Homec4science
Diffusion lammps (master) Browse

lammps/doc/fix_lb_rigid_pc_sphere.txt
1cada0b49692master

Last Change
Raw File

fix_lb_rigid_pc_sphere.txt
View Options

"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)
:line
fix lb/rigid/pc/sphere command :h3
[Syntax:]
fix ID group-ID lb/rigid/pc/sphere bodystyle args keyword values ... :pre
ID, group-ID are documented in "fix"_fix.html command :ulb,l
lb/rigid/pc/sphere = style name of this fix command :l
bodystyle = {single} or {molecule} or {group} :l
{single} args = none
{molecule} args = none
{group} args = N groupID1 groupID2 ...
N = # of groups
zero or more keyword/value pairs may be appended :l
keyword = {force} or {torque} or {innerNodes}
{force} values = M xflag yflag zflag
M = which rigid body from 1-Nbody (see asterisk form below)
xflag,yflag,zflag = off/on if component of center-of-mass force is active
{torque} values = M xflag yflag zflag
M = which rigid body from 1-Nbody (see asterisk form below)
xflag,yflag,zflag = off/on if component of center-of-mass torque is active
{innerNodes} values = innergroup-ID
innergroup-ID = ID of the atom group which does not experience a hydrodynamic force from the lattice-Boltzmann fluid :pre
:ule
[Examples:]
fix 1 spheres lb/rigid/pc/sphere
fix 1 all lb/rigid/pc/sphere force 1 0 0 innerNodes ForceAtoms :pre
[Description:]
This fix is based on the "fix rigid"_fix_rigid.html command, and was
created to be used in place of that fix, to integrate the equations of motion
of spherical rigid bodies when a lattice-Boltzmann fluid is present with a user-specified value of the force-coupling constant.
The fix uses the integration algorithm described in "Mackay et al."_#Mackay [1] to update
the positions, velocities, and orientations of a set of spherical rigid bodies experiencing
velocity dependent hydrodynamic forces. The spherical bodies are assumed to rotate as
solid, uniform density spheres, with moments of inertia calculated using the combined sum of the masses
of all the constituent particles (which are assumed to be point particles).
:line
By default, all of the atoms that this fix acts on experience a hydrodynamic force due to the
presence of the lattice-Boltzmann fluid. However, the {innerNodes} keyword allows the user
to specify atoms belonging to a rigid object which do not interact with the lattice-Boltzmann
fluid (i.e. these atoms do not feel a hydrodynamic force from the lattice-Boltzmann fluid).
This can be used to distinguish between atoms on the surface of a non-porous object, and
those on the inside.
This feature can be used, for example, when implementing a hard sphere interaction
between two spherical objects. Instead of interactions occurring between the particles on
the surfaces of the two spheres, it is desirable simply to place an atom at the center of
each sphere, which does not contribute to the hydrodynamic force, and have these central
atoms interact with one another.
:line
Apart from the features described above, this fix is very similar to the rigid fix (although
it includes fewer optional arguments, and assumes the constituent atoms are point particles); see "fix_rigid"_fix_rigid.html
for a complete documentation.
[Restart, fix_modify, output, run start/stop, minimize info:]
No information about the {rigid} and {rigid/nve} fixes are written to
"binary restart files"_restart.html.
Similar to the "fix_rigid"_fix_rigid.html command:
" The rigid fix computes a global scalar which can be
accessed by various "output commands"_Section_howto.html#howto_15.
The scalar value calculated by these fixes is "intensive". The scalar
is the current temperature of the collection of rigid bodies. This is
averaged over all rigid bodies and their translational and rotational
degrees of freedom. The translational energy of a rigid body is 1/2 m
v^2, where m = total mass of the body and v = the velocity of its
center of mass. The rotational energy of a rigid body is 1/2 I w^2,
where I = the moment of inertia tensor of the body and w = its angular
velocity. Degrees of freedom constrained by the {force} and {torque}
keywords are removed from this calculation."
"All of these fixes compute a global array of values which can be
accessed by various "output commands"_Section_howto.html#howto_15.
The number of rows in the array is equal to the number of rigid
bodies. The number of columns is 15. Thus for each rigid body, 15
values are stored: the xyz coords of the center of mass (COM), the xyz
components of the COM velocity, the xyz components of the force acting
on the COM, the xyz components of the torque acting on the COM, and
the xyz image flags of the COM, which have the same meaning as image
flags for atom positions (see the "dump" command). The force and
torque values in the array are not affected by the {force} and
{torque} keywords in the fix rigid command; they reflect values before
any changes are made by those keywords.
The ordering of the rigid bodies (by row in the array) is as follows.
For the {single} keyword there is just one rigid body. For the
{molecule} keyword, the bodies are ordered by ascending molecule ID.
For the {group} keyword, the list of group IDs determines the ordering
of bodies.
The array values calculated by these fixes are "intensive", meaning
they are independent of the number of atoms in the simulation.
No parameter of these fixes can be used with the {start/stop} keywords
of the "run"_run.html command. These fixes are not invoked during
"energy minimization"_minimize.html. "
[Restrictions:]
Can only be used if a lattice-Boltzmann fluid has been created via the
"fix lb/fluid"_fix_lb_fluid.html command, and must come after this command.
Should only be used if the force coupling constant used in "fix lb/fluid"_fix_lb_fluid.html has been set by the user; this integration fix cannot be used if the force coupling constant is set by default.
This fix can only be used if LAMMPS was built with the
"fluid" package. See the "Making
LAMMPS"_Section_start.html#2_3 section for more info on packages.
[Related commands:]
"fix lb/fluid"_fix_lb_fluid.html, "fix lb/pc"_fix_lb_pc.html
[Default:]
The defaults are force * on on on, and torque * on on on.
:line
:link(Mackay)
[[1] (Mackay et al.)] 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.
c4science · Help