In the couette flow example I wanted to fix the velocity of the top wall to 1.0 and hence have used the velocity command. And, also as mentioned by Steve in some old posts have used setforce and set it to 0. But when I writing the average velocity of the molecules using fix ave/spatial the values near the wall exceed 1.0. I am unable to figure out if I am modelling the problem or interpreting the results wrongly.
Would appreciate if you could help me clarify as to why the values exceed the prescribed velocity value.
BR?joseph
The input file:
# 3-d LJ flow simulation
units lj
dimension 3
boundary p s p
atom_style atomic
neighbor 0.3 bin
neigh_modify delay 5
# create geometry
region fluid block 0 12.51 3.0 27.57 -3.61 3.61 units box
region btm_wall block 0 12.51 0 3.0 -3.61 3.61 units box
region top_wall block 0 12.51 27.57 30.57 -3.61 3.61 units box
region simulation_box union 3 btm_wall fluid top_wall units box
create_box 3 simulation_box
lattice fcc 0.81 orient x 1 1 -2 orient y 1 1 1 orient z 1 -1 0
create_atoms 1 region fluid
lattice fcc 0.81 orient x -1 1 0 orient y 1 1 1 orient z 1 1 -2
create_atoms 2 region btm_wall
lattice fcc 0.81 orient x -1 1 0 orient y 1 1 1 orient z 1 1 -2
create_atoms 3 region top_wall
mass 1 1.0
mass 2 1.0
mass 3 1.0
# LJ potentials
pair_style lj/cut 2.2
pair_coeff * * 1.0 1.0
pair_coeff 1 2*3 0.6 1.0
pair_modify shift yes
# define groups
region 1 block INF INF INF 3.0 INF INF units box
group lower region 1
region 2 block INF INF 27.57 INF INF INF units box
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 temp
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 1.0 0.0 0.0
fix 3 boundary setforce 0.0 0.0 0.0
#fix 4 all enforce2d
group tp_wall region top_wall
#fluid adjacent to bottom wall
fix 5 flow ave/spatial 1 7000 7500 x center 2.0 z center 2.0 y 3.0 2.0 vx file vel_fb.profile
fix 6 tp_wall ave/spatial 1 7000 7500 x center 2.0 z center 2.0 y 3.0 2.0 vx file vel_tw.profile
# Poiseuille flow
#velocity boundary set 0.0 0.0 0.0
#fix 3 lower setforce 0.0 0.0 0.0
#fix 4 upper setforce 0.0 NULL 0.0
#fix 5 upper aveforce 0.0 -1.0 0.0
#fix 6 flow addforce 0.5 0.0 0.0
#fix 7 all enforce2d
# Run
timestep 0.005
thermo 500
thermo_modify temp mobile
dump 1 all atom 50 dump.flow
run 15000
Part of the velocity data:
31 12.0414 2.78084 0 0 0
32 12.0414 2.78084 5.89905 44.4549 0.116262
33 12.0414 2.78084 11.7981 48.5744 0.240347
34 12.0414 2.78084 17.6972 45.2464 0.609851
35 12.0414 2.78084 23.5962 40.7083 1.24065
36 12.0414 2.78084 29.4953 8.07929 1.90468