Page Menu
Home
c4science
Search
Configure Global Search
Log In
Files
F115567551
fix_wall_piston.html
No One
Temporary
Actions
Download File
Edit File
Delete File
View Transforms
Subscribe
Mute Notifications
Award Token
Subscribers
None
File Metadata
Details
File Info
Storage
Attached
Created
Sun, Jun 1, 20:02
Size
5 KB
Mime Type
text/html
Expires
Tue, Jun 3, 20:02 (2 d)
Engine
blob
Format
Raw Data
Handle
26500078
Attached To
rLAMMPS lammps
fix_wall_piston.html
View Options
<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 0.0 0.0 vel 0.0 0.0 10.0 units box
fix top all wall/piston zlo vel 0.0 0.0 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
</P>
<P><A HREF = "fix_append_atoms.html">fix append/atoms</A> command
</P>
<P><B>Default:</B> <I>pos</I> = 0, <I>vel</I> = 0, <I>units</I> = lattice.
</P>
<HR>
</HTML>
Event Timeline
Log In to Comment