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<HTML>
<CENTER><A HREF = "http://www.cfdem.com">LIGGGHTS WWW Site</A> - <A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LIGGGHTS Documentation</A> - <A HREF = "Section_commands.html#comm">LIGGGHTS Commands</A>
</CENTER>
<HR>
<H3>pair_style gran command
</H3>
<P><B>Syntax:</B>
</P>
<PRE>pair_style model_type model_name model_keyword model_value
</PRE>
<LI>zero or more model_type/model_name pairs may be appended. They must be appended in the following order (!)
<PRE> <I>model</I> values = described <A HREF = "Section_gran_models.html">here</A>
<I>tangential</I> values = described <A HREF = "Section_gran_models.html">here</A>
<I>cohesion</I> values = described <A HREF = "Section_gran_models.html">here</A>
<I>rolling_friction</I> values = described <A HREF = "Section_gran_models.html">here</A>
<I>surface</I> values = described <A HREF = "Section_gran_models.html">here</A>
</PRE>
<LI>following the model_type/model_name pairs, zero or more model_keyword/model_valuezero pairs may be appended in arbitrary order
<PRE> model_type/model_name pairs = described for each model separately <A HREF = "Section_gran_models.html">here</A>
</PRE>
</UL>
<P><B>Examples:</B>
</P>
<PRE>pair_style gran model hooke tangential history
pair_style gran model hertz tangential history rolling_friction cdt
pair_style gran model hertz tangential no_history cohesion sjkr
</PRE>
<P><B>LIGGGHTS vs. LAMMPS Info:</B>
</P>
<P>This LIGGGHTS command offers the following improvements vs. LAMMPS: The stiffness and
damping coefficients k_n, k_t, gamma_n, gamma_t can now be derived from the material
properties. Also, a new model for macroscopic cohesion is introduced.
</P>
<P><B>General description:</B>
</P>
<P>The <I>gran</I> styles use the following formula for the frictional force between two granular
particles, when the distance r between two particles of radii Ri and Rj is less than their
contact distance d = Ri + Rj. Typically, there is no force between the particles when r > d.
</P>
<CENTER><IMG SRC = "Eqs/pair_gran_html_60b8ced2.png">
</CENTER>
<P>The quantities in the equations are as follows:
</P>
<UL><LI>delta_n = d - r = overlap distance of 2 particles
<LI>k_n = elastic constant for normal contact
<LI>k_t = elastic constant for tangential contact
<LI>gamma_n = viscoelastic damping constant for normal contact
<LI>gamma_t = viscoelastic damping constant for tangential contact
<LI>delta_t = tangential displacement vector between 2 spherical particles
</UL>
<P>In the first term is the normal force between the two particles and the second term is the
tangential force. The normal force has 2 terms, a contact force and a damping force. The
tangential force also has 2 terms: a shear force and a damping force. The shear force is
a "history" effect that accounts for the tangential displacement ("tangential overlap")
between the particles for the duration of the time they are in contact.
</P>
<P>The concrete implementation for k_n, k_t, gamma_n, gamma_t and the shear history
depend on the concrete models as chosen by the user. They are described on separate
doc pages <A HREF = "Section_gran_models.html">here</A>
</P>
<P>Also, other models may add additional forces or torques on the particles, such as
cohesive or rolling friction forces. These are also described on separate
doc pages <A HREF = "Section_gran_models.html">here</A>
</P>
<P>IMPORTANT NOTE: The order of model keywords is important, you have to stick
to the order as outlined in the "Syntax" section of this doc page.
</P>
<P><B>General comments:</B>
</P>
<P>For granular styles there are no additional coefficients to set for each pair of atom types
via the <A HREF = "pair_coeff.html">pair_coeff</A> command. All settings are global and are made via the
pair_style command. However you must still use the <A HREF = "pair_coeff.html">pair_coeff</A> for all pairs
of granular atom types. For example the command
</P>
<PRE>pair_coeff * *
</PRE>
<P>should be used if all atoms in the simulation interact via a granular potential
(i.e. one of the pair styles above is used). If a granular potential is used as a sub-style
of <A HREF = "pair_hybrid.html">pair_style hybrid</A>, then specific atom types can be used in the pair_coeff
command to determine which atoms interact via a granular potential.
</P>
<P><B>Mixing, shift, table, tail correction, restart, rRESPA info:</B>
</P>
<P>The <A HREF = "pair_modify.html">pair_modify</A> mix, shift, table, and tail options are not
relevant for granular pair styles.
</P>
<P>These pair styles write their information to <A HREF = "restart.html">binary restart files</A>,
so a pair_style command does not need to be specified in an input script that reads a restart file.
</P>
<P>IMPORTANT NOTE: The material properties are not written to restart files! Thus, if you
restart a simulation, you have to re-define them (by using the fixes mentioned above).
</P>
<P>These pair styles can only be used via the pair keyword of the <A HREF = "run_style.html">run_style respa</A>
command. They do not support the inner, middle, outer keywords.
</P>
<P><B>Restrictions:</B>
</P>
<P>These pair styles require that atoms store torque and angular velocity (omega) as
defined by the <A HREF = "atom_style.html">atom_style</A>. They also require a per-particle radius is
stored. The <I>sphere</I> or <I>granular</I> atom style does all of this.
</P>
<P>This pair style requires you to use the <A HREF = "communicate.html">communicate vel yes</A> option so that
velocites are stored by ghost atoms.
</P>
<P>Only unit system that are self-consistent (si, cgs, lj) can be used with this pair style.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "pair_coeff.html">pair_coeff</A>
Models for use with this command are described <A HREF = "Section_gran_models.html">here</A>
</P>
<P><B>Default:</B>
</P>
<P><I>model</I> = 'hertz'
<I>tangential</I> = 'history'
<I>rolling_friction</I> = 'off'
<I>cohesion</I> = 'off'
<I>surface</I> = 'default'
</P>
</HTML>
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