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### fix gravity command

Syntax:

    fix ID group gravity style magnitude args

- ID, group are documented in fix command
- gravity = style name of this fix command
- magnitude = size of acceleration (force/mass units)
- style = chute or spherical or gradient or vector

        chute args = angle
          angle = angle in +x away from -z or -y axis in 3d/2d (in degrees)
        spherical args = phi theta
          phi = azimuthal angle from +x axis (in degrees)
          theta = angle from +z or +y axis in 3d/2d (in degrees)
        gradient args = phi theta phi_grad theta_grad
          phi = azimuthal angle from +x axis (in degrees)
          theta = angle from +z or +y axis in 3d/2d (in degrees)
          phi_grad = rate of change of angle phi (full rotations per time unit)
          theta_grad = rate of change of angle theta (full rotations per time unit)
        vector args = x y z
          x y z = vector direction to apply the acceleration

Examples:

    fix 1 all gravity 1.0 chute 24.0
    fix 1 all gravity 1.0 spherical 0.0 -180.0
    fix 1 all gravity 1.0 gradient 0.0 -180.0 0.0 0.1
    fix 1 all gravity 100.0 vector 1 1 0

Description:

Impose an additional acceleration on each particle in the group. This fix is typically used with granular systems to include a "gravity" term acting on the macroscopic particles. More generally, it can represent any kind of driving field, e.g. a pressure gradient inducing a Poiseuille flow in a fluid. Note that this fix operates differently than the fix addforce command. The addforce fix adds the same force to each atom, independent of its mass. This command imparts the same acceleration to each atom (force/mass).

The magnitude of the acceleration is specified in force/mass units. For granular systems (LJ units) this is typically 1.0. See the units command for details.

Style chute is typically used for simulations of chute flow where the specified angle is the chute angle, with flow occurring in the +x direction. For 3d systems, the tilt is away from the z axis; for 2d systems, the tilt is away from the y axis.

Style spherical allows an arbitrary 3d direction to be specified for the acceleration vector. Phi and theta are defined in the usual spherical coordinates. Thus for acceleration acting in the -z direction, theta would be 180.0 (or -180.0). Theta = 90.0 and phi = -90.0 would mean acceleration acts in the -y direction. For 2d systems, phi is ignored and theta is an angle in the xy plane where theta = 0.0 is the y-axis.

Style gradient is the same as style spherical except that the direction of the acceleration vector is time dependent. The units of the gradient arguments are in full rotations per time unit. E.g. a timestep of 0.001 and a gradient of 0.1 means the acceleration vector would rotate thru 360 degrees every 10,000 timesteps. For the time-dependent case, the initial direction of the acceleration vector is set to phi,theta when the fix is specified and evolves thereafter. For 2d systems, phi and phi_grad are ignored.

Style vector imposes an acceleration in the vector direction given by (x,y,z). For 2d systems, the z component is ignored.

Restart, fix_modify, output, run start/stop, minimize info:

No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global scalar or vector or per-atom quantities are stored by this fix for access by various output commands. No parameter of this fix can be used with the start/stop keywords of the run command. This fix is not invoked during energy minimization.

Restrictions: none

Related commands:

atom_style granular, fix addforce

Default: none