Persistent DT < DT_MIN. Recovery not possible! error

MFiX
#1

Dear All,

I have built a TFM simulation project. But it is constantly encountering the above popped error though when it is running the remained time shows.

Another phenomenon is that though I have set up the initial volume fraction of particles in some regions, they are not visualized at the very beginning as the same as that of other tutorial examples with similar configuration and simulation method. I would quite appreciate if you could help me check my .mfx file, and indicate its potential errors if they were not bugs. Thank you.

builtin_sphere1.mfx (19.1 KB)
BUILTIN_SPHERE1.msh (34.3 MB)

Nt
Nt1615×958 177 KB

#2

Hi Ju. We will take a look at your case when we get a chance. Right now I’m trying to put the 21.3 release together :slight_smile:

You may find this FAQ section on non-convergence helpful:

https://mfix.netl.doe.gov/doc/mfix/21.2/html/reference/faq.html#what-do-i-do-if-a-run-does-not-converge

– Charles

#3

Thank you Charles, it is weird that everything seems normal when the simulation begins. The mesh seems normal as well. But after 1-2mins, it pops up the same error as follows.

weird
weird1451×796 170 KB

And I can not find conspicuous errors in generated mesh for bounds and regions either. Thank you.

mesh1.PNG
mesh1.PNG1289×811 254 KB

#4

And I have the following three questions if you do not mind though they do not inhibit the start of the simulation.

  1. The red warning words in the right bottom corner
    Number of SMP threads: 16
  • Running on all addresses.
    WARNING: This is a development server. Do not use it in a production deployment.
    MFiX (21.2 ) simulation on host: BT4N3Z2
    Run name: BUILTIN_SPHERE1 Time: 11: 7 Date: 9-28-2021
    Project version: 48

  1. WARNING check_data\check_gas_phase.f:143
    Message 2000: MW_AVG is not needed when RO_g0 is specified.

  2. I want to know how to trade off the relative sizes of the particles and cells. Whether it is mandatory to set the size of grid cells less than that of the particles. Thank you.

#5

The boundary mesh is beautiful, but the DT<DT_MIN problem happens persistently.

bmesh1
bmesh1810×805 293 KB

#6

This time I change the density of gas, and the simulation lasts for 0.028s and DT<DT_MIN happens again.

persist.PNG
persist.PNG1304×814 222 KB

#7

Items 1 and 2 can be ignored. We will remove these warning so they no longer cause confusion.

#8

I will take a look at you files when I get a chance. Regarding the grid vs particle size, the rule of thumb for TFM is the grid size should be around 10 particle diameters. This is just a rough estimate, sometimes it is not feasible for very small particles and/or very large systems. Typically, people do a grid sensitivity analysis to see the effect of grid size and make sure grid convergence is achieved.