Dear MFiX Developers：
I am currently running a simple 3D single-phase gas flow simulation and have encountered an immediate solver crash issue (DT＜DT_MIN Recovery not possible) that has plagued me for over a week. I have tried multiple troubleshooting approaches without success and am writing to seek your assistance.
My original case features a regular rectangular prism geometry with dimensions X=11 m, Y=16 m, Z=0.16 m (only the Z-direction width is adjustable). I initially used a uniform structured grid of 4 cm × 4 cm × 4 cm, resulting in approximately 440,000 total cells. The boundary conditions are as follows: a 1 m × 0.16 m velocity inlet on the left face with an inlet velocity of 0.85 m/s, a pressure outlet at the upper right face set to 101.125 kPa, a mass flow outlet at the bottom right face, and no-slip walls on all remaining surfaces. I set the time step to 5×10⁻³ s, but the simulation crashes immediately after startup with no convergence history or intermediate results output .
To identify the root cause, I conducted two sets of controlled experiments. In the first approach, I kept the grid size constant and gradually reduced the Y-direction geometry dimension. When Y=8 m with 220,000 cells, the solver still crashed. However, when Y was reduced to 2.5 m with 68,750 cells, the simulation ran successfully and produced expected flow field results. I initially hypothesized that excessive total cell count was causing the cras h.
To verify this hypothesis, in the second approach, I maintained the original X=11 m, Y=16 m geometry and progressively coarsened the grid to reduce total cell count. I sequentially tested grids of 8 cm × 8 cm × 4 cm (110,000 cells), 10 cm × 10 cm × 4 cm (70,400 cells), and 20 cm × 20 cm × 4 cm (17,600 cells). All of these cases resulted in immediate solver crash es.
These two sets of experimental results present a clear contradiction. They demonstrate that the issue is not caused by excessive total grid count, but rather is related to the Y-direction geometry dimension itself or the combination of geometry and boundary conditions. I must retain the original X=11 m, Y=16 m dimensions for my study. I have uploaded the complete bug file for the 20 cm × 20 cm × 4 cm grid c ase.
I sincerely request your help in identifying the source of this problem. Any guidance or suggestions would be greatly appreciated. Thank you very much for your time and assist ance.
The error message is as fo llows:
