Moving wall and compression test

MFiX
1

Dear support,

I was trying to set up a simple uniaxial compression test, but I am having problems when moving the walls. I have a prismatic geometry with cyclic X and Z BCs (to avoid wall effects). The top boundary is supposed to move downwards and compress the particles, but it does not seem to be moving when setting Moving Wall options. The idea would be to later monitor force vs. displacement, which I guess could be done in the bottom wall. I attach the very simple case (MFiX 25.2.1). I did a couple of trials with and without Keyframe, but the wall is not moving. Am I missing something?

compression_bed_2025-08-22T155718.708788.zip (3.4 MB)

I tested using a STL as a wall and it does not work. I tried a Movable STL and it seems to move, but I am wondering why the other alternatives do not. Two questions also arose:

  • Is there a way to visualize the moving wall itself? I did not seem to be able to see the .stl moving.

  • In this kind of settings, how can we avoid the particles from going through the .stl when they are very compressed? As displacement is imposed, the only way for the system to keep converging would be for the particles to penetrate the .stl wall. Although this would also happen in the “pure” wall (particle dissapearing). Am I right?

Thank you for your kind help and kind regards!
Eduardo

2

Only STL files can be used as moveable geometry. These are internal surfaces that the fluid phase doesn’t see. Regular BC walls cannot be moved. The GUI should not allow this, we will fix for the next release. You can visualize the moving wall by selecting “geometry data” as the Output type when defining VTK outputs. Please see the 3D mixer with moving STL surfaces for an example.

We impose a rigid motion to the moving walls so they do not see resistance as they compress particles. You could modify the code to monitor the force on the wall and adjust the wall motion accordingly. We are planning to add the STL force to monitor data in the 25.3 release.

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3

Thank you for your detailed answer!