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# 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 using the fix lb/fluid. This should be used in conjunction with one of the built-in LAMMPS integrators, such as fix NVE or fix rigid.

This fix adds a viscous force to each atom to cause it move with the same velocity as the fluid (an equal and opposite force is applied to the fluid via fix lb/fluid). When fix lb/fluid is called with the noise option, the atoms will also experience random forces which will thermalize them to the same temperature as the fluid. In this way, the combination of this fix with fix lb/fluid and a LAMMPS integrator like fix NVE is analogous to fix langevin except here the fluid is explicit. The temperature of the particles can be monitored via the scalar output of fix lb/fluid.

For details of this fix, as well as descriptions and results of several test runs, see Denniston 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 LATBOLTZ package. It is only enabled if LAMMPS was built with that package. See the Build package 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.

## Related commands

fix lb/fluid

## Default

none

(Denniston et al.) Denniston, C., Afrasiabian, N., Cole-Andre, M.G., Mackay, F. E., Ollila, S.T.T., and Whitehead, T., LAMMPS lb/fluid fix version 2: Improved Hydrodynamic Forces Implemented into LAMMPS through a lattice-Boltzmann fluid, Computer Physics Communications 275 (2022) 108318 .