forked from lijiext/lammps
75 lines
2.7 KiB
Plaintext
75 lines
2.7 KiB
Plaintext
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"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 to the args :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. In granular simulations
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this can be useful for draining the kinetic energy from the system in
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a controlled fashion. If used without additional thermostatting (to
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add kinetic energy to the system), it has the effect of slowly (or
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rapidly) freezing the system; hence it is a simple energy minimization
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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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In a Brownian dynamics context, gamma = kT / mD, where k = Bolztmann's
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constant, T = temperature, m = particle mass, and D = particle
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diffusion coefficient. D can be written as kT / (6 pi eta d), where
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eta = viscosity of the frictional fluid and d = diameter of particle.
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This means gamma = 6 pi eta d, and thus is proportional to the
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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 (in centiPoise or some other units), gamma is specified
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directly in force/velocity units. If needed, gamma can be adjusted
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for atoms of different sizes (i.e. sigma) by using the {scale}
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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 adds both a viscous damping term
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and this random force to each atom; hence if using fix {langevin} you
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do not typically need to use fix {viscous}.
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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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