97 lines
3.0 KiB
ReStructuredText
97 lines
3.0 KiB
ReStructuredText
.. index:: fix smd/setvel
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fix smd/setvel command
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======================
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Syntax
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""""""
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.. code-block:: LAMMPS
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fix ID group-ID smd/setvel vx vy vz keyword value ...
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* ID, group-ID are documented in :doc:`fix <fix>` command
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* smd/setvel = style name of this fix command
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* vx,vy,vz = velocity component values
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* any of vx,vy,vz can be a variable (see below)
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* zero or more keyword/value pairs may be appended to args
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* keyword = *region*
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.. parsed-literal::
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*region* value = region-ID
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region-ID = ID of region particles must be in to have their velocities set
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Examples
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""""""""
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.. code-block:: LAMMPS
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fix top_velocity top_group smd/setvel 1.0 0.0 0.0
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Description
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"""""""""""
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Set each component of velocity on each particle in the group to the specified
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values vx,vy,vz, regardless of the forces acting on the particle. This command can
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be used to impose velocity boundary conditions.
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Any of the vx,vy,vz values can be specified as NULL which means do not
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alter the velocity component in that dimension.
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This fix is indented to be used together with a time integration fix.
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Any of the 3 quantities defining the velocity components can be specified
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as an equal-style or atom-style :doc:`variable <variable>`, namely *vx*,
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*vy*, *vz*\ . If the value is a variable, it should be specified as
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v_name, where name is the variable name. In this case, the variable
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will be evaluated each timestep, and its value used to determine the
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force component.
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Equal-style variables can specify formulas with various mathematical
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functions, and include :doc:`thermo_style <thermo_style>` command
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keywords for the simulation box parameters and timestep and elapsed
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time. Thus it is easy to specify a time-dependent velocity field.
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Atom-style variables can specify the same formulas as equal-style
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variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent velocity
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field with optional time-dependence as well.
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If the *region* keyword is used, the particle must also be in the
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specified geometric :doc:`region <region>` in order to have its velocity set by this command.
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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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Currently, no part of MACHDYN supports restarting nor minimization
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None of the :doc:`fix_modify <fix_modify>` options
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are relevant to this fix.
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This fix computes a global 3-vector of forces, which can be accessed
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by various :doc:`output commands <Howto_output>`. This is the total
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force on the group of atoms. The vector values calculated by this fix
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are "extensive".
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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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Restrictions
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""""""""""""
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This fix is part of the MACHDYN 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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Related commands
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""""""""""""""""
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none
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Default
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"""""""
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none
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