forked from lijiext/lammps
96 lines
3.5 KiB
HTML
96 lines
3.5 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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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group gravity style 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>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 axis (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 axis (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 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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<I>vector</I> 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
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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 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
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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. 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 <A HREF = "fix_addforce.html">fix addforce</A> 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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</P>
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<P>The first 3 styles apply to granular systems. Style <I>chute</I> 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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<I>spherical</I> allows an arbitrary 3d direction to be specified for the
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gravity vector. Style <I>gradient</I> 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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</P>
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<P>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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</P>
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<P>Style <I>vector</I> 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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</P>
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<P>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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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>Styles <I>chute</I>, <I>spherical</I>, and <I>gradient</I> can only be used with
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atom_style granular. Style <I>vector</I> can only be used with
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non-granular systems.
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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 granular</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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