remove outdated .txt file

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

:link(lws,http://lammps.sandia.gov)

:link(ld,Manual.html)

:link(lc,Commands_all.html)

:line

fix wall/reflect/stochastic command :h3

[Syntax:]

fix ID group-ID wall/reflect/stochastic rstyle seed face args ... keyword value ... :pre

ID, group-ID are documented in "fix"_fix.html command :ulb,l

wall/reflect/stochastic = style name of this fix command :l

rstyle = diffusive or maxwell or ccl :l

seed = random seed for stochasticity (positive integer) :l

one or more face/args pairs may be appended :l

face = {xlo} or {xhi} or {ylo} or {yhi} or {zlo} or {zhi} :l

  args = pos temp velx vely velz accomx accomy accomz

    pos = EDGE or constant

      EDGE = current lo or hi edge of simulation box

      constant = number like 0.0 or 30.0 (distance units)

    temp = wall temperature (temperature units)

    velx,vely,velz = wall velocity in x,y,z directions (velocity units)

    accomx,accomy,accomz = accommodation coeffs in x,y,z directions (unitless)

      not specified for rstyle = diffusive

      single accom coeff specified for rstyle maxwell

      all 3 coeffs specified for rstyle cll :pre 

zero or more keyword/value pairs may be appended :l

keyword = {units} :l

  {units} value = {lattice} or {box}

    {lattice} = the wall position is defined in lattice units

    {box} = the wall position is defined in simulation box units :pre

:ule

[Examples:]

fix zwalls all wall/reflect/stochastic diffusive 23424 zlo EDGE 300 0.1 0.1 0 zhi EDGE 200 0.1 0.1 0

fix ywalls all wall/reflect/stochastic maxwell 345533 ylo 5.0 300 0.1 0.0 0.0 0.8 yhi 10.0 300 0.1 0.0 0.0 0.8

fix xwalls all wall/reflect/stochastic cercignanilampis 2308 xlo 0.0 300 0.0 0.1 0.9 0.8 0.7 xhi EDGE 300 0.0 0.1 0 0.9 0.8 0.7 units box :pre

[Description:]

Bound the simulation with one or more walls which reflect particles

in the specified group when they attempt to move through them.

Reflection means that if an atom moves outside the wall on a timestep

(e.g. due to the "fix nve"_fix_nve.html command), then it is put back

inside the wall with a changed velocity.

This fix models treats the wall as a moving solid boundary with a

finite temperature, which can exchange energy with particles that

collide with it.  This is different than the simpler "fix

wall/reflect"_fix_wall_reflect.html command which models mirror

reflection.  For this fix, the post collision velocity of each

particle is treated stochastically.  The randomness can come from many

sources: thermal motion of the wall atoms, surface roughness, etc.

Three stochastic reflection models are currently implemented.

For rstyle {diffusive}, particles are reflected diffusively. Their

velocity distribution corresponds to an equilibrium distribution of

particles at the wall temperature.  No accomondation coeffs are

specified.

For rstyle {maxwell}, particle reflection is Maxwellian which means

partially diffusive and partially specular ("Maxwell"_#Maxwell).  A

single accommodation coeff is specified which must be between 0.0 and

1.0 inclusive.  It determines the fraction of the collision which is

diffusive versus specular.  An accomodation coeff of 1.0 is fully

diffusive; a coeff of 0.0 is fully specular.

For rstyle {cll}, particle collisons are computed by the

Cercignani/Lampis model.  See "CL"_#CL and "To"_#To for details.

Three accommodations coefficient are specified.  Each must be between

0.0 and 1.0 inclusive.  Two are velocity accommodation coefficients;

one is a normal kinetic energy accommodation.  The normal coeff is the

one corresponding to the normal of the wall itself.  For example if

the wall is {ylo} or {yhi}, {accomx} and {accomz} are the tangential

velocity accommodation coefficients, and {accomy} is the normal

kinetic energy accommodation coefficient.

The optional {units} keyword determines the distance units used to

define a wall position.  A {box} value selects standard distance units

as defined by the "units"_units.html command, e.g. Angstroms for units

= real or metal.  A {lattice} value means the distance units are in

lattice spacings. The "lattice"_lattice.html command must have been

previously used to define the lattice spacings.

:line

[Restrictions:]

This fix has the same limitations as the "fix

wall/reflect"_fix_wall_reflect.html command.  Any dimension (xyz) that

has a wall must be non-periodic.  It should not be used with rigid

bodies such as those defined by the "fix rigid"_fix_rigid.html

command.  The wall velocity must lie on the same plane as the wall

itself.

This fix is part of the USER-MISC package.  It is only enabled if

LAMMPS was built with that package.  See the "Build

package"_Build_package.html doc page for more info.

[Related commands:]

"fix wall/reflect"_fix_wall_reflect.html

[Default:]

The default for the units keyword is lattice.

:line

:link(Maxwell)

[(Maxwell)] J.C. Maxwell, Philos. Tans. Royal Soc. London, 157: 49-88

(1867).

:link(CL)

[(Cercignani)] C. Cercignani and M. Lampis. Trans. Theory

Stat. Phys. 1, 2, 101 (1971).

:link(To)

[(To)] Q.D. To, V.H. Vu, G. Lauriat, and

C. Leonard. J. Math. Phys. 56, 103101 (2015).