forked from lijiext/lammps
149 lines
6.9 KiB
HTML
149 lines
6.9 KiB
HTML
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<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 lb/rigid/pc/sphere 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 lb/rigid/pc/sphere bodystyle args 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>lb/rigid/pc/sphere = style name of this fix command
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<LI>bodystyle = <I>single</I> or <I>molecule</I> or <I>group</I>
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<P> <I>single</I> args = none
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<I>molecule</I> args = none
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<I>group</I> args = N groupID1 groupID2 ...
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N = # of groups
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</P>
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<LI>zero or more keyword/value pairs may be appended
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<PRE>keyword = <I>force</I> or <I>torque</I> or <I>innerNodes</I>
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<I>force</I> values = M xflag yflag zflag
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M = which rigid body from 1-Nbody (see asterisk form below)
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xflag,yflag,zflag = off/on if component of center-of-mass force is active
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<I>torque</I> values = M xflag yflag zflag
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M = which rigid body from 1-Nbody (see asterisk form below)
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xflag,yflag,zflag = off/on if component of center-of-mass torque is active
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<I>innerNodes</I> values = innergroup-ID
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innergroup-ID = ID of the atom group which does not experience a hydrodynamic force from the lattice-Boltzmann fluid
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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 spheres lb/rigid/pc/sphere
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fix 1 all lb/rigid/pc/sphere force 1 0 0 innerNodes ForceAtoms
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This fix is based on the <A HREF = "fix_rigid.html">fix rigid</A> command, and was
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created to be used in place of that fix, to integrate the equations of motion
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of spherical rigid bodies when a lattice-Boltzmann fluid is present with a user-specified value of the force-coupling constant.
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The fix uses the integration algorithm described in <A HREF = "#Mackay">Mackay et al.</A> [1] to update
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the positions, velocities, and orientations of a set of spherical rigid bodies experiencing
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velocity dependent hydrodynamic forces. The spherical bodies are assumed to rotate as
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solid, uniform density spheres, with moments of inertia calculated using the combined sum of the masses
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of all the constituent particles (which are assumed to be point particles).
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</P>
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<HR>
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<P>By default, all of the atoms that this fix acts on experience a hydrodynamic force due to the
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presence of the lattice-Boltzmann fluid. However, the <I>innerNodes</I> keyword allows the user
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to specify atoms belonging to a rigid object which do not interact with the lattice-Boltzmann
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fluid (i.e. these atoms do not feel a hydrodynamic force from the lattice-Boltzmann fluid).
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This can be used to distinguish between atoms on the surface of a non-porous object, and
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those on the inside.
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</P>
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<P>This feature can be used, for example, when implementing a hard sphere interaction
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between two spherical objects. Instead of interactions occurring between the particles on
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the surfaces of the two spheres, it is desirable simply to place an atom at the center of
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each sphere, which does not contribute to the hydrodynamic force, and have these central
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atoms interact with one another.
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</P>
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<HR>
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<P>Apart from the features described above, this fix is very similar to the rigid fix (although
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it includes fewer optional arguments, and assumes the constituent atoms are point particles); see <A HREF = "fix_rigid.html">fix_rigid</A>
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for a complete documentation.
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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 the <I>rigid</I> and <I>rigid/nve</I> fixes are written to
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<A HREF = "restart.html">binary restart files</A>.
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</P>
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<P>Similar to the <A HREF = "fix_rigid.html">fix_rigid</A> command:
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" The rigid fix computes a global scalar which can be
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accessed by various <A HREF = "Section_howto.html#howto_15">output commands</A>.
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The scalar value calculated by these fixes is "intensive". The scalar
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is the current temperature of the collection of rigid bodies. This is
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averaged over all rigid bodies and their translational and rotational
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degrees of freedom. The translational energy of a rigid body is 1/2 m
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v^2, where m = total mass of the body and v = the velocity of its
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center of mass. The rotational energy of a rigid body is 1/2 I w^2,
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where I = the moment of inertia tensor of the body and w = its angular
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velocity. Degrees of freedom constrained by the <I>force</I> and <I>torque</I>
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keywords are removed from this calculation."
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</P>
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<P>"All of these fixes compute a global array of values which can be
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accessed by various <A HREF = "Section_howto.html#howto_15">output commands</A>.
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The number of rows in the array is equal to the number of rigid
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bodies. The number of columns is 15. Thus for each rigid body, 15
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values are stored: the xyz coords of the center of mass (COM), the xyz
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components of the COM velocity, the xyz components of the force acting
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on the COM, the xyz components of the torque acting on the COM, and
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the xyz image flags of the COM, which have the same meaning as image
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flags for atom positions (see the "dump" command). The force and
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torque values in the array are not affected by the <I>force</I> and
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<I>torque</I> keywords in the fix rigid command; they reflect values before
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any changes are made by those keywords.
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</P>
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<P>The ordering of the rigid bodies (by row in the array) is as follows.
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For the <I>single</I> keyword there is just one rigid body. For the
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<I>molecule</I> keyword, the bodies are ordered by ascending molecule ID.
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For the <I>group</I> keyword, the list of group IDs determines the ordering
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of bodies.
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</P>
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<P>The array values calculated by these fixes are "intensive", meaning
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they are independent of the number of atoms in the simulation.
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</P>
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<P>No parameter of these fixes can be used with the <I>start/stop</I> keywords
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of the <A HREF = "run.html">run</A> command. These fixes are not invoked during
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<A HREF = "minimize.html">energy minimization</A>. "
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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>Can only be used if a lattice-Boltzmann fluid has been created via the
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<A HREF = "fix_lb_fluid.html">fix lb/fluid</A> command, and must come after this command.
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Should only be used if the force coupling constant used in <A HREF = "fix_lb_fluid.html">fix lb/fluid</A> has been set by the user; this integration fix cannot be used if the force coupling constant is set by default.
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</P>
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<P>This fix can only be used if LAMMPS was built with the
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"fluid" package. See the <A HREF = "Section_start.html#2_3">Making
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LAMMPS</A> section for more info on packages.
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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_lb_fluid.html">fix lb/fluid</A>, <A HREF = "fix_lb_pc.html">fix lb/pc</A>
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</P>
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<P><B>Default:</B>
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</P>
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<P>The defaults are force * on on on, and torque * on on on.
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</P>
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<HR>
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<A NAME = "Mackay"></A>
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<P><B>[1] (Mackay et al.)</B> Mackay, F. E., Ollila, S.T.T., and Denniston, C., Hydrodynamic Forces Implemented into LAMMPS through a lattice-Boltzmann fluid, Computer Physics Communications 184 (2013) 2021-2031.
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</P>
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</HTML>
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