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### fix wall/gran command

Syntax:

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

- ID, group-ID are documented in fix command
- wall/gran = style name of this fix command
- style = xplane or yplane or zplane or zcylinder
- args = list of arguments for a particular style

        xplane or yplane or zplane args = lo hi gamma xmu
          lo, hi = position of lower and upper plane (either can be NULL)
          gamman = damping coeff for normal direction collisions with wall
          xmu = friction coeff for the wall
        zcylinder args = radius gamma xmu
          radius = cylinder radius (distance units)
          gamman = damping coeff for normal direction collisions with wall
          xmu = friction coeff for the wall

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

        keyword = wiggle
          values = dim amplitude period
            dim = x or y or z
            amplitude = size of oscillation (distance units)
            period = time of oscillation (time units)

Examples:

    fix 1 all wall/gran xplane -10.0 10.0 50.0 0.5
    fix 2 all wall/gran zcylinder 15.0 50.0 0.5 wiggle z 3.0 2.0
    fix 1 all wall/gran zplane 0.0 NULL 100.0 0.5

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 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. For all wallstyles, a damping and friction coefficient for particle-wall interactions are also specified.

Optionally, a wall can be oscillated, similar to the oscillations of frozen particles specified by the fix_wiggle command. This is useful in packing simulations of granular particles. If the keyword wiggle is appended to the argument list, then a dimension for the motion, as well as it's amplitude and period is specified. Each timestep, the position of the wall in the appropriate dim is set according to this equation:

    position = pos0 + A - A cos (omega * delta)

where pos0 is the position at the time the fix was specified, A is the amplitude, omega is 2 PI / period, and delta is the elapsed time since the fix was specified. The velocity of the wall is also set to the derivative of this expression.

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, so that a simulation can continue correctly if granular potentials with shear "history" effects are being used. See the 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.

None of the fix_modify options are relevant to this fix. No global scalar or vector or per-atom quantities are stored by this fix for access by various output commands. 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:

This fix is part of the "granular" package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

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

A zcylinder wall can only be oscillated in the z dimension.

Related commands:

fix_wiggle

Default: none