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

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

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fix wall/gran/region command :h3

[Syntax:]

fix ID group-ID wall/gran/region fstyle Kn Kt gamma_n gamma_t xmu dampflag wallstyle regionID :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l
wall/region = style name of this fix command :l
fstyle = style of force interactions between particles and wall :l
  possible choices: hooke, hooke/history, hertz/history :pre
Kn = elastic constant for normal particle repulsion (force/distance units or pressure units - see discussion below) :l
Kt = elastic constant for tangential contact (force/distance units or pressure units - see discussion below) :l
gamma_n = damping coefficient for collisions in normal direction (1/time units or 1/time-distance units - see discussion below) :l
gamma_t = damping coefficient for collisions in tangential direction (1/time units or 1/time-distance units - see discussion below) :l
xmu = static yield criterion (unitless value between 0.0 and 1.0e4) :l
dampflag = 0 or 1 if tangential damping force is excluded or included :l
wallstyle = region (see "fix wall/gran"_fix_wall_gran.html for options for other kinds of walls) :l
region-ID = region whose boundary will act as wall :l,ule

[Examples:]

fix wall all wall/gran/region hooke/history 1000.0 200.0 200.0 100.0 0.5 1 region myCone :pre

[Description:]

Treat the surface of the geometric region defined by the {region-ID}
as a bounding frictional wall which interacts with nearby finite-size
granular particles when they are close enough to touch the wall.  See
the "fix wall/region"_fix_wall_region.html and "fix
wall/gran"_fix_wall_gran.html commands for related kinds of walls for
non-granular particles and simpler wall geometries, respectively.

Here are snapshots of example models using this command.
Corresponding input scripts can be found in examples/granregion.
Click on the images to see a bigger picture.  Movies of these
simulations are "here on the Movies
page"_http://lammps.sandia.gov/movies.html#granregion of the
LAMMPS web site.

:image(JPG/gran_funnel_small.jpg,JPG/gran_funnel.png)
:image(JPG/gran_mixer_small.jpg,JPG/gran_mixer.png)

:line

The distance between a particle and the region boundary is the
distance to the nearest point on the region surface.  The force the
wall exerts on the particle is along the direction between that point
and the particle center, which is the direction normal to the surface
at that point.  Note that if the region surface is comprised of
multiple "faces", then each face can exert a force on the particle if
it is close enough.  E.g. for "region_style block"_region.html, a
particle in the interior, near a corner of the block, could feel wall
forces from 1, 2, or 3 faces of the block.

Regions are defined using the "region"_region.html command.  Note that
the region volume can be interior or exterior to the bounding surface,
which will determine in which direction the surface interacts with
particles, i.e. the direction of the surface normal. The exception to
this is if one or more {open} options are specified for the region
command, in which case particles interact with both the interior and
exterior surfaces of regions.

Regions can either be primitive shapes (block, sphere, cylinder, etc)
or combinations of primitive shapes specified via the {union} or
{intersect} region styles.  These latter styles can be used to
construct particle containers with complex shapes.  Regions can also
move dynamically via the "region"_region.html command keywords (move)
and {rotate}, or change their shape by use of variables as inputs to
the "region"_region.html command.  If such a region is used with this
fix, then the region surface will move in time in the corresponding
manner.

NOTE: As discussed on the "region"_region.html command doc page,
regions in LAMMPS do not get wrapped across periodic boundaries.  It
is up to you to ensure that the region location with respect to
periodic or non-periodic boundaries is specified appropriately via the
"region"_region.html and "boundary"_boundary.html commands when using
a region as a wall that bounds particle motion.

NOTE: For primitive regions with sharp corners and/or edges (e.g. a
block or cylinder), wall/particle forces are computed accurately for
both interior and exterior regions.  For {union} and {intersect}
regions, additional sharp corners and edges may be present due to the
intersection of the surfaces of 2 or more primitive volumes.  These
corners and edges can be of two types: concave or convex.  Concave
points/edges are like the corners of a cube as seen by particles in
the interior of a cube.  Wall/particle forces around these features
are computed correctly.  Convex points/edges are like the corners of a
cube as seen by particles exterior to the cube, i.e. the points jut
into the volume where particles are present.  LAMMPS does NOT compute
the location of these convex points directly, and hence wall/particle
forces in the cutoff volume around these points suffer from
inaccuracies.  The basic problem is that the outward normal of the
surface is not continuous at these points.  This can cause particles
to feel no force (they don't "see" the wall) when in one location,
then move a distance epsilon, and suddenly feel a large force because
they now "see" the wall.  In a worst-case scenario, this can blow
particles out of the simulation box.  Thus, as a general rule you
should not use the fix wall/gran/region command with {union} or
{interesect} regions that have convex points or edges resulting from
the union/intersection (convex points/edges in the union/intersection
due to a single sub-region are still OK).

NOTE: Similarly, you should not define {union} or {intersert} regions
for use with this command that share an overlapping common face that
is part of the overall outer boundary (interior boundary is OK), even
if the face is smooth.  E.g. two regions of style block in a {union}
region, where the two blocks overlap on one or more of their faces.
This is because LAMMPS discards points that are part of multiple
sub-regions when calculating wall/particle interactions, to avoid
double-counting the interaction.  Having two coincident faces could
cause the face to become invisible to the particles.  The solution is
to make the two faces differ by epsilon in their position.

The nature of the wall/particle interactions are determined by the
{fstyle} setting.  It can be any of the styles defined by the
"pair_style granular"_pair_gran.html commands.  Currently this is
{hooke}, {hooke/history}, or {hertz/history}.  The equation for the
force between the wall and particles touching it is the same as the
corresponding equation on the "pair_style granular"_pair_gran.html doc
page, but the effective radius is calculated using the radius of the
particle and the radius of curvature of the wall at the contact point.

Specifically, delta = radius - r = overlap of particle with wall,
m_eff = mass of particle, and RiRj/Ri+Rj is the effective radius, with
Rj replaced by the radius of curvature of the wall at the contact
point.  The radius of curvature can be negative for a concave wall
section, e.g. the interior of cylinder.  For a flat wall, delta =
radius - r = overlap of particle with wall, m_eff = mass of particle,
and the effective radius of contact is just the radius of the
particle.

The parameters {Kn}, {Kt}, {gamma_n}, {gamma_t}, {xmu} and {dampflag}
have the same meaning and units as those specified with the
"pair_style granular"_pair_gran.html commands.  This means a NULL can
be used for either {Kt} or {gamma_t} as described on that page.  If a
NULL is used for {Kt}, then a default value is used where {Kt} = 2/7
{Kn}.  If a NULL is used for {gamma_t}, then a default value is used
where {gamma_t} = 1/2 {gamma_n}.

Note that you can choose a different force styles and/or different
values for the 6 wall/particle coefficients than for particle/particle
interactions.  E.g. if you wish to model the wall as a different
material.

[Restart, fix_modify, output, run start/stop, minimize info:]

Similar to "fix wall/gran"_fix_wall_gran.html command, this fix
writes the shear friction state of atoms interacting with the wall to
"binary restart files"_restart.html, so that a simulation can continue
correctly if granular potentials with shear "history" effects are
being used.  This fix also includes info about a moving region in the
restart file.  See the "read_restart"_read_restart.html command for
info on how to re-specify a fix in an input script that reads a
restart file, so that the operation of the fix continues in an
uninterrupted fashion.

NOTE: Information about region definitions is NOT included in restart
files, as discussed on the "read_restart"_read_restart.html doc page.
So you must re-define your region and if it is a moving region, define
its motion attributes in a way that is consistent with the simulation
that wrote the restart file.  In particular, if you want to change the
region motion attributes (e.g. its velocity), then you should ensure
the position/orientation of the region at the initial restart
timestep is the same as it was on the timestep the restart file was
written.  If this is not possible, you may need to ignore info in the
restart file by defining a new fix wall/gran/region command in your
restart script, e.g. with a different fix ID.  Or if you want to keep
the shear history info but discard the region motion information, you
can use the same fix ID for fix wall/gran/region, but assign it a
region with a different region ID.

None of the "fix_modify"_fix_modify.html options are relevant to this
fix.  No global or per-atom quantities are stored by this fix for
access by various "output commands"_Section_howto.html#howto_15.  No
parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.  This fix is not invoked during "energy
minimization"_minimize.html.

[Restrictions:]

This fix is part of the GRANULAR package.  It is only enabled if
LAMMPS was built with that package.  See the "Making
LAMMPS"_Section_start.html#start_3 section for more info.

[Related commands:]

"fix_move"_fix_move.html,
"fix wall/gran"_fix_wall_gran.html,
"fix wall/region"_fix_wall_region.html,
"pair_style granular"_pair_gran.html,
"region"_region.html

[Default:] none

