velocity profile for couette flow

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1

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

2

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.

your dump file contains no position information for the atoms. how can
you even assign them to spatial bins to determine your spatial
velocity profile?
apart from that, why not use the spatial averaging feature in LAMMPS
directly via compute atom/chunk bin1d and fix ave/chunk?

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

your thinking is wrong. this is a case of PEBCAC.

sure. How can I have more atoms in group flow?

by changing the group boundaries accordingly. LAMMPS does what you are
asking it to do. PEBCAC, too.

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

you need to revisit your approach to problem solving. a problem is not
solved when you trick the simulation software into producing the
output that you want (as is evident from your various commented out
hacks), but when you *understand* what is causing the difference
between what you see and what you expect. just ask any medical doctor
about the difference between suppressing the symptoms and curing the
disease...

axel.