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asd June 22, 2007 03:19

Airfoil y+ for Validation Purposes
I am validating an airfoil at Re=6 million at mach 0.32 within FLUENT. I have found over the five grids examined thus far that a grid with approx. 92,000 cells at y+ about 150-300 produces lift and drag results that are within 3-4% of experimental data, over an AOA range of -1 to 12 deg.

Since i am using the SST k-w model, i would like to further test result convergene by increasing the number of cells such that y+ of 1 can be obtained. I am limited in my computer resources so have been able to achieve a y+ of 25, yet my results are now worst off, than they were with a y+ of 150-300. I have maintained a cell grid aspect ratio of ~20 within the BL, and have about 340 grid points on the airfoil surface. I am confused as to why my results are worst off now; i would have expected similar results or slight improvement given that the y+ has been reduced to capture the visoucs sub-layer. Would they deteoriate even further if i was to achive a y+ of 1? Any advice???

Neil June 22, 2007 08:28

Re: Airfoil y+ for Validation Purposes
the accuracy of the results depends on what region of the boundary layer your are trying to resolve and the wall functions you employ to do this. If using the standard or non equuilibrium wall functions your y+ should be between 30-300 where a y+=30 would be ideal. These wall functions will resolve the turbulent region of the boundary layer where y+=30 is at the edge of the turbulent region before the buffer region. If you want to resolve the viscous sublayer you should use the enhanced wall function where your y+ should be ideally 1 but should be in the range of 1-4. Your decreased accuracy is probably due to you using the standard wall function which in combination with a y+=25 is trying to resolve the buffer region of the boundary layer which it is not formulated to do.

Hope this makes things clearer

asd June 23, 2007 01:01

Re: Airfoil y+ for Validation Purposes
Hi Neil, Thanks for your reply. I am using the k-w SST which uses the enhanced wall treatment model; my intial mesh provided a y+ of about 200 and also very good lift and drag agreement. From what i understand, a mesh with y+ 25 is unsuitable since it lies in the buffer region (y+ = 5~30). Based on this analysis it seems that for my problem (Re=6 million at Mach 0.32), the k-w SST can be used within the log region and for best results the viscous sub-layer can be resolved with a fine mesh, although the buffer region is not accpetable with the adopted model? Is that correct or am i missing the point somewhere?? Appreciate your feedback

Neil June 26, 2007 09:27

Re: Airfoil y+ for Validation Purposes
Spot on, good luck with getting the y+ down and not increasing the cell count too drastically

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