112 lines
3.9 KiB
ReStructuredText
112 lines
3.9 KiB
ReStructuredText
.. index:: fix viscous/sphere
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fix viscous/sphere command
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==========================
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Syntax
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""""""
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.. parsed-literal::
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fix ID group-ID viscous/sphere gamma keyword values ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* viscous/sphere = style name of this fix command
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* gamma = damping coefficient (torque/angular velocity units)
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* zero or more keyword/value pairs may be appended
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.. parsed-literal::
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keyword = *scale*
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*scale* values = *type ratio* or *v_name*
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type = atom type (1-N)
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ratio = factor to scale the damping coefficients by
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v_name = reference to atom style variable *name*
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix 1 flow viscous/sphere 0.1
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fix 1 damp viscous/sphere 0.5 scale 3 2.5
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fix 1 damp viscous/sphere 0.5 scale v_radscale
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Description
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"""""""""""
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Add a viscous damping torque to finite-size spherical particles in the group
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that is proportional to the angular velocity of the atom. In granular
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simulations this can be useful for draining the rotational kinetic energy from
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the system in a controlled fashion. If used without additional thermostatting
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(to add kinetic 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 minimization
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technique.
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The damping torque :math:`T_i` is given by :math:`T_i = - \gamma \omega_i`.
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The larger the coefficient, the faster the rotational kinetic energy is reduced.
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If the optional keyword *scale* is used, :math:`\gamma` can be scaled up
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or down by the specified factor for atoms. This factor can be set for
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different atom types and thus the *scale* keyword used multiple times
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followed by the atom type and the associated scale factor. Alternately
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the scaling factor can be computed for each atom (e.g. based on its
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radius) by using an :doc:`atom-style variable <variable>`.
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.. note::
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You should specify gamma in torque/angular velocity units. This is not
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the same as mass/time units, at least for some of the LAMMPS
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:doc:`units <units>` options like "real" or "metal" that are not
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self-consistent.
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In the current implementation, rather than have the user specify a viscosity,
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:math:`\gamma` is specified directly in torque/angular velocity units.
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If needed, :math:`\gamma` can be adjusted for atoms of different sizes
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(i.e. :math:`\sigma`) by using the *scale* keyword.
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----------
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Restart, fix_modify, output, run start/stop, minimize info
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"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
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No information about this fix is written to :doc:`binary restart files
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<restart>`. None of the :doc:`fix_modify <fix_modify>` options are
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relevant to this fix. No global or per-atom quantities are stored by
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this fix for access by various :doc:`output commands <Howto_output>`.
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No parameter of this fix can be used with the *start/stop* keywords of
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the :doc:`run <run>` command.
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The :doc:`fix_modify <fix_modify>` *respa* option is supported by this
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fix. This allows to set at which level of the :doc:`r-RESPA <run_style>`
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integrator the fix is modifying torques. Default is the outermost level.
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The torques due to this fix are imposed during an energy minimization,
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invoked by the :doc:`minimize <minimize>` 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 :doc:`min_style <min_style>` command
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for details.
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Restrictions
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""""""""""""
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This fix is part of the EXTRA-FIX package. It is only enabled if
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LAMMPS was built with that package. See the :doc:`Build package <Build_package>` page for more info.
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This fix requires that atoms store torque and angular velocity (omega)
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and a radius as defined by the :doc:`atom_style sphere <atom_style>`
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command.
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All particles in the group must be finite-size spheres. They cannot
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be point particles.
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Related commands
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""""""""""""""""
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:doc:`fix viscous <fix_viscous>`, :doc:`fix damping/cundall <fix_damping_cundall>`
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Default
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"""""""
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none
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