# How to solve the problem of time_step.f:195, DT<DT_MIN

Hello, everyone!
When I run the mfix-TFM, I met the following problem.

Nit P0 U0 V0 P1 U1 V1 Max res
1 1.0E+01 0. 0. 2. 0. 0. P0
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 4 K = 2 M = 1
Eps = 0.62855 Us = -0.39467 Vs = 125.97 Ws = -0.30707
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 9 K = 2 M = 1
Eps = 0.62787 Us = -0.90384E-01Vs = 390.09 Ws = -0.67717E-01
2 1.0E+01 8.1E-09 1.3E+03 1.0E+01 6.6E-04 3.0E+09 V1
t= 0.0000 Dt= 0.1825E-06 NIT= 2MbErr%=-0.1145E-08: Run diverged/stalled Recovered: Dt= 0.16423E-06 Time = 0.0000 Dt = 0.16423E-06
Nit P0 U0 V0 P1 U1 V1 Max res
1 1.0E+01 0. 0. 2. 0. 0. P0
Message from: CHECK_VEL_BOUND
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 4 K = 2 M = 1
Eps = 0.62868 Us = -0.27892 Vs = 125.19 Ws = -0.21624
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 9 K = 2 M = 1
Eps = 0.62793 Us = -0.90969E-01Vs = 402.12 Ws = -0.68191E-01
2 1.0E+01 1.1E-08 1.2E+03 1.0E+01 7.3E-04 3.2E+09 V1
t= 0.0000 Dt= 0.1642E-06 NIT= 2MbErr%=-0.7466E-09: Run diverged/stalled Recovered: Dt= 0.14781E-06 Time = 0.0000 Dt = 0.14781E-06
Nit P0 U0 V0 P1 U1 V1 Max res
1 1.0E+01 0. 0. 2. 0. 0. P0
Message from: CHECK_VEL_BOUND
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 4 K = 2 M = 1
Eps = 0.62882 Us = -0.23058 Vs = 122.95 Ws = -0.17972
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 9 K = 2 M = 1
Eps = 0.62799 Us = -0.51652E-01Vs = 411.35 Ws = -0.39481E-01
2 1.0E+01 1.2E-08 1.1E+03 1.0E+01 8.9E-04 3.0E+09 V1
t= 0.0000 Dt= 0.1478E-06 NIT= 2MbErr%=-0.4779E-09: Run diverged/stalled Recovered: Dt= 0.13303E-06 Time = 0.0000 Dt = 0.13303E-06
Nit P0 U0 V0 P1 U1 V1 Max res
1 1.0E+01 0. 0. 2. 0. 0. P0
Message from: CHECK_VEL_BOUND
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 4 K = 2 M = 1
Eps = 0.62896 Us = -0.16399 Vs = 118.57 Ws = -0.12762
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 9 K = 2 M = 1
Eps = 0.62805 Us = -0.40040E-01Vs = 417.22 Ws = -0.30926E-01
2 1.0E+01 5.8E-09 9.8E+02 1.0E+01 5.3E-04 2.9E+09 V1
t= 0.0000 Dt= 0.1330E-06 NIT= 2MbErr%=-0.3006E-09: Run diverged/stalled Recovered: Dt= 0.11973E-06 Time = 0.0000 Dt = 0.11973E-06
Nit P0 U0 V0 P1 U1 V1 Max res
1 1.0E+01 0. 0. 2. 0. 0. P0
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 4 K = 2 M = 1
Eps = 0.62910 Us = -0.11165 Vs = 112.33 Ws = -0.86901E-01
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 9 K = 2 M = 1
Eps = 0.62812 Us = -0.29436E-01Vs = 419.68 Ws = -0.22454E-01
2 1.0E+01 1.2E-08 8.9E+02 1.0E+01 7.0E-04 3.2E+09 V1
t= 0.0000 Dt= 0.1197E-06 NIT= 2MbErr%=-0.1861E-09: Run diverged/stalled Recovered: Dt= 0.10775E-06 Time = 0.0000 Dt = 0.10775E-06
Nit P0 U0 V0 P1 U1 V1 Max res
1 1.0E+01 0. 0. 2. 0. 0. P0
Message from: CHECK_VEL_BOUND
Message from: CHECK_VEL_BOUND
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 4 K = 2 M = 1
Eps = 0.62924 Us = -0.79770E-01Vs = 104.40 Ws = -0.62234E-01
WARNING: velocity higher than maximum allowed velocity: 90.000
in this cell: I = 2 J = 9 K = 2 M = 1
Eps = 0.62819 Us = -0.17378E-01Vs = 418.76 Ws = -0.13574E-01
2 1.0E+01 7.8E-09 8.1E+02 1.0E+01 5.4E-04 3.2E+09 V1
t= 0.0000 Dt= 0.1078E-06 NIT= 2MbErr%=-0.1135E-09: Run diverged/stalled ERROR time_step.f:193
DT < DT_MIN. Recovery not possible!

Fatal error reported on one or more processes. The .LOG file
MPI Terminated.

I tried many of the mthods mentioned in the forum, but they did not work. Can anyone helps me? Thanks.

yanzheng1.mfx (14.5 KB)
these are the related documents, thanks! And I am using mfix-21.1.4 under linux system.

The “Bed” region Initial Condition is overpacked. Initial solids fraction is 0.63, i.e. initial void fraction is 0.37, while the packed bed void fraction (Solids>TFM tab) is set to 0.42. Either decrease the packed bed void fraction below 0.37 or decrease the initial solids fraction below 0.58.

Hi Jeff! Thanks for such a quick reply!
I’ve tried to reduce the packed bed void fraction to 0.36 or even smaller, but the calculation still stops at the same simulation time of 0.003s and shows DT<DT_MIN, I don’t know what to do anymore, can you give me some more advice, thanks a lot!
Best wishes.
yanzheng1.mfx (14.9 KB)

You actually increased the packed bed void fraction to 0.7, and you disabled close packing.

You need to set the packed bed void fraction to 0.36 (Solids>TFM pane), and enable close packing by unchecking the “disable close pack” box in the Solids>Materials pane.

Hi, Jeff! Sorry to bother you again, I followed your method to set it up, but this time the calculation was interrupted again at simulation time of 0.001s with DT<DT_MIN, why is that?
Again, my sincere thanks and apologies!
yanzheng1.mfx (14.9 KB)