[brightfame3]

Good evening everyone.

I am performing an internal combustion engine simulation using Fluent and I have encountered the following error, over and over again:

"Divergence detected in AMG solver: pressure correction -> Increasing relaxation sweeps!"

I followed pretty much every tutorial I found on IC engines, however I will give more information:

I am using a k-e standard model, pressure-inlet and outlet for boundary conditions, PISO is used for the pressure-velocity coupled scheme, Presto! for pressure, second-order upwind for the rest of the variables with the exception of k and epsilon, since it has some dificutlties converging in the first time-steps. Initially, the valves are closed and before they do open, the solution converges in the iterations. Afterwards, epsilon, again, has great difficulties converging, however I increase the number of iterations and it eventually converges, but very slowly. After i increase the time-step and under-relaxation factors, epsilon does not show signs of diveregence, and nontheless the error shown above appears. So I suppose it is not related to epsilon but yes with pressure or density. Should i adjust the solution-method for either or change the under-relaxation factor? I have been messing around the AMG controls: increasing pre-sweeps to 2, I changed epsilon cycle to V (which helped) and stabilization method to BCGSTAB.

Also, the maximum equi-angle skewness is about 0.79 and and minimum ortho quality is 0.25.

I have about a year of experience in Fluent, however most of what I know has been reading topics and tutorials. I would appreciate any suggestion or opinion. If you need anymore info, feel free to ask.

Thank you very much for your attention.

[sampathevs]

decrease your pressure relaxation factor to 0.6 from 0.3 and momentum to 0.4 from 0.7

if u have enabled energy, decrease that as well to 0.5 from 1

[brightfame3]

Thank you for your reply.

My URF's are currently:
Pressure - 0.3
Density - 1
Body Forces - 1
Momentum - 0.5
k - 0.4
epsilon - 0.4
energy - 1

You are suggesting for me to decrease the energy to 0.5, is that correct?
Thank you once again.
Best regards

[sampathevs]

yea..
just try like this and post the update

Pressure - 0.5
Density - 0.6
Body Forces - 0.6
Momentum - 0.4
k - 0.4
epsilon - 0.4
energy - 0.5

[brightfame3]

Will do. Thank you for your help. Let's if this time it works!

Best regards

[brightfame3]

Greatings!

I have tried your suggestion, however without success. I have no idea nor clue what is causing this divergence...
Thanjs for your help

[Yuhan]

Hi, I am doing the similar study, modeling engine spray combustion using FLUENT! My urf is as followings:
Pressure 0.1
TKE 0.2
TDR 0.2
energy 0.2
However, I still have the same problem. Have you solved it?

[lehoanganh07]

Please check your mesh, It usually cause by mesh, also reduce the time step size and URF will also help

[brightfame3]

Hello Yuhan!
I previously was using PISO for pressure-velocity coupling, however attempted changing it to Coupled and it helped, but not completely. My problem occured a few degrees after the valve was activated. Now the divergence occurs due to temperature reaching its minimum limit in some cells. I have read that, as lehoanganh07 said, it can be the mesh.

Thanks for your reply!