Dear lammps users,

I hope I can get some help, this is my first time using lammps. I am trying to get the velocity profile for couette flow. I get the velocities using the code below, then calculate averages and plot them, I don’t get the linear velocity profile. Velocities are wrong. I tried several attempts, hence some lines are commented out.

Also, when I run the simulation, I am getting 10000 atoms in group boundary and only 1200 atoms in group flow, I think this might be the problem but not sure. How can I have more atoms in group flow?

If anyone can tell me what am I doing wrong and how can I get the linear velocity profile, I would really appreciate.

Thank you all

Haz

# 2-d LJ flow simulation

dimension 2

boundary p s p

atom_style atomic

#units real

neighbor 0.3 bin

neigh_modify delay 5

# create geometry

lattice hex 0.7

region box block 0 80 0 70 -0.25 0.25

create_box 3 box

create_atoms 1 box

mass 1 1.0

mass 2 1.0

mass 3 1.0

# LJ potentials

pair_style lj/cut 1.12246

pair_coeff * * 1.0 1.0 1.12246

# define groups

region 1 block INF INF INF 1.25 INF INF

group lower region 1

region 2 block INF INF 8.75 INF INF INF

group upper region 2

group boundary union lower upper

group flow subtract all boundary

set group lower type 2

set group upper type 3

# initial velocities

#compute mobile flow vacf

compute mobile flow temp

#compute mobile flow temp/profile 1 0 0 xy 20

#velocity flow create 50.0 482748 sum yes dist gaussian

#velocity flow scale 50.0 sum yes dist gaussian

velocity flow ramp vx 0 3 y 0 70 dist gaussian

#velocity flow ramp vx 0 3 y 0 70 dist gaussian

#fix 1 all nve

fix 1 all nvt temp 1.0 1.0 1.0

#fix 2 flow temp/rescale 1 50.0 50.0 0.02 1.0

#fix_modify 2 temp mobile

# Couette flow

velocity lower set 0.0 0.0 0.0

velocity upper set 3.0 0.0 0.0

fix 3 boundary setforce 0.0 0.0 0.0

fix 4 all enforce2d

# Run

timestep 0.003

thermo 500

#thermo_modify temp mobile

dump 1 all custom 500 velocities1.txt type vx vy vz

#dump 1 all image 100 image.*.jpg type type &

zoom 1.6 adiam 1.2

#dump_modify 1 pad 5

run 10000