I am trying to simulate a Couette flow in which the velocity of the wall is constantly increased. I want to be able to keep the temperature of the system constant, but I am unable to do so. I have used the same thermostating procedure for a constant wall velocity Couette flow problem and it worked, even for much higher wall velocities. I am unable to find what I am doing wrong or if i just need to use a better thermostat. Please find the input file below and the log file attached.
Thank you.
Appreciate your help.
BR/Joseph
3-d LJ flow simulation
units lj
dimension 3
boundary p s p
atom_style atomic
neighbor 0.3 bin
neigh_modify delay 1
create geometry
region simulation_box block 0.0 12.0 0.0 28.9 0.0 144.49 units box
create_box 3 simulation_box
region fluid block INF INF 2.94 25.96 INF INF units box
region btm_wall block INF INF INF 1.5 INF INF units box
region top_wall block INF INF 27.4 INF INF INF units 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 units box
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 units box
create_atoms 3 region top_wall units box
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 0.1 1.0
pair_modify shift yes
define groups
group lower region btm_wall
group upper region top_wall
group boundary union lower upper
group flow subtract all boundary
set group flow type 1
set group lower type 2
set group upper type 3
initial velocities
compute mobile flow temp
velocity flow create 1.10 482748 temp mobile units box
thermo_modify temp mobile
thermostating
compute mobile_thermo all temp/partial 0 0 1
fix 2 all langevin 1.10 1.10 1.0 699483
fix 1 flow nve
fix 1_1 lower nve
fix_modify 2 temp mobile_thermo
thermo_modify temp mobile_thermo
#Run
Equilbration run
timestep 0.005
thermo 50000
Production run
Couette flow
ramping wall
variable step equal step
variable VelRamp equal step*0.000000025
variable y_Vel equal 0.0
variable z_Vel equal 0.0
velocity lower set 0.0 0.0 0.0 units box
fix wl upper move variable NULL NULL NULL v_VelRamp v_y_Vel v_z_Vel units box
fix 3 boundary setforce 0.0 0.0 0.0
print wall speed
fix 41 all print 50000 “wall speed {step} {VelRamp}”
outputing velocity
fix 5 all ave/spatial 1 1000 100000 y 27.0285 1.0218092 vx region simulation_box file vel_fb.profile units box
compute atomstress all stress/atom NULL
fix 17 flow ave/spatial 1 1000 100000 y 27.0285 1.0218092 c_atomstress[1] c_atomstress[2] c_atomstress[3] c_atomstress[4] c_atomstress[5] c_atomstress[6] file ptens.dat units box
Run
restart 50000000 restart_ramp_Case_1.*.equil
run 800000000
log.lammps (824 KB)