Residuals spikes even when physical flow field is correct

[shezanghamat]

Hello there,

I am simulating a 3D combustion problem and I noticed large residual spikes as shown in the image. I checked all possible reasons of that which I could find online such as mesh quality, check physical flow field, under relaxation factor, etc. My mesh reports min orthogonal quality ~0.17 which I believe is fine, the physical flow field is as expected (not perfectly stable but flame is establishing correctly), tried reducing under relaxation factors, but no change. Since, I kept it overnight so I actually don't know exactly after what iteration these spikes started. But I was running another simulation with exact same setup but different mesh, observing that, I assume spikes started after 1500 iteration. One more important note is that, I previously ran few same setup simulation with different meshes but I never encountered spikes in that. Can you please suggest something on this?

Method: Steady-state, pressure based, absolute, gravity on
Models: Standard k-epsilon, partially premixed combustion with FGM
Scheme: SIMPLE (tried with SIMPLEC also, but still same issue)
Fuel/Ox: H2/Air

[Rob]

How do the other solution monitors look? Residuals are only part of the checking process. Were the other meshes coarser?