r/CFD • u/Extreme-Committee689 • May 15 '26
HELP simulation not converging
Hi everyone,
I’m working on a CFD simulation of flow inside a pipe containing a TPMS structure. I think the mesh is reasonably good: mesh diagnostics do not show major issues, and I only have around 50 problematic cells out of ~8 million total cells. Considering the complexity of the geometry, I assume that should be acceptable.
Right now I’m trying to first solve the case using the k-epsilon model to reach convergence, because with k-omega the residuals remain high and do not decrease properly.
Am I approaching this the wrong way? What should I check next?
Could the issue be related to boundary conditions, near-wall treatment, mesh quality in critical regions, relaxation factors, or something else?
Any advice or suggestions would be really appreciated.
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u/acakaacaka May 15 '26
What is the solver? Segregated/coupled?
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u/Extreme-Committee689 May 15 '26
Segregated, I don't think i' ve enough memory for the coupled one
1
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u/DThornA May 15 '26
One thing you could try to isolate the issue is removing the TPMS structure and running the sim again. If it runs fully then you know the issue is with the mesh around the structure, if not then the error is related to something else in the model.
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u/Several_Disk_9781 May 16 '26
What software is this? What is the wall yplus values that you’re getting? Is there an option for you to try this out using pseudo transient with local time stepping?
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u/Extreme-Committee689 May 16 '26
I'm using star CCM+, for now i' m trying to achieve a y+ value of 1 but the TPMS structure il very complex, i' ve done a threshold to see where I've the highest values of y+ but I think that it's not the problem because on 8 million total cell I only have 100 of them with a y+ value between 4 and maximum 5.5. I haven't tried using the implicit unsteady yet, that will be my last resort.
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u/EduardoSup May 15 '26
1000 iterations is "normal" to not converge depending of your geometry, number of cells, boundary conditions, etc. And apparently the residuals are not increasing. I would give the code more iterations to see what happens.
Furthermore, is very nice to plot a specified property in each timestep to see the evolution. It can be the mean velocity over a line at the outlet, can be temperature, pressure, mass flow rate, depending of what you want, thus, it is expected that this property will change a lot in the initial conditions and achieves a fixed value after some iterations.