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[Aoxi Zhang <question701438@xxxxxxxxxxxxxxxxxxxxx>]

New question #701438 on Yade:
https://answers.launchpad.net/yade/+question/701438

Hi,
I was trying to simulate a bender element test using DEM, and I'd like to ask that why the evolution of particle velocity varies with time step, even dt is small enough compared to the dt obtained from PWaveTimeStep().

The attached MWE can illustrate my question, the MWE consists of 3 scripts, which can be found from [1].

(1) phase1.py is for sample generation and stress relaxation, it takes around 7 minutes if you want to reproduce the results.

The idea is: firstly having a sample according to Bruno's example[2], then closing triax engine and increasing damping to quickly reduce unb. phase1 stops when unb reaches a small value (e.g.,1e-7). 

(2) phase2.py is the test under different dt, if you want to reproduce the resutls, you may use yade-batch phase2_batchParam.txt phase2.py, it takes around 8 minutes in general.

The idea of this test is: firstly load the sample generated from phase1, then turn damping to 0, close time stepper, select some particles whose velocity will be recorded. phase2 will terminate when the time reaches 3e-4.

(3) phase3.py is for plotting the results to get a figure which compared the results under different dt. You can directly find it in the attached files.

The MWE above leads to two questions:

(a) When I run phase2 in batch mode, not all simulations terminate when the time reaches 3e-4. Some of them terminated in advance. This can be seen from the figure.

(b) As you can see from the figure, different dt leads to different velocity of the same group of particles. If I understood correctly, when dt is small enough, different dt should not affect the results. Do you have any ideas about it?

Thank you very much!
Aoxi

[1]https://we.tl/t-B5s6rc6dRP
[2]https://gitlab.com/yade-dev/trunk/blob/master/examples/triax-tutorial/script-session1.py

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