optimal coupling interval time

Submitted by paulaalejandrayo on Wed, 03/01/2017 - 14:42

Hello! I am really confused, I am running a really simple simulation , a square box filled with particles (that can't leave the box), inlet in a lateral wall, outlet on the opposite wall.
I am interested in to measure the difference of pressure between inlet and outlet walls.
Things work well, but if I change the coupling time-step from 100 to 10 (and the OF time-step accordingly) the pressure just increases crazily (almost 5 times) and the particles inside the square box start to be compressed against the outlet wall.
I am not changing anything else that the coupling interval time and OF time-step.
Why could that be happening? I will appreciate any hint that could help me to understand this behaviour. Thank you in advance.

j-kerbl's picture

j-kerbl | Wed, 03/08/2017 - 17:26

Hi Paula,

this is a bit confusing. I'd expect it exactly the other way around. Typically with higher CFD ts and bigger CouplingInterval the less exact you resolve a particles acceleration and you might overshoot the velocity for a couple timesteps (e.g. resolving acceleration during free-fall to terminal velocity).
What happens in your case, if both cases have the same CFDts and different coupling interval only?

Cheers,
Josef

paulaalejandrayo | Fri, 03/10/2017 - 12:09

Hello Josef, thank you very much for your answer. I am still going around this and still cannot understand. If I keep the same CFD and use different coupling interval the simulation stops after a few time-steps (the courant number become huge). As a matter of fact I did not know that to do that was possible, I thought that the rule was always DT_OpenFoam=DT_liggghts x coupling_interval. What is the system doing if coupling interval is bigger than OF time-step?
I tested the same system with several more coupling times in the range from 1 to 100 (and OF time-step according with these) and the behaviour is the same, as smaller the coupling interval as higher the pressure. The system is dense, maybe this is affecting the relation between the two time-steps. But I cannot figure out yet why. Any idea?
Thank you very much in advance.
Paula

paulaalejandrayo | Fri, 03/17/2017 - 16:32

Hello,

I am sorry for being insisting, I kept testing this and still do not find a way in which I could get some convergent behaviour. I freeze the grains and tested different coupling times again and I have the exactly same behaviour. I mean, it is something I thing is happening in OF side, (DEM part looks behave well) Could any one give me a suggestion about how can I test this in a simple OF case? I am using IB solver and static mesh. Thank you very much.
Best regards,
Paula

AndresMM | Tue, 11/07/2017 - 09:28

Hi Paula,

I am having the same experience. I have a constrained packing of particles in a cube with one inlet and outlet.

If I decrese the CouplingInterval (which should give better results) the courant number explodes and the particles leave the domain. If I use a bigger coupling interval howerver the simulation runs with no problems, even with a finer mesh.

Did you solve this problem?

Cheers,
Andrés