Randomize particle orientation issue in 2D MFIX-23 to MFIX-25

MFiX
6

Hi @ebreard6
Thanks for submitting this case, I found some very interesting things while running it.

  1. There seem to be some problems with SQP seeding. With the default settings for DEM seeding parameters (Hexagonal lattice, 0.05 particle spacing) it looks like the particles are interpenetrating. (This has nothing to do with the randomization, I have that disabled here)

    default_spacing_hex_norand
    default_spacing_hex_norand1042×190 49.9 KB

  2. I created a patch (more about this below) to force the randomization to only rotate in the x-y plane, for 2D cases. With this enabled, the interpenetration is even more visible:

default_spacing_hex_rand
default_spacing_hex_rand1042×363 226 KB

This should have caused some warnings from the SQP particle collision detection code, I’m not sure why that isn’t happening :frowning:

  1. I got around this issue by using a cubic lattice and increasing the particle spacing to 1. I had to move the pebbles from the “IC” region to the “Background” since there was no longer room for them in the bottom region, but this should be OK because they fall down anyhow. I specified “particle count” rather than “volume fraction” to get the desired number of pebbles.
    shot-2025-04-04_10-08-25
    shot-2025-04-04_10-08-25681×869 83 KB

With these settings (and my patch) the initial seeding looks like this, with random orientation enabled:

cubic_spacing_1
cubic_spacing_1593×315 87.6 KB

  1. Here’s the code change to enable 2-D randomization.
    patch.txt (20.2 KB)

To apply it, save the patch to a file (say, /tmp/patch.txt), then go to the MFiX source directory ($CONDA_PREFIX/share/mfix/src if you are using a Conda installation), and run
patch -p1 < /tmp/patch.txt
Now you can build custom solvers, which will pick up the new code, or to rebuild the default solver,

$ cd $CONDA_PREFIX/bin
$ build_mfixsolver -j; build_mfixsolver --smp -j; build_mfixsolver --dmp -j

to rebuild all 3 flavors of the default solver.

  1. I need to discuss this change with the MFiX team to see if it’s something we want to add to the official code. I have a number of questions about 2D cases and how they work, which I need to answer before merging this. If particles are spherical and all start in the same plane, collision forces should also be in that plane. But for SQP particles this is no longer the case, and I’m not sure what constrains them to stay in the plane, and if this is physically realistic. I will follow up on this topic as I find out more.

– Charles

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