Dear lammps users,
I am modeling a simple 2D model without any body force and temperature gradient. ‘p’ and ‘f’ style boundary condition are applied in x and y direction respectively. 270 fluid atoms are confined by two solid atoms wall that are perpendicular to y direction. Each wall has 3 layers and per layer includes 20 solid atoms. Every wall atoms is fixed to it’s initial position by using ‘fix spring/self’ command. In the whole simulation process, temperature for fluid atoms or solid atoms is kept constant value at 200K. However, I find that, by dividing fluid atoms region into many chunks, temperature for each chunk is far more less or higher than the specified temperature (200K). I will appreciate for any comments. My input file and temperature sample value are shown below.
Input file:
units nano
variable T_wall equal 200
variable T_fluid equal 200
variable spring_constant equal 70.0e3
dimension 2
boundary p f p
atom_style atomic
neighbor 1.0 bin
neigh_modify delay 1 check yes
#-------------------------model----------------------------
lattice sq 0.4
region box block 0 20 0 20 -0.25 0.25
create_box 2 box
#-------------------------region----------------------------
region r_bot_wall block INF INF INF 2 INF INF
region r_top_wall block INF INF 17.0 INF INF INF
region r_flow block INF INF 2.6 16.6 INF INF
#--------------------------create atoms----------------------
create_atoms 1 region box
delete_atoms region r_flow
group g_bot_wall region r_bot_wall
group g_top_wall region r_top_wall
group g_boundary union g_bot_wall g_top_wall
create_atoms 2 random 270 987654 r_flow
group g_flow region r_flow
mass 1 323.98306/1.0e6
mass 2 66.424/1.0e6
#-----------------------------LJ potential------------------
pair_style lj/cut 0.85125
pair_coeff 1 1 83.5 0.2475 0.85125
pair_coeff 2 2 1.657 0.3405 0.85125
pair_coeff 1 2 11.763 0.2940 0.85125
#------------------------initialization velocity------------
velocity g_boundary create ${T_wall} 987654 dist gaussian
velocity g_flow create ${T_wall} 987654 dist gaussian
#-------------------------spring model for wall atoms----------------------------------------------
fix spring_boundary_wall g_boundary spring/self ${spring_constant}
#-------------------------energy minimization-------------------------------------
minimize 1.0e-4 1.0e-6 10000 20000
min_style cg
#-------------------------thermostat for boundary---------------------------------
compute th_com_wall g_bot_wall temp
fix th_fix_wall g_bot_wall temp/rescale 1 {T_wall} {T_wall} 0.1 1
fix_modify th_fix_wall temp th_com_wall
compute th_com_wall_0 g_top_wall temp
fix th_fix_wall_0 g_top_wall temp/rescale 1 {T_wall} {T_wall} 0.1 1
fix_modify th_fix_wall_0 temp th_com_wall
#-------------------------thermostat for liquid------------------------------------
compute th_com_flow g_flow temp
fix th_scale_flow g_flow temp/rescale 1 {T_wall} {T_wall} 0.1 1
fix_modify th_scale_flow temp th_com_flow
fix 1 g_flow nve
fix spring_recenter_0 g_bot_wall recenter INIT INIT INIT
fix spring_recenter_1 g_top_wall recenter INIT INIT INIT
fix 2D all enforce2d
#------------------------------------sampling---------------------------------------
compute ps_com_20 g_flow chunk/atom bin/2d x lower 0.4 y lower 0.4 units box
fix ps_rusult_20 g_flow ave/chunk 2 450000 1000000 ps_com_20 vx vy density/number temp file flow_result_20.out format %20.16g
fix 2D all enforce2d
#------------------------------------equilibrium------------------------------------
thermo 50000
timestep 1e-5
run 1200000
Result file:
Chunk-averaged data for fix ps_rusult_20 and group vy
Timestep Number-of-chunks Total-count
Chunk Coord1 Coord2 Ncount vx vy density/number temp
1000000 400 270
1 0.2 0.2 0 0 0 0 0
2 0.2 0.6 0 0 0 0 0
3 0.2 1 0.849518 0.02352804448664468 -0.01301563323264512 5.30948611111111 472.0724828598343
4 0.2 1.4 0.835733 0.02778265445903207 0.002484448054801916 5.223333333333333 231.8115759493574
5 0.2 1.8 0.765669 -0.08681604323864593 -0.08960753577980518 4.785430555555554 255.7800832261954
6 0.2 2.2 0.651207 0.05774551308321196 -0.1011650063128498 4.070041666666666 295.817685251791
7 0.2 2.6 0.95758 -0.01612331854840349 -0.05656651574723375 5.984874999999999 232.1320425641624
8 0.2 3 0.678713 -0.1106450264499378 -0.05156627925166036 4.241958333333332 211.9923597647906
9 0.2 3.4 0.976262 0.02317112972051899 -0.03692316737165308 6.101638888888888 164.1864107184842
10 0.2 3.8 1.04875 0.005903985893984654 -0.02502700824055246 6.554708333333332 147.4709464387095
11 0.2 4.2 1.02764 -0.08828839589997028 -0.05857859491781743 6.42273611111111 146.1221720173032
12 0.2 4.6 1.01364 -0.02081578341295853 -0.05502178697296155 6.33523611111111 202.1554337989851
13 0.2 5 0.922111 -0.01015158915211346 0.005847000316195504 5.763194444444443 143.8013536391488
14 0.2 5.4 1.03162 0.01090064654920931 0.01906690753196229 6.44761111111111 143.5376657735961
15 0.2 5.8 0.958351 0.02335967671758488 -0.008381007332411464 5.989694444444443 119.3280655059463
16 0.2 6.2 0.605938 0.0157236117560291 0.001028595226501184 3.78711111111111 267.1929246342699
17 0.2 6.6 0.983867 0.0158147489503501 0.001105043228880838 6.149166666666666 281.1074527327147
18 0.2 7 0 0 0 0 0
19 0.2 7.4 0 0 0 0 0
20 0.2 7.8 0 0 0 0 0
21 0.6 0.2 0 0 0 0 0
22 0.6 0.6 0 0 0 0 0
23 0.6 1 0.951907 0.05172156724970964 0.001537519539081735 5.949416666666665 271.7508361325796
24 0.6 1.4 0.539631 0.01883793283066265 -0.05909856665974594 3.372694444444444 285.5369986608534
25 0.6 1.8 0.75654 -0.05936042233465616 -0.0864925773257037 4.728374999999999 167.6098919036001
26 0.6 2.2 0.838131 0.005788201658283289 -0.0691722535753259 5.238319444444444 190.3178900976716
27 0.6 2.6 0.66644 -0.09478387610074612 -0.04597171037207621 4.165249999999999 187.7235525702241
28 0.6 3 1.08922 0.006225641030681069 -0.03965648323285471 6.807624999999999 186.0507455276578
29 0.6 3.4 0.946067 0.0003923845041151799 -0.09130854932594615 5.912916666666666 170.8021623627343
30 0.6 3.8 0.880638 -0.009052697095403652 -0.05022956476833229 5.50398611111111 280.7950327296847
31 0.6 4.2 0.948231 -0.08923973798846932 0.003395978920757619 5.926444444444443 119.0612160886134
32 0.6 4.6 0.88118 -0.05547087357098041 -0.004540757339456147 5.507374999999999 207.7102084607978
33 0.6 5 1.28102 -0.07054393141508182 -0.01642128027470105 8.006374999999998 143.6596514674557
34 0.6 5.4 0.999156 0.03619045639749988 -0.003712629483356373 6.244722222222221 115.5324706098614
35 0.6 5.8 0.31664 0.04024401974051375 0.06954972690469866 1.979 188.2840310493752
36 0.6 6.2 0.936398 0.001023289130979687 0.03276297151422664 5.85248611111111 197.5639529151157
37 0.6 6.6 0.97152 -0.0001972680684431195 0.01505331050389747 6.071999999999999 377.7859140320545
38 0.6 7 0 0 0 0 0
39 0.6 7.4 0 0 0 0 0
40 0.6 7.8 0 0 0 0 0
…
Best wishes,
Qiangqiang Sun