yade-dev team mailing list archive

[Christian Jakob <806944@xxxxxxxxxxxxxxxxxx>]

Zitat von Chiara Modenese <c.modenese@xxxxxxxxx>:

> So the problem is not the tension on or off. Can you check that the damping
> coefficients are defined in the same way for both codes? That could be the
> cause of the difference then.
>


As far as I know the viscous damping slows the particles down, that means that the relative velocity of the two contacting particles influences the damping force D_n = c_n * abs(v_n). In PFC manual there are two formulas for the normal force F_n = k_n * u_n and the damping force, that I joined together :

F_n_damped = F_n - D_n
F_n_damped = k_n * u_n - c_n * abs(v_n)

k_n - normal stiffness
u_n - overlap (positive if particles overlap)
c_n - normal damping constant 
v_n - relative velocity at the contact

(see page 1-9 and 1-26 in Itasca manual Theory and Background PFC 4.0)

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

So with the command "damp viscous normal visc_damp_normal notension on"
I can set beta_n. This should be the same as in YADE, when I set
"ContactModel.betan=visc_damp_normal" .... or not ?!


> In case 2 and 3 we should get the same result, I think. Am not sure what
> could be the cause.
> Hint 1) Can you try to compare the numbers step by step, having exactly the
> same configuration and time step?


I have the same configuration in both programs, same step width, same properties, same constants, as you can see when you are looking into the scripts.


> Hint 2) Can you try to do the same test but with the simple linear elastic
> law and no damping at all?
> Also I would just suggest to do a much simpler test where you just study the
> contact between two balls and record velocities, positions etc in every time
> step. Let me know.


I will do so, but it takes some time ...


> Bruno, what do you think about the above numbers?
>
>>
>> ...hope it helps ...
>>
>> I do not understand where the attractive force (in PFC notension off) comes
>> from, but the influence on the results is very low (see case1).
>>
> The attractive force comes from the damper.


Ok, but can you explain what the damper does? Is there a difference in the formula (see above)?


> Christian, did you write the model or you are taking the one already
> implemented in PFC? In the last case, I can have a look at the manual and
> see what sort of viscous damping they implement. Let me know.


I used the Hertz model, that is implemented in the standard PFC package (command "model hertz" in line 36 in two-spheres-jumping.dat)

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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