3D Poiseuille flow

Hi all,

I am using lammps to simulate the 3D Poiseuille flow. My model is a cubic and I use the fix addforce command to give each atom a force which is along with x direction. Unlike the other 3D poiseuille flow, there are two walls used to confine the fluid zone in y and z direction respectively.
However, when my code is run about 500,000 timesteps and there is an error reminding me that some atoms are lost. I will be grateful if you may help me fix this problem. Some commands of my input file is shown as follows:

dimension 3
boundary p f f
atom_style atomic
neighbour 2.0 bin
neigh_modify delay 1 check yes
#geometry create
lattice fcc 0.545
region box block 0 20 0 16 0 16
create_box 2 box
create_atoms 1 box
#define groups
region 1 block 0 20 3 13 3 13
group flow region 1
group boundary subtract all flow
#initial velocities
velocity boundary create {T_wall} 99999 dist gaussian mom yes fix wall_force boundary spring/self 70e3 velocity flow create {T_fluid} 99999 dist gaussian mom yes

Best wishes,
Qiangqiang Sun

in.flow (1.79 KB)