You are right. I didn’t check that, sorry.
I can’t think of any other way of achieving a region with a constant velocity. My guess is that you’ll have to create a new fix for this. It should be pretty straightforward to do based on fix setforce.
Fix move doesn’t allow for dynamic groups b/c it
could be fatal to not move an atom for a while
then when it becomes part of the group, move
it halfway across the box.
However, the case you want is to use fix move
with its variable option, and set x,y,z = NULL
and vx,vy,vz to some constant vector. That
could work with a dynamic group since there
is no discontinuity in the motino. We can enable
dynamic groups for that option.
There are other problems with the scheme, like the absence of thermal motion in the inflow fluid. One solution is to apply a langevin thermostat to the inflow particles with a very strong coupling. This will randomize the velocities at the desired temperature, with the correct average streaming velocity. The simulation you are aiming for has been done many times in LAMMPS. See: