Learning Forum

[eric1234598765]

Hi,

I had read the Fluent theory guide about y+ insensitive wall treatment, but I still didn't get it.

In theory guide/Spalart-Allmaras model, it said that this model has been extended to y + insensitive wall treatment, and the grids under y+ = 30 is fine.

My questions are:

1.Is that mean first layer height mesh smaller than y+= 30 can still get the fine result? And the result is more accurate with the smaller mesh?

2. Is this statements in Spalart-Allmaras section is also applicable to all turbulence models using y+ insensitive wall treatment or enhance wall treatment?

Thanks!

[Federico Alzamora Previtali]

Hello Eric, 

It will generally be more accurate to resolve the viscous sublayer with a finer grid (y+ ~1), especially for cases involving flow separation or heat transfer. However, for simple flow geometries with attached flow, you can still get accurate results with proper wall treatment.

What the wall treatment does is that it will blend between viscous sublayer formulation and law of the wall formulation based on the local y+ value. How this is achieved will vary between different turbulence models but is general for all wall treatment methods.

[eric1234598765]

 

 

 

Hi, Previtali

Thanks for you concern! But I want to know if first layer grid with y plus between 1 and 30 is work with insensitive wall treatment, since user guide said:

When wall functions are used, it is essential to avoid meshes with values lower than ~30 as the wall shear stress and the wall heat transfer can and will seriously deteriorate under such conditions. For this reason, the usage of y+-insensitive wall treatments is recommended.

So it recommended the  y+-insensitive wall treatments for the corase mesh when wall function is used, but it did not mention if y+-insensitive wall treatments will also “Deteriorate” the boundary layer result when y+ <30 ?

Thanks again!

 

 

 

[Federico Alzamora Previtali]

If the guide says "y+ insensitive" for the model you are chosing, this means that the solver will automatically adjust the formulation between the 2 models depending on the local y+ value.