# [lammps-users] question about couette flow

Hello all,

============= wall -->v
particles
============= wall <–v

I want to simulate particle in couette shear flow with nvt system in Nose/Hoover temperature thermostat .

I use fix nvt to the particles , fix nve/noforce to wall and fix ave/spatial to get the averaged atoms velocity profile.

Couette flow geometry was considered with dimensions 40 * 40 * 20. the interaction of the particles is LJ potential at 2^(1/6) of cutoff distance. the distance of the two walls is 20 unit, and I want to get the shear rate which is about 1.0. so I set the velocity of walls is +10/-10. The question is the range of particles’ velocity is about +2/-2 when I get the velocity profile of steady state .

What’s the matter? How can I do if I get the shear rate is 1.0?

main code as follows:

group ion type 1 2 3
group water type 4
group outsys union poly ion
group system union outsys water
group upwall type 5
group lowall type 6
group wall union upwall lowall

pair_style lj/cut 1.12246
pair_coeff * * 1.0 1.0

# Couette flow-for steay

reset_timestep 0
compute mobile system temp/partial 0 1 1
velocity system create 1.0 98748 units box

velocity lowall set -10.0 0.0 0.0 units box
velocity upwall set 10.0 0.0 0.0 units box

fix 1 system nvt 1.0 1.0 1.0 drag 0.2
fix 3 wall nve/noforce
thermo 100
thermo_modify temp mobile

fix 4 system ave/spatial 50 200 10000 z 0.0 2.0 vx fx density/number units box ave running file vel.profile
run 800000

Hello all,

============= wall -->v /+10
particles
============= wall <–v /-10

I want to simulate particle in couette shear flow with nvt system in Nose/Hoover temperature thermostat .

I use fix nvt to the particles , fix nve/noforce to wall and fix ave/spatial to get the averaged atoms velocity profile.

Couette flow geometry was considered with dimensions 40 * 40 * 20. the interaction of the particles is LJ potential at 2^(1/6) of cutoff distance. the distance of the two walls is 20 unit, and I want to get the shear rate which is about 1.0. so I set the velocity of walls is +10/-10. The question is the range of particles’ velocity is about +2/-2 when I get the velocity profile of steady state .

What’s the matter? How can I do if I get the shear rate is 1.0?

Any suggestion? Thanks a million.

main code :

group ion type 1 2 3
group water type 4
group outsys union poly ion
group system union outsys water
group upwall type 5
group lowall type 6
group wall union upwall lowall

pair_style lj/cut 1.12246
pair_coeff * * 1.0 1.0

# Couette flow-for steay

reset_timestep 0
compute mobile system temp/partial 0 1 1
velocity system create 1.0 98748 units box

velocity lowall set -10.0 0.0 0.0 units box
velocity upwall set 10.0 0.0 0.0 units box

fix 1 system nvt 1.0 1.0 1.0 drag 0.2
fix 3 wall nve/noforce
thermo 100
thermo_modify temp mobile

fix 4 system ave/spatial 50 200 10000 z 0.0 2.0 vx fx density/number units box ave running file vel.profile
run 10000000

I would start with fix langevin for thermostatting and make sure you subtract
off the velocity profile (e.g. fix temp/ramp) before thermostatting. Fix nvt
may have problems applying a global thermostat if the profile isn't
established.

Steve