Syntax:
fix ID group-ID wall/gran wallstyle args keyword values ...
xplane or yplane or zplane args = lo hi gamma xmu lo,hi = position of lower and upper plane (distance units), 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
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)
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, 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 frozen particles specified by the fix_wiggle 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 = 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 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.
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.
Related commands:
Default: none