"LIGGGHTS WWW Site"_liws - "LAMMPS WWW Site"_lws - "LIGGGHTS Documentation"_ld - "LIGGGHTS Commands"_lc :c

:link(liws,http://www.cfdem.com)
:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)

:line

pair_style gran command :h3

[Syntax:]

pair_style model keyword value :pre

zero or more model_keyword/value pairs may be appended :l
  {model} values = described "here"_Section_gran_models.html
  {tangential} values = described "here"_Section_gran_models.html
  {rolling_friction} values = described "here"_Section_gran_models.html
  {cohesion} values = described "here"_Section_gran_models.html
  {surface} values = described "here"_Section_gran_models.html :pre
:ule

[Examples:]

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

[LIGGGHTS vs. LAMMPS Info:]

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.

[General description:]

The {gran} 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.

:c,image(Eqs/pair_gran_html_60b8ced2.png)

The quantities in the equations are as follows:

delta_n = d - r = overlap distance of 2 particles  :ulb,l
k_n = elastic constant for normal contact :l
k_t = elastic constant for tangential contact :l
gamma_n = viscoelastic damping constant for normal contact :l
gamma_t = viscoelastic damping constant for tangential contact :l
delta_t = tangential displacement vector between 2 spherical particles :l
:ule

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. 

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 "here"_Section_gran_models.html

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 "here"_Section_gran_models.html

[General comments:]

For granular styles there are no additional coefficients to set for each pair of atom types 
via the "pair_coeff"_pair_coeff.html command. All settings are global and are made via the 
pair_style command. However you must still use the "pair_coeff"_pair_coeff.html for all pairs 
of granular atom types. For example the command

pair_coeff * * :pre

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 "pair_style hybrid"_pair_hybrid.html, then specific atom types can be used in the pair_coeff 
command to determine which atoms interact via a granular potential. 


[Mixing, shift, table, tail correction, restart, rRESPA info:]

The "pair_modify"_pair_modify.html mix, shift, table, and tail options are not 
relevant for granular pair styles.

These pair styles write their information to "binary restart files"_restart.html, 
so a pair_style command does not need to be specified in an input script that reads a restart file.

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).

These pair styles can only be used via the pair keyword of the "run_style respa"_run_style.html 
command. They do not support the inner, middle, outer keywords. 

[Restrictions:]

These pair styles require that atoms store torque and angular velocity (omega) as 
defined by the "atom_style"_atom_style.html. They also require a per-particle radius is 
stored. The {sphere} or {granular} atom style does all of this.

This pair style requires you to use the "communicate vel yes"_communicate.html option so that 
velocites are stored by ghost atoms.

Only unit system that are self-consistent (si, cgs, lj) can be used with this pair style.

[Related commands:]

"pair_coeff"_pair_coeff.html
Models for use with this command are described "here"_Section_gran_models.html

[Default:] 

{model} = 'hertz'
{tangential} = 'history'
{rolling_friction} = 'off'
{cohesion} = 'off'
{surface} = 'default'

