yade-dev team mailing list archive

[Chiara Modenese <c.modenese@xxxxxxxxx>]

You (not PFC at this point) are defining the effective radius in the wrong
way. And well, to my opinion _hn (and _hs) should NOT be user defined as
their values are well established by Hertz (and Mindlin) theory itself.
Please input the same constant coefficients that you find in Yade if you
still aim at making a meaningful comparison. You can get them like:

O.interactions[0,1].phys.kno
O.interactions[0,1].phys.kso

Those should be equal to your PFC variable _hn and _hs respectively.
Please, close this bug if you find the same results. You should always check
that the formulations you are comparing are the same before going any
deeper.

Chiara


On 14 July 2011 13:49, Christian Jakob <806944@xxxxxxxxxxxxxxxxxx>
wrote:

> In the udm_hertz the user has to modify contacts manually, as you can
> see here:
>
> def modify_cp
>
>  bp1 = c_ball1(cp)
>  ;ball pointers
>  bp2 = c_ball2(cp)
>
>  c_model(cp) = 'udm_hertz'
>
>  _fricset = friction_coeff
>
>  if pointer_type(bp2) = 100  ; Ball-Ball contact
>
>    _meff =
> b_realmass(bp1)*b_realmass(bp2)/(b_realmass(bp1)+b_realmass(bp2));
>
>    _rbar = 2.0 * b_rad(bp1) * b_rad(bp2) / (b_rad(bp1) + b_rad(bp2))
>
>  else                         ; Ball-Wall contact
>
>    _meff = b_realmass(bp1)
>
>    _rbar = b_rad(bp1)
>
>    _fricset = min(_fricset,w_fric(bp2))
>
>  end_if
>
>  _hn = 2.0 * sqrt(2.0*_rbar) * shear_modulus / (3.0*(1.0 -
> poisson_ratio));
>
>  temp = shear_modulus * shear_modulus * 3.0 * (1.0 - poisson_ratio) *
> _rbar;
>
>  _hs = 2.0 * exp((1.0/3.0)*ln(temp)) / (2.0 - poisson_ratio);
>
>  c_prop(cp,'hertz_meff') = _meff
>
>  c_prop(cp,'hertz_hn')   = _hn
>
>  c_prop(cp,'hertz_hs')   = _hs
>
>  c_prop(cp,'hertz_fric') = _fricset
>
>  c_prop(cp,'hertz_vn')   = viscous_normal
>
>  c_prop(cp,'hertz_vs')   = viscous_shear
>
>  c_prop(cp,'hertz_nt')   = _nt
>
>  c_prop(cp,'hertz_hssd') = _hssd
>
> end
>
> to answer your question:
>
> normal stiffness:
>  _hn = 2.0 * sqrt(2.0*_rbar) * shear_modulus / (3.0*(1.0 - poisson_ratio));
>
> shear stiffness:
>  temp = shear_modulus * shear_modulus * 3.0 * (1.0 - poisson_ratio) *
> _rbar;
>  _hs = 2.0 * exp((1.0/3.0)*ln(temp)) / (2.0 - poisson_ratio);
>
> --
> You received this bug notification because you are a member of Yade
> developers, which is the registrant for Yade.
> https://bugs.launchpad.net/bugs/806944
>
> Title:
>  different behavoir of Hertz model while comparing PFC and YADE
>
> Status in Yet Another Dynamic Engine:
>   Confirmed
>
> 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
>
> To manage notifications about this bug go to:
> https://bugs.launchpad.net/yade/+bug/806944/+subscriptions
>
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-- 
You received this bug notification because you are a member of Yade
developers, which is the registrant for Yade.
https://bugs.launchpad.net/bugs/806944

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

Status in Yet Another Dynamic Engine:
  Confirmed

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

To manage notifications about this bug go to:
https://bugs.launchpad.net/yade/+bug/806944/+subscriptions