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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 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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