2006-09-22 00:22:34 +08:00
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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
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</H3>
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<P><B>Syntax:</B>
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</P>
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2009-01-09 07:18:29 +08:00
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<PRE>fix ID group-ID wall/gran Kn Kt gamma_n gamma_t xmu dampflag wallstyle args keyword values ...
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2006-09-22 00:22:34 +08:00
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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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2009-01-09 07:18:29 +08:00
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<LI>Kn = elastic constant for normal particle repulsion (force/distance units or pressure units - see discussion below)
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<LI>Kt = elastic constant for tangential contact (force/distance units or pressure units - see discussion below)
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<LI>gamma_n = damping coefficient for collisions in normal direction (1/time units or 1/time-distance units - see discussion below)
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<LI>gamma_t = damping coefficient for collisions in tangential direction (1/time units or 1/time-distance units - see discussion below)
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<LI>xmu = static yield criterion (unitless fraction between 0.0 and 1.0)
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<LI>dampflag = 0 or 1 if tangential damping force is excluded or included
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<LI>wallstyle = <I>xplane</I> or <I>yplane</I> or <I>zplane</I> or <I>zcylinder</I>
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2006-09-22 00:22:34 +08:00
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<LI>args = list of arguments for a particular style
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2009-01-09 08:14:52 +08:00
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<PRE> <I>xplane</I> or <I>yplane</I> or <I>zplane</I> args = lo hi
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2008-04-10 01:29:51 +08:00
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lo,hi = position of lower and upper plane (distance units), either can be NULL)
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2009-01-09 07:18:29 +08:00
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<I>zcylinder</I> args = radius
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2009-01-09 08:14:52 +08:00
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radius = cylinder radius (distance units)
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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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2006-09-22 00:22:34 +08:00
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2008-04-10 01:29:51 +08:00
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<LI>keyword = <I>wiggle</I> or <I>shear</I>
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<PRE> <I>wiggle</I> 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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<I>shear</I> values = dim vshear
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dim = <I>x</I> or <I>y</I> or <I>z</I>
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vshear = magnitude of shear velocity (velocity units)
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2006-09-22 00:22:34 +08:00
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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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2009-01-09 07:18:29 +08:00
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<PRE>fix 1 all wall/gran 200000.0 NULL 50.0 NULL 0.5 0 xplane -10.0 10.0
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fix 1 all wall/gran 200000.0 NULL 50.0 NULL 0.5 0 zplane 0.0 NULL
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fix 2 all wall/gran 100000.0 20000.0 50.0 30.0 0.5 1 zcylinder 15.0 wiggle z 3.0 2.0
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2006-09-22 00:22:34 +08:00
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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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2009-01-09 07:18:29 +08:00
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<P>The first set of parameters (Kn, Kt, gamma_n, gamma_t, xmu, and
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dampflag) have the same meaning as those specified with the
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<A HREF = "pair_gran.html">pair_style granular</A> force fields. This means a NULL
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can be used for either Kt or gamma_t as described on that page. If a
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NULL is used for Kt, then a default value is used where Kt = 2/7 Kn.
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If a NULL is used for gamma_t, then a default value is used where
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gamma_t = 1/2 gamma_n.
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</P>
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<P>The nature of the wall/particle interactions are determined by which
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pair_style is used in your input script: <I>hooke</I>, <I>hooke/history</I>, or
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<I>hertz/history</I>. The equation for the force between the wall and
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particles touching it is the same as the corresponding equation on the
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<A HREF = "pair_gran.html">pair_style granular</A> doc page, in the limit of one of
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the two particles going to infinite radius and mass (flat wall).
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I.e. delta = radius - r = overlap of particle with wall, m_eff = mass
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of particle, and sqrt(RiRj/Ri+Rj) becomes sqrt(radius of particle).
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The units for Kn, Kt, gamma_n, and gamma_t are as described on that
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doc page. The meaning of xmu and dampflag are also as described on
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that page. Note that you can choose different values for these 6
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wall/particle coefficients than for particle/particle interactions, if
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you wish your wall to interact differently with the particles, e.g. if
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the wall is a different material.
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</P>
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<P>IMPORTANT NOTE: As discussed on the doc page for <A HREF = "pair_gran.html">pair_style
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granular</A>, versions of LAMMPS before 9Jan09 used a
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different equation for Hertzian interactions. This means Hertizian
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wall/particle interactions have also changed. They now include a
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sqrt(radius) term which was not present before. Also the previous
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versions used Kn and Kt from the pairwise interaction and hardwired
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dampflag to 1, rather than letting them be specified directly. This
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means you can set the values of the wall/particle coefficients
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appropriately in the current code to reproduce the results of a
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prevoius Hertzian monodisperse calculation. For example, for the
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common case of a monodisperse system with particles of diameter 1, Kn,
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Kt, gamma_n, and gamma_s should be set sqrt(2.0) larger than they were
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previously.
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</P>
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2006-09-22 00:22:34 +08:00
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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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</P>
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2008-04-10 01:29:51 +08:00
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<P>Optionally, the wall can be moving, if the <I>wiggle</I> or <I>shear</I>
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keywords are appended. Both keywords cannot be used together.
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</P>
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<P>For the <I>wiggle</I> keyword, the wall oscillates sinusoidally, similar to
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2010-06-29 01:44:48 +08:00
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the oscillations of particles which can be specified by the
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<A HREF = "fix_move.html">fix_move</A> command. This is useful in packing
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2008-04-10 01:29:51 +08:00
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simulations of granular particles. The arguments to the <I>wiggle</I>
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keyword specify a dimension for the motion, as well as it's
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<I>amplitude</I> and <I>period</I>. Note that if the dimension is in the plane
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of the wall, this is effectively a shearing motion. If the dimension
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is perpendicular to the wall, it is more of a shaking motion. A
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<I>zcylinder</I> wall can only be wiggled in the z dimension.
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</P>
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<P>Each timestep, the position of a wiggled wall in the appropriate <I>dim</I>
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is set according to this equation:
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2006-09-22 00:22:34 +08:00
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</P>
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2009-12-04 02:37:34 +08:00
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<PRE>position = coord + A - A cos (omega * delta)
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2006-09-22 00:22:34 +08:00
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</PRE>
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2009-12-04 02:37:34 +08:00
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<P>where <I>coord</I> is the specified initial position of the wall, <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 time
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elapsed since the fix was specified. The velocity of the wall is set
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to the derivative of this expression.
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2008-04-10 01:29:51 +08:00
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</P>
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<P>For the <I>shear</I> keyword, the wall moves continuously in the specified
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dimension with velocity <I>vshear</I>. The dimension must be tangential to
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walls with a planar <I>wallstyle</I>, e.g. in the <I>y</I> or <I>z</I> directions for
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an <I>xplane</I> wall. For <I>zcylinder</I> walls, a dimension of <I>z</I> means the
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cylinder is moving in the z-direction along it's axis. A dimension of
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<I>x</I> or <I>y</I> means the cylinder is spinning around the z-axis, either in
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the clockwise direction for <I>vshear</I> > 0 or counter-clockwise for
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<I>vshear</I> < 0. In this case, <I>vshear</I> is the tangential velocity of
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the wall at whatever <I>radius</I> has been defined.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-10-11 06:28:11 +08:00
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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2007-06-26 08:03:39 +08:00
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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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2010-04-03 00:51:06 +08:00
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fix. No global or per-atom quantities are stored by this fix for
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access by various <A HREF = "Section_howto.html#4_15">output commands</A>. No
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parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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2007-06-26 08:03:39 +08:00
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</P>
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2006-09-22 00:22:34 +08:00
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<P><B>Restrictions:</B>
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</P>
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2007-06-26 08:03:39 +08:00
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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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2007-03-08 09:01:08 +08:00
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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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2006-09-22 00:22:34 +08:00
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<P><B>Related commands:</B>
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</P>
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2010-06-29 01:44:48 +08:00
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<P><A HREF = "fix_move.html">fix_move</A>, <A HREF = "pair_gran.html">pair_style granular</A>
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2006-09-22 00:22:34 +08:00
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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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