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