[giammy92]

Hi foamers,
i'm simulating a flow around an airfoil on which calculate force coefficients to vary the angle of attack. I'm meeting difficulties in find correct bc for nut in a kklomega. My first test is been with nut nutLowReWallFunction for wall and calculated for farfield.
This give good results until 10-11° but for 13°, near angle of stall, i have the first discrepancies according experimental data that are very accentuated for unsteady simulation.
On forum I've founded that for nut at wall is incorrect my wall function and it have to be set zeroGradient, but for farfield? Is correct calculated?
My other bc are:

Wall:
-kt = fixedValue 1e-12
-kl = fixedValue 1e-12
-omega = zeroGradient
-nut = zeroGradient
-p = zeroGraditent
-U = fixedValue ( 0 0 0)

Farfield:

-kt = freestream 0.0025847
-kl = freestream 0
-omega = freestream 1769.45
-nut = calculated 1.4506 e-6
-p = freestreamPressure
-U = freestream

[mzzmrt]

Hi,

You have not mentioned the version used but there are important differences between kkLOmega implementations in OpenFOAM.

If you are using a version prior to 2.4.0 what you get seems as expected...

[giammy92]

my version is 3.0.1

[mzzmrt]

When I have done a validation test for kkLOmega I could get good agreement on full pre stall region (naca0012 - Ogrid - 2.4.0 - steady state) with following values:

Wall:

-kt = fixedValue 0
-kl = fixedValue 0
-omega = zeroGradient
-nut = nutLowReWallFunction / value uniform 0
-p = zeroGradient
-U = fixedValue ( 0 0 0)

Farfield:

-kt = freestreamValue (estimated k value)
-kl = freestreamValue 0
-omega = freestream (estimated w value)
-nut = calculated (no initial value)
-p = freestreamPressure
-U = freestream

Very similar to your set up. You can also check the initial values for k,w and nut. Mesh quality and size may also be an issue...

[giammy92]

initial values of turbulence's parameter are been validated from previous results obtained from k-w SST. Mesh instead is been imported and converted from a file .msh that is been used for a precedent simulation with Fluent using transitional SST that gives better results than my kklomega mostly for unsteady simulation. So i wish implement transition SST for OpenFOAM for version 2.3.1

[malv83]

There are a few problems with the k-kl-omega model in the farfield. One of them is the growth of Laminar Kinetic energy when separation occurs. Lopez and Walters have a paper (have not been published yet) correcting this issue:

Maurin Lopez. D. K. Walters. “A recommended correction to the k-kl-omega transition sensitive eddy-viscosity model”. Journal of Fluid Engineering.

This correction has to be made to the 2008 k-kl-omega model from now on.

Now, Lopez and Walters also developed a new transitional model (k-omega-v2) as an alternative to the k-kl-omega one. This new model has more capabilities (it is more reliable) than the k-kl-omega model, especially in the farfield computations. Fortunately the paper for this new model is already publish.

Maurin Lopez. D. K. Walters. “Prediction of transitional and fully turbulent free shear flows using an alternative to the laminar kinetic energy approach”. Journal of Turbulence, Vol 17, Iss. 3, 2016.

If you see the papers, you will immediately see how the k-kl-omega model is not good for free shear flows, and how the new model corrects all those issues. From now on, k-kl-omega users have to start using the new k-omega-v2 model.

Hope this helps

[giammy92]

But this model is been implemented for OpenFOAM? I have version 2.3.1