fix speedcontrol command¶

Syntax¶

fix ID group-ID speedcontrol vx vy vz keyword value ...

ID, group-ID are documented in fix command
speedcontrol = style name of this fix command
vx,vy,vz = set-velocity component values (velocity units)
```
any of vx,vy,vz can be a variable (see below)
```
zero or more keyword/value pairs may be appended to args

keyword = region or gain

region value = region-ID
  region-ID = ID of region atoms must be in to have added force
gain value = gain
  gain = proportional gain of the proportional controller

Examples¶

fix kick flow speedcontrol 1.0 0.0 0.0
fix kick flow speedcontrol 1.0 0.0 v_oscillate region force_reg
fix kick flow speedcontrol 0.0 NULL v_push gain v_gain

Description¶

Add vx,vy,vz to the corresponding component of the set-velocity for each atom in the group. This command can be used to constrain the velocity of the atoms in a simulation, e.g. at an outlet with a well-defined massflow (screw conveyor).

This fix applies a force to each atom which is proportional to the controller gain times the difference between the atom’s velocity and the set-velocity.

Any of the 3 quantities defining the set-velocity can be specified as an equal-style or atom-style variable, namely vx, vy, vz. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and its value(s) used to determine the set-velocity component. Any of the x,y,z values can be specified as NULL which means do not include that dimension in the application of the velocity controller.

Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent force field.

Atom-style variables can specify the same formulas as equal-style variables but can also include per-atom values, such as atom coordinates. Thus it is easy to specify a spatially-dependent force field with optional time-dependence as well.

If the region keyword is used, the atom must also be in the specified geometric region in order to have its velocity controlled.

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

No information about this fix is written to binary restart files.

No parameter of this fix can be used with the start/stop keywords of the run command.

Restrictions¶

none

Default¶

The option defaults are gain = 0.1