# fix lb/viscous command

## Syntax

fix ID group-ID lb/viscous

• ID, group-ID are documented in fix command
• lb/viscous = style name of this fix command

## Examples

fix 1 flow lb/viscous

## Description

This fix is similar to the fix viscous command, and is to be used in place of that command when a lattice-Boltzmann fluid is present, and the user wishes to integrate the particle motion using one of the built in LAMMPS integrators.

This fix adds a force, F = - Gamma*(velocity-fluid_velocity), to each atom, where Gamma is the force coupling constant described in the fix lb/fluid command (which applies an equal and opposite force to the fluid).

Note

This fix should only be used in conjunction with one of the built in LAMMPS integrators; it should not be used with the fix lb/pc or fix lb/rigid/pc/sphere integrators, which already include the hydrodynamic forces. These latter fixes should only be used if the force coupling constant has been set by the user (instead of using the default value); if the default force coupling value is used, then this fix provides the only method for adding the hydrodynamic forces to the particles.

For further details, as well as descriptions and results of several test runs, see Mackay et al.. Please include a citation to this paper if this fix is used in work contributing to published research.

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

As described in the fix viscous documentation:

“No information about this fix is written to binary restart files. None of the fix_modify options are relevant to this fix. No global 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.

The forces due to this fix are imposed during an energy minimization, invoked by the minimize command. This fix should only be used with damped dynamics minimizers that allow for non-conservative forces. See the min_style command for details.”

## Restrictions

This fix is part of the USER-LB package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.

Can only be used if a lattice-Boltzmann fluid has been created via the fix lb/fluid command, and must come after this command.

This fix should not be used if either the fix lb/pc or fix lb/rigid/pc/sphere integrator is used.