.. index:: fix ave/euler

fix ave/euler command
=====================

Syntax
""""""

.. parsed-literal::

   fix ID group-ID ave/euler nevery N cell_size_relative c parallel par keywords values

* ID, group-ID are documented in :doc:`fix <fix>` command
* ave/euler = style name of this fix command
* nevery = obligatory keyword
* n = calculate average values every this many timesteps
* cell_size_relative = obligatory keyword
* c = cell size in multiples of max cutoff
* parallel = obligatory keyword
* par = "yes" or "no
* zero or more keyword/value pairs may be appended
* keyword = *basevolume_region*
.. parsed-literal::

     *basevolume_region* values = reg-ID
       region-ID = correct grid cell volume based on this region



Examples
""""""""

.. parsed-literal::

   fix 1 all ave/euler nevery 100 cell_size_relative 4.5

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

Calculate cell-based averages of velocity, radius, volume fraction,
and pressure (-1/3 * trace of the stress tensor) every few timesteps,
as specified by the *nevery* keyword. The size of the cells is calculated
as multiple of the maximum cutoff, via the *cell_size_relative*.
Note that at least a relative cell size of 3 is required.

Note that velocity is favre (mass) averaged, whereas radius is arithmetically
averaged. To calculate the stress, this command internally uses a
:doc:`compute stress/atom <compute_stress_atom>` . It includes the convective
term correctly for granular particles with non-zero average velocity
(which is not included  in :doc:`compute stress/atom <compute_stress_atom>`).
However, it does not include bond, angle, diahedral or kspace contributions
so that the stress tensor finally reads

.. image:: Eqs/stress_tensor_granular.png
   :align: center

where vave is the (cell-based) average velocity.
The first term is a kinetic energy contribution for atom *I*.  The
second term is a pairwise energy contribution where *n* loops over the
*Np* neighbors of atom *I*, *r1* and *r2* are the positions of the 2
atoms in the pairwise interaction, and *F1* and *F2* are the forces on
the 2 atoms resulting from the pairwise interaction.

The *parallel* option determines if every process allocates
its own local grid for postprocessing (for *parallel* = yes),
or each proc contributes to one single global grid (for *parallel* = no).
This will be slower since it requires parallel communication,
but will ensure that the grid cells do not move over time (e.g.
in case of a moving boundary)

The *basevolume_region* option allows to specify a region that
represents the volume which can theoretically be filled with
particles. This will then be used to correct the basis of the averaging
volume for each cell in the grid. For example, if you use a cylindrical
wall, it makes sense to use an identical cylindrical region for 
the *basevolume_region* option, and the command will correctly
calculate the volume fraction in the near-wall cells.
the calculation of overlap between grid cells and the region
is done using a Monte-Carlo approach.


----------


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

No information about this fix is written to :doc:`binary restart files <restart>`.  None of the :doc:`fix_modify <fix_modify>` options
are relevant to this fix.

This fix computes the above-mentioned quantities for output via a
:doc:`dump euler/vtk <dump>` command.  The values can
only be accessed on timesteps that are multiples of *nevery* since that
is when calculations are performed.

No parameter of this fix can be used with the *start/stop* keywords of
the :doc:`run <run>` command.  This fix is not invoked during :doc:`energy minimization <minimize>`.

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


Volume fractions and stresses are calculated based on the assumption
of a structured (equidistant regular) grid, so volume fractions and
stresses near walls that are not alligned with the grid will be
incorrect.

**Coarse-graining information:**

Using :doc:`coarsegraining <coarsegraining>` in
combination with this command might lead to
different dynamics or system state and thus to 
inconsistancies. However, the influence of this
model on the global dynamics or system state might 
be small so in some cases the results may be valid. 
This has to be reviewed by a specialist on a 
case-by-case basis.

.. note::

   :doc:`Coarsegraining <coarsegraining>` may or 
   may not be available in LIGGGHTS(R)-PUBLIC.

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

:doc:`compute <compute>`, :doc:`compute stress/atom <compute_stress_atom>`,
:doc:`fix ave/atom <fix_ave_atom>`, :doc:`fix ave/histo <fix_ave_histo>`,
:doc:`fix ave/time <fix_ave_time>`, :doc:`fix ave/spatial <fix_ave_spatial>`,

**Default:** none


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