Question on Steady vs Unsteady RANS in Code_Saturne

[ximeng]

Dear Yvan,

I would like to clarify how steady and unsteady RANS are selected in Code_Saturne. Am I correct that choosing Constant or Adaptive time step corresponds to unsteady simulations, while Steady (local time step) corresponds to a steady simulation (But why does the Steady option still require input of adaptive time-step parameters, is it the “pseudo-time”)?

In my case, I used the Adaptive time step option for a wall-bounded square cylinder. The flow should be unsteady, but the results appear nearly steady: the time history shows almost no variation, and the snapshots do not reveal vortex shedding. Do you have any suggestions on how to address this? Is there a specific option I need to enable for URANS?

For reference, I am using Code_Saturne 8.0 with an ABL inlet profile and wall functions at the wall. My setup.xml file is attached.

Best regards,
Ximeng

[Yvan Fournier]

Hello,

The "steady" option is actually (as mentioned) "pseudo-steady", because it is not based on a steady algorithm, but on an unsteady algorithm with a local time step which allows faster convergence to steady state.

In most cases, the fact that a RANS computation leads to stable or fluctuating results depend on a combination of that case's physical behavior, time step/CFL aspects, numerical options, and mesh complexity and quality.

When you do observe fluctuations, they should be compared to the turbulence model's local time scale, to determine whether this may make physical sense (intepret with caution), or have a period which is smaller than that ime scale, in which case they can be considered as numerical artifacts/parasites.

If a case is very stable, this might simply be because the actual physical behavior is stable,

Regards,

Yvan