Delete fix_wall_Maxwell.txt

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fix wall/Maxwell command :h3

fix wall/Maxwell/kk command :h3

[Syntax:]

fix ID group-ID wall/Maxwell face pos temp velx vely velz keyword value ... :pre

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

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

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

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

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

    EDGE = current lo edge of simulation box

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

    variable = "equal-style variable"_variable.html like v_x or v_wiggle

    temp = wall temperature

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

    accom = accommodation coefficient

  {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)

    variable = "equal-style variable"_variable.html like v_x or v_wiggle 

    temp = wall temperature 

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

    accom = accommodation coefficient :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

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[Examples:]

fix ywalls all wall/Maxwell xlo EDGE 300 0.1 0.1 0 0.8 xhi EDGE 200 0.1 0.1 0 0.8

fix ywalls all wall/Maxwell xlo 300 0.1 0.1 0 ylo 300 0.1 0.1 0 0.9 units box :pre

[Description:]

In the USER-GWBC package, the three fixes wall/diffusive, wall/ACL, wall/Maxwell are implementations of stochastic wall models. The acronym GWBC stands for Gas-Wall Boundary Conditions. Like fix wall/reflect, these fixes bound the simulation with one or more walls by puting back atoms inside whenever they cross through the wall, i.e collision event. While the wall/reflect corrsponds to mirror reflection where the reflected velocity is deterministic depending on the incoming velocity, the reflected velocity in the stochastic wall models is random. The randomness accounts for the nature of the wall, the thermal motion of the wall atoms or the surface roughness. These models are useful for simulation gas flows without describing atomistically the wall boundary and thus save significantly computation cost. 

This model is the mixture of two fixes (and two wall models) wall/reflect and wall/diffusive, proposed by Maxwell in his paper on the kinetic theory of gases. An atom can be either fully thermalized by the wall and reflected diffusively or reflected specularly. In this case, the accommodation coefficient (ACCOM) lying between 0 and 1, is introduced to account for the diffusive reflection portion. As noted, when ACCOM = 0, the reflection is purely specular and when ACCOM = 1, the reflection is purely diffusive. The use of this stochastic model is almost the same as wall/reflect but with five additional parameters the wall temperature and the wall velocity along 3 directions x, y, z, and the accommodation coefficient.

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 its properties, this fix has 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. 

[Related commands:]

"fix wall/reflect"_fix_wall_reflect.html, "fix wall/diffusive"_fix_wall_diffusive.html, "fix wall/ACL"_fix_wall_ACL.html

[Default:] none

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