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[Christian Jakob <806944@xxxxxxxxxxxxxxxxxx>]

>> c_n = beta_n * c_crit = beta_n * 2 * sqrt(m * k_n)
>>
>> beta_n - critical damping ratio
>> c_crit - critical damping constant
>> m - average mass of the two contacting spheres
>>
>This may be different in Yade. If one of the two bodies is fixed, like in
>your case, then only the mass of the moving body is considered. I do not
>know how is PFC in this regard, please check it out. 

I looked into the manual again and I found two mistakes, that I did.

1.) m is not the average mass, but

m = (m_1 * m_2) / (m_1 + m_1)

(see PFC manual page 1-27, formula 1.53)

In the manual it is not said what happen, when one ball is fixed ...
This could be the difference between damping in YADE and damping in PFC.

2.) c_crit = 2 * sqrt(m * k_i), where k_i is the contact tangent
stiffness

So it is the same as you said:
"In case of Hertz
law I take the current tangential stiffness to compute the critical damping
coefficients."

for local damping (page 1-22) it is said (only translational
components):

F_d = - alpha * abs(F) * sign(v)

alpha - local damping constant
F - force from force-displacement law
v - velocity of the particle

Christian

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https://bugs.launchpad.net/bugs/806944

Title:
  different behavoir of Hertz model while comparing PFC and YADE

Status in Yet Another Dynamic Engine:
  New

Bug description:
  Hello,

  During verifiing my model I compared output from PFC 4.0 with the
  output from YADE (bzr2877 on Debian Squeeze 64bit). There I found a
  different behavior between these two programs. I dont know if this is
  a bug or not, but I hope someone can explain the different behavior.

  The model itself is very simple. There are two spheres, an upper one and a lower one. The lower one is fixed and the upper one is falling down to the lower one, collides and jumps back. The value of interest is the flyback height of the upper particle.
  In PFC I measure higher values of flyback height, then in YADE (see log-files in the attachment).

  Can someone tell me why there is a difference in the flyback height?
  (input parameters are the same for both programs ... see caller_two-spheres-jumping.dat for PFC and two-spheres.py for YADE)

  Regards,

  Christian Jakob

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