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171 lines
6.0 KiB
Plaintext
171 lines
6.0 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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pair_style lubricate command :h3
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pair_style lubricate/poly command :h3
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[Syntax:]
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pair_style style mu flaglog flagfld cutinner cutoff :pre
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style = {lubricate} or {lubricate/poly}
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mu = dynamic viscosity (dynamic viscosity units)
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flaglog = 0/1 log terms in the lubrication approximation off/on
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flagfld = 0/1 to include/exclude Fast Lubrication Dynamics effects
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cutinner = inner cutoff distance (distance units)
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cutoff = outer cutoff for interactions (distance units) :ul
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[Examples:] (all assume radius = 1)
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pair_style lubricate 1.5 1 1 2.01 2.5
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pair_coeff 1 1 2.05 2.8
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pair_coeff * * :pre
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pair_style lubricate 1.5 1 1 2.01 2.5
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pair_coeff * *
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variable mu equal ramp(1,2)
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fix 1 all adapt 1 pair lubricate mu * * v_mu :pre
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[Description:]
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Styles {lubricate} and {lubricate/poly} compute hydrodynamic
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interactions between mono-disperse spherical particles in a pairwise
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fashion. The interactions have 2 components. The first is
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Ball-Melrose lubrication terms via the formulas in "(Ball and
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Melrose)"_#Ball
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:c,image(Eqs/pair_lubricate.jpg)
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which represents the dissipation W between two nearby particles due to
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their relative velocities in the presence of a background solvent with
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viscosity mu. Note that this is dynamic viscosity which has units of
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mass/distance/time, not kinematic viscosity.
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The Asq (squeeze) term is the strongest and is always included. It
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scales as 1/gap where gap is the separation between the surfaces of
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the 2 particles. The Ash (shear) and Apu (pump) terms are only
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include if {flaglog} is set to 1. Thy are the next strongest
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interactions, and the only other singular interaction, and scale as
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log(gap). The Atw (twist) term is currently not included. It is
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typically a very small contribution to the lubrication forces.
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{Cutinner} sets the minimum center-to-center separation that will be
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used in calculations irrespective of the actual separation. {Cutoff}
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is the maximum center-to-center separation at which an interaction is
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computed. Using a {cutoff} less than 3 radii is recommended if
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{flaglog} is set to 1.
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The other component is due to the Fast Lubrication Dynamics (FLD)
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approximation, described in "(Kumar)"_#Kumar, which can be
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represented by the following equation
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:c,image(Eqs/fld.jpg)
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where U represents the velocities and angular velocities of the
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particles, U^{infty} represents the velocity and the angular velocity
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of the undisturbed fluid, and E^{infty} represents the rate of strain
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tensor of the undisturbed fluid with viscosity mu. Again, note that
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this is dynamic viscosity which has units of mass/distance/time, not
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kinematic viscosity.
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IMPORTANT NOTE: When using the FLD terms, these pair styles are
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designed to be used with explicit time integration and a
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correspondingly small timestep. Thus either "fix
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nve/sphere"_fix_nve_sphere.html or "fix
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nve/asphere"_fix_nve_asphere.html should be used for time integration.
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To perform implicit FLD, see the "pair_style
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lubricateU"_pair_lubricateU.html command.
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Style {lubricate} requires monodisperse spherical particles; style
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{lubricate/poly} allows for polydisperse spherical particles.
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The viscosity mu can be varied in a time-dependent manner over the
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course of a simluation, in which case in which case the pair_style
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setting for mu will be overridden. See the "fix adapt"_fix_adapt.html
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command for details.
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If the suspension is sheared via the "fix deform"_fix_deform.html
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command then the pair style uses the shear rate to adjust the
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hydrodynamic interactions accordingly.
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Since lubrication forces are dissipative, it is usually desirable to
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thermostat the system at a constant temperature. If Brownian motion
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(at a constant temperature) is desired, it can be set using the
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"pair_style brownian"_pair_brownian.html command. These pair styles
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and the {brownian} style should use consistent parameters for {mu},
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{flaglog}, {flagfld}, {cutinner}, and {cutoff}.
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:line
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The following coefficients must be defined for each pair of atoms
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types via the "pair_coeff"_pair_coeff.html command as in the examples
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above, or in the data file or restart files read by the
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"read_data"_read_data.html or "read_restart"_read_restart.html
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commands, or by mixing as described below:
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cutinner (distance units)
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cutoff (distance units) :ul
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The two coefficients are optional. If neither is specified, the two
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cutoffs specified in the pair_style command are used. Otherwise both
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must be specified.
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:line
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[Mixing, shift, table, tail correction, restart, rRESPA info]:
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For atom type pairs I,J and I != J, the two cutoff distances for this
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pair style can be mixed. The default mix value is {geometric}. See
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the "pair_modify" command for details.
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This pair style does not support the "pair_modify"_pair_modify.html
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shift option for the energy of the pair interaction.
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The "pair_modify"_pair_modify.html table option is not relevant
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for this pair style.
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This pair style does not support the "pair_modify"_pair_modify.html
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tail option for adding long-range tail corrections to energy and
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pressure.
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This pair style writes its information to "binary restart
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files"_restart.html, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.
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This pair style can only be used via the {pair} keyword of the
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"run_style respa"_run_style.html command. It does not support the
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{inner}, {middle}, {outer} keywords.
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:line
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[Restrictions:]
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These styles are part of the FLD package. They are only enabled if
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LAMMPS was built with that package. See the "Making
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LAMMPS"_Section_start.html#2_3 section for more info.
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Only spherical monodisperse particles are allowed for pair_style
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lubricate.
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Only spherical particles are allowed for pair_style lubricate/poly.
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[Related commands:]
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"pair_coeff"_pair_coeff.html, "pair_style
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lubricateU"_pair_lubricateU.html
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[Default:] none
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:line
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:link(Ball)
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[(Ball)] Ball and Melrose, Physica A, 247, 444-472 (1997).
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:link(Kumar)
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[(Kumar)] Kumar and Higdon, Phys Rev E, 82, 051401 (2010).
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