What is DT<DTmin?

When I did the 2D fluidized bed DEM tutorial, there was an error, I don’t know why this error appeared, I followed the tutorial step by step and tried several times, and still did not solve it.

Looking forward to any possible answer, thanks!

This is usually due to lack of convergence or unphysical results. Is this the tutorial from the documentation or the existing sample file from the GUI? Please attach your file and provide details about your system (OS, fortran compiler, MFiX version).

dt is the discrete timestep sized used in the simulation. It is adjusted dynamically. If the model is converging well (residuals are low) then dt can be increased to make the simulation run faster. Conversely, if there are convergence issues (residuals too high) then dt is dynamically decreased and the iteration is tried again.

dt_min and dt_max govern the acceptable range of dt. If the dynamic adjustment gets to the point where dt is less than dt_min and the iteration still does not converge, there is nothing else that can be done and the solver gives up.

The solution is to either decrease dt_min or try to understand why the model is not converging. This can be due to many factors, most commonly poor mesh quality.

Thanks.It is the tutorial from the documentation like the picture.And my file is as follows.

2d_fluidbed.mfx (9.2 KB)

Thank you, but I don’t know how to adjust dt_min,can you teach me?

It’s in the “Run” pane.
If you can’t find a control in MFiX, try clicking on the “search” icon

You forgot to set a value for the inlet gas velocity. If you want to use the same as in the tutorial (42m/s), you also need to use a much smaller inlet width (from x=0.07m to x=0.08m), otherwise it will blow the particles away.

1 Like

Thank you, there was indeed a phenomenon of particles being blown away, but with your help this problem was successfully solved. So I would like to ask how to determine parameters such as the width of the entrance, do you have any reference book recommendations?

This is not a simulation parameter, it is a design option. You can have a uniform velocity at the bottom over the entire width, you can have a central jet, or multiple distribution points.