# fix wall/gran command

## Syntax

    fix ID group-ID style model_type model_name wallstyle wallstyleargs general_keywords general_values model_keywords model_values  ...

- ID, group-ID are documented in fix command
- style = wall/gran
- zero or more model_type/model_name pairs may be appended. They must be appended in the following order

    model values = described here
    tangential values = described here
    cohesion values = described here
    rolling_friction values = described here
    surface values = described here

- wallstyle = mesh or primitive
- wallstyle args for wallstyle mesh = n_meshes and meshes

    n_meshes value = nm
      nm = # of meshes (see fix mesh/surface) to use for the wall (positive integer)
    meshes values = meshlist
      meshlist =  id(s) of the mesh(es) (see fix mesh/surface) to be used. These must be defined before

- wallstyle args for wallstyle primitive = type or xplane or yplane or zplane or xcylinder or ycylinder or zcylinder

    type args = tp
      tp = atom_type (material type) of the wall
    xplane or yplane or zplane args = pos
      pos = position plane (distance units)
    xcylinder or ycylinder or zcylinder args = radius c1 c2
      radius = cylinder radius (distance units)
      c1,c2 = coordinates of center axis in other 2 dims (distance units)

- zero or more general_keyword/value pairs may be appended
- general_keyword = shear or store_force or store_force_contact or store_force_contact_stress

    shear values = dim vshear
      dim = x or y or z
      vshear = magnitude of shear velocity (velocity units)
    temperature value = T0
      T0 = Temperature of the wall (temperature units)
    contact_area values = 'overlap' or 'constant [value]' or 'projection'
    store_force value = 'yes' or 'no'
      yes, no = determines if the wall force exerted on the particles is stored in a fix property/atom with id force_(ID), where (ID) is the id of the fix wall/gran command.
    store_force_contact value = 'yes' or 'no'
      yes, no = determines if the force for each particle-wall contact is stored in a fix property/atom with id contactforces_(ID), where (ID) is the id of the fix wall/gran command.
    store_force_contact_stress value = 'yes' or 'no'
      yes, no = determines if the force and contact point for each particle-wall contact is stored in a fix property/atom with id contactforces_stress_(ID), where (ID) is the id of the fix wall/gran command.

- following the general_keyword/value pairs, zero or more model_keyword/model_value pairs may be appended in arbitrary order model_keyword/model_value pairs = described for each model separately here

## Examples

    fix zwalls all wall/gran model hertz tangential history primitive type 1 zplane 0.15
    fix zwalls all wall/gran model hertz tangential history primitive type 1 zplane 0.15 surface superquadric
    fix meshwalls all wall/gran model hertz tangential history mesh n_meshes 2 meshes cad1 cad2

## 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 equation for the force between the wall and particles touching it is the same as the corresponding equation on the pair_style granular doc page, in the limit of one of the

> two particles going to infinite radius and mass (flat wall).

You must choose the models matching the pair style used, otherwise an error is created. As with pair_style granular, you have to define the mechanical properties for each material you are using in the simulation with fix property commands. See pair_style gran for more details and the model doc page for details.

For wallstyle mesh, fix_id1, fix_id2 etc. must be IDs of valid fixes of type fix mesh/surface, defining the granular mesh to be used for the wall. Triangle-particle neighbor lists are built to efficiently track particle-triangle contacts. Particle-tri neighbor list build is triggered if any particle moves more than half the skin distance or (in case of moving mesh) if the mesh itself moves more than half the skin distance since the last build. A warning is generated if a dangerous particle-tri neigh list build is detected (e.g. if particles are inserted too close to a wall, see section ‘Restrictions’). For style mesh, the atom_type (material type) is inherited from the atom style provided in the fix mesh/surface command.

For wallstyle primitive, the atom_type (material type) has to be provided via keyword type. Primitve walls can be xplane or yplane or zplane or cylindrical. The 3 planar options specify a single wall in a dimension. Wall positions are given by values for lo and hi. For an xcylinder, ycylinder or zcylinder, the radius and the cylinder axis in the other two dims is specified.

Optionally, primitive walls can be moving, if the shear keyword is appended.

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 same applies to xcylinder and ycylinder accordingly.

Note

The keywords wiggle or shear can NOT be used for wallstyle mesh. For a moving a granular wall with wallstyle mesh, use the more flexible command fix move/mesh, or use the keywords velocity or angular_velocity in fix mesh/surface.

The keyword temperature is used to assign a constant temperature to the wall. This setting gets effective in conjunction with heat conduction via fix heat/gran/conduction. For wallstyle mesh, the value for the temperature given in this command is ignored and the temperature value is specified per mesh via fix mesh/surface. Contact area calculation can be controlled by keyword contact_area in the same manner as for fix heat/gran/conduction.

By specifying store_force = ‘yes’, you can instruct the command to store the wall force exerted on the particles in a fix property/atom with id force_(ID), where (ID) is the id of the fix wall/gran command.

store_force_contact does the same for contact forces between particle and wall.

store_force_contact_stress does the same for contact forces and contact point between particle and wall. In general this keyword will be set automatically if a continuum/weighted fix is used. It needs to be set explicitly only if a new wall is created after the such a fix is specified. This functionality may not be available in your version of LIGGGHTS.

The effect of keyword rolling_friction, cohesion, tangential_damping, viscous and absolute_damping is explanted in pair gran

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

If applicable, contact history is written to binary restart files so simulations can continue properly. None of the fix_modify options are relevant to this fix. No global scalar or vector or are stored by this fix. If store_force = ‘yes’ is specified, the per-atom wall force can be accessed by the various output commands via f_force(ID)**1**, f_force(ID)**2**, f_force(ID)**3**. (ID) is the id of the fix wall/gran command. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during `energy minimization` .

## Restrictions

There can be only one fix wall/gran command with style mesh. Note that this is not really a restriction because you can include multiple fixes of type fix mesh/surface in the fix wall/gran command.

store_force_contact is available in selected versions only; it is not available in the PUBLIC version.

When using style style mesh, you have to use the style bin for the neighbor command.

Style mesh can not be used in conjunction with triclinic simulation boxes.

When using style mesh‘ in combination with a particle insertion command, you always have to keep a minimum distance between the wall and the insertion region that is equal to maximum particle radius + half the skin defined in the neighbor command. Otherwise, newly inserted particles interpenetrate the walls before a triangle neighbor list is built the first time.

The keyword shear can NOT be used for style mesh. For moving granular wall with style mesh, use fix move/mesh.

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

## Related commands

fix mesh/surface, fix_move_mesh, pair_style granular Models for use with this command are described here

## Default

model = ‘hertz’ tangential = ‘history’ rolling_friction = ‘off’ cohesion = ‘off’ surface = ‘default’