forked from lijiext/lammps
132 lines
5.3 KiB
HTML
132 lines
5.3 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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<H3>fix viscous/cuda 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 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, 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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</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>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 <A HREF = "units.html">units</A> options like "real" or "metal" that are not
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self-consistent.
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</P>
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<P>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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</P>
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<P>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 <I>scale</I> 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 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 <I>langevin</I> you do not typically need to use
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fix <I>viscous</I>. Also note that the gamma of fix viscous is related to
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the damping parameter of <A HREF = "fix_langevin.html">fix langevin</A>, 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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</P>
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<HR>
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<P>Styles with a <I>cuda</I> suffix are functionally the same as the
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corresponding style without the suffix. They have been optimized to
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run faster, depending on your available hardware, as discussed in
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<A HREF = "Section_accelerate.html">this section</A> of the manual. The accelerated
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styles take the same arguments and should produce the same results,
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except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA package. They are
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only enabled if LAMMPS was built with that package. See the <A HREF = "Section_start.html#start_3">Making
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LAMMPS</A> section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_6">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">this section</A> of the manual for more
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instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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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 or per-atom quantities are stored
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by this fix for access by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. No parameter of this fix can
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be used with the <I>start/stop</I> keywords of the <A HREF = "run.html">run</A> command.
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</P>
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<P>The forces due to this fix are imposed during an energy minimization,
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invoked by the <A HREF = "minimize.html">minimize</A> command. This fix should only
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be used with damped dynamics minimizers that allow for
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non-conservative forces. See the <A HREF = "min_style.html">min_style</A> command
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for details.
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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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