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wall function and 1st grid

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Old   February 18, 2017, 08:59
Default wall function and 1st grid
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bye bye my blue
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bye bye my blue is an unknown quantity at this point
I'm using kEpsilon on 2d airfoil naca 0015 and boundary layer (0.03m).

my questions.

1) yPlus (postprocess -func yPlus)

I don't know meaning of min, max, average...

2) 1st grid

To use wall function, 1st grid is 30<y+<100. So I will use calculator

(https://geolab.larc.nasa.gov/APPS/YPlus/) But when I used it, Cd had big different value with xfoil.

can I use it for y+ value???

3) viscous layer length

I got boundary layer using B.L for re>10^6 on textbook. this results was 0.023m. So I'm using 0.03 on V.L. is it right??


Thanks a lot
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Old   February 18, 2017, 13:02
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> I got boundary layer using B.L for re>10^6 on textbook. this results was 0.023m. So I'm using 0.03 on V.L. is it right??


The viscous boundary layer reaches to y+~30. Up to this wall distance the relation between y+ und u+ (and therewith between y und U_tangential) is more or less proportional. This is the reason for selecting y+=30 for the first mesh layer.

If your textbook says, the boundary layer thickness is 1e-6 meters you need an first layer in the region of 3e-5 meters.

y+ depends on the "free stram velocity". That is the turbulent velocity away from the wall, let's say y+>100. This value may change over your geometry, and therwith y+. The mesh has to take this into account.
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