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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>fix wall/gran command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>fix ID group-ID wall/gran wallstyle args keyword values ...
</PRE>
<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
<LI>wall/gran = style name of this fix command
<LI>style = <I>xplane</I> or <I>yplane</I> or <I>zplane</I> or <I>zcylinder</I>
<LI>args = list of arguments for a particular style
<PRE> <I>xplane</I> or <I>yplane</I> or <I>zplane</I> 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
<I>zcylinder</I> args = radius gamma xmu
radius = cylinder radius (distance units)
gamman = damping coeff for normal direction collisions with wall
xmu = friction coeff for the wall
</PRE>
<LI>zero or more keyword/value pairs may be appended to args
<PRE> keyword = <I>wiggle</I>
values = dim amplitude period
dim = <I>x</I> or <I>y</I> or <I>z</I>
amplitude = size of oscillation (distance units)
period = time of oscillation (time units)
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>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
</PRE>
<P><B>Description:</B>
</P>
<P>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.
</P>
<P>The <I>wallstyle</I> can be planar or cylindrical. The 3 planar options
specify a pair of walls in a dimension. Wall positions are given by
<I>lo</I> and <I>hi</I>. Either of the values can be specified as NULL if a
single wall is desired. For a <I>zcylinder</I> 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.
</P>
<P>Optionally, a wall can be oscillated, similar to the oscillations of
frozen particles specified by the <A HREF = "fix_wiggle.html">fix_wiggle</A>
command. This is useful in packing simulations of granular particles.
If the keyword <I>wiggle</I> is appended to the argument list, then a
dimension for the motion, as well as it's <I>amplitude</I> and <I>period</I> is
specified. Each timestep, the position of the wall in the appropriate
<I>dim</I> is set according to this equation:
</P>
<PRE>position = pos0 + A - A cos (omega * delta)
</PRE>
<P>where <I>pos0</I> is the position at the time the fix was specified, <I>A</I> is
the <I>amplitude</I>, <I>omega</I> is 2 PI / <I>period</I>, and <I>delta</I> is the
elapsed time since the fix was specified. The velocity of the wall is
also set to the derivative of this expression.
</P>
<P><B>Restrictions:</B>
</P>
<P>Any dimension (xyz) that has a granular wall must be non-periodic.
</P>
<P>This fix can only be used if LAMMPS was built with the "granular"
package and with atom_style granular. A zcylinder wall can only be
oscillated in the z dimension.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "fix_wiggle.html">fix_wiggle</A>
</P>
<P><B>Default:</B> none
</P>
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