CFDEM simulation stopped with a sudden change of Courant number and particle number

Hi,

I am now facing a strange problem with my simulation. I have successfully run my CFDEM code for more than 12 sec with the courant number to be about 0.01. However, in the following step after time 12.2965s, the courant number suddenly increased to more than 1000 and stopped after several step and my particles also left the calculation region in a few steps. I didn't change any boundary conditions during this simulation, why could this sudden stop happen?

I am using the latest LIGGGHTS version 3.8.0, but I use the the OpenFOAM 3.0.x because I have wrote my code using the old script rules. This is not an official supported one (The official one should be OpenFOAM 5.0), but I have used this version for several simulations and there seem to be no problem.

Could anyone please give me any clues about this sudden stop? Any suggestions are greatly appreciated!

Best,
Rachel

***** Attached is part of my log file indicating the courant suddenly changed from 0.01 to 1000+ *****

Time = 12.2965

Courant Number mean: 0.00462484 max: 0.017621

Coupling...
Starting up LIGGGHTS
Executing command: 'run 100 '
run 100
Setting up run at Wed Jan 24 02:04:54 2018

Memory usage per processor = 20.9767 Mbytes
Step Atoms KinEng 1 Volume tscheck[ tscheck[ centerOf explDrag explDrag explDrag cfd2[1] cfd2[2] cfd2[3]
32593002 6951 1.7284393e-10 2.4527497e-10 9.6e-05 0.14559429 0.029633105 0.020961084 -0.0065086976 0.0054973266 0.28438671 -0.0065086976 0.0054973266 0.28438671
CFD Coupling established at step 32593100
32593102 6951 2.3199549e-10 2.3913056e-10 9.6e-05 0.14559429 0.029633105 0.020961085 -0.0065086976 0.0054973266 0.28438671 -0.0065086976 0.0054973266 0.28438671
Loop time of 0.232954 on 16 procs for 100 steps with 6951 atoms, finish time Wed Jan 24 02:04:54 2018

Pair time (%) = 0.0440159 (18.8947)
Neigh time (%) = 0 (0)
Comm time (%) = 0.00673512 (2.89118)
Outpt time (%) = 8.34763e-05 (0.0358338)
Other time (%) = 0.18212 (78.1783)

Nlocal: 434.438 ave 1607 max 0 min
Histogram: 8 0 0 3 2 1 0 1 0 1
Nghost: 638.938 ave 1722 max 0 min
Histogram: 8 0 0 0 1 1 1 2 2 1
Neighs: 14006.4 ave 64098 max 0 min
Histogram: 8 0 4 2 0 0 0 1 0 1

Total # of neighbors = 224102
Ave neighs/atom = 32.2403
Neighbor list builds = 0
Dangerous builds = 0
Executing command: 'delete_atoms region deleteArea compress yes '
delete_atoms region deleteArea compress yes
Deleted 0 atoms, new total = 6951
Executing command: 'write_restart ../DEM/post/restart/liggghts.restartCFDEM'
write_restart ../DEM/post/restart/liggghts.restartCFDEM
System init for write_restart ...
LIGGGHTS finished

timeStepFraction() = 1

Solver level total Eulerian momentum exchange:
TotalForceImp: (-0.000925955 0.00767524 -0.097385)
Warning, these values are based on latest Ksl and Us but prev. iteration U!

DILUPBiCG: Solving for Ux, Initial residual = 0.00495177, Final residual = 5.01546e-06, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.00478385, Final residual = 5.61765e-06, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.000205532, Final residual = 1457.53, No Iterations 1001
DICPCG: Solving for p, Initial residual = 1, Final residual = 0.0718173, No Iterations 4
DICPCG: Solving for p, Initial residual = 0.0356457, Final residual = 0.0034, No Iterations 9
time step continuity errors : sum local = 90494.2, global = 13025, cumulative = 13025
DICPCG: Solving for p, Initial residual = 0.832433, Final residual = 0.0595029, No Iterations 4
DICPCG: Solving for p, Initial residual = 0.222455, Final residual = 0.0216217, No Iterations 9
time step continuity errors : sum local = 92020, global = -14842.6, cumulative = -1817.58
DICPCG: Solving for p, Initial residual = 0.25916, Final residual = 0.0234269, No Iterations 10
DICPCG: Solving for p, Initial residual = 0.24407, Final residual = 0.0227205, No Iterations 9
time step continuity errors : sum local = 1529.67, global = -150.673, cumulative = -1968.26
DICPCG: Solving for p, Initial residual = 0.19799, Final residual = 0.00969306, No Iterations 11
DICPCG: Solving for p, Initial residual = 0.73671, Final residual = 6.58381e-07, No Iterations 27
time step continuity errors : sum local = 0.000101788, global = -6.66061e-07, cumulative = -1968.26
ExecutionTime = 125288 s ClockTime = 126018 s

Time = 12.2966

Courant Number mean: 46.0522 max: 1018.46

Coupling...
Starting up LIGGGHTS
Executing command: 'run 100 '
run 100
Setting up run at Wed Jan 24 02:04:54 2018

Memory usage per processor = 20.9767 Mbytes
Step Atoms KinEng 1 Volume tscheck[ tscheck[ centerOf explDrag explDrag explDrag cfd2[1] cfd2[2] cfd2[3]
32593102 6951 2.3199549e-10 2.3913056e-10 9.6e-05 0.14559429 0.029633105 0.020961085 -0.0048705403 0.0085464214 0.28258701 -0.0048705403 0.0085464214 0.28258701
CFD Coupling established at step 32593200
32593202 6951 2.1256608e-10 2.8529695e-10 9.6e-05 0.14559429 0.029633105 0.020961088 -0.0048705403 0.0085464214 0.28258701 -0.0048705403 0.0085464214 0.28258701
Loop time of 0.2332 on 16 procs for 100 steps with 6951 atoms, finish time Wed Jan 24 02:04:54 2018

Pair time (%) = 0.0440751 (18.9001)
Neigh time (%) = 0 (0)
Comm time (%) = 0.00675862 (2.89821)
Outpt time (%) = 8.443e-05 (0.036205)
Other time (%) = 0.182282 (78.1654)

Nlocal: 434.438 ave 1607 max 0 min
Histogram: 8 0 0 3 2 1 0 1 0 1
Nghost: 638.938 ave 1722 max 0 min
Histogram: 8 0 0 0 1 1 1 2 2 1
Neighs: 14006.5 ave 64098 max 0 min
Histogram: 8 0 4 2 0 0 0 1 0 1

Total # of neighbors = 224104
Ave neighs/atom = 32.2405
Neighbor list builds = 0
Dangerous builds = 0
Executing command: 'delete_atoms region deleteArea compress yes '
delete_atoms region deleteArea compress yes
Deleted 0 atoms, new total = 6951
Executing command: 'write_restart ../DEM/post/restart/liggghts.restartCFDEM'
write_restart ../DEM/post/restart/liggghts.restartCFDEM
System init for write_restart ...
LIGGGHTS finished

timeStepFraction() = 1

Solver level total Eulerian momentum exchange:
TotalForceImp: (-296819 324668 -827393)
Warning, these values are based on latest Ksl and Us but prev. iteration U!

DILUPBiCG: Solving for Ux, Initial residual = 0.728993, Final residual = 0.0510983, No Iterations 2
DILUPBiCG: Solving for Uy, Initial residual = 0.551545, Final residual = 0.0323495, No Iterations 3
DILUPBiCG: Solving for Uz, Initial residual = 0.912234, Final residual = 0.074493, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.946107, Final residual = 0.0649456, No Iterations 10
DICPCG: Solving for p, Initial residual = 0.00700255, Final residual = 0.000553143, No Iterations 7
time step continuity errors : sum local = 0.0771105, global = -0.0385528, cumulative = -1968.29
DICPCG: Solving for p, Initial residual = 0.0591874, Final residual = 0.00531657, No Iterations 9
DICPCG: Solving for p, Initial residual = 0.00686325, Final residual = 0.00049976, No Iterations 10
time step continuity errors : sum local = 0.053846, global = 0.000511906, cumulative = -1968.29
DICPCG: Solving for p, Initial residual = 0.0321575, Final residual = 0.00243829, No Iterations 3
DICPCG: Solving for p, Initial residual = 0.00253539, Final residual = 0.000252316, No Iterations 11
time step continuity errors : sum local = 0.0251114, global = 0.00558073, cumulative = -1968.29
DICPCG: Solving for p, Initial residual = 0.0221084, Final residual = 0.0016909, No Iterations 3
DICPCG: Solving for p, Initial residual = 0.00163854, Final residual = 9.70763e-07, No Iterations 20
time step continuity errors : sum local = 9.75561e-05, global = -1.58668e-05, cumulative = -1968.29
ExecutionTime = 125288 s ClockTime = 126018 s

Time = 12.2966

Courant Number mean: 27.7118 max: 315.941

Coupling...
Starting up LIGGGHTS
Executing command: 'run 100 '
run 100
Setting up run at Wed Jan 24 02:04:54 2018