git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14950 f3b2605a-c512-4ea7-a41b-209d697bcdaa

doc/html/_sources/fix_viscous.txt

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+.. index:: fix viscous
+
+fix viscous command
+===================
+
+fix viscous/cuda command
+========================
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   fix ID group-ID viscous gamma keyword values ...
+
+* ID, group-ID are documented in :doc:`fix <fix>` command
+* viscous = style name of this fix command
+* gamma = damping coefficient (force/velocity units)
+* zero or more keyword/value pairs may be appended
+.. parsed-literal::
+
+   keyword = *scale*
+     *scale* values = type ratio
+       type = atom type (1-N)
+       ratio = factor to scale the damping coefficient by
+
+
+
+Examples
+""""""""
+
+.. parsed-literal::
+
+   fix 1 flow viscous 0.1
+   fix 1 damp viscous 0.5 scale 3 2.5
+
+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, i.e. the no-slip Stokes
+drag on a spherical particle.  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 can also be used as 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.
+
+.. note::
+
+   You should specify gamma in force/velocity units.  This is not
+   the same as mass/time units, at least for some of the LAMMPS
+   :doc:`units <units>` options like "real" or "metal" that are not
+   self-consistent.
+
+In a Brownian dynamics context, gamma = Kb T / D, where Kb =
+Boltzmann's constant, T = temperature, and D = particle diffusion
+coefficient.  D can be written as Kb T / (3 pi eta d), where eta =
+dynamic viscosity of the frictional fluid and d = diameter of
+particle.  This means gamma = 3 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, 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 :doc:`fix langevin <fix_langevin>` command does this.  It has the same
+viscous damping term as fix viscous and adds a random force to each
+atom.  The random force term is proportional to the sqrt of the chosen
+thermostatting temperature.  Thus if you use fix langevin with a
+target T = 0, its random force term is zero, and you are essentially
+performing the same operation as fix viscous.  Also note that the
+gamma of fix viscous is related to the damping parameter of :doc:`fix langevin <fix_langevin>`, however the former is specified in units
+of force/velocity and the latter in units of time, so that it can more
+easily be used as a thermostat.
+
+
+----------
+
+
+Styles with a *cuda* suffix are functionally the same as the
+corresponding style without the suffix.  They have been optimized to
+run faster, depending on your available hardware, as discussed in
+:doc:`Section_accelerate <Section_accelerate>` of the manual.  The
+accelerated styles take the same arguments and should produce the same
+results, except for round-off and precision issues.
+
+These accelerated styles are part of the USER-CUDA package.  They are
+only enabled if LAMMPS was built with that package.  See the :ref:`Making LAMMPS <start_3>` section for more info.
+
+You can specify the accelerated styles explicitly in your input script
+by including their suffix, or you can use the :ref:`-suffix command-line switch <start_7>` when you invoke LAMMPS, or you can
+use the :doc:`suffix <suffix>` command in your input script.
+
+See :doc:`Section_accelerate <Section_accelerate>` of the manual for
+more instructions on how to use the accelerated styles effectively.
+
+
+----------
+
+
+Restart, fix_modify, output, run start/stop, minimize info
+""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
+
+No information about this fix is written to :doc:`binary restart files <restart>`.  None of the :doc:`fix_modify <fix_modify>` options
+are relevant to this fix.  No global or per-atom quantities are stored
+by this fix for access by various :ref:`output commands <howto_15>`.  No parameter of this fix can
+be used with the *start/stop* keywords of the :doc:`run <run>` command.
+
+The forces due to this fix are imposed during an energy minimization,
+invoked by the :doc:`minimize <minimize>` command.  This fix should only
+be used with damped dynamics minimizers that allow for
+non-conservative forces.  See the :doc:`min_style <min_style>` command
+for details.
+
+Restrictions
+""""""""""""
+ none
+
+Related commands
+""""""""""""""""
+
+:doc:`fix langevin <fix_langevin>`
+
+**Default:** none
+
+
+.. _lws: http://lammps.sandia.gov
+.. _ld: Manual.html
+.. _lc: Section_commands.html#comm