# fix wall/gran command

Accelerator Variants: *wall/gran/kk*

## Syntax

    fix ID group-ID wall/gran fstyle fstyle_params wallstyle args keyword values ...

-   ID, group-ID are documented in [fix](fix) command

-   wall/gran = style name of this fix command

-   fstyle = style of force interactions between particles and wall

        possible choices: hooke, hooke/history, hertz/history, granular

-   fstyle_params = parameters associated with force interaction style

        For *hooke*, *hooke/history*, and *hertz/history*, *fstyle_params* are:
              Kn = elastic constant for normal particle repulsion (force/distance units or pressure units - see discussion below)
              Kt = elastic constant for tangential contact (force/distance units or pressure units - see discussion below)
              gamma_n = damping coefficient for collisions in normal direction (1/time units or 1/time-distance units - see discussion below)
              gamma_t = damping coefficient for collisions in tangential direction (1/time units or 1/time-distance units - see discussion below)
              xmu = static yield criterion (unitless value between 0.0 and 1.0e4)
              dampflag = 0 or 1 if tangential damping force is excluded or included
              optional keyword = *limit_damping*, limit damping to prevent attractive interaction

        For *granular*, *fstyle_params* are set using the same syntax as for the *pair_coeff* command of `pair_style granular <pair_granular>`__

-   wallstyle = *xplane* or *yplane* or *zplane* or *zcylinder*

-   args = list of arguments for a particular style

        *xplane* or *yplane* or *zplane* args = lo hi
          lo,hi = position of lower and upper plane (distance units), either can be NULL)
        *zcylinder* args = radius
          radius = cylinder radius (distance units)

-   zero or more keyword/value pairs may be appended to args

-   keyword = *wiggle* or *shear* or *contacts* or *temperature*

        *wiggle* values = dim amplitude period
          dim = *x* or *y* or *z*
          amplitude = size of oscillation (distance units)
          period = time of oscillation (time units)
        *shear* values = dim vshear
          dim = *x* or *y* or *z*
          vshear = magnitude of shear velocity (velocity units)
        *contacts* value = none
          generate contact information for each particle
        *temperature* value = temperature
          specify temperature of wall

## Examples

``` LAMMPS
fix 1 all wall/gran hooke  200000.0 NULL 50.0 NULL 0.5 0 xplane -10.0 10.0
fix 1 all wall/gran hooke/history 200000.0 NULL 50.0 NULL 0.5 0 zplane 0.0 NULL
fix 2 all wall/gran hooke 100000.0 20000.0 50.0 30.0 0.5 1 zcylinder 15.0 wiggle z 3.0 2.0
fix 3 all wall/gran granular hooke 1000.0 50.0 tangential linear_nohistory 1.0 0.4 damping velocity region myBox
fix 4 all wall/gran granular jkr 1e5 1500.0 0.3 10.0 tangential mindlin NULL 1.0 0.5 rolling sds 500.0 200.0 0.5 twisting marshall region myCone
fix 5 all wall/gran granular dmt 1e5 0.2 0.3 10.0 tangential mindlin NULL 1.0 0.5 rolling sds 500.0 200.0 0.5 twisting marshall damping tsuji heat 10 region myCone temperature 1.0
fix 6 all wall/gran hooke  200000.0 NULL 50.0 NULL 0.5 0 xplane -10.0 10.0 contacts
```

## Description

Bound the simulation domain of a granular system with a frictional wall.
All particles in the group interact with the wall when they are close
enough to touch it.

The nature of the wall/particle interactions are determined by the
*fstyle* setting. It can be any of the styles defined by the [pair_style
gran/\*](pair_gran) or the more general [pair_style
granular](pair_granular) commands. Currently the options are *hooke*,
*hooke/history*, or *hertz/history* for the former, and *granular* with
all the possible options of the associated *pair_coeff* command for the
latter. The equation for the force between the wall and particles
touching it is the same as the corresponding equation on the [pair_style
gran/\*](pair_gran) and [pair_style granular](pair_granular) doc pages,
in the limit of one of the two particles going to infinite radius and
mass (flat wall). Specifically, delta = radius - r = overlap of particle
with wall, m_eff = mass of particle, and the effective radius of contact
= RiRj/Ri+Rj is set to the radius of the particle.

The parameters *Kn*, *Kt*, *gamma_n*, *gamma_t*, *xmu*, *dampflag*, and
the optional keyword *limit_damping* have the same meaning and units as
those specified with the [pair_style gran/\*](pair_gran) commands. This
means a NULL can be used for either *Kt* or *gamma_t* as described on
that page. If a NULL is used for *Kt*, then a default value is used
where *Kt* = 2/7 *Kn*. If a NULL is used for *gamma_t*, then a default
value is used where *gamma_t* = 1/2 *gamma_n*.

All the model choices for cohesion, tangential friction, rolling
friction and twisting friction supported by the [pair_style
granular](pair_granular) through its *pair_coeff* command are also
supported for walls. These are discussed in greater detail on the doc
page for [pair_style granular](pair_granular).

Note that you can choose a different force styles and/or different
values for the wall/particle coefficients than for particle/particle
interactions. E.g. if you wish to model the wall as a different
material.

:::: note
::: title
Note
:::

As discussed on the page for [pair_style gran/\*](pair_gran), versions
of LAMMPS before 9Jan09 used a different equation for Hertzian
interactions. This means Hertizian wall/particle interactions have also
changed. They now include a sqrt(radius) term which was not present
before. Also the previous versions used Kn and Kt from the pairwise
interaction and hardwired dampflag to 1, rather than letting them be
specified directly. This means you can set the values of the
wall/particle coefficients appropriately in the current code to
reproduce the results of a previous Hertzian monodisperse calculation.
For example, for the common case of a monodisperse system with particles
of diameter 1, Kn, Kt, gamma_n, and gamma_s should be set sqrt(2.0)
larger than they were previously.
::::

The effective mass *m_eff* in the formulas listed on the [pair_style
granular](pair_gran) page is the mass of the particle for particle/wall
interactions (mass of wall is infinite). If the particle is part of a
rigid body, its mass is replaced by the mass of the rigid body in those
formulas. This is determined by searching for a [fix rigid](fix_rigid)
command (or its variants).

The *wallstyle* can be planar or cylindrical. The 3 planar options
specify a pair of walls in a dimension. Wall positions are given by *lo*
and *hi*. Either of the values can be specified as NULL if a single wall
is desired. For a *zcylinder* wallstyle, the cylinder\'s axis is at x =
y = 0.0, and the radius of the cylinder is specified.

Optionally, the wall can be moving, if the *wiggle* or *shear* keywords
are appended. Both keywords cannot be used together.

For the *wiggle* keyword, the wall oscillates sinusoidally, similar to
the oscillations of particles which can be specified by the [fix
move](fix_move) command. This is useful in packing simulations of
granular particles. The arguments to the *wiggle* keyword specify a
dimension for the motion, as well as it\'s *amplitude* and *period*.
Note that if the dimension is in the plane of the wall, this is
effectively a shearing motion. If the dimension is perpendicular to the
wall, it is more of a shaking motion. A *zcylinder* wall can only be
wiggled in the z dimension.

Each timestep, the position of a wiggled wall in the appropriate *dim*
is set according to this equation:

    position = coord + A - A cos (omega \* delta)

where *coord* is the specified initial position of the wall, *A* is the
*amplitude*, *omega* is 2 PI / *period*, and *delta* is the time elapsed
since the fix was specified. The velocity of the wall is set to the
derivative of this expression.

For the *shear* keyword, the wall moves continuously in the specified
dimension with velocity *vshear*. The dimension must be tangential to
walls with a planar *wallstyle*, e.g. in the *y* or *z* directions for
an *xplane* wall. For *zcylinder* walls, a dimension of *z* means the
cylinder is moving in the z-direction along it\'s axis. A dimension of
*x* or *y* means the cylinder is spinning around the z-axis, either in
the clockwise direction for *vshear* \> 0 or counter-clockwise for
*vshear* \< 0. In this case, *vshear* is the tangential velocity of the
wall at whatever *radius* has been defined.

The *temperature* keyword is used to assign a temperature to the wall.
The following value can either be a numeric value or an equal-style
[variable](variable). If the value is a variable, it should be specified
as v_name, where name is the variable name. In this case, the variable
will be evaluated each timestep, and its value used to determine the
temperature. This option must be used in conjunction with a heat
conduction model defined in [pair_style granular](pair_granular), [fix
property/atom](fix_property_atom) to store temperature and a heat flow,
and [fix heat/flow](fix_heat_flow) to integrate heat flow.

------------------------------------------------------------------------

Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are
functionally the same as the corresponding style without the suffix.
They have been optimized to run faster, depending on your available
hardware, as discussed on the [Accelerator packages](Speed_packages)
page. The accelerated styles take the same arguments and should produce
the same results, except for round-off and precision issues.

These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and
OPT packages, respectively. They are only enabled if LAMMPS was built
with those packages. See the [Build package](Build_package) page for
more info.

You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the [-suffix command-line
switch](Run_options) when you invoke LAMMPS, or you can use the
[suffix](suffix) command in your input script.

See the [Accelerator packages](Speed_packages) page for more
instructions on how to use the accelerated styles effectively.

------------------------------------------------------------------------

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

This fix writes the shear friction state of atoms interacting with the
wall to [binary restart files](restart), so that a simulation can
continue correctly if granular potentials with shear \"history\" effects
are being used. See the [read_restart](read_restart) command for info on
how to re-specify a fix in an input script that reads a restart file, so
that the operation of the fix continues in an uninterrupted fashion.

If the `contacts` option is used, this fix generates a per-atom array
with 8 columns as output, containing the contact information for owned
particles (nlocal on each processor). All columns in this per-atom array
will be zero if no contact has occurred. The values of these columns are
listed in the following table:

+------+-----------------------------------------------+--------------+
| I    | Value                                         | Units        |
| ndex |                                               |              |
+======+===============================================+==============+
| > 1  | 1.0 if particle is in contact with wall, 0.0  |              |
|      | otherwise                                     |              |
+------+-----------------------------------------------+--------------+
| > 2  | Force $f_x$ exerted by the wall               | force units  |
+------+-----------------------------------------------+--------------+
| > 3  | Force $f_y$ exerted by the wall               | force units  |
+------+-----------------------------------------------+--------------+
| > 4  | Force $f_z$ exerted by the wall               | force units  |
+------+-----------------------------------------------+--------------+
| > 5  | $x$-coordinate of contact point on wall       | distance     |
|      |                                               | units        |
+------+-----------------------------------------------+--------------+
| > 6  | $y$-coordinate of contact point on wall       | distance     |
|      |                                               | units        |
+------+-----------------------------------------------+--------------+
| > 7  | $z$-coordinate of contact point on wall       | distance     |
|      |                                               | units        |
+------+-----------------------------------------------+--------------+
| > 8  | Radius $r$ of atom                            | distance     |
|      |                                               | units        |
+------+-----------------------------------------------+--------------+

None of the [fix_modify](fix_modify) options are relevant to this fix.
No parameter of this fix can be used with the *start/stop* keywords of
the [run](run) command. This fix is not invoked during [energy
minimization](minimize).

## Restrictions

This fix is part of the GRANULAR package. It is only enabled if LAMMPS
was built with that package. See the [Build package](Build_package) page
for more info.

Any dimension (xyz) that has a granular wall must be non-periodic.

## Related commands

[fix move](fix_move), [fix wall/gran/region](fix_wall_gran_region),
[pair_style gran/\*](pair_gran) [pair_style granular](pair_granular)

## Default

none