particle explosion

Submitted by mschramm on Thu, 02/20/2020 - 16:07

I am working on coupling the fiber model with CFD and have hit a snag with a simulation.
I have a long fiber fixed at one end (using fix freeze) while all other atoms are free to interact.
I have 0.5 m/s air traveling up from the bottom of the domain and all other boundaries are zero gradient.

After a few moments an explosion occurs at the spheres and I do not know where to start to look to find the problem.
I have tried IB solvers and the piso solver but both cause this type of problem.

video of what is happening in the simulation

I thought maybe it was by grid size but I do not think this is the case as the grid in the video is fairly fine (far right side of video is the grid).

I don't think it has to do with the the bonding by its self. I know that care has to be taken with multisphere particles but there are no overlaps
in the spheres that are bonded.

Any ideas on where I should start looking would be a great help.

Thank you.
Attached are the input files. If you wish to run this, you will need my fiber model code off of github.

Binary Data fiberinthewind.tar_.gz842.98 KB
Image icon highomega.png371.27 KB
Image icon whiping.png98.13 KB

achuth1992 | Mon, 02/24/2020 - 10:22


I think the problem might be with the physics involved. If you considering the rotational velocities of the particles as well in the
calculation of the particle velocity in the CFD side, it might lead to some errors. This is because the individual particles will have different
angular velocities and does not represent the actual angular velocity of the fiber structure is such as scenario.


mschramm | Tue, 02/25/2020 - 16:13

I have been running a few more tests and I do not know if it has something to do with adding angular velocity to the particle velocity or if it has something to do with the fix freeze command.
I have had some success with getting the original simulation to run and over the weekend I tried using multiple fibers. In my original post, I have added a picture of the end of the simulation.
You can see that the angular velocity next to the fix freeze is going out of control. I have been able to successfully run simulations of fibers "floating" with little to no problems so I think this has
to deal with how fix freeze interacts with things.

However I have another simulation where I have tried to recreate my cantilever beam problem to see how much air drag dampens the fiber as compared to the internal damping.
In that simulation I pre-deflect the fiber before going to CFD, but as soon as I start the CFD-DEM simulation, the fiber turns into a whip and curls up onto itself (attached in first post).

I will keep looking through the source code but I am not 100% sure where I am looking. I may try adding an stl file to act as the "clamp" and stop using fix freeze to see if that fixes the problem.