forked from lijiext/lammps
104 lines
4.2 KiB
Plaintext
104 lines
4.2 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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fix gravity command :h3
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[Syntax:]
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fix ID group gravity style magnitude args :pre
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ID, group are documented in "fix"_fix.html command :ulb,l
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gravity = style name of this fix command :l
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magnitude = size of acceleration (force/mass units) :l
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style = {chute} or {spherical} or {gradient} or {vector} :l
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{chute} args = angle
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angle = angle in +x away from -z or -y axis in 3d/2d (in degrees)
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{spherical} args = phi theta
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phi = azimuthal angle from +x axis (in degrees)
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theta = angle from +z or +y axis in 3d/2d (in degrees)
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{gradient} args = phi theta phi_grad theta_grad
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phi = azimuthal angle from +x axis (in degrees)
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theta = angle from +z or +y axis in 3d/2d (in degrees)
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phi_grad = rate of change of angle phi (full rotations per time unit)
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theta_grad = rate of change of angle theta (full rotations per time unit)
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{vector} args = x y z
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x y z = vector direction to apply the acceleration :pre
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:ule
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[Examples:]
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fix 1 all gravity 1.0 chute 24.0
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fix 1 all gravity 1.0 spherical 0.0 -180.0
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fix 1 all gravity 1.0 gradient 0.0 -180.0 0.0 0.1
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fix 1 all gravity 100.0 vector 1 1 0 :pre
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[Description:]
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Impose an additional acceleration on each particle in the group. This
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fix is typically used with granular systems to include a "gravity"
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term acting on the macroscopic particles. More generally, it can
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represent any kind of driving field, e.g. a pressure gradient inducing
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a Poiseuille flow in a fluid. Note that this fix operates differently
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than the "fix addforce"_fix_addforce.html command. The addforce fix
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adds the same force to each atom, independent of its mass. This
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command imparts the same acceleration to each atom (force/mass).
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The {magnitude} of the acceleration is specified in force/mass units.
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For granular systems (LJ units) this is typically 1.0. See the
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"units"_units.html command for details.
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Style {chute} is typically used for simulations of chute flow where
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the specified angle is the chute angle, with flow occurring in the +x
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direction. For 3d systems, the tilt is away from the z axis; for 2d
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systems, the tilt is away from the y axis.
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Style {spherical} allows an arbitrary 3d direction to be specified for
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the acceleration vector. Phi and theta are defined in the usual
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spherical coordinates. Thus for acceleration acting in the -z
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direction, theta would be 180.0 (or -180.0). Theta = 90.0 and phi =
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-90.0 would mean acceleration acts in the -y direction. For 2d
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systems, {phi} is ignored and {theta} is an angle in the xy plane
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where theta = 0.0 is the y-axis.
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Style {gradient} is the same as style {spherical} except that the
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direction of the acceleration vector is time dependent. The units of
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the gradient arguments are in full rotations per time unit. E.g. a
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timestep of 0.001 and a gradient of 0.1 means the acceleration vector
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would rotate thru 360 degrees every 10,000 timesteps. For the
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time-dependent case, the initial direction of the acceleration vector
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is phi,theta at the beginning of the simulation. For 2d systems,
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{phi} and {phi_grad} are ignored. See the note below about making the
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acceleration vector rotate continuously across multiple runs.
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Style {vector} imposes an acceleration in the vector direction given
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by (x,y,z). For 2d systems, the z component is ignored.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html. None of the "fix_modify"_fix_modify.html options
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are relevant to this fix. No global scalar or vector or per-atom
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quantities are stored by this fix for access by various "output
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commands"_Section_howto.html#4_15.
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This fix can change the direction of the gravity vector, due to the
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time-dependent {gradient} keyword, continuously over multiple runs,
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using the {start} and {stop} keywords of the "run"_run.html command.
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If you do not do this, the direction of the gravity vector will be
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reset to its initial value at the beginning of each run.
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This fix is not invoked during "energy minimization"_minimize.html.
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[Restrictions:] none
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[Related commands:]
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"atom_style granular"_atom_style.html, "fix addforce"_fix_addforce.html
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[Default:] none
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