lammps/doc/fix_viscous.txt

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fix viscous command :h3

[Syntax:]

fix ID group-ID viscous gamma keyword values ... :pre
  
ID, group-ID are documented in "fix"_fix.html command :ulb,l
viscous = style name of this fix command :l
gamma = damping coefficient (force/velocity units) :l
zero or more keyword/value pairs can be appended :l
keyword = {b} or {a} or {t} or {m} :l
zero or more keyword/value pairs may be appended :l
keyword = {scale}
  {scale} values = type ratio
    type = atom type (1-N)
    ratio = factor to scale the damping coefficient by :pre
:ule

[Examples:]

fix 1 flow viscous 0.1
fix 1 damp viscous 0.5 scale 3 2.5 :pre

[Description:]

Add a viscous damping force to atoms in the group that is proportional
to the velocity of the atom.  The added force can be thought of as a
frictional interaction with implicit solvent.  In granular simulations
this can be useful for draining the kinetic energy from the system in
a controlled fashion.  If used without additional thermostatting (to
add kinetic energy to the system), it has the effect of slowly (or
rapidly) freezing the system; hence it is a simple energy minimization
technique.

The damping force F is given by F = - gamma * velocity.  The larger
the coefficient, the faster the kinetic energy is reduced.  If the
optional keyword {scale} is used, gamma can scaled up or down by the
specified factor for atoms of that type.  It can be used multiple
times to adjust gamma for several atom types.

In a Brownian dynamics context, gamma = kT / mD, where k = Bolztmann's
constant, T = temperature, m = particle mass, and D = particle
diffusion coefficient.  D can be written as kT / (6 pi eta d), where
eta = viscosity of the frictional fluid and d = diameter of particle.
This means gamma = 6 pi eta d, and thus is proportional to the
viscosity of the fluid and the particle diameter.

In the current implementation, rather than have the user specify a
viscosity (in centiPoise or some other units), gamma is specified
directly in force/velocity units.  If needed, gamma can be adjusted
for atoms of different sizes (i.e. sigma) by using the {scale}
keyword.

Note that Brownian dynamics models also typically include a randomized
force term to thermostat the system at a chosen temperature.  The "fix
langevin"_fix_langevin.html command adds both a viscous damping term
and this random force to each atom; hence if using fix {langevin} you
do not typically need to use fix {viscous}.

[Restart, fix_modify, thermo output, run start/stop, minimize info:]

No information about this fix is written to "binary restart
files"_restart.html.  None of the "fix_modify"_fix_modify.html options
are relevant to this fix.  No quantities calculated by this fix can be
output by the "thermo_style custom"_thermo_style.html command.  No
parameter of this fix can be used with the {start/stop} keywords of
the "run"_run.html command.  This fix is not invoked during "energy
minimization"_minimize.html.

[Restrictions:] none

[Related commands:]

"fix langevin"_fix_langevin.html

[Default:] none