forked from lijiext/lammps
97 lines
3.5 KiB
HTML
97 lines
3.5 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>fix viscous command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID viscous gamma keyword values ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>viscous = style name of this fix command
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<LI>gamma = damping coefficient (force/velocity units)
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<LI>zero or more keyword/value pairs can be appended
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<LI>keyword = <I>b</I> or <I>a</I> or <I>t</I> or <I>m</I>
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<LI>zero or more keyword/value pairs may be appended
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<PRE>keyword = <I>scale</I>
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<I>scale</I> values = type ratio
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type = atom type (1-N)
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ratio = factor to scale the damping coefficient by
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 1 flow viscous 0.1
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fix 1 damp viscous 0.5 scale 3 2.5
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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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</P>
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<P>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 <I>scale</I> 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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</P>
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<P>In a Brownian dynamics context, gamma = kT / mD, where k = Boltzmann'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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</P>
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<P>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 <I>scale</I>
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keyword.
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</P>
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<P>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 <A HREF = "fix_langevin.html">fix
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langevin</A> command adds both a viscous damping term
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and this random force to each atom; hence if using fix <I>langevin</I> you
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do not typically need to use fix <I>viscous</I>.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> 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 <A HREF = "Section_howto.html#4_15">output
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commands</A>. No parameter of this fix can be
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used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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This fix is not invoked during <A HREF = "minimize.html">energy minimization</A>.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "fix_langevin.html">fix langevin</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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