forked from lijiext/lammps
87 lines
3.2 KiB
Plaintext
87 lines
3.2 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 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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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 axis (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 axis (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 axis (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
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(full rotations per time unit)
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{vector} args = magnitude x y z
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magnitude = size of acceleration (force/mass units)
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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 chute 24.0
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fix 1 all gravity spherical 0.0 -180.0
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fix 1 all gravity gradient 0.0 -180.0 0.0 0.1
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fix 1 all gravity vector 100.0 1 1 0 :pre
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[Description:]
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Impose an additional acceleration on each particle in the group. For
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granular systems the magnitude is chosen so as to be due to gravity.
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For non-granular systems the magnitude of the acceleration is
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specified, so it can be any kind of driving field desired (e.g. a
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pressure gradient inducing a Poisselle flow). Note that this is
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different from what the "fix addforce"_fix_addforce.html command does,
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since it adds the same force to each atom, independent of its mass.
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This command adds the same acceleration to each atom (force/mass).
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The first 3 styles apply to granular systems. Style {chute} is
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typically used for simulations of chute flow where the specified angle
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is the chute angle, with flow occurring in the +x direction. Style
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{spherical} allows an arbitrary 3d direction to be specified for the
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gravity vector. Style {gradient} allows the direction of the gravity
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vector to be time dependent. The units of the gradient arguments are
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in full rotations per time unit. E.g. a timestep of 0.001 and a
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gradient of 0.1 means the gravity vector would rotate thru 360 degrees
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every 10,000 timesteps. For the time-dependent case, the initial
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direction of the gravity vector is phi,theta at the time the fix is
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specified.
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Phi and theta are defined in the usual spherical coordinates. Thus
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for gravity acting in the -z direction, theta would be specified as
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180.0 (or -180.0). Theta = 90.0 and phi = -90.0 would mean gravity
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acts in the -y direction.
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Style {vector} is used for non-granular systems. An acceleration of
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the specified magnitude is applied to each atom in the group in the
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vector direction given by (x,y,z).
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The strength of the acceleration due to gravity is 1.0 in LJ units,
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which are the only allowed units for granular systems.
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[Restrictions:]
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Styles {chute}, {spherical}, and {gradient} can only be used with
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atom_style granular. Style {vector} can only be used with
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non-granular systems.
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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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