[Gabreeal]

Dear Sir/Madam,

I had been simulating flow over OneraM6 wing at transonic regime. The y+ value for komega SST is preferably 1, for K epsilon should be 30<y+<300.

Currently I have used Spalart Allmaras because of doing validation. Now I have heard that Spalart Allmaras is insensitive to y+ value, so the mesh layer height will get automatically adjusted (correct me if I am wrong).
The boundary layer thickness is nearly 0.0170, so have put this distance in the mesh size of 1st cell.

This again to be taken into account, for viscous sublayer y+ is below 5 and more than 30 for logarithmic. (No idea is range of 300 can be considered here or not)

I ran the simulation, the plots for Cp, M, velocity are understandable. Now If I plot the y+ over surface (from additional data file quantities in Ansys CFD post), there is wide variation of y+ over wing surface. Legend shows from 0.2 to 500. I am attaching a picture below. Kindly explain how to interpret this plot and why is y+ value more than 300, around 500 for Spalart Allmaras model. If this contour is incorrect, then why other contours of pressure and velocity parameters are near about matching the values of validation data?

Regards
Gabreeal

[LuckyTran]

Plot a contour of the wall shear stress and then look at the definition of wall y+.

[Gabreeal]

Hello LuckyTran,

I did what you said. The y+ is showing a behavioral nature with wall shear stress, as moving away from root to tip of wing.
This is still unclear. Can you please explain.


Regards

[LuckyTran]

And the formula for wall y+?

[Gabreeal]

Hi,

The y+ is "(y*ut)/v". "ut" is friction velocity and given by sqrt(Shear stress/ density). This means 'ut' is proportional to sqrt(shear stress), same with y+, y+ is proportional to sqrt(shear stress). Which again means that y+ will increase as shear stress increases.

Or the graph says that both quantities are directly proportional but at some point, y+ fluctuates a little.

[FMDenaro]

Originally Posted by Gabreeal

Hi,

The y+ is "(y*ut)/v". "ut" is friction velocity and given by sqrt(Shear stress/ density). This means 'ut' is proportional to sqrt(shear stress), same with y+, y+ is proportional to sqrt(shear stress). Which again means that y+ will increase as shear stress increases.

Or the graph says that both quantities are directly proportional but at some point, y+ fluctuates a little.

Generally U_tau is the statistical averaging, not a local value.