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-   -   some quiries about how to set a "low_Re" kOmegaSST model? (http://www.cfd-online.com/Forums/openfoam-solving/117252-some-quiries-about-how-set-low_re-komegasst-model.html)

immortality May 5, 2013 12:13

some quiries about how to set a "low_Re" kOmegaSST model?
 
I want to have a turbulent kOmegaSST model with yplus<=1 to be sure that separation regions and adverse pressure gradients are calculated and included truely not through some "wall-function rough estimations".
he has a low-Re grid mesh.
1)is my approach(stablishment of a very fine mesh near the wall to capture precisely separation and adverse pressure gradients there) true?
2)how have to set kOmegaSST to have a low-Re model satisfies 1?
3)I then need "y plus" to be certain that mesh is fine near wall with y+<=1 and only "y plus" not "y star" or any other thing instead of "y plus" (yPlusRAS tool calculates y-star based on wall-functions).

ignacio May 6, 2013 03:44

Hello Ehsan,

1) Yes, Low Reynolds kOmegaSST model includes a set of damping factors and constants that make it possible to use to integrate through the boundary layer. It doesn't need wall functions.

2) You need to define patches for k, omega and mut (compressible) / nut (incompressible)
k --> fixedValue, uniform 1e-10
mut/nut -- > compressible::mutLowReWallFunction, uniform 0
omega --> compressible::omegaWallFunction, uniform 0

3) yPlusRAS in OpenFOAM does not calculate the yPlus for Low Re. It uses a correlation for the friction coefficient which is only valid for high Reynolds numbers.
You will need to extract the velocity data at the first cell in the boundary layer and calculate it by yourself (approximate the derivatives in the wall shear stress to small increments)


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