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["Alexander Eulitz \[Eugen\]" <question232351@xxxxxxxxxxxxxxxxxxxxx>]

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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