[lammps-users] why the velocity profile is not linear?

LAMMPS LAMMPS Mailing List Mirror
1

Dear Steve,

I have run the example file for the NEMD simulation as bellow. I have added a line to moniter the velocity profile along the flow gradient direction. However, I found that the velocity profile was not linear as we expected (see attachment). How did this happen?

Thanks.

Input file:

2d NEMD simulation

units lj
atom_style atomic
dimension 2d

lattice sq2 0.8442
region box prism 0 10 0 8 -0.5 0.5 0 0 0
create_box 2 box
create_atoms 1 box
mass * 1.0

velocity all create 1.44 87287 loop geom

region slice block 4 6 INF INF INF INF
set region slice type 2

pair_style lj/cut 2.5
pair_coeff * * 1.0 1.0 1.0

neighbor 0.3 bin
neigh_modify delay 0 every 1

fix 1 all nvt/sllod temp 1.0 1.0 1.0 tchain 1
fix 2 all deform 1 xy erate 0.01 remap v
fix 11 all ave/spatial 1 10 10000 y lower 0.1 vx units reduced norm sample file vel.profile

dump 1 all custom 500 dump.nemd id type x y z

thermo 1000

run 50000

Graph1.jpg

2

Could be any number of reasons - maybe you aren't averaging
enough samples. It looks like you only have 10 samples. Have
you tried visualiing the system - is it flowing correctly?

Steve

3

Dear Steve,

I have run the same simulation with a large sample size 1000. However, the velocity profile is still not linear as shown in the attachment. I have visualized the results. The box moves as under shear, which is also given in the attachment.

Best,
Ying Li

Picture3.emf (24.6 KB)

snap.bmp (706 KB)

4

If the atoms are moving with the box, and you use
fix ave/spatial correctly, then you will get a roughly linear
profile.

Steve