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[Dominik Boemer <question253045@xxxxxxxxxxxxxxxxxxxxx>]

Question #253045 on Yade changed:
https://answers.launchpad.net/yade/+question/253045

    Status: Answered => Open

Dominik Boemer is still having a problem:
Hey Anton,

sorry for my late answer.

There seem to be three errors, one in the paper and two in the code:

1.  The forth equation of (22)[Pournin2001] should be the following:

 ct = - 2 / (tc * (1/meff + R1^2/I1 + R2^2/I2)) * ln(et)

Why?  First, the differential equations in (21)[Pournin2001] is similar
(i.e. it has the same form) to the second one and the initial conditions
of both equations are also similar.  Thus, the results should be
similar.  Why is the second equation of (22)[Pournin2001] correct?
Because this equation is the same than the one in [2], which seems to be
correct.

[2] Nagurka & Huang_A, Mass-Spring-Damper Model of a Bouncing
Ball_19_Ijee1697


2.  Taking into account the previous correction, the equations of [Pournin2001] are correct.  This is the reason why 

cs1 = cs2 = -2.0/7.0 /Tc * log(Et)*massR; [3]

should be replaced by

cs1 = cs2 = -4.0/7.0 /Tc * log(Et)*massR;

[3]
https://github.com/yade/trunk/blob/master/pkg/dem/ViscoelasticPM.cpp#L194


3.  As the equations of [Pournin2001] are correct (with the correction), 

const Real massR = 2*mass1*mass2/(mass1+mass2); [4]

should be replaced by

const Real massR = mass1*mass2/(mass1+mass2);

according to [Pournin2001, just below equation (19)].

[4]
https://github.com/yade/trunk/blob/master/pkg/dem/ViscoelasticPM.cpp#L173

Alright?  I hope I am not missing anything.

Regards,
Dominik

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