Hi, Can someone confirm my understanding of scaleable wall functions and the importance of y+ values. Is it true that there is no recommended minimum y+ value? So y+ values of around 1 to 3 are ok? If I want to resolve the flow profile in the boundary layer, then I should just make sure there are sufficient nodes, right?

Thanks in advance!

Mark

Hi, Mark

To my understanding, yes, you are right. Scalable wall function can overcome the problems of inconsistencies with fine mesh (the first node is down to the sub-layer). If you are trying to resolve the boundary including the sub-layer, sub-layer models with y+ values lower than 2 are recommended.

Regards, Forrest

Thanks!

This is not the whole story.

k-e with scalable wall functions was a first step towards "y+ independent" meshes. In case of a y+ value below 11 anywhere in the model, y+ is set to 11 locally. So the node is shifted virtually out of the viscous sublayer. So wall functions are applied everywhere. This is a pragmatic approach but it leads to results better than ordinary k-e with meshes where y+ locally lies outside a range between 20 and 200, the well known y+ criterion.

The next step was the introduction of the SST model in which the wall treatment is managed automatically. Where y+ locally is below 11 the SST model switches from a wall function formulation to a low-Re formulation where the flow field is integrated down to the wall (no wall functions are used)

If you have a mesh where y+ is around 1 allover (which is better regarding accuracy), you also can use SST because the model then behaves as a full low-Re model

Hi Helge,

I just look at the solver theory. It used Y* instead of Y+. ie. the alternative velocity scale is used instead of friction velocity in Y+.

I wonder if Y+ & Y* are the same. If I used friction velocity in the Y+ formula, can I still follow the criteria stated in the manual (i.e. 20<y*<100 if wall function approach is used)?

Thanks for advice.