[lammps-users] viscous damping + force


I’m doing a friction simulation and I want to reduce bouncing in my tip by applying some damping to it’s motion. I’m using the following fixes to the tip:

fix 1 topfix viscous 0.01
fix 2 topfix setforce 0.0 0.0 NULL
fix 3 topfix aveforce NULL NULL -0.05

If I run using this series of fixes, my force starts at -0.05 / atom then drops to zero with small oscillations. Basically it is damped like I want but with the wrong average force. If I run with the fix viscous off, it has large oscillations around -0.05 / atom.

I think what I want is Fatom = Fave - velgamma so I get a constant, nonzero applied force. What I am getting appears to be F = -velgamma only. Is there a way to get around this?

Thanks, Dave

These 3 fixes all adjust the force at the same point in the
timestep. Which means they will act in the order you
specify them. So you might try listing the viscous fix
third, instead of first, if you want its term to be finally added.