId2());
}

I tried but the problem is that at the next iteration the contact gets 
deleted and so I am not on time to record the value i.phys.contact_time.

Now, how do I output those numbers directly to an output file from Yade 
c++ code? Is there any example in Yade I can look at? It is also a 
general question which would be good to know.

Or I could create a list from a template where I keep the information of 
initial/final/total contact time for each contact. How to do that in 
Yade? Any example in the code?

Thanks for help!
Chiara


On 03/06/12 09:22, Launchpad Janitor wrote:
> Question #197576 on Yade changed:
> https://answers.launchpad.net/yade/+question/197576
>
>  Status: Open =>  Expired
>
> Launchpad Janitor expired the question:
> This question was expired because it remained in the 'Open' state
> without activity for the last 15 days.
>

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Re: [Yade-users] [Question #199585]: how can i simulate rolling friction?

[Chiara Modenese]

Question #199585 on Yade changed:
https://answers.launchpad.net/yade/+question/199585

Chiara Modenese proposed the following answer:

On 6 Jun 2012, at 15:10, Christian Jakob wrote:

> Question #199585 on Yade changed:
> https://answers.launchpad.net/yade/+question/199585
> 
> Christian Jakob posted a new comment:
> @chiara:
> 
> Thanks, will
> 
> ...
> #define material:
> id_SphereMat=O.materials.append(FrictMat(young=young_modulus,poisson=poisson_ratio,density=rho_p,frictionAngle=angle))
> ...
> InteractionLoop(
>  ...
>  [Ip2_FrictMat_FrictMat_MindlinCapillaryPhys(krot=roll_stiff)],
>  [Law2_ScGeom_MindlinPhys_Mindlin(includeMoment=True)]
>  ),
> 
> work in my case?
It should, indeed. Mind that you can also define eta parameter if you want to 
set a maximum plastic moment.
Chiara

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Re: [Yade-users] [Question #199585]: how can i simulate rolling friction?

[Chiara Modenese]

Question #199585 on Yade changed:
https://answers.launchpad.net/yade/+question/199585

Chiara Modenese proposed the following answer:
Hi Jacob,

As for point 1:
I introduced the rolling friction in HM Law some time ago. Please have a look 
at the documentation, in particular at:
https://yade-dem.org/doc/yade.wrapper.html?highlight=includemoment#yade.wrapper.Law2_ScGeom_MindlinPhys_Mindlin.includeMoment
https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.Ip2_FrictMat_FrictMat_MindlinPhys.eta
https://yade-dem.org/doc/yade.wrapper.html?highlight=krot#yade.wrapper.Ip2_FrictMat_FrictMat_MindlinCapillaryPhys.krot

@Luc: It is not about combining two contact laws but everything can be
dealt with inside one contact model (resistance to rolling is a separate
additional feature, in fact).

As for point 2:
It is entirely up to the calibration procedure.

Chiara


-- 
Chiara Modenese, BSc MSc
DPhil(PhD) Candidate in Engineering Science
Department of Engineering Science
Parks Road, Oxford, OX1 3PJ, UK
email: chiara.moden...@eng.ox.ac.uk
website: http://www.eng.ox.ac.uk/derg/people/chiara-modenese-1



On 6 Jun 2012, at 14:11, Christian Jakob wrote:

> New question #199585 on Yade:
> https://answers.launchpad.net/yade/+question/199585
> 
> Hi,
> 
> I want to model a sand with Hertz-Mindlin-Contact law and rolling friction.
> 
> 1. Is it possible to do so in yade and if yes, how?
> 
> I tried to combine a material with rolling stiffness and hertz model:
> 
> ...
> #define material:
> id_SphereMat=O.materials.append(CohFrictMat(young=young_modulus,poisson=poisson_ratio,density=rho_p,frictionAngle=angle,alphaKr=roll_stiff))
> ...
>   InteractionLoop(
>   ...
>   [Ip2_FrictMat_FrictMat_MindlinCapillaryPhys()],
>   [Law2_ScGeom_MindlinPhys_Mindlin()]
>   ),
> ...
> 
> but this seems to have no effect on behavior of the particles.
> 
> 2. What would be a good value for lets say a well rounded sand? Has anyone 
> experience with rolling stiffness?
> 
> Thanks in advance,
> 
> Christian
> 
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Re: [Yade-users] [Question #199540]: Who is usung these contact laws?

[Chiara Modenese]

Question #199540 on Yade changed:
https://answers.launchpad.net/yade/+question/199540

Chiara Modenese proposed the following answer:

On 6 Jun 2012, at 13:55, Chareyre wrote:

> Question #199540 on Yade changed:
> https://answers.launchpad.net/yade/+question/199540
> 
> Chareyre posted a new comment:
>> Maybe we could keep it for future reference in case that kind of use of
> quaternions became useful.
> 
> It's in svn/bzr/git history anyway. Nothing will be lost.
Great. In which case, I suggest to remove it so to avoid confusion and/or 
malfunctioning due to poor testing.
Chiara


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Re: [Yade-users] [Question #199540]: Who is usung these contact laws?

[Chiara Modenese]

Question #199540 on Yade changed:
https://answers.launchpad.net/yade/+question/199540

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 6 Jun 2012, at 11:41, Chareyre wrote:

> Question #199540 on Yade changed:
> https://answers.launchpad.net/yade/+question/199540
> 
>Status: Answered => Open
> 
> Chareyre is still having a problem:
>> This "irreversible" behaviour happens in all the Yade Laws where the
> rolling is implemented so it is not specific to
> Law2_SCG_MomentPhys_CohesionlessMomentRotation.
> 
> I was not sure of that.
OK.


> 
>> The details of the implementation, however, are different (use of
> quaternions versus incremental implementation). Do you think that this
> could be a good reason for keeping it?
> 
> Maybe not, if the difference is only in implementation.

I believe that is the only difference but I have not tested much because
I started to use the incremental formulation (which I did test but
versus other implementations like Plassiard's formulation - still
incremental). Maybe we could keep it for future reference in case that
kind of use of quaternions became useful.


> Boon is not
> using SMR?
I think he has used it a long time ago for some background analyses but nothing 
more than that (not sure if he still reads the mailing list so I will have to 
ask him about it and then let you know).

Chiara


> 
> Bruno
> 
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Re: [Yade-users] [Question #199540]: Who is usung these contact laws?

[Chiara Modenese]

Question #199540 on Yade changed:
https://answers.launchpad.net/yade/+question/199540

Chiara Modenese proposed the following answer:

On 6 Jun 2012, at 10:40, Chareyre wrote:

> Question #199540 on Yade changed:
> https://answers.launchpad.net/yade/+question/199540
> 
> Chareyre posted a new comment:
> Chiara: I was remembering the discussion on elastic unloading for rolling 
> (what I called "irreversible") but I didn't know in which law it had been 
> implemented. You confirm that it is in 
> Law2_SCG_MomentPhys_CohesionlessMomentRotation. In that case the law has 
> something specific and shouldn't be removed, I think.

OK. Yes, there was a discussion but I cannot find the link to it right
now. This "irreversible" behaviour happens in all the Yade Laws where
the rolling is implemented so it is not specific to
Law2_SCG_MomentPhys_CohesionlessMomentRotation. The details of the
implementation, however, are different (use of quaternions versus
incremental implementation). Do you think that this could be a good
reason for keeping it?

> Concerning twist_creep, it is relatively well tested (by us). It has
similarities and served as an example for rolling IIRC, but it is very
different, it's the normal component of torque and it defines a viscous
creep.

Yes, I understood it is different but with a similar form.

Cheers,
Chiara

> 
> 
> Anton: Ok.
> 
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Re: [Yade-users] [Question #199540]: Who is usung these contact laws?

[Chiara Modenese]

Question #199540 on Yade changed:
https://answers.launchpad.net/yade/+question/199540

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi Bruno,

If I am not wrong:

Law2_SCG_MomentPhys_CohesionlessMomentRotation is a duplicate of 
Law2_ScGeom6D_CohFrictPhys_CohesionMoment (but with less features than the 
latter)
Law2_SCG_MomentPhys_CohesionlessMomentRotation is the only Law that implements 
the rolling resistance using quaternions (actually there is something similar 
about twist_creep in Law2_ScGeom6D_CohFrictPhys_CohesionMoment) which I would 
not recommend as I am not sure it always gives the expected result (has anybody 
tested it properly?)
In principle the rolling resistance implemented in 
Law2_SCG_MomentPhys_CohesionlessMomentRotation does not seem to be doing 
anything different than what is already implemented in 
Law2_ScGeom6D_CohFrictPhys_CohesionMoment which, however, uses an incremental 
fashion (a copy of what I introduced in the Law2_ScGeom_MindlinPhys_Mindlin 
Law) that seems to work fine from what I gathered
Why do you say it implements irreversible rolling friction?

Finally, I would remove it if nobody is using it.

Thanks!
Chiara


On 6 Jun 2012, at 09:01, Chareyre wrote:

> New question #199540 on Yade:
> https://answers.launchpad.net/yade/+question/199540
> 
> Hi all,
> 
> I'd like to remove some of the contact laws which are in Yade. Some of them 
> are unmaintained and/or broken and/or duplicates, and too many contact laws 
> is only making the choice uneasy.
> If you are using some of the contact laws listed below, please reply to this 
> message, else it will be removed eventually.
> 
> Law2_Dem3Dof_CSPhys_CundallStrack
> Law2_Dem3DofGeom_FrictPhys_CundallStrack
> Law2_SCG_MomentPhys_CohesionlessMomentRotation
> Law2_Dem3DofGeom_RockPMPhys_Rpm
> Law2_L3Geom_FrictPhys_ElPerfPl
> Law2_L6Geom_FrictPhys_Linear
> 
> More details at https://blueprints.launchpad.net/yade/+spec/cleancontactlaws
> 
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Re: [Yade-users] [Question #198989]: Symmetric boundary

[Chiara Modenese]

Question #198989 on Yade changed:
https://answers.launchpad.net/yade/+question/198989

Status: Answered => Open

Chiara Modenese is still having a problem:

On 31 May 2012, at 21:10, Chareyre wrote:

> Question #198989 on Yade changed:
> https://answers.launchpad.net/yade/+question/198989
> 
>Status: Open => Answered
> 
> Chareyre proposed the following answer:
> I'm thinking to a plane too. What does "sufficient" mean?
OK, thanks. I was just checking with you that that was appropriate. I will try 
it and let you know if I discover problems.
Cheers, 
Chiara
> 
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Re: [Yade-users] [Question #198989]: Symmetric boundary

[Chiara Modenese]

Question #198989 on Yade changed:
https://answers.launchpad.net/yade/+question/198989

Status: Answered => Open

Chiara Modenese is still having a problem:
On 31/05/12 17:15, Sergei Dorofeenko wrote:
> Your question #198989 on Yade changed:
> https://answers.launchpad.net/yade/+question/198989
>
>  Status: Open =>  Answered
>
> Sergei Dorofeenko proposed the following answer:
> For example, setup plane with zero friction.
>
I had a thought about that. Is that sufficient you think? Have you 
already tried?
Thanks! Chiara

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[Yade-users] [Question #198989]: Symmetric boundary

[Chiara Modenese]

New question #198989 on Yade:
https://answers.launchpad.net/yade/+question/198989

Hi there,

Do you know how to model a symmetric boundary in Yade?

Thanks for ideas,
Chiara

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Re: [Yade-users] [Question #198183]: clump packing in 2D

[Chiara Modenese]

Question #198183 on Yade changed:
https://answers.launchpad.net/yade/+question/198183

Status: Open => Answered

Chiara Modenese proposed the following answer:
On 23/05/12 13:06, ceguo wrote:
> Question #198183 on Yade changed:
> https://answers.launchpad.net/yade/+question/198183
>
>  Status: Answered =>  Open
>
> ceguo is still having a problem:
> Thanks Jerome and Luc,
>
> Actually I did some 2d simulations with yade using discs. We had some
> discussions before (e.g. #174453). The solution is some modification in
> makeCloud which tolerates zero size in one dimension. And I blocked DOFs
> as 'zXY'. For the internal calculation, as I set the third dimension of
> O.cell.hSize equal to 1, the volume is equal to area. So the stress calc
> is correct. Particle masses are also set manually.
>
> Now my problem is I want to extend it to clumps. The function
> makeClumpCloud cannot work correctly with zero size in one dimension. I
> guess it can be solved similarly to that in makeCloud but I'm not
> familiar with the code and the algorithm. So I hope anyone can help me.
> More ideally, I wish I can save the clump packing and use ymport to
> import it for later use.
>
> Ning
Hi Ning,

I think you need to play with the axis of orientation of your clumps and 
modify the function such that clumps are generated on a plane. It should 
be easy to fix but you do need to look into the c++ code of 
makeClumpCloud. IICR, this function generates clumps with random 
orientation. You need to make sure that that axis is contained on the 
plane. Also, make sure your clumps "develop" in 2D.

Chiara

>

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Re: [Yade-users] [Question #197438]: No interactions after erasing body

[Chiara Modenese]

Question #197438 on Yade changed:
https://answers.launchpad.net/yade/+question/197438

Chiara Modenese proposed the following answer:
Sounds good. Thanks!


On 18 May 2012, at 18:15, Chareyre  wrote:

> Question #197438 on Yade changed:
> https://answers.launchpad.net/yade/+question/197438
> 
> Chareyre posted a new comment:
> If interactions->dirty, then the collider runs at next iteration (including 
> bounds dispatching).
> Then the new bound will be defined immediatly (I guess it was defined only 
> after N iterations before the fix).
> Do you confirm Anton?
> 
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Re: [Yade-users] [Question #197438]: No interactions after erasing body

[Chiara Modenese]

Question #197438 on Yade changed:
https://answers.launchpad.net/yade/+question/197438

Chiara Modenese proposed the following answer:
Hi, 
It is not very clear to me how this was fixed. Could you please say something 
more about it, Anton (or Bruno)?
Thank you very much,
Chiara


On 18 May 2012, at 14:25, Chareyre wrote:

> Question #197438 on Yade changed:
> https://answers.launchpad.net/yade/+question/197438
> 
> Chareyre proposed the following answer:
> This is the new record for fixing a bug. ;)
> 
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Re: [Yade-users] [Question #197576]: Contact time duration

[Chiara Modenese]

Question #197576 on Yade changed:
https://answers.launchpad.net/yade/+question/197576

Status: Answered => Open

Chiara Modenese is still having a problem:


On 18 May 2012, at 14:15, Chareyre wrote:

> Your question #197576 on Yade changed:
> https://answers.launchpad.net/yade/+question/197576
> 
>Status: Open => Answered
> 
> Chareyre proposed the following answer:
> Interaction::iterMadeReal will probably help.
Right, I think that can help to track the moment at which the interaction is 
created.

> Tracking death of the contact is more difficult. You may have to hack
> the collider to make it dump deletion data, or write a relatively
> complex python script that compares lists of interactions to see if some
> of them disappear...
Maybe it could be easily done inside the contact law? In fact, it might be hard 
to do that in python now that I think more about it.

> 
> If you know the current number of contact and their lifetimes with
> iterMadeReal, isn't there a logical way to deduce the statistics of
> death-times?...


> 
> Bruno
> 
> -- 
> If this answers your question, please go to the following page to let us
> know that it is solved:
> https://answers.launchpad.net/yade/+question/197576/+confirm?answer_id=0
> 
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/yade/+question/197576
> 
> You received this question notification because you asked the question.

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[Yade-users] [Question #197576]: Contact time duration

[Chiara Modenese]

New question #197576 on Yade:
https://answers.launchpad.net/yade/+question/197576

Hi there,

I want to compute an average contact time duration. I would like some 
suggestions from you on which is the best way of doing it in Yade. Basically I 
need to know when a contact forms (T_initial) and when it is erased (T_final). 
Any hints?

Thank you,
Chiara

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Re: [Yade-users] the results are different between parallel and non parallel running.

[Chiara Modenese]

Hi Liqing,

I think you should specify how big is the difference. If the difference 
is negligible, that is an expected result as explained in Yade 
documentation and as Bruno points out. If the difference is significant, 
that is most likely a bug in the code. I guess you are in the former case?


Chiara


On 10/05/2012 10:08, Liqing Jiao wrote:

Dear all,

When the same script runs on the same version of yade, but one is 
single core, another one is multi-cores, the results are different. I 
use yade trunk version, which is recommended by Anton. It's good. But 
when I run the same script, the result is different. You can try the 
concrete uniax.py, which is in examples folder, you will see the 
different lines printed out on the screen. And I tried my model both 
using parallel and non parallel running, the results are also 
different. Thank you for your attention. Could you please reply me if 
you know how to fix it? Thank you in advance.


Cheers, Liqing




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Re: [Yade-users] Enquiry

[Chiara Modenese]

Hi Francois,

first you should add the label parameter to your engine so that you can easily 
call it at need. Something like: 
O.engines=[...,
NewtonIntegrator(damping=0.7,label='Integrator')]
Then:
Integrator.damping=0.2 # to change the value of the damping parameter

By subtract I suppose you mean to get rid of that engine. You can do something 
like:
Integrator.dead=True # it will deactivate the engine next time step

Chiara


On 2 May 2012, at 15:37, Rioual François wrote:

>Hello,
> 
> I have two questions regarding some modifications in engines:
> 
> - I would like to modify afterwards the value of the damping
> parameter in the NewtonIntegrator: how can I write it ?
> 
> - I would like to substract an engine with PyRunner: how can I
>  do it ?
> 
>  Thank's for your help,
> 
> 
>   Francois
> 
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Re: [Yade-users] [Question #194761]: is particle distribution true or close to real sand simulated by yade

[Chiara Modenese]

Question #194761 on Yade changed:
https://answers.launchpad.net/yade/+question/194761

Chiara Modenese proposed the following answer:
Hi Wangxiaoliang,
The closer you get to the physics of the real material with your 
numerical model, the easier the calibration should become, up to the 
point where no calibration at all is needed (currently not possible due 
to the limit on the computational time, as Jerome said). The fact that 
you need to calibrate your model against experimental results does not 
mean that the model is wrong. With finite elements, you still need to 
calibrate the parameters of your mathematical constitutive model, for 
instance.

Suggestion:
Before deciding on the assumptions you want to make, you need to clarify 
what is the goal of your model and how you are going to do it. One 
example aong others: it is a bad idea to scale up the size of your 
particles if you are going to include attractive forces which scale with 
the particle size (the scaling would be incorrect then and the results 
difficult to interpret). Otherwise it should be safe doing it.

Chiara


On 26/04/2012 09:35, jduriez wrote:
> Question #194761 on Yade changed:
> https://answers.launchpad.net/yade/+question/194761
>
>  Status: Open =>  Answered
>
> jduriez proposed the following answer:
> Briefly : for practical reasons computer simulations can not reproduce
> the reality as it is, as you understood it. However, if you agree that
> "good calibration over the dem model with experimental models" can be
> reached, you should not see any problem in the method... (in the sense
> that the results it gives are ok, even if it is "simple")
>
> So welcome in, and enjoy, the numerical world ! :-)
>
> There might be something in common with the real one...
>

-- 
Chiara Modenese BSc MSc(Eng)
DPhil(PhD) Candidate in Engineering Science
Department of Engineering Science
University of Oxford
Parks Road, Oxford, OX1 3PJ, UK

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Re: [Yade-users] [Question #191800]: convergence very very slow

[Chiara Modenese]

Question #191800 on Yade changed:
https://answers.launchpad.net/yade/+question/191800

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 28/03/2012 04:05, wangxiaoliang wrote:
> Question #191800 on Yade changed:
> https://answers.launchpad.net/yade/+question/191800
>
>  Status: Answered =>  Open
>
> wangxiaoliang is still having a problem:
> thanks all. First of all, wangxiaoliang is my Chinese name (Chinese
> style name) In English version( Xiaoliang Wang)
>
> as the number of particles, now I just have about 1100 in my triaxial
> test engine.
>
> Increasing mass density is useless to my ques.
It is not useless if your question was how to speed up the calculation 
(obviously you need to test it). This is not only my opinion, but a 
method that is quite often applied in the literature (e.g. Thornton, 2000).

Chiara
> that you said decreasing the stiffness maybe a way.
>
> I will have a test first
>

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Re: [Yade-users] [Question #191800]: convergence very very slow

[Chiara Modenese]

Question #191800 on Yade changed:
https://answers.launchpad.net/yade/+question/191800

Chiara Modenese proposed the following answer:
On 27/03/12 13:35, Chareyre wrote:
> Question #191800 on Yade changed:
> https://answers.launchpad.net/yade/+question/191800
>
> Chareyre proposed the following answer:
> Of course, stiffness is a physical parameter and changing it may affect
> (or not...) the results. Ultimately the number of particle is the only
> parameter you can play with.
I agree. I would also suggest to wangxiaoliang to always check that the 
results are not affected by the change in stiffness or any other 
physical parameters.
> However, if wangxiaoliang is targeting a final strain of 0.3, he is probably 
> not focusing on the elastic regime.
> If the confining pressure is small compared to the stiffness of silicates 
> (10e11), stiffness can most likely be decreased by a few orders of magnitude 
> without any change in peak strength, residual strength, dilatancy, etc. It's 
> all a matter of modelling methodology and wise compromize. The best model is 
> useless if it can't be computed...
Correct... But then wangxiaoliang (btw, is this your name?) should 
reconsider the use of Hertzian law. The sensitivity with this law is 
high to the stiffness level, in terms of strains (small and large), 
dilatancy, peak strength etc. If you are interested, there is some nice 
published work by C. Thornton where the behaviour of soft and hard 
spheres (Hertzian type) is compared and discussed (700MPa versus 70GPa 
of Young's modulus).

Cheers,
Chiara

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Re: [Yade-users] [Question #191800]: convergence very very slow

[Chiara Modenese]

Question #191800 on Yade changed:
https://answers.launchpad.net/yade/+question/191800

Chiara Modenese proposed the following answer:

On 27 Mar 2012, at 12:05, Chareyre wrote:

> Question #191800 on Yade changed:
> https://answers.launchpad.net/yade/+question/191800
> 
> Chareyre proposed the following answer:
>> a high rigid modulus (usually quartz GPa), induce a high P wave speed,
> and a smaller size makes dt = r/ vp even much more smaller than the
> usual sample in the tutorial
> 
> This statement is correct.
> The thing that puzzles me in your question is you don't mention the number of 
> particles, which is for sure the first parameter to consider.
> 
>> So I do not know how to deal with this ques.
> 
> Use a smaller number of particles?
I agree that this could be one possible solution.
> 
> The number of particles aside, changing mass and/or strain rate does not help 
> in general (unless strain rate is excessively small initially, of course), 
> because higher mass needs slower strain if you don't want to get inertial 
> effects in your results (similarly, faster strain needs smaller mass).
> The only way to speed things up is to decrease stiffness.
I have to disagree with this last statement. Decreasing the inter-particle 
stiffness (the Young's modulus, in case of Hertzian contacts) won't solve your 
problem because your results are likely going to be affected by the reduced 
stiffness. In other words, you would be simulating a different material. This 
is the case for shear type tests which I suppose are what you are after? In 
other scenarios, I reckon that the stiffness is less important (like in flow 
kinematics).

Chiara

> 
> "Then how can I simulate 10e10 particles with stiffness 10e12", you may
> ask. You can't, or you have to be very patient.
> 
> Also note that GlobalStiffnessTimeStepper will help you to find the
> largest possible timestep, while PWave only gives a rough estimate.
> 
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Re: [Yade-users] [Question #191800]: convergence very very slow

[Chiara Modenese]

Question #191800 on Yade changed:
https://answers.launchpad.net/yade/+question/191800

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi,

I hope to interpret correctly what you are saying. Your time step is too
small and thus your test runs very slowly. It is a well known issue and
you can workaround it by increasing the mass of your particles, for
instance.

HTH,
Chiara 


On 27 Mar 2012, at 03:55, wangxiaoliang wrote:

> New question #191800 on Yade:
> https://answers.launchpad.net/yade/+question/191800
> 
> Hi all
> 
> In example , the box is usually 10*10*10 or 1*1*1, and particles size is 
> usually a bit larger than the size of real sand.
> 
> However, when I tried to model grenoble sand (d50 = 0.33mm), I use two 
> contact model
> 
> 1, hertz model
> 
> 2, Moment material model
> 
> in both model, after the isotropic compression, in the deviatoric compression 
> process, usually the strain response is 
> 
> very very small. 1000 step just make the strain in y direction go forward 
> about 1e-6 or 1e-7.
> 
> So it needs maybe month or year to  achieve a final strain in y of 0.3, say.
> 
> In my opinion, a high rigid modulus (usually quartz GPa), induce a  high P 
> wave speed, and a smaller size makes dt = r/ vp even
> 
> much more smaller than the usual sample in the tutorial(usually mod. = 
> 1e5~6Pa, and particle size .5m)
> 
> So I do not know how to deal with this ques.
> 
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Re: [Yade-users] [Question #191489]: How to translate a periodic cell ?

[Chiara Modenese]

Question #191489 on Yade changed:
https://answers.launchpad.net/yade/+question/191489

Chiara Modenese proposed the following answer:
On 23/03/12 12:50, Kneib François wrote:
> Question #191489 on Yade changed:
> https://answers.launchpad.net/yade/+question/191489
>
> Kneib François posted a new comment:
> Reasons why I want to do that are to long to explain here.
I do not see any particular reason for that.
> By moving the cell's origin I would save lot of time. According to your 
> answer I supposed that it's not possible so I found an other (more 
> complicated) solution.
It is not possible, I think.

Chiara
> François
>


-- 
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DPhil Candidate in Engineering Science
Department of Engineering Science, Parks Road, Oxford, OX1 3PJ, UK
Mobile: +44 7760915539
Email: chiara.moden...@eng.ox.ac.uk

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Re: [Yade-users] [Question #191412]: import module problem

[Chiara Modenese]

Question #191412 on Yade changed:
https://answers.launchpad.net/yade/+question/191412

Chiara Modenese proposed the following answer:
On 23/03/12 13:30, Rinaldi Giuseppe wrote:
> Question #191412 on Yade changed:
> https://answers.launchpad.net/yade/+question/191412
>
> Rinaldi Giuseppe posted a new comment:
> I'm trying to import simulation I saved in daily-yade ("Yade 
> 1+2978+27~oneiric1")  and I'm trying to run it on daily-yade ("Yade 
> 1+3053+27~lucid1").
> they have different version of Ubuntu (11.10 in the old computer and 10.04.3 
> in the new computer)
>
> But I installed daily-yade on both the computers.
See my reply. If the xml format is still not supported, you will have to 
run the test once again.

Cheers.
Chiara
>
> Giuseppe
>

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Re: [Yade-users] [Question #191412]: import module problem

[Chiara Modenese]

Question #191412 on Yade changed:
https://answers.launchpad.net/yade/+question/191412

Chiara Modenese proposed the following answer:
On 23/03/12 13:05, Anton Gladky wrote:
> Question #191412 on Yade changed:
> https://answers.launchpad.net/yade/+question/191412
>
> Anton Gladky proposed the following answer:
> You are trying to import the simulation, that you saved in 0.50 version.
> Right? So in this case you should really use 0.50
>
> Anton
>
On different machines, there can be conflicts even while using the same 
version (I have some experience with that).

1. As Anton suggests, you should install the same version you used to 
create your simulation.
2. You should also make sure that the simulation is saved as 
"bla.xml.bz2" other than "bla.yade.gz" as this could avoid the error you 
get now (just reload the test on your original machine and save it in 
xml format).
3. Finally, if this still does not work, you should simply run the test 
again on the new machine.

Chiara

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Re: [Yade-users] [Question #191489]: How to translate a periodic cell ?

[Chiara Modenese]

Question #191489 on Yade changed:
https://answers.launchpad.net/yade/+question/191489

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 23/03/2012 10:10, Kneib François wrote:
> New question #191489 on Yade:
> https://answers.launchpad.net/yade/+question/191489
>
> Hi all,
>
> I'm trying to translate a periodic cell, but not successfully.
Hi François,
what are you trying to achieve by doing that? I mean, is there any 
advantage if you do that? Whether the origin of your cell is at (0,0,0) 
or any other point, should only be a matter of convention.

Chiara

>   The only cell I'm able to do takes its origin at (0,0,0).
> Is there a way to do that ?
>
> Thanks
>

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Re: [Yade-users] [Question #190529]: deleting some liquid bridges in Law2_ScGeom_CapillaryPhys_Capillarity

[Chiara Modenese]

Question #190529 on Yade changed:
https://answers.launchpad.net/yade/+question/190529

Status: Open => Answered

Chiara Modenese proposed the following answer:

On 20/03/2012 17:30, Christian Jakob wrote:
> Question #190529 on Yade changed:
> https://answers.launchpad.net/yade/+question/190529
>
>  Status: Answered =>  Open
>
> Christian Jakob is still having a problem:
> I implemented this isBroken flag in Law2...Capillarity and in the header 
> files of CapillaryPhys.hpp and Ip2_FrictMat_FrictMat_MindlinCapillaryPhys.hpp
> But deleting single liquid bridges is still not working, i tried:
>
> for i in O.interactions:
>z_int=i.geom.contactPoint[2]   #z coordinate of interaction
>...
>if ((z_int<  water_height) and (i.isReal)):
The problem is most likely with the condition you set: (z_int < 
water_height)
Does it work if you only said i.isReal?

Chiara
>  i.isBroken=True  #delete liquid bridges
>
> if i look into simulation inspection isBroken flag is false. What i can
> do now is deleting all bridges with
>
> Ip2_FrictMat_FrictMat_MindlinCapillaryPhys.isBroken=True
>
> but how can i delete single bridges from O.interactions ?
>

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Re: [Yade-users] [Question #191084]: clumps - particle size distribution

[Chiara Modenese]

Question #191084 on Yade changed:
https://answers.launchpad.net/yade/+question/191084

Chiara Modenese gave more information on the question:
On 19/03/12 12:45, Chiara Modenese wrote:
> Question #191084 on Yade changed:
> https://answers.launchpad.net/yade/+question/191084
>
> Chiara Modenese gave more information on the question:
> On 19/03/12 12:11, Christian Jakob wrote:
> Thank you, Christian, it makes sense to me. I will try to do it in python.
> Cheers, Chiara
>> Question #191084 on Yade changed:
>> https://answers.launchpad.net/yade/+question/191084
>>
>> Christian Jakob posted a new comment:
>> Hi Chiara,
>>
>> I am also interested in how to do this in Yade.
>> I have experience how this can be done with PFC. The procedure is as follows:
>>
>> 1. Generate your packing with spheres.
>> 2. Create templates of your clumps.
>> 3. Replace spheres with clump templates (clumps should have same real volume 
>> as your spheres).
As to point 3., do you also account for the overlapping volume between 
the spheres somehow? Or do you neglect it?
>> for example I send you the fish-code of my model:
>>
>> ;make a template of a clump with 2 overlapping spheres:
>> clump template make dyad 2 radii 0.5 0.5 pos (0.25, 0.5, 0.25) (-0.25, 
>> -0.25, -0.25)
>>
>>
>> ;replace 5 percent of the spheres by clumps with my template called dyad:
>> clump replace 1 dyad 0.05
>>
>> Regards,
>>
>> Christian
>>

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Re: [Yade-users] [Question #191084]: clumps - particle size distribution

[Chiara Modenese]

Question #191084 on Yade changed:
https://answers.launchpad.net/yade/+question/191084

Chiara Modenese gave more information on the question:
On 19/03/12 12:11, Christian Jakob wrote:
Thank you, Christian, it makes sense to me. I will try to do it in python.
Cheers, Chiara
> Question #191084 on Yade changed:
> https://answers.launchpad.net/yade/+question/191084
>
> Christian Jakob posted a new comment:
> Hi Chiara,
>
> I am also interested in how to do this in Yade.
> I have experience how this can be done with PFC. The procedure is as follows:
>
> 1. Generate your packing with spheres.
> 2. Create templates of your clumps.
> 3. Replace spheres with clump templates (clumps should have same real volume 
> as your spheres).
>
> for example I send you the fish-code of my model:
>
> ;make a template of a clump with 2 overlapping spheres:
> clump template make dyad 2 radii 0.5 0.5 pos (0.25, 0.5, 0.25) (-0.25, -0.25, 
> -0.25)
>
>
> ;replace 5 percent of the spheres by clumps with my template called dyad:
> clump replace 1 dyad 0.05
>
> Regards,
>
> Christian
>

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[Yade-users] [Question #191084]: clumps - particle size distribution

[Chiara Modenese]

New question #191084 on Yade:
https://answers.launchpad.net/yade/+question/191084

Hi guys,
do you think is possible to follow a specific particle size distribution while 
using clumps? Does anybody have experience with that and maybe can suggest a 
simple method how to do it?

Thank you for suggestions,
Chiara

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Re: [Yade-users] [Question #190529]: deleting some liquid bridges in Law2_ScGeom_CapillaryPhys_Capillarity

[Chiara Modenese]

Question #190529 on Yade changed:
https://answers.launchpad.net/yade/+question/190529

Status: Open => Answered

Chiara Modenese proposed the following answer:
On 19/03/12 07:30, Christian Jakob wrote:
> Question #190529 on Yade changed:
> https://answers.launchpad.net/yade/+question/190529
>
>  Status: Answered =>  Open
>
> Christian Jakob is still having a problem:
> Something is very strange in my model. My periodic cell is from (0,0,0)
> to (.002,.002.0042). When I look to the model view, every particle is
> inside the cell (except mirrored particles). When I print z-positions of
> my spheres, I get very strange values, that do not agree with the model
> view. Also I get negative z-positions, which should be impossible!
Hi Christian,
remember that particles position are saved as absolute values in Yade, 
not as relative to the main period (let us say). If you want to know 
particles position relative to the cell, you need to wrap them into the 
cell first.

Chiara
> Has anyone an idea, whats wrong here?
>

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Re: [Yade-users] [Question #190776]: too many fluctuations in 2d simulation

[Chiara Modenese]

Question #190776 on Yade changed:
https://answers.launchpad.net/yade/+question/190776

Status: Open => Answered

Chiara Modenese proposed the following answer:
On 15/03/12 14:55, ceguo wrote:
> New question #190776 on Yade:
> https://answers.launchpad.net/yade/+question/190776
>
> Hi,
>
> Due to my problem scale, I have to use a relatively small packing to 
> represent a RVE. So the particle number is about 1000~2000. But I found the 
> behaviour is too fluctuated. I've seen some published results using the 
> similar number of particles but get relatively smooth results. After some 
> failed try on tuning parameters (particle properties, compression speed, 
> etc.), I am wondering if anyone has experience on the 2d simulation with 
> yade. Below is my script:
Hi,
1. Are you trying to compare results under the same grain size 
distribution and contact model parameters?
2. If you increase the number of particles, you should see a reduction 
of the fluctuations; does it occur in your case?

Chiara
> 
> from yade import pack,plot
>
> O.materials.append(FrictMat(young=1e9,poisson=.3,frictionAngle=.7,density=2.6e3))
>
> sp = pack.SpherePack()
> sp.makeCloud(minCorner=(0,0,0),maxCorner=(3,3,0),rMean=0.03,rRelFuzz=.5,periodic=True,seed=1)
> sp.toSimulation()
> O.cell.hSize = Matrix3(3,0,0, 0,3,0, 0,0,1)
> print len(O.bodies)
> for p in O.bodies:
> p.state.blockedDOFs = 'zXY'
>
> def saveAddData():
> plot.addData(
>stress_xx = biax.stress[0],
>stress_yy = biax.stress[1],
>strain_xx = biax.strain[0],
>strain_yy = biax.strain[1]
> )
>
> O.dt = .5*utils.PWaveTimeStep()
> ##consolidation
> O.engines = [
> ForceResetter(),
> InsertionSortCollider([Bo1_Sphere_Aabb()]),
> InteractionLoop(
>[Ig2_Sphere_Sphere_Dem3DofGeom()],
>[Ip2_FrictMat_FrictMat_FrictPhys()],
>[Law2_Dem3DofGeom_FrictPhys_CundallStrack()]
> ),
> Peri3dController(
>goal=(-1.e6,-1.e6,0,0,0,0),
>stressMask=3,
>nSteps=2,
>maxStrain=1.,
>label='cons'
> ),
> NewtonIntegrator(damping=.3)
> ]
> O.run();O.wait()
>
> O.saveTmp(); O.loadTmp()
> O.materials[0].frictionAngle=.5
> ##undrained biaxial compression
> O.engines = [
> ForceResetter(),
> InsertionSortCollider([Bo1_Sphere_Aabb()]),
> InteractionLoop(
>[Ig2_Sphere_Sphere_Dem3DofGeom()],
>[Ip2_FrictMat_FrictMat_FrictPhys()],
>[Law2_Dem3DofGeom_FrictPhys_CundallStrack()]
> ),
> Peri3dController(
>goal=(1.e-1,-1.e-1,0,0,0,0),
>stressMask=0,
>nSteps=20,
>maxStrain=1.,
>label='biax',
>stressIdeal=Vector6(-1.e6,-1.e6,0,0,0,0)
> ),
> NewtonIntegrator(damping=.3),
> PyRunner(command='saveAddData()',iterPeriod=1000)
> ]
> O.run();O.wait()
>
> plot.saveDataTxt('result_undrained.txt',vars=('stress_xx','stress_yy','strain_xx','strain_yy','porosity'))
>

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Re: [Yade-users] [Question #190237]: install yade on a virtual machine

[Chiara Modenese]

Question #190237 on Yade changed:
https://answers.launchpad.net/yade/+question/190237

Chiara Modenese proposed the following answer:
Hi,
as Anton suggested you can install Yade from packages following this link:
https://answers.launchpad.net/yade/+faq/1789
for stable versions. However, the difference being that in this case you 
won't be able to modify the source code.

Chiara

On 10/03/2012 13:10, wangxiaoliang wrote:
> Question #190237 on Yade changed:
> https://answers.launchpad.net/yade/+question/190237
>
>  Status: Answered =>  Open
>
> wangxiaoliang is still having a problem:
> Hi Anton, I do not understand your comments.
> I have installed (using scons) in the yade fold unpackage from the compressed 
> file downloaded from the offical website. It works in my ubuntu system, but 
> not works in virtual ubuntu.
>

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Re: [Yade-users] [Question #190075]: About NewtonIntegrator(damping) and utils.PWaveTimeStep()

[Chiara Modenese]

Question #190075 on Yade changed:
https://answers.launchpad.net/yade/+question/190075

Status: Open => Answered

Chiara Modenese proposed the following answer:
Hi,

1. The damping in NewtonIntegrator is an artificial parameter. If you
want to use it you need to make sure that it does not influence your
results to some level of approximation. You can employ it to accelerate
the achievement of quasi-static equilibrium. If you run tests under
dynamic conditions you should avoid to use it. I guess you can start
using the default value and try to increase it but without much
influence on the results (this is what you should check).

2. To prevent numerical instability you generally select a coefficient for the 
time step computed by PWaveTimeStep() less than 1, something like 0.2 or so. I 
would not choose a coefficient bigger than 1 as the solution might become 
unstable (but you should check that too). For the time step definition you can 
also use another engine in Yade, called 
https://yade-dem.org/doc/yade.wrapper.html?highlight=timeste#yade.wrapper.GlobalStiffnessTimeStepper,
 which updates the time step according to the current interactions (see 
documentation for more details). The time step definition is related to the 
properties (stiffness, mass and size) of your particles (the formula tells you 
how those properties are accounted for). 
To answer your question, I guess you could run parametric studies for different 
values of xxx and identify the threshold per which the numerical solution 
converges.

Chiara


On 8 Mar 2012, at 15:35, jimtianxi wrote:

> New question #190075 on Yade:
> https://answers.launchpad.net/yade/+question/190075
> 
> I'm researching the soil sampling process using yade. I want to know how to 
> choose the damping parameter of the NewtonIntegrator(damping=) and how it 
> influence the simulation speed. Additionally, I don't know how to choose the 
> simulation step. Usually, O.dt=xxx*utils.PWaveTimeStep(),where xxx is 
> sometimes less than 1, and sometimes more than 1, I want to know how to 
> choose it, and is it related to the property of the soil. Thank you very 
> much!!
> 
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Re: [Yade-users] [Question #189100]: contact point with periodic boundary

[Chiara Modenese]

Question #189100 on Yade changed:
https://answers.launchpad.net/yade/+question/189100

Status: Answered => Solved

Chiara Modenese confirmed that the question is solved:
Thanks Chareyre, that solved my question.

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[Yade-users] [Question #189100]: contact point with periodic boundary

[Chiara Modenese]

New question #189100 on Yade:
https://answers.launchpad.net/yade/+question/189100

Hi there (Bruno?),
when periodic boundary is employed, is the position of the contact point (which 
can be accessed from i.geom) always inside the main periodic cell or can it be 
in other locations too? I know that particles position should be wrapped if one 
wants to refer them to the main cell but am wondering now what about the 
contact point.

Thanks for answer,
Chiara

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Re: [Yade-users] [Question #188478]: quaternions

[Chiara Modenese]

Question #188478 on Yade changed:
https://answers.launchpad.net/yade/+question/188478

Status: Answered => Solved

Chiara Modenese confirmed that the question is solved:
Thanks Chareyre, that solved my question.

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Re: [Yade-users] [Question #188478]: quaternions

[Chiara Modenese]

Question #188478 on Yade changed:
https://answers.launchpad.net/yade/+question/188478

Chiara Modenese posted a new comment:
OK, thanks Bruno for clarification! 
C.

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Re: [Yade-users] [Question #188478]: quaternions

[Chiara Modenese]

Question #188478 on Yade changed:
https://answers.launchpad.net/yade/+question/188478

Status: Answered => Open

Chiara Modenese is still having a problem:

On 22 Feb 2012, at 11:55, Chareyre wrote:

> Your question #188478 on Yade changed:
> https://answers.launchpad.net/yade/+question/188478
> 
>Status: Open => Answered
> 
> Chareyre proposed the following answer:
> By "difference", do you mean q1*q2^-1 or q1-q2?
> The first case should be safe now if eigen fixed the problem (I think they 
> should but I didn't keep exact track of what they did), else the isnan() test 
> is a workaround.
Of course I meant the first case. OK, so the workaround is still necessary, I 
just wanted confirmation of that.

What about this second line?
if (aa.angle() > Mathr::PI) aa.angle() -= Mathr::TWO_PI;   // angle is between 
0 and 2*pi, but should be between -pi and pi

Thanks,
Chiara

> The second case: does it make sense mathematicaly?
> 
> -- 
> If this answers your question, please go to the following page to let us
> know that it is solved:
> https://answers.launchpad.net/yade/+question/188478/+confirm?answer_id=0
> 
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/yade/+question/188478
> 
> You received this question notification because you asked the question.

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[Yade-users] [Question #188478]: quaternions

[Chiara Modenese]

New question #188478 on Yade:
https://answers.launchpad.net/yade/+question/188478

Hi there,
if I take the difference between two quaternions in Yade should I worry about 
anything at all? I recall these two lines in ScGeom.cpp (which I find difficult 
to gather -- maybe there is an old discussion about it?)

#else
if (isnan(aa.angle())) aa.angle()=0;
#endif
   if (aa.angle() > Mathr::PI) aa.angle() -= Mathr::TWO_PI;   // angle is 
between 0 and 2*pi, but should be between -pi and pi


Thanks for advice! 
Chiara

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Re: [Yade-users] [Question #188297]: Particles orientations

[Chiara Modenese]

Question #188297 on Yade changed:
https://answers.launchpad.net/yade/+question/188297

Status: Answered => Solved

Chiara Modenese confirmed that the question is solved:
Thanks Chareyre, that solved my question.

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