yade-dev team mailing list archive

[Christian Jakob <806944@xxxxxxxxxxxxxxxxxx>]

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);

-- 
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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

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