2006-09-22 00:22:34 +08:00
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<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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2012-04-06 22:28:14 +08:00
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<PRE>fix ID group-ID wall/gran Kn Kt gamma_n gamma_t xmu dampflag style 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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2012-04-06 22:28:14 +08:00
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<LI>one or more style/arg pairs may be appended
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<LI>style = <I>xlo</I> or <I>xhi</I> or <I>ylo</I> or <I>yhi</I> or <I>zlo</I> or <I>zhi</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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2012-04-06 22:28:14 +08:00
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<PRE> <I>xlo</I> or <I>xhi</I> or <I>ylo</I> or <I>yhi</I> or <I>zlo</I> or <I>zhi</I> args = coord vwall
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coord = position of wall = constant or variable
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constant = number like 0.0 or -30.0 (distance units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_x or v_wiggle
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vwall = velocity of wall in normal direction = constant or variable
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constant = number like 0.0 or 2.0 (velocity units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_vx or v_wiggle
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<I>zcylinder</I> args = rad vrad
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rad = radius of cylinder = constant or variable
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constant = number like 0.0 or -30.0 (distance units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_rad or v_wiggle
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vwall = velocity of radius in normal direction = constant or variable
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constant = number like 0.0 or 2.0 (velocity units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_vrad or v_wiggle
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2009-01-09 08:14:52 +08:00
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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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2012-04-06 22:28:14 +08:00
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<LI>keyword = <I>shear</I> or <I>piston</I>
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2008-04-10 01:29:51 +08:00
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2012-04-06 22:28:14 +08:00
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<PRE> <I>shear</I> values = dim vshear
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dim = <I>x</I> or <I>y</I> or <I>z</I> or <I>theta</I>
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vshear = shear velocity = constant or variable
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constant = number like 2.0 or -2.0 (velocity units or radians/time units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_vshear or v_wiggle
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<I>piston</I> values = pload pmass
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pload = external load on piston = constant or variable
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constant = number like 0.0 or 30.0 (force units)
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variable = <A HREF = "variable.html">equal-style variable</A> like v_push or v_wiggle
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pmass = piston mass (mass 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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2012-04-06 22:28:14 +08:00
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<PRE>fix 1 all wall/gran 200000.0 NULL 50.0 NULL 0.5 0 xlo -10.0 0 xhi 10.0 0
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fix 1 all wall/gran 200000.0 NULL 50.0 NULL 0.5 0 zhi v_squeeze shear x v_ramp
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fix 2 all wall/gran 100000.0 20000.0 50.0 30.0 0.5 1 zcylinder 15.0 0.0 piston -5000.0 1000.0
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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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2012-04-06 22:28:14 +08:00
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<P>Bound the simulation domain of a granular system with one or more
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frictional walls. All particles in the group interact with the wall
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when they are close enough to touch it.
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2006-09-22 00:22:34 +08:00
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</P>
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2012-04-06 22:28:14 +08:00
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<P>The first set of parameters (<I>Kn</I>, <I>Kt</I>, <I>gamma_n</I>, <I>gamma_t</I>, <I>xmu</I>,
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and <I>dampflag</I>) have the same meaning as those specified with the
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2009-01-09 07:18:29 +08:00
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<A HREF = "pair_gran.html">pair_style granular</A> force fields. This means a NULL
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2012-04-06 22:28:14 +08:00
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can be used for either <I>Kt</I> or <I>gamma_t</I> as described on that page.
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If a NULL is used for <I>Kt</I>, then a default value is used where <I>Kt</I> =
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2/7 <I>Kn</I>. If a NULL is used for <I>gamma_t</I>, then a default value is
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used where <I>gamma_t</I> = 1/2 <I>gamma_n</I>.
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2009-01-09 07:18:29 +08:00
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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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2012-04-06 22:28:14 +08:00
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The units for <I>Kn</I>, <I>Kt</I>, <I>gamma_n</I>, and <I>gamma_t</I> are as described on
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that doc page. The meaning of xmu and dampflag are also as described
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on that page. Note that you can choose different values for these 6
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2009-01-09 07:18:29 +08:00
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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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2012-04-06 22:28:14 +08:00
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<HR>
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<P>One or more wall <I>styles</I> can be specified. The <I>xlo</I>, <I>xhi</I>, <I>ylo</I>,
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<I>yhi</I>, <I>zlo</I>, <I>zhi</I> styles are planar faces that push particles back
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into the box from one of their sides. E.g. the <I>xlo</I> wall should be
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positioned to the left of all particles in the x dimension, so that it
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pushes particles to the right, and conversely for the <I>xhi</I> wall. The
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<I>zcylinder</I> style is a cylindrical shell with its axis along the
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z-axis, centered at x = y = 0.0. The interior surface of the cylinder
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pushes particles towards the x = y = 0.0 center axis. The <I>zcylinder</I>
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style cannot be specified with any of the <I>xlo</I>, <I>xhi</I>, <I>ylo</I>, <I>yhi</I>
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styles, but if can be with the <I>zlo</I> or <I>zhi</I> styles.
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</P>
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<HR>
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<P>For the planer wall styles, <I>coord</I> and <I>vwall</I> arguments are
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specified. These are the position and normal velocity of the wall. A
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positive velocity means the wall is moving in the positive direction
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in the normal dimension, e.g. +x for a <I>xlo</I> or <I>xhi</I> wall. Either or
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both of the <I>coord</I> and <I>vwall</I> arguments can be specified as a
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constant value or a variable. Note that the force exerted by a wall
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on granular particles depends on the velocity of the wall, in both the
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normal and tangential directions, so it is important to set <I>vwall</I>
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consistent with the <I>coord</I> argument.
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</P>
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<P>If a numeric constant is specified for <I>coord</I> then the wall is placed
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at that position in the appropriate dimension (x, y, or z) and will
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never move. Likewise if a constant is specified for <I>vwall</I>, then the
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normal velocity of the wall is set to that value and will never
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change.
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</P>
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<P>If <I>coord</I> or <I>vwall</I> is a variable, it should be specified as v_name,
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where name is an <A HREF = "variable.html">equal-style variable</A> name. In this
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case the variable is evaluated each timestep and the result becomes
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the current position or normal velocity of the wall. Equal-style
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variables can specify formulas with various mathematical functions,
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and include <A HREF = "thermo_style.html">thermo_style</A> command keywords for the
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simulation box parameters and timestep and elapsed time. Thus it is
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easy to specify a time-dependent wall position and velocity.
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</P>
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<P>If <I>coord</I> is specified as a constant, then <I>vwall</I> must be specified
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as 0.0. If <I>coord</I> is specified as a variable, then <I>vwall</I> must be a
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non-zero constant or a variable. If <I>vwall</I> is specified as a
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non-zero constant or variable and <I>coord</I> as a constant, then <I>coord</I>
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is treated as the initial position of the wall, and the wall position
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is updated each timestep by dt*v_current, where dt = the timestep, and
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v_current = the current normal velocity.
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</P>
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<HR>
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<P>For the <I>zcylinder</I> wall style, <I>rad</I> and <I>vrad</I> arguments are
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specified. These are the radius and radial velocity of the cylinder.
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A positive velocity means the radius is growing. Either or both of
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the <I>rad</I> and <I>vrad</I> arguments can be specified as a constant value or
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a variable. Note that the force exerted by the wall of the cylinder
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on granular particles depends on the velocity of the wall, in both the
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normal (radial) and tangential directions, so it is important to set
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<I>vrad</I> consistent with the <I>rad</I> argument.
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</P>
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<P>If a numeric constant is specified for <I>rad</I> then the cylinder will
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have a fixed radius. Likewise if a constant is specified for <I>vrad</I>,
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then the radial velocity of the cylinder is set to that value and will
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never change.
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</P>
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<P>If <I>rad</I> or <I>vrad</I> is a variable, it should be specified as
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v_name, where name is an <A HREF = "variable.html">equal-style variable</A> name.
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In this case the variable is evaluated each timestep and the result
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becomes the current radius or radial velocity of the cylinder.
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Equal-style variables can specify formulas with various mathematical
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functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent cylinder radius and
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velocity.
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</P>
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<P>If <I>rad</I> is specified as a constant, then <I>vrad</I> must be specified as
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0.0. If <I>rad</I> is specified as a variable, then <I>vrad</I> must be a
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non-zero constant or a variable. If <I>vrad</I> is specified as a non-zero
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constant or variable and <I>rad</I> as a constant, then <I>rad</I> is treated as
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the initial radius of the wall, and the radius is updated each
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timestep by dt*v_current, where dt = the timestep, and v_current = the
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current radial velocity.
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</P>
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<HR>
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<P>Here are examples of variable definitions that move the wall position
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in a time-dependent fashion using equal-style
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<A HREF = "variable.html">variables</A>. They also specify the correct normal
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velocity of the wall based on the time derivative of the position.
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</P>
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<PRE>variable ramp equal ramp(0,10)
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variable vnorm equal 10/(10000*dt) # 10000 step run
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fix 1 all wall/gran ... xlo v_ramp v_vnorm
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</PRE>
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<PRE>variable linear equal vdisplace(0,20)
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fix 1 all wall/gran ... xlo v_linear 20
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2006-09-22 00:22:34 +08:00
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</PRE>
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2012-04-06 22:28:14 +08:00
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<PRE>variable omega equal 2*PI/3.0
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variable wiggle equal swiggle(0.0,5.0,3.0)
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variable vwiggle equal "5.0 * v_omega * cos(v_omega*(timestep-startstep)*dt)"
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fix 1 all wall/gran ... xlo v_wiggle v_vwiggle
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</PRE>
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<PRE>variable omega equal 2*PI/3.0
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variable wiggle equal cwiggle(0.0,5.0,3.0)
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variable vwiggle equal "5.0 * v_omega * sin(v_omega*(timestep-startstep)*dt)"
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fix 1 all wall/gran ... xlo v_wiggle v_vwiggle
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</PRE>
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<P>The ramp(lo,hi) function adjusts the wall position linearly from lo to
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hi over the course of a run. The vdisplace(c0,velocity) function does
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something similar using the equation position = c0 + velocity*delta,
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where delta is the elapsed time.
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</P>
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<P>The swiggle(c0,A,period) function causes the wall position to
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oscillate sinusoidally according to this equation, where omega = 2 PI
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/ period:
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2006-09-22 00:22:34 +08:00
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</P>
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2012-04-06 22:28:14 +08:00
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<PRE>position = c0 + A sin(omega*delta)
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</PRE>
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<P>The cwiggle(c0,A,period) function causes the wall position to
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oscillate sinusoidally according to this equation, which will have an
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initial wall velocity of 0.0, and thus may impose a gentler
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perturbation on the particles:
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</P>
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<PRE>position = c0 + A (1 - cos(omega*delta))
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</PRE>
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<HR>
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<P>The optional keywords <I>shear</I> and <I>piston</I> are applied to all the
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specified wall styles. If you need to apply different options to
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different wall styles, then you should use multiple fix wall/gran
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commands.
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</P>
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<HR>
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<P>The <I>shear</I> keyword allows specification of one or two tangential
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velocities for planar or <I>zcylinder</I> walls. The <I>dim</I> value specifies
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the velocity direction. <I>X</I>, <I>y</I>, and <I>z</I> can be used for planar
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walls, but not for the direction normal to the wall. <I>Z</I> and <I>theta</I>
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can be used for a <I>zcylinder</I> wall. <I>Theta</I> is the azimuthal angle in
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the xy plane, i.e. a rotation of the cylinder about its vertical z
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axis. Note that a positive theta velocity means increasing theta,
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i.e. rotation in a counter-clockwise direction when looking down on
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the xy plane from above.
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</P>
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<P><I>Vshear</I> is the magnitude of the shear velocity and can be specified
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as a numeric constant or variable. The units of <I>vshear</I> are velocity
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units for dim = <I>x</I>, <I>y</I>, or <I>z</I>. They are units of radians/time for
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<I>dim</I> = <I>theta</I>.
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</P>
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<P>If a numeric constant is used, then then the shear velocity will never
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change. If <I>vshear</I> is a variable, it should be specified as v_name,
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where name is an <A HREF = "variable.html">equal-style variable</A> name. In this
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case the variable is evaluated each timestep and the result becomes
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the current shear velocity. Equal-style variables can specify
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formulas with various mathematical functions, and include
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<A HREF = "thermo_style.html">thermo_style</A> command keywords for the simulation
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box parameters and timestep and elapsed time. Thus it is easy to
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specify a time-dependent shear velocity.
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</P>
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<HR>
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<P>The <I>piston</I> keyword allows specification of an external loading force
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on the wall, which is opposed by granular particles pushing back
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against the load. The wall position and normal velocity adjust
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dynamically in response to these 2 opposing forces.
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</P>
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<P><I>Pload</I> is the magnitude of the loading force and can be specified as
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a numeric constant or variable. This force must have a value less
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than 0.0 which means that it is directed inward towards the particles.
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Thus for a <I>xlo</I> wall, a negative values is a force in the +x
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direction. For a <I>xhi</I> wall a negative values is a force in the -x
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direction. For a <I>zcylinder</I> wall, a negative value is a force
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towards the center z axis of the cylinder.
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</P>
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<P>If a numeric constant is used for <I>pload</I>, then then the load force
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will never change. If <I>pload</I> is a variable, it should be specified
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as v_name, where name is an <A HREF = "variable.html">equal-style variable</A> name.
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In this case the variable is evaluated each timestep and the result
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becomes the current load force. Equal-style variables can specify
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formulas with various mathematical functions, and include
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<A HREF = "thermo_style.html">thermo_style</A> command keywords for the simulation
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box parameters and timestep and elapsed time. Thus it is easy to
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specify a time-dependent load force.
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</P>
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<P>When the <I>piston</I> keyword is used, the <I>coord</I> and <I>vwall</I> arguemnts
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for each wall style (or <I>rad</I> and <I>vrad</I> arguments for a <I>zcylinder</I>
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style) must be numeric constants and are treated as initial values of
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the position/radius and velocity/radial-velocity. The initial values
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are updated every timestep by integrating these equations in a
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velocity-Verlet fashion:
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</P>
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<PRE>vwall_new = vwall + dt * (Fload + Fparticles) / pmass
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xwall_new = xwall + dt * vwall_new
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</PRE>
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<P>Fload and Fparticles are the force on the wall due to the external
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load and interactions with particles respectively. Fparticles is
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always a positive value, representing particles pushing outward on the
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wall (-x direction for a <I>xlo</I> wall, +x direction for a <I>xhi</I> wall,
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radially outward for a <I>zylinder</I> wall). Thus Fload and Fparticles
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will always have opposite signs. <I>Pmass</I> is the mass of the wall, as
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specified with the <I>piston</I> keyword. Xwall and Vwall are the wall
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position (or radius) and normal velocity (or radial velocity) on the
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previous timestep. Xwall_new and vwall_new are the updated values.
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</P>
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<HR>
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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>
|
2012-04-06 22:28:14 +08:00
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<P>The fix does not write any information about the walls (e,g, position
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or velocity) to the restart file. It is up to you to insure that fix
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wall/gran commands you re-specify in a restart input script are
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consistent with the previous simulation.
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</P>
|
2007-06-26 08:03:39 +08:00
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<P>None of the <A HREF = "fix_modify.html">fix_modify</A> options are relevant to this
|
2012-04-06 22:28:14 +08:00
|
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fix.
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</P>
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<P>This fix computes a global vector of values, which can be accessed by
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various <A HREF = "Section_howto.html#howto_15">output commands</A>. The length of
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the vector is 4*Nstyle, where Nstyle is the number of styles
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specified.
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|
</P>
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<P>The 4 values (per style) are as follows:
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</P>
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<PRE>(1) position (or radius) of wall (distance units)
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(2) normal (or radial) velocity of wall (velocity or radians/time units)
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(3) positive force on wall due to particles (force units)
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(4) negative force on wall due to external load (force units)
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</PRE>
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<P>The sign conventions for these values are as described above. The 4th
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value will be 0.0 if the <I>piston</I> keyword is not used. The vector
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values calculated by this fix are "extensive".
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|
</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
|
2010-04-03 00:51:06 +08:00
|
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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>.
|
2007-06-26 08:03:39 +08:00
|
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</P>
|
2006-09-22 00:22:34 +08:00
|
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|
<P><B>Restrictions:</B>
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|
</P>
|
2011-08-27 02:53:00 +08:00
|
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|
<P>This fix is part of the GRANULAR package. It is only enabled if
|
2011-08-26 00:46:23 +08:00
|
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|
LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
|
2007-06-26 08:03:39 +08:00
|
|
|
LAMMPS</A> section for more info.
|
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|
</P>
|
2007-03-08 09:01:08 +08:00
|
|
|
<P>Any dimension (xyz) that has a granular wall must be non-periodic.
|
|
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|
</P>
|
2006-09-22 00:22:34 +08:00
|
|
|
<P><B>Related commands:</B>
|
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|
|
</P>
|
2010-06-29 01:44:48 +08:00
|
|
|
<P><A HREF = "fix_move.html">fix_move</A>, <A HREF = "pair_gran.html">pair_style granular</A>
|
2006-09-22 00:22:34 +08:00
|
|
|
</P>
|
2012-04-06 22:28:14 +08:00
|
|
|
<P><B>Default:</B>
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|
|
</P>
|
|
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|
|
<P>The default shear velocities are 0.0.
|
2006-09-22 00:22:34 +08:00
|
|
|
</P>
|
|
|
|
|
</HTML>
|
