forked from lijiext/lammps
94 lines
3.7 KiB
Plaintext
94 lines
3.7 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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fix viscous command :h3
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[Syntax:]
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fix ID group-ID viscous gamma keyword values ... :pre
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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viscous = style name of this fix command :l
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gamma = damping coefficient (force/velocity units) :l
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zero or more keyword/value pairs can be appended :l
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keyword = {b} or {a} or {t} or {m} :l
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zero or more keyword/value pairs may be appended :l
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keyword = {scale}
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{scale} values = type ratio
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type = atom type (1-N)
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ratio = factor to scale the damping coefficient by :pre
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:ule
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[Examples:]
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fix 1 flow viscous 0.1
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fix 1 damp viscous 0.5 scale 3 2.5 :pre
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[Description:]
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Add a viscous damping force to atoms in the group that is proportional
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to the velocity of the atom. The added force can be thought of as a
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frictional interaction with implicit solvent, i.e. the no-slip Stokes
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drag on a spherical particle. In granular simulations this can be
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useful for draining the kinetic energy from the system in a controlled
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fashion. If used without additional thermostatting (to add kinetic
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energy to the system), it has the effect of slowly (or rapidly)
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freezing the system; hence it can also be used as a simple energy
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minimization technique.
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The damping force F is given by F = - gamma * velocity. The larger
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the coefficient, the faster the kinetic energy is reduced. If the
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optional keyword {scale} is used, gamma can scaled up or down by the
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specified factor for atoms of that type. It can be used multiple
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times to adjust gamma for several atom types.
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IMPORTANT NOTE: You should specify gamma in force/velocity units.
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This is not the same as mass/time units, at least for some of the
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LAMMPS "units"_units.html options like "real" or "metal" that are not
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self-consistent.
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In a Brownian dynamics context, gamma = Kb T / D, where Kb =
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Boltzmann's constant, T = temperature, and D = particle diffusion
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coefficient. D can be written as Kb T / (3 pi eta d), where eta =
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dynamic viscosity of the frictional fluid and d = diameter of
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particle. This means gamma = 3 pi eta d, and thus is proportional to
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the viscosity of the fluid and the particle diameter.
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In the current implementation, rather than have the user specify a
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viscosity, gamma is specified directly in force/velocity units. If
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needed, gamma can be adjusted for atoms of different sizes
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(i.e. sigma) by using the {scale} keyword.
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Note that Brownian dynamics models also typically include a randomized
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force term to thermostat the system at a chosen temperature. The "fix
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langevin"_fix_langevin.html command does this. It has the same
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viscous damping term as fix viscous and adds a random force to each
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atom. Hence if using fix {langevin} you do not typically need to use
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fix {viscous}. Also note that the gamma of fix viscous is related to
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the damping parameter of "fix langevin"_fix_langevin.html, except that
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the units of gamma are force/velocity and the units of damp are time,
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so that it can more easily be used as a thermostat.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix. No global scalar or vector or per-atom
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quantities are stored by this fix for access by various "output
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commands"_Section_howto.html#4_15. No parameter of this fix can be
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used with the {start/stop} keywords of the "run"_run.html command.
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This fix is not invoked during "energy minimization"_minimize.html.
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[Restrictions:] none
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[Related commands:]
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"fix langevin"_fix_langevin.html
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[Default:] none
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