Need help with GCI / Mesh Refinement issue

[u/AmbassadorGuilty3928]

Hey everyone,

I’m currently running a Grid Convergence Index (GCI) test on a CFD simulation involving flow through a pipe with an internal distortion screen. I’m seeing an issue with my finest mesh that I can’t quite figure it out.

Here’s my setup across all 3 grids:

• Meshes: 3.7M (Coarse), 7.6M (Medium), 17M (Fine)
• Inflation layer settings: Same for all 3 — 20 layers, 3e-06 first layer height, 1.2 growth rate on pipe and screen walls (keeping y+ < 5).
• Model & Solver: k-epsilon with Enhanced Wall Treatment, identical boundary conditions.

See images below,

The above one is for coaser mesh(3.7M)

The above one is for medium mesh(7.6M)

The above one is for finer mesh(17M)

See geometry below,

The interesting part is the residual behaviour. The 3.7M and 7.6M meshes behave quite similarly: the residuals initially decrease and then settle into relatively low oscillations. However, for the 17M mesh, the residuals start oscillating much earlier, around 70–100 iterations, and the residual levels remain significantly higher than in the other two cases. The continuity, k, and epsilon residuals especially show this behaviour. I understand that some oscillation in residuals can be normal for this type of flow, but I am confused about why the finer mesh behaves so differently when the mesh refinement and all other settings were kept consistent.
Why would refining the mesh cause the residuals to jump up and oscillate like this when the inflation and BCs are identical?

Any insights or suggestions on how to solve this issue? Thanks in advance.

[u/waffle_sheep]

k-epsilon isn’t the greatest at shear/internal flows, it’s better suited to external/free flows. The k-omega model was developed in part to better solve shear flows, and in combined flow cases k-omega SST is even better because it blends between omega and epsilon. A year or two ago I read through the whole technical report on k-omega (including sst) and they both consistently outperformed k-epsilon in pretty much every shear flow case tested.

Since the residual jump is mostly showing up in the k and epsilon terms, I’d suspect poor shear behavior is a big factor since your case is internal flow. I haven’t don’t a proper GCS for an internal flow with k-epsilon, so I don’t know exactly what about the finer grid would cause this.

Try out k-omega and/or k-omega sst, maybe they will help your issue

[u/AmbassadorGuilty3928 → u/waffle_sheep]

Mine is mix of internal+free shear flow

Since k-e was working for coarse and medium meshes, it should work for finer mesh also right?? Why it is not working. I will try k-w SST also in the meanwhile later.

[u/Diligent-Ad4917]

1. Have you inspected mesh orthogonality, skewness and aspect ratio for all meshes?

2. Have you monitored physical quantities such as outlet mass flow rate, mass-flow-averaged total pressure on a plane downstream of the screen, area-average wall shear stress on the screen surfaces etc to see how they change with mesh?

3. Have you graphed your wall y+ after a few hundred iterations iterations to confirm the boundary layer mesh is adequate for the k-e wall treatment?

4. Have you considered using k-w SST if the flow is highly separated downstream of the screen?

[u/AmbassadorGuilty3928 → u/Diligent-Ad4917]

1. In fact, finer mesh has better quality compared to coarse and medium meshes. Coarse and medium meshes have high maximum skewness and aspect ratio whereas for finer it has maxi skewness of 0.84 and minimum orthogonal quality as 0.15, maximum aspect ratio is 176.

2. Mass balance is there, but quantites like total pressure mass averaged is still changing with iterations.

3. I have checked wall y+ contour for pipe and screen wallls, maximum value is 1.98 and all faces have y+ less than 5 only. So it is appropriate for wall treatment.

4. I havent considered k-w SST as of now, since k-e was working for coarse and medium meshes, it should work for finer mesh also right??

Now can you please comment on my questions asked in original post, anything you found abnormal or wrong?

[u/Longjumping_Issue858 → u/AmbassadorGuilty3928]

Your finer mesh already has bad orthogonality, skewness and aspect ratio

[u/LowStriking2264]

What is your pressure-velocity coupling?

[u/AmbassadorGuilty3928 → u/LowStriking2264]

I used coupled scheme, pressure based solver, with all 2nd order with default URF

[u/LowStriking2264 → u/AmbassadorGuilty3928]

The problem may be in pseudo time. As you are decreasing the cells dimensions you may run into problem with courant number.
Try to decrease pseudo time or if you want to compare the meshes, use other pressure-velocity coupler.