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[Chiara Modenese <question232351@xxxxxxxxxxxxxxxxxxxxx>]

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] <
question232351@xxxxxxxxxxxxxxxxxxxxx> 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: bounces@xxxxxxxxxxxxx [mailto:bounces@xxxxxxxxxxxxx] 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] <
> question232351@xxxxxxxxxxxxxxxxxxxxx> 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.wrap
> > per.Ip2_FrictMat_FrictMat_MindlinPhys
> > [2]
> > https://github.com/yade/trunk/blob/master/pkg/dem/HertzMindlin.cpp?sou
> > rce=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?sou
> > rce=cc#L86
> >
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