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Validatation NACA 0012 OFv2006 lack of precision

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Old   January 8, 2021, 06:55
Question Validation NACA 0012 OFv2006 lack of precision
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Ivan
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Hi everyone,

I'm using OpenFoamv2006.
I'm trying to validate the case "Turbulent flow over NACA0012 airfoil (2D)". Basically, it's a NACA0012 with Re of 6*10^6.

The details of the case can be found in: https://www.openfoam.com/documentati...irfoil-2d.html

I'm focus on the lift coefficient. I tried the AoA [3,6,9,12].
Usually, The results are deviated around 10 % of the real ones. To show that, in case AoA of 3 and 6:
Cl (3) [Real] = 0.3234
Cl (3) [obtained = 0.2991

Cl (6) [Real] = 0.6648
Cl (6) [obtained = 0.59627

I tried two differents turbulent models, Spalart-Allmaras and KwSST and the results are quite similar.

I have some experience meshing. I tried at least 6 different meshes, because usually the error it's there, but the results doesn't improve (2-3 % of change).

About how I calculated the force:

-----------------

force{
pitchAxis (0 0 1);
magUInf 1.0;
rhoName rhoInf;
rhoInf 1.0;
patches (wallS);
// wallS is the patch who represent the NACA profile.
CofR (0 0 0);
liftDir (-0 1 0);
lRef 1.;
writeControl timeStep;
Aref 1.;
UName U;
pName p;
U U;
rho rhoInf;
p p;
libs ("libforces.so");
dragDir (1 0 0);
type forceCoeffs;
writeInterval 1;
}

-------

I attached the case just in case anyone one to glance at it.
https://drive.google.com/file/d/1p2P...pWkuxhT4y/view

It's ready to be run with "simpleFoam".

Any kind of help is more than welcome.

Thank for your time.

Last edited by Ivangzp; January 8, 2021 at 09:18.
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Old   January 10, 2021, 13:14
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You have to declare lift and drag direction accordingly as in the flow vector direction, this is your main problem in your setup.

I have tested your case with this direction correction and got Cl (3) = 0.326 just in a few hundred steps.
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Old   January 10, 2021, 13:35
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Ivan
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First of all, thank you so much por replying.

I tried that and it didn't work.

I usually project the values of Cl and Cd, after obtaining from OpenFoam, to get the "real ones", in that way I don't have to change the direction of Lift and Drag all the time.
For example, for 3:

Cl (real) = Cl* cos(3)-Cd*SENO(3), being Cl* and Cd*, the values obtained in OpenFoam.

Let me ask you 2 question:

- Just in case, which direction have you declared for lift and drag ? (copy and paste from the controlDict, would be awesome).

- Which version of OF have you used?
I asked you that, because with the same setting in OpenFoam4.0, I have the "correct" result, really close to Cl (3) = 0.326.
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Old   January 11, 2021, 03:22
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I have only changed this line:
dragDir (1 0 0);
with this:
dragDir (0.998629534754574 0.0523359562429438 0);
to test your setup.
it must also be changed the following line accordingly indeed:
liftDir (-0 1 0);
with
liftDir (0.0523359562429438 0.998629534754574 0);


and yes, I have used OF4.1!
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naca0012, openfoamv2006, precission, turbulence, validation

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