forked from lijiext/lammps
114 lines
4.4 KiB
HTML
114 lines
4.4 KiB
HTML
<HTML>
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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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<HR>
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<H3>fix wall/gran command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID wall/gran wallstyle args keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>wall/gran = style name of this fix command
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<LI>style = <I>xplane</I> or <I>yplane</I> or <I>zplane</I> or <I>zcylinder</I>
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<LI>args = list of arguments for a particular style
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<PRE> <I>xplane</I> or <I>yplane</I> or <I>zplane</I> args = lo hi gamma xmu
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lo, hi = position of lower and upper plane (either can be NULL)
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gamman = damping coeff for normal direction collisions with wall
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xmu = friction coeff for the wall
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<I>zcylinder</I> args = radius gamma xmu
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radius = cylinder radius (distance units)
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gamman = damping coeff for normal direction collisions with wall
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xmu = friction coeff for the wall
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</PRE>
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<LI>zero or more keyword/value pairs may be appended to args
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<PRE> keyword = <I>wiggle</I>
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values = dim amplitude period
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dim = <I>x</I> or <I>y</I> or <I>z</I>
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amplitude = size of oscillation (distance units)
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period = time of oscillation (time units)
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 all wall/gran xplane -10.0 10.0 50.0 0.5
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fix 2 all wall/gran zcylinder 15.0 50.0 0.5 wiggle z 3.0 2.0
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fix 1 all wall/gran zplane 0.0 NULL 100.0 0.5
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Bound the simulation domain of a granular system with a frictional
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wall. All particles in the group interact with the wall when they are
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close enough to touch it.
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</P>
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<P>The <I>wallstyle</I> can be planar or cylindrical. The 3 planar options
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specify a pair of walls in a dimension. Wall positions are given by
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<I>lo</I> and <I>hi</I>. Either of the values can be specified as NULL if a
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single wall is desired. For a <I>zcylinder</I> wallstyle, the cylinder's
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axis is at x = y = 0.0, and the radius of the cylinder is specified.
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For all wallstyles, a damping and friction coefficient for
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particle-wall interactions are also specified.
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</P>
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<P>Optionally, a wall can be oscillated, similar to the oscillations of
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frozen particles specified by the <A HREF = "fix_wiggle.html">fix_wiggle</A>
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command. This is useful in packing simulations of granular particles.
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If the keyword <I>wiggle</I> is appended to the argument list, then a
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dimension for the motion, as well as it's <I>amplitude</I> and <I>period</I> is
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specified. Each timestep, the position of the wall in the appropriate
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<I>dim</I> is set according to this equation:
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</P>
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<PRE>position = pos0 + A - A cos (omega * delta)
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</PRE>
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<P>where <I>pos0</I> is the position at the time the fix was specified, <I>A</I> is
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the <I>amplitude</I>, <I>omega</I> is 2 PI / <I>period</I>, and <I>delta</I> is the
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elapsed time since the fix was specified. The velocity of the wall is
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also set to the derivative of this expression.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>This fix writes the shear friction state of atoms interacting with the
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wall to <A HREF = "restart.html">binary restart files</A>, so that a simulation can
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continue correctly if granular potentials with shear "history" effects
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are being used. See the <A HREF = "read_restart.html">read_restart</A> command for
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info on how to re-specify a fix in an input script that reads a
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restart file, so that the operation of the fix continues in an
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uninterrupted fashion.
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</P>
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<P>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
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fix. No global scalar or vector or per-atom quantities are stored by
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this fix for access by various <A HREF = "Section_howto.html#4_15">output
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commands</A>. No parameter of this fix can be
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used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>This fix is part of the "granular" package. It is only enabled if
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LAMMPS was built with that package. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>Any dimension (xyz) that has a granular wall must be non-periodic.
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</P>
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<P>A zcylinder wall can only be oscillated in the z dimension.
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_wiggle.html">fix_wiggle</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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