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Wed, Nov 27, 12:40
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rLAMMPS lammps
fix_wall_piston.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>
fix wall/piston command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
fix ID group-ID wall/piston face ... keyword value ...
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"fix.html"
>
fix
</A>
command
<LI>
wall/piston = style name of this fix command
<LI>
face =
<I>
zlo
</I>
<LI>
zero or more keyword/value pairs may be appended
<LI>
keyword =
<I>
pos
</I>
or
<I>
vel
</I>
or
<I>
ramp
</I>
or
<I>
units
</I>
<PRE>
<I>
pos
</I>
args = z
z = z coordinate at which the piston begins (distance units)
<I>
vel
</I>
args = vz
vz = final velocity of the piston (velocity units)
<I>
ramp
</I>
= use a linear velocity ramp from 0 to vz
<I>
temp
</I>
args = target damp seed extent
target = target velocity for region immediately ahead of the piston
damp = damping paramter (time units)
seed = random number seed for langevin kicks
extent = extent of thermostated region (distance units)
<I>
units
</I>
value =
<I>
lattice
</I>
or
<I>
box
</I>
<I>
lattice
</I>
= the wall position is defined in lattice units
<I>
box
</I>
= the wall position is defined in simulation box units
</PRE>
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
fix xwalls all wall/piston zlo
fix walls all wall/piston zlo pos 1.0 vel 10.0 units box
fix top all wall/piston zlo vel 10.0 ramp
</PRE>
<P><B>
Description:
</B>
</P>
<P>
Bound the simulation with a moving wall which reflect particles in the
specified group and drive the system with an effective infinite-mass
piston capable of driving shock waves.
</P>
<P>
A momentum mirror technique is used, which means that if an atom (or
the wall) moves such that an atom is outside the wall on a timestep by
a distance delta (e.g. due to
<A
HREF =
"fix_nve.html"
>
fix nve
</A>
), then it is put
back inside the face by the same delta, and the velocity relative to
the moving wall is flipped in z. For instance, a stationary particle
hit with a piston wall with velocity vz, will end the timestep with a
velocity of 2*vz.
</P>
<P>
Currently the
<I>
face
</I>
keyword can only be
<I>
zlo
</I>
. This creates a piston
moving in the positive z direction. Particles with z coordinate less
than the wall position are reflected to a z coordinate greater than
the wall position. If the piston velocity is vpz and the particle
velocity before reflection is vzi, the particle velocity after
reflection is -vzi + 2*vpz.
</P>
<P>
The initial position of the wall can be specified by the
<I>
pos
</I>
keyword.
</P>
<P>
The final velocity of the wall can be specified by the
<I>
vel
</I>
keyword
</P>
<P>
The
<I>
ramp
</I>
keyword will cause the wall/piston to adjust the velocity
linearly from zero velocity to
<I>
vel
</I>
over the course of the run. If
the
<I>
ramp
</I>
keyword is omitted then the wall/piston moves at a constant
velocity defined by
<I>
vel
</I>
.
</P>
<P>
The
<I>
temp
</I>
keyword will cause the region immediately in front of the
wall/piston to be thermostated with a Langevin thermostat. This
region moves with the piston. The damping and kicking are measured in
the reference frame of the piston. So, a temperature of zero would
mean all particles were moving at exactly the speed of the
wall/piston.
</P>
<P>
The
<I>
units
</I>
keyword determines the meaning of the distance units used
to define a wall position, but only when a numeric constant is used.
</P>
<P>
A
<I>
box
</I>
value selects standard distance units as defined by the
<A
HREF =
"units.html"
>
units
</A>
command, e.g. Angstroms for units = real or metal.
A
<I>
lattice
</I>
value means the distance units are in lattice spacings.
The
<A
HREF =
"lattice.html"
>
lattice
</A>
command must have been previously used to
define the lattice spacings.
</P>
<HR>
<P><B>
Restart, fix_modify, output, run start/stop, minimize info:
</B>
</P>
<P>
No information about this fix is written to
<A
HREF =
"restart.html"
>
binary restart
files
</A>
. None of the
<A
HREF =
"fix_modify.html"
>
fix_modify
</A>
options
are relevant to this fix. No global or per-atom quantities are stored
by this fix for access by various
<A
HREF =
"Section_howto.html#howoto_15"
>
output
commands
</A>
. No parameter of this fix can
be used with the
<I>
start/stop
</I>
keywords of the
<A
HREF =
"run.html"
>
run
</A>
command.
This fix is not invoked during
<A
HREF =
"minimize.html"
>
energy minimization
</A>
.
</P>
<P><B>
Restrictions:
</B>
</P>
<P>
This fix style is part of the SHOCK package. It is only enabled if
LAMMPS was built with that package. See the
<A
HREF =
"Section_start.html#start_3"
>
Making
LAMMPS
</A>
section for more info.
</P>
<P>
The face that has the wall/piston must be boundary type 's'
(shrink-wrapped). The opposing face can be
any boundary type other than periodic.
</P>
<P>
A wall/piston should not be used with rigid bodies such as those
defined by a "fix rigid" command. This is because the wall/piston
displaces atoms directly rather than exerting a force on them.
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"fix_wall.html"
>
fix wall/reflect
</A>
command,
<A
HREF =
"fix_append_atoms.html"
>
fix
append/atoms
</A>
command
</P>
<P><B>
Default:
</B>
</P>
<P>
The keyword defaults are pos = 0, vel = 0, units = lattice.
</P>
</HTML>
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