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[Robert Caulk <question670765@xxxxxxxxxxxxxxxxxxxxx>]

Question #670765 on Yade changed:
https://answers.launchpad.net/yade/+question/670765

Robert Caulk proposed the following answer:
Hello,

You have two problems that require two solutions. One of your problems
is an interesting phenomenon that needs to be documented, so I went
ahead and answered Bruno's question for you ;-)

<< First problem >> negative volume for ordinary pore:

Example reply to Bruno: I notice that when I add the small particles to
the script, I get this error. But when I only use the large particles,
the script is functional.

Solution: Reduce the time step. Smaller particles require smaller time
steps. Try an order of magnitude less:

O.dt=0.1e-4

<< Second problem >> Vh==NULL!!:

Example reply to Bruno: When I use stressMask=7 (triaxialStress goals)
with the Z-direction pressure gradient
(flow.bndCondValue=[0,0,0,0,100,0], I notice the packing drifts in the
Z-direction until the particles become uncontained and fly away,
resulting in the Vh==NULL!! error.

Explanation: Your imposed pressure gradient is pushing on the particles
in the Z-direction, those particles are pushing on your walls in the
Z-direction. You commanded your walls to maintain constant stress, so
they move to accommodate the changing stress.

Solution: The problem can be corrected by locking one of the walls [2]
associated with the pressure boundary condition:

triax.wall_back_activated=False

Cheers,

Robert

[1]https://answers.launchpad.net/yade/+question/634175
[2]https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.TriaxialStressController.wall_back_activated

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