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 or general_plane or general_cylinder or general_cone

  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)
  general_plane args = px py pz nx ny nz
    px,py,pz = coordinates of a point on the plane
    nx,ny,nz = components of a vector orthogonal to the plane
  general_cylinder args = radius px py pz axisx axisy axisz
    radius = cylinder radius (distance units)
    px,py,pz = coordinates of a point on cylinder axis
    axisx,axisy,axisz = components of a vector parallel to cylinder axis
  general_cone args = radius1 x1 y1 z1 x2 y2 z2 radius2
    radius1,radius2 = radii corresponding to xyz1 and xyz2 (distance units)
    x1,y1,z1 = coordinates of center of bounding circle w/ radius radius1 (distance units)
    x2,y2,z2 = coordinates of center of bounding circle w/ radius radius2 (distance units)

• zero or more general_keyword/value pairs may be appended

• general_keyword = shear or store_force or store_force_contact

  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.

• 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 meshwalls all wall/gran model hertz tangential history mesh n_meshes 2 meshes cad1 cad2

LIGGGHTS vs. LAMMPS Info:

This command has been improved in accordance with the overhauled pair gran styles. Furthermore, it offers the new wallstyle mesh, which enables fix wall/gran to handle complex wall geometries imported from CAD.

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 used 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. Primitive walls can be xplane or yplane or zplane or or xcylinder or ycylinder or zcylinder or general_plane or general_cylinder or general_cone. The three axis-aligned planar options specify a single wall in a dimension. For an xcylinder, ycylinder or zcylinder, the radius and the cylinder axis in the other two dims are specified. The general_plane is an arbitrarily oriented plane, where a point on the plane and a vector orthogonal to the plane need to be specified. The vector does not need to be a unit vector. The general_cylinder is an arbitrarily oriented cylinder, where the radius, a point on and a direction vector parallel to the cylinder axis need to be specified. The direction vector does not need to be normalized. general_cone defines an open (frustum of a) cone described by the two endpoints of the cone (frustum) axis and the radii at these points.

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.

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.

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

Styles with a cuda, gpu, 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 in Section_accelerate of the manual. 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 USER-CUDA, GPU, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the Making LAMMPS section 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 when you invoke LAMMPS, or you can use the suffix command in your input script.

See Section_accelerate of the manual for more instructions on how to use the accelerated styles effectively.

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.

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”fix_move_mesh.html.

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’