# fix wall/gran/region command

## Syntax

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

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

-   wall/region = 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

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

-   wallstyle = region (see [fix wall/gran](fix_wall_gran) for options
    for other kinds of walls)

-   region-ID = region whose boundary will act as wall

-   keyword = *contacts* or *temperature*

        *contacts* value = none
           generate contact information for each particle
        *temperature* value = temperature
           specify temperature of wall

## Examples

``` LAMMPS
fix wall all wall/gran/region hooke/history 1000.0 200.0 200.0 100.0 0.5 1 region myCone
fix 3 all wall/gran/region granular hooke 1000.0 50.0 tangential linear_nohistory 1.0 0.4 damping velocity region myBox
fix 4 all wall/gran/region 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/region 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 region myCone
fix wall all wall/gran/region hooke/history 1000.0 200.0 200.0 100.0 0.5 1 region myCone contacts
```

## Description

Treat the surface of the geometric region defined by the *region-ID* as
a bounding frictional wall which interacts with nearby finite-size
granular particles when they are close enough to touch the wall. See the
[fix wall/region](fix_wall_region) and [fix wall/gran](fix_wall_gran)
commands for related kinds of walls for non-granular particles and
simpler wall geometries, respectively.

Here are snapshots of example models using this command. Corresponding
input scripts can be found in examples/granregion. Movies of these
simulations are [here on the Movies
page](https://www.lammps.org/movies.html#granregion)\_ of the LAMMPS
website.

![wallgran1](img/gran_funnel.png){width="48.0%"}
![wallgran2](img/gran_mixer.png){width="48.0%"}

Click on the images to see a bigger picture.

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

The distance between a particle and the region boundary is the distance
to the nearest point on the region surface. The force the wall exerts on
the particle is along the direction between that point and the particle
center, which is the direction normal to the surface at that point. Note
that if the region surface is comprised of multiple \"faces\", then each
face can exert a force on the particle if it is close enough. E.g. for
[region_style block](region), a particle in the interior, near a corner
of the block, could feel wall forces from 1, 2, or 3 faces of the block.

Regions are defined using the [region](region) command. Note that the
region volume can be interior or exterior to the bounding surface, which
will determine in which direction the surface interacts with particles,
i.e. the direction of the surface normal. The exception to this is if
one or more *open* options are specified for the region command, in
which case particles interact with both the interior and exterior
surfaces of regions.

Regions can either be primitive shapes (block, sphere, cylinder, etc) or
combinations of primitive shapes specified via the *union* or
*intersect* region styles. These latter styles can be used to construct
particle containers with complex shapes.

Regions can also move dynamically via the [region](region) command
keywords (move) and *rotate*, or change their shape by use of variables
as inputs to the [region](region) command. If such a region is used with
this fix, then the region surface will move in time in the corresponding
manner.

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

As discussed on the [region](region) command doc page, regions in LAMMPS
do not get wrapped across periodic boundaries. It is up to you to ensure
that the region location with respect to periodic or non-periodic
boundaries is specified appropriately via the [region](region) and
[boundary](boundary) commands when using a region as a wall that bounds
particle motion.
::::

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

For primitive regions with sharp corners and/or edges (e.g. a block or
cylinder), wall/particle forces are computed accurately for both
interior and exterior regions. For *union* and *intersect* regions,
additional sharp corners and edges may be present due to the
intersection of the surfaces of 2 or more primitive volumes. These
corners and edges can be of two types: concave or convex. Concave
points/edges are like the corners of a cube as seen by particles in the
interior of a cube. Wall/particle forces around these features are
computed correctly. Convex points/edges are like the corners of a cube
as seen by particles exterior to the cube, i.e. the points jut into the
volume where particles are present. LAMMPS does NOT compute the location
of these convex points directly, and hence wall/particle forces in the
cutoff volume around these points suffer from inaccuracies. The basic
problem is that the outward normal of the surface is not continuous at
these points. This can cause particles to feel no force (they don\'t
\"see\" the wall) when in one location, then move a distance epsilon,
and suddenly feel a large force because they now \"see\" the wall. In a
worst-case scenario, this can blow particles out of the simulation box.
Thus, as a general rule you should not use the fix wall/gran/region
command with *union* or *interesect* regions that have convex points or
edges resulting from the union/intersection (convex points/edges in the
union/intersection due to a single sub-region are still OK).
::::

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

Similarly, you should not define *union* or *intersert* regions for use
with this command that share an overlapping common face that is part of
the overall outer boundary (interior boundary is OK), even if the face
is smooth. E.g. two regions of style block in a *union* region, where
the two blocks overlap on one or more of their faces. This is because
LAMMPS discards points that are part of multiple sub-regions when
calculating wall/particle interactions, to avoid double-counting the
interaction. Having two coincident faces could cause the face to become
invisible to the particles. The solution is to make the two faces differ
by epsilon in their position.
::::

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,
but the effective radius is calculated using the radius of the particle
and the radius of curvature of the wall at the contact point.

Specifically, delta = radius - r = overlap of particle with wall, m_eff
= mass of particle, and RiRj/Ri+Rj is the effective radius, with Rj
replaced by the radius of curvature of the wall at the contact point.
The radius of curvature can be negative for a concave wall section, e.g.
the interior of cylinder. For a flat wall, delta = radius - r = overlap
of particle with wall, m_eff = mass of particle, and the effective
radius of contact is just 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 6 wall/particle coefficients than for particle/particle
interactions. E.g. if you wish to model the wall as a different
material.

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.

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

Similar to [fix wall/gran](fix_wall_gran) command, 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. This
fix also includes info about a moving region in the restart file. 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.

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

Information about region definitions is NOT included in restart files,
as discussed on the [read_restart](read_restart) doc page. So you must
re-define your region and if it is a moving region, define its motion
attributes in a way that is consistent with the simulation that wrote
the restart file. In particular, if you want to change the region motion
attributes (e.g. its velocity), then you should ensure the
position/orientation of the region at the initial restart timestep is
the same as it was on the timestep the restart file was written. If this
is not possible, you may need to ignore info in the restart file by
defining a new fix wall/gran/region command in your restart script, e.g.
with a different fix ID. Or if you want to keep the shear history info
but discard the region motion information, you can use the same fix ID
for fix wall/gran/region, but assign it a region with a different region
ID.
::::

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.

## Related commands

[fix_move](fix_move), [fix wall/gran](fix_wall_gran), [fix
wall/region](fix_wall_region), [pair_style granular](pair_gran),
[region](region)

## Default

none