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"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)

:line

fix wall/stochastic command :h3

[Syntax:]

fix ID group-ID wall/stochastic reflectionstyle face pos temp velx vely velz accomx accomy accomz ... :pre

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

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

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

reflectionstyle = diffusive or maxwell or cercignanilampis

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

  {xlo},{ylo},{zlo} pos = EDGE or constant 

    EDGE = current lo edge of simulation box

    constant = number like 0.0 or -30.0 (distance units)

    temp = wall temperature

    velx, vely, velz = wall velocity along x, y, z directions

    accomx, accomy, accomz = accommodation coefficients along x, y, z directions (only for maxwell and cercignanilampis reflection style)

  {xhi},{yhi},{zhi} pos = EDGE or constant or variable

    EDGE = current hi edge of simulation box

    constant = number like 50.0 or 100.3 (distance units)

    temp = wall temperature 

    velx, vely, velz = wall velocity along x, y, z directions

    accomx, accomy, accomz = accommodation coefficients along x, y, z directions (only for maxwell and cercignanilampis style)

    :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/stochastic diffusive zlo EDGE 300 0.1 0.1 0 zhi EDGE 200 0.1 0.1 0

fix ywalls all wall/stochastic maxwell 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/stochastic cercignanilampis 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:]

Different from the command fix wall/reflect which is equivalent to the mirror reflection, the post collision velocity of the atoms in the stochastic wall is random. The randomness can come from many sources: thermal motion of solid atoms, the surface roughness, etc... Three stochastic wall models are implemented. 

Diffusive wall (reflectionstyle = diffusive): all the gas atoms are reflected diffusively. Their velocity distribution corresponds to the equilibrium distribution at the wall temperature. 

Maxwell model (reflectionstyle = maxwell): the reflection is partially diffusive and partially specular. The accommodation coefficient is the portion of the diffusive reflection "Maxwell"_#Maxwell. 

Cercignani Lampis model: 3 accommodations coefficients are used, two velocity accommodation coefficients and one normal kinetic energy accommodation "CL"_#CL, "To"_#To. The code will recognize automatically the normal direction of the wall and the other two tangential directions. For example, if accomx = 0.5, accomy = 0.2, accomz = 0.3 and the wall is normal to direction y, accomx and accomz are the tangential velocity accommodation coefficients and accomy is the normal kinetic energy accommodation coefficients.

The {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.

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[Restrictions:]

Due to the similarities, this fix has the same limitation as the the

wall/reflect. Any dimension (xyz) that has a wall must be non-periodic. It

shoudld not be used with rigid bodies such as those defined by a "fix rigid"

command. Furthermore, the wall velocity must lie on the same plane as the wall. Package USER-MISC is required to use this fix.

[Related commands:]

"fix wall/reflect"_fix_wall_reflect.html, 

[Default:] none

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:link(Maxwell)  [(Maxwell)] J. C. Maxwell. Philos. Trans. Royal Soc. London. 157: 49–88 (1867).

:link(CL)  [(CL)] C. Cercignani and M. Lampis. Transp. 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).

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