k-omega SST is not applicable for roughness study in Saturne

[Minh Quan Nguyen]

Hello everyone,

I am currently using the code Saturne to study the effect of roughness on the aerodynamics
performance of a 2D airfoil, application in prediction mean power loss in wind turbine.

Numerical parameters are then :

unsteady flow
turbulent model : k-omega SST
mesh 2D with hexa-hedral elements.

I found out that the prediction of lift and drag coefficients are very weird : there is a discontinuity when passing from smooth surface to rough surface following by an evolution rule abnormal, i.e. lift increases and drag decrease when roughness is added as shown in figure1 , file pdf attached.

I took a look in the CodeSaturne practical user's guide, and understood that on one hand, in the k-omega model of Saturne the mesh does not need to respect the y+ criteria likes k-epsilon, etc. in spite of the fact that my mesh did anyway. The code is supposed to calculate itself the wall
distance and adapting the wall function. (“CodeSaturne practical user's guide” page 128,144,143).

On the other hand, the roughness in Saturne is taken into account by shifting the wall virtually.

Therefore, I would like to guess that because of this double and conflicting modification, we should not use k-omega SST for studying roughness effect in Code Saturne.

I also tried to prove my hypothesis by performing the same simulation with k-epsilon linear and got a likely correct one as shown in figure2, file pdf attached.

Lastly, I would like to contribute my observation to the forum, expect receiving the confirmation from the development team and share the useful experience to other novice users like me.

Best regards,

Quan.