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] < [email protected]> 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 > > -- > You received this question notification because you are a member of > yade-users, which is an answer contact for Yade. > > _______________________________________________ > Mailing list: https://launchpad.net/~yade-users > Post to : [email protected] > Unsubscribe : https://launchpad.net/~yade-users > More help : https://help.launchpad.net/ListHelp > -- You received this question notification because you are a member of yade-users, which is an answer contact for Yade. _______________________________________________ Mailing list: https://launchpad.net/~yade-users Post to : [email protected] Unsubscribe : https://launchpad.net/~yade-users More help : https://help.launchpad.net/ListHelp
