.. index:: fix multicontact/halfspace

fix multicontact/halfspace command
==================================

Syntax
""""""

.. parsed-literal::

   fix ID group multicontact/halfspace geometric_prefactor gp_value

* ID, group are documented in :doc:`fix <fix>` command
* multicontact/halfspace = style name of this fix command
* geometric_prefactor/gp_value = optional keyword-value pair
.. parsed-literal::

     *geometric_prefactor* gp_value = gamma
       gamma is an empirical factor accounting for the geometry



Examples
""""""""

.. parsed-literal::

   fix mc all multicontact/halfspace
   fix mc all multicontact/halfspace geometric_prefactor 1.8

Description
"""""""""""

Implements the multicontact model by :ref:`(Brodu et al.) <Brodu>` . This model computes a per-contact deformation for each particle based on the other contacts this particles has. A particle *i* with contact *ij* has the following new radius (when computing the contact laws with particle *j*):

.. parsed-literal::

   r_i + sum_k delta_{ij->ik}

where *r_i* is the default radius of particle *i*, *sum_k* is the sum over all particles in contact with *i* (*k*!=*j*). The delta value is given by

.. parsed-literal::

   delta_{ij->ik} = - gamma (1 + nu) F_ik / (2 pi Y d_{ik->ij}) * [(**n**_ik . **u**_{ik->ij})(**n**_ij . **u**_{ik->ij}) + (3 - 4 nu) **n**_ik . **n**_ij - (1 - 2 nu) (**n**_ik + **u**_{ik->ij}) . **n**_ij) / (1 + **n**_ik **u**_{ik->ij}) ]

where

* gamma = geometric prefactor
* nu = Poisson ratio
* F_ik = absolute value of normal force acting at contact *ik*
* Y = Youngs modulus
* d_{ik->ij} = distance from contact *ik* to contact *ij*
* **u**_{ik->ij} = unit vector pointing from contact *ik* to contact *ij*
* **n**_ik = normal vector of contact *ik*
* **n**_ij = normal vector of contact *ij*



----------


Restart, fix_modify, output, run start/stop, minimize info
""""""""""""""""""""""""""""""""""""""""""""""""""""""""""

No information about this fix is written to :doc:`binary restart files <restart>`.

:doc:`fix_modify <fix_modify>` cannot be used on the parameter of this fix.

Restrictions
""""""""""""


Requires the use of the :doc:`gran surface model multicontact <gran_surface_multicontact>`

Related commands
""""""""""""""""

:doc:`gran surface multicontact <gran_surface_multicontact>`

**Default:** none

**References:**

.. _Brodu:



**(Brodu)** Brodu, Dijksman, Behringer, Multiple-contact discrete-element model for simulating dense granular media, Phys. Rev. E (2015).


.. _liws: http://www.cfdem.com
.. _ld: Manual.html
.. _lc: Section_commands.html#comm