Testing the Kevin-Helmhotz instability with USER-SPH package


I am just playing around with LAMMPS - specifically the USER-SPH package. I am trying to create the Kevin-Helmhotz instability by having two flows in opposite direction with the SPH. The results is pretty close to what we should see (or so I hope), but if you can see at the top and bottom I have some particles somehow managed to penetrate the walls and get re-entried at the other end.

Video of the clip: https://goo.gl/ZsbMNF

In case the video clip is not clear enough, here is the phenomenom: https://goo.gl/eoxcya

Here is the script I used for this case: https://pastebin.com/grt1SJZc

Am I doing something wrong, even fundamentally?

Thanks in advance for your time.



your script did not specify box boundary type so it used the default periodic boundary conditions as shown in the manual:


A few particles seem to be getting through the bottom edge and are being re-mapped to the top.