forked from lijiext/lammps
218 lines
8.8 KiB
Plaintext
218 lines
8.8 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 lubricateU command :h3
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pair_style lubricateU/poly command :h3
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[Syntax:]
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pair_style style mu flaglog cutinner cutoff gdot flagHI flagVF :pre
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style = {lubricateU} or {lubricateU/poly}
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mu = dynamic viscosity (dynamic viscosity units)
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flaglog = 0/1 to exclude/include log terms in the lubrication approximation
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cutinner = inner cut off distance (distance units)
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cutoff = outer cutoff for interactions (distance units)
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gdot = shear rate (1/time units)
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flagHI (optional) = 0/1 to exclude/include 1/r hydrodynamic interactions
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flagVF (optional) = 0/1 to exclude/include volume fraction corrections in the long-range isotropic terms :ul
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[Examples:] (all assume radius = 1)
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pair_style lubricateU 1.5 1 2.01 2.5 0.01 1 1
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pair_coeff 1 1 2.05 2.8
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pair_coeff * * :pre
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[Description:]
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Styles {lubricateU} and {lubricateU/poly} compute velocities and
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angular velocities such that the hydrodynamic interaction balances the
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force and torque due to all other types of interactions.
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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 included as long as
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{flagHI} is set to 1 (default). It scales as 1/gap where gap is the
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separation between the surfaces of the 2 particles. The Ash (shear)
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and Apu (pump) terms are only included if {flaglog} is set to 1. They
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are the next strongest interactions, and the only other singular
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interaction, and scale as log(gap). Note that {flaglog} = 1 and
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{flagHI} = 0 is invalid, and will result in a warning message, after
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which {flagHI} will be set to 1. The Atw (twist) term is currently not
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included. It is typically a very small contribution to the lubrication
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forces.
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The {flagHI} and {flagVF} settings are optional. Neither should be
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used, or both must be defined.
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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. The equation being
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solved to balance the forces and torques is
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:c,image(Eqs/fld2.jpg)
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where U represents the velocities and angular velocities of the
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particles, U^{infty} represents the velocities and the angular
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velocities of the undisturbed fluid, and E^{infty} represents the rate
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of strain tensor of the undisturbed fluid flow with viscosity
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{mu}. Again, note that this is dynamic viscosity which has units of
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mass/distance/time, not kinematic viscosity. Volume fraction
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corrections to R_FU are included if {flagVF} is set to 1 (default).
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F{rest} represents the forces and torques due to all other types of
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interactions, e.g. Brownian, electrostatic etc. Note that this
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algorithm neglects the inertial terms, thereby removing the
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restriction of resolving the small interial time scale, which may not
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be of interest for colloidal particles. This pair style solves for
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the velocity such that the hydrodynamic force balances all other types
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of forces, thereby resulting in a net zero force (zero inertia limit).
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When defining this pair style, it must be defined last so that when
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this style is invoked all other types of forces have already been
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computed. For the same reason, it won't work if additional non-pair
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styles are defined (such as bond or Kspace forces) as they are
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calculated in LAMMPS after the pairwise interactions have been
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computed.
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IMPORTANT NOTE: When using these styles, the these pair styles are
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designed to be used with implicit time integration and a
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correspondingly larger timestep. Thus either "fix
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nve/noforce"_fix_nve_noforce.html should be used for spherical
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particles defined via "atom_style sphere"_atom_style.html or "fix
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nve/asphere/noforce"_fix_nve_asphere_noforce.html should be used for
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spherical particles defined via "atom_style
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ellipsoid"_atom_style.html. This is because the velocity and angular
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momentum of each particle is set by the pair style, and should not be
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reset by the time integration fix.
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Style {lubricateU} requires monodisperse spherical particles; style
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{lubricateU/poly} allows for polydisperse spherical particles.
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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. Volume changes due to fix
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deform are accounted for when computing the volume fraction
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corrections to R_FU.
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When computing the volume fraction corrections to R_FU, the presence
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of walls (whether moving or stationary) will affect the volume
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fraction available to colloidal particles. This is currently accounted
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for with the following types of walls: "wall/lj93"_fix_wall.html,
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"wall/lj126"_fix_wall.html, "wall/colloid"_fix_wall.html, and
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"wall/harmonic_fix_wall.html". For these wall styles, the correct
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volume fraction will be used when walls do not coincide with the box
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boundary, as well as when walls move and thereby cause a change in the
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volume fraction. To use these wall styles with pair_style {lubricateU}
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or {lubricateU/poly}, the {fld yes} option must be specified in the
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fix wall command.
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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}, {cutoff}, {flagHI} and {flagVF}.
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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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Currently, these pair styles assume that all other types of
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forces/torques on the particles have been already been computed when
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it is invoked. This requires this style to be defined as the last of
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the pair styles, and that no fixes apply additional constraint forces.
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One exception is the "fix wall/colloid"_fix_wall.html commands, which
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has an "fld" option to apply their wall forces correctly.
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Only spherical monodisperse particles are allowed for pair_style
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lubricateU.
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Only spherical particles are allowed for pair_style lubricateU/poly.
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For sheared suspensions, it is assumed that the shearing is done in
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the xy plane, with x being the velocity direction and y being the
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velocity-gradient direction. In this case, one must use "fix
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deform"_fix_deform.html with the same rate of shear (erate).
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[Related commands:]
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"pair_coeff"_pair_coeff.html, "pair_style
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lubricate"_pair_lubricate.html
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[Default:]
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The default settings for the optional args are flagHI = 1 and flagVF =
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1.
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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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