[lammps-users] can't get linear velocity profile in couette flows

since the system scrubbed the attached files, the script is as
follows. you advice is gratefully appreciated.

# 3-d LJ flow simulation

dimension 3
boundary p p s

atom_style atomic
units lj
neighbor 0.3 bin
neigh_modify delay 5

# create geometry

lattice fcc 0.81
region box block 0 20 0 4 0 28
create_box 3 box
create_atoms 2

mass 1 1.0
mass 2 1.0
mass 3 1.0

# LJ potentials

pair_style lj/cut 2.5
pair_coeff * * 1.0 1.0 2.5

# define groups

region 1 block INF INF INF INF INF 2
group lower region 1
region 2 block INF INF INF INF 26 INF
group upper region 2
group boundary union lower upper
group flow subtract all boundary

set lower atom 1
set upper atom 3

# initial velocities

compute mobile flow temp/partial 0 1 1
velocity flow create 1.1 482748 temp mobile
fix 1 all nve
fix 2 flow temp/rescale 200 1.1 1.1 0.02 1.0
fix_modify 2 temp mobile

# Couette flow

velocity lower set 0.0 0.0 0.0
velocity upper set 0.0 0.0 0.0
fix 3 boundary setforce 0.0 0.0 0.0

# Run
run 10000

timestep 0.005
velocity upper set 1.0 0.0 0.0
fix 4 flow ave/spatial 2 2000 z center 1 grid3.plt atom vx norm sample
dump 1 all atom 1000 dump.flow
run 50000