[lammps-users] Ramped viscous boundary conditions

Hello All,

I am doing atomistic simulations for studying crack propagation in metals. In the literature I have seen people applying a ramped viscous damping to a “skin” region few angstroms inside the box boundaries (for absorbing strain waves emitted from the crack tip). The sample is divided in to two regions inner and the skin region. The temperature (in the skin region) is slowly ramped from its value in the inner region (say 10K), to those (say 0K) at the outer boundary of the skin region.

I want to use fix viscous with a thermostat (fix temp/rescale or any other thermostat with fix nve) for applying ramped viscous boundary conditions. I am not sure how to do this in lammps. Any help will be deeply appreciated.

Thank you



If you mean you want to ramp the famma value given to fix
viscous over time, since it is proportional to T, then here
are some ideas. Fix viscous will work on a group
of atoms. For a solid, you could define a geometric region
and then use the group command. The region and group
will remain basically identical for a solid. You would have
to modify the fix viscous command to ramp its gamma value
over time, like the fix nvt command does for its temperature.
This would be a small change to the fix_viscous.cpp file.