Poor cylinder wake results on unstructured meshes in Code_Saturne 9.1

[wayne_cai]

Hello,

We are testing the flow around cylinder case with Code_Saturne 9.1.

With structured meshes, the case works correctly in Code_Saturne 9.1. However, with unstructured meshes, we could not obtain reasonable results.

We tested:

- 2D / quasi-2D unstructured mesh
- 3D unstructured mesh
- Re = 200
- Re ≈ 3900

The observed problems in Code_Saturne are:

- No clear vortex shedding / Karman vortex street
- Poor wake field
- Abnormal velocity or pressure distribution near the outlet
- Outlet backflow messages such as `Incoming flow detained ... outlet faces`

For comparison, we ran the same meshes and the same Reynolds numbers in OpenFOAM, and the results were normal, with the expected wake and vortex shedding behavior.

So the issue seems related to Code_Saturne when using unstructured meshes for the cylinder wake case. Structured meshes do not show this problem.

Could you please advise whether this is a known limitation, a setup issue, or a possible bug? Are there recommended settings for unstructured cylinder wake simulations, especially regarding:

- outlet boundary condition
- gradient reconstruction
- extended neighborhood settings
- turbulence model setup for Re ≈ 3900
- mesh requirements near the outlet

Thank you.