forked from lijiext/lammps
146 lines
6.0 KiB
HTML
146 lines
6.0 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<HR>
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<H3>fix gravity command
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</H3>
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<H3>fix gravity/cuda command
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</H3>
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<H3>fix gravity/omp command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group gravity style magnitude args
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</PRE>
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<UL><LI>ID, group are documented in <A HREF = "fix.html">fix</A> command
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<LI>gravity = style name of this fix command
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<LI>magnitude = size of acceleration (force/mass units)
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<LI>style = <I>chute</I> or <I>spherical</I> or <I>gradient</I> or <I>vector</I>
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<PRE> <I>chute</I> 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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<I>spherical</I> 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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<I>gradient</I> 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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<I>vector</I> args = x y z
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x y z = vector direction to apply the acceleration
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>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
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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 <A HREF = "fix_addforce.html">fix addforce</A> 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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</P>
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<P>The <I>magnitude</I> 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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<A HREF = "units.html">units</A> command for details.
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</P>
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<P>Style <I>chute</I> 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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</P>
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<P>Style <I>spherical</I> 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, <I>phi</I> is ignored and <I>theta</I> is an angle in the xy plane
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where theta = 0.0 is the y-axis.
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</P>
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<P>Style <I>gradient</I> is the same as style <I>spherical</I> 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 set to phi,theta when the fix is specified and evolves thereafter.
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For 2d systems, <I>phi</I> and <I>phi_grad</I> are ignored.
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</P>
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<P>Style <I>vector</I> 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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</P>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the gravitational potential energy of the system to the
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system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>.
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</P>
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<P>This fix computes a global scalar which can be accessed by various
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<A HREF = "Section_howto.html#howto_15">output commands</A>. This scalar is the
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gravitational potential energy of the particles in the defined field,
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namely mass * (g dot x) for each particles, where x and mass are the
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particles position and mass, and g is the gravitational field. The
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scalar value calculated by this fix is "extensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "atom_style.html">atom_style sphere</A>, <A HREF = "fix_addforce.html">fix addforce</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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