How to set constant time step in MFIX-DEM?

MFiX
1

I am performing DEM simulation in MFix with pure granular flow. The time step is automatically calculated as 5 x 10^-8. But I need to set constant time step manually. How to do that? Do I need to use user defined function for that? If so please let me know the procedure.

Note: The particle characteristics or the parameters cannot be changed

2

You can set the solids time step in the Solids>DEM pane:

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However, if the calculated time step is 5E-8 s, DEM may not be the right model for your application. You may be able to increase it a bit, but if it is too large, you are going to miss collisions or have excessive overlap which will likely lead to failure of the simulation.

3

Yes I see. Some particles are coming out of the domain when time step is greater than 1E-6.

Is there any other solution for this problem?

For mass inflow condition, should I reduce the volume fraction of particles?

4

Hi @jeff.dietiker,

Can we also use it to set a constant time for CFD-DEM?

5

I don’t think reducing the mass fraction at the inlet will help. If you specify a constant velocity, this will inject fewer particles. If you specify a mass flow rate, it will still inject the same amount of particles. In both cases, you will still miss collisions at some point.

6

The constant solids time step can be applied without or with the gas phase. You can set a constant fluid time step too by setting a time step factor of 1.0, but I would not recommend that.

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7

Thank you very much @jeff.dietiker.

8

When I decrease the maximum steps between neighbor search from 25 to 5 then very few particles pass through the wall. Can this be an effective solution?

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9

Yes, if it is fixing the issue it looks like you found a solution!

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