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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<HR>
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<H3>pair_style gran/hertizian command
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</H3>
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<H3>pair_style gran/history command
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</H3>
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<H3>pair_style gran/no_history command
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</H3>
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<P><B>Syntax:</B>
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</P>
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2006-11-18 03:13:19 +08:00
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<PRE>pair_style style Kn gamma_n xmu dampflag
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2006-09-22 00:22:34 +08:00
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</PRE>
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<UL><LI>style = <I>gran/hertzian</I> or <I>gran/history</I> or <I>gran/no_history</I>
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<LI>Kn = spring constant for particle repulsion
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<PRE> (mg/d units where m is mass, g is the gravitational constant, d is diameter of a particle)
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</PRE>
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<LI>gamma_n = damping coefficient for normal direction collisions (sqrt(g/d) units)
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<LI>xmu = static yield criterion
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<LI>dampflag = flag (0/1) for whether to (no/yes) include tangential damping
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style gran/history 200000.0 0.5 1.0 1
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>The <I>gran</I> styles use the following formula <A HREF = "#Silbert">(Silbert)</A> for
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frictional force between two granular particles that are a distance r
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apart when r is less than the contact distance d.
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</P>
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<CENTER><IMG SRC = "Eqs/pair_granular.jpg">
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</CENTER>
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<P>The 1st term is a normal force and the 2nd term is a tangential force.
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The other quantites are as follows:
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</P>
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<UL><LI>delta = d - r
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<LI>f(x) = 1 for Hookean contacts used in pair styles <I>history</I> and <I>no_history</I>
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<LI>f(x) = sqrt(x) for pair style <I>hertzian</I>
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<LI>Kn = elastic constant for normal contact
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<LI>Kt = elastic constant for tangential contact = 2/7 of Kn
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<LI>gamma_n = viscoelastic constants for normal contact
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<LI>gamma_t = viscoelastic constants for tangential contact = 1/2 of gamma_n
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<LI>m_eff = Mi Mj / (Mi + Mj) = effective mass of 2 particles of mass Mi and Mj
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<LI>Delta St = tangential displacement vector between the 2 spherical particles which is truncated to satisfy a frictional yield criterion
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<LI>n = a unit vector along the line connecting the centers of the 2 particles
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<LI>Vn = normal component of the relative velocity of the 2 particles
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<LI>Vt = tangential component of the relative velocity of the 2 particles
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</UL>
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<P>The Kn and gamma_n coefficients are set as parameters to the
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pair_style command. Xmu is also specified which is the upper limit of
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the tangential force through the Coulomb criterion Ft = xmu*Fn. The
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tangential force between 2 particles grows according to a tangential
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spring and dash-pot model until Ft/Fn = xmu and then is held at Ft =
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Fn*xmu until the particles lose contact.
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</P>
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<P>For granular styles there are no individual atom type coefficients
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2007-06-26 08:03:39 +08:00
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that can be set via the <A HREF = "pair_coeff.html">pair_coeff</A> command. All
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global settings are made via the pair_style command.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-06-26 08:03:39 +08:00
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<P>See the citation below for more discussion of granular potentials.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-06-26 08:03:39 +08:00
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<HR>
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2007-10-04 08:21:14 +08:00
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<P><B>Mixing, shift, table, tail correction, per-atom energy/stress,
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restart, rRESPA info</B>:
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2007-06-26 08:03:39 +08:00
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</P>
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<P>The <A HREF = "pair_modify.html">pair_modify</A> mix, shift, table, and tail options
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are not relevant for granular pair styles.
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</P>
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2007-10-04 08:21:14 +08:00
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<P>These pair styles do not calculate per-atom energy and stress, as used
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2007-06-26 08:03:39 +08:00
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by the <A HREF = "compute_epair_atom.html">compute epair/atom</A>, <A HREF = "compute_stress_atom.html">compute
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stress/atom</A>, and <A HREF = "dump.html">dump custom</A>
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commands.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-10-04 08:21:14 +08:00
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<P>These pair styles write their information to <A HREF = "restart.html">binary restart
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files</A>, so a pair_style command does not need to be
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2007-06-26 08:03:39 +08:00
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specified in an input script that reads a restart file.
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</P>
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2007-10-04 08:21:14 +08:00
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<P>These pair styles can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. They do not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords of the <A HREF = "run_style.html">run_style
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command</A>.
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</P>
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2007-06-26 08:03:39 +08:00
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<HR>
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<P><B>Restrictions:</B> none
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2006-09-22 00:22:34 +08:00
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</P>
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2007-06-26 08:03:39 +08:00
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<P>All the granular pair styles are part of the "granular" package. It
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is only enabled if LAMMPS was built with that package. See the
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<A HREF = "Section_start.html#2_3">Making LAMMPS</A> section for more info.
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2006-09-22 00:22:34 +08:00
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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 = "pair_coeff.html">pair_coeff</A>
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</P>
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<P><B>Default:</B> none
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</P>
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<HR>
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<A NAME = "Silbert"></A>
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<P><B>(Silbert)</B> Silbert, Ertas, Grest, Halsey, Levine, Plimpton, Phys Rev
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E, 64, p 051302 (2001).
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</P>
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</HTML>
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