Airfoil NACA 0006 with alpha -4° is not going as expected, why?
 

hi everyone,
I'm focusing on solving an AIRFOIL NACA 0006 in 2D, I have to find the correct Coefficient of DRUG and LIFT, but after several times I haven't get the clue yet.

I've got a section angle of attack about -4 degrees, which means according to the tables a value for Cl=-0.4,-0.5 and Cd=0.010,0.014 i suppose?! (CORRECT ME IF IS WRONG, PLEASE)

https://www.cfd-online.com/Forums/at...1&d=1507370187

I'm using this function objects for the calculation of the Coeffs::

functions
{ forces
{ type forceCoeffs;
functionObjectLibs ("libforces.so");
outputControl timeStep;
outputInterval 1;
patches (profiloalare);
pName p;
UName U;
rhoName rhoInf;
log true;
rhoInf 1.2;
liftDir (0.069756474 0.99756405 0);
dragDir (0.99756405 -0.069756474 0);
//liftDir (0 1 0);
//dragDir (1 0 0);
CofR (1.0375 0 0);
pitchAxis (0 0 -1);
magUInf 21.6867;
lRef 4.15;
Aref 4.15;
} }

I think I'm using a good mesh, (i used youtube videos to create the best mesh possible)

https://www.cfd-online.com/Forums/at...1&d=1507370171

I settled all the named section, wall for the airfoil, inlet and outlet for the edges.

I had post as system settings:

divSchemes
{
default none;
div(phi,U) bounded Gauss linearUpwind grad(U);
div(phi,nuTilda) bounded Gauss linearUpwind grad(nuTilda);
div((nuEff*dev2(T(grad(U))))) Gauss linear;
}

relaxationFactors
{
fields
{
p 0.3;

}
equations
{
U 0.7;
nuTilda 0.7;
}
}

simulationType RAS;

RAS
{
RASModel SpalartAllmaras;

turbulence on;

printCoeffs on;
}

transportModel Newtonian;

rho [1 -3 0 0 0 0 0] 1.2;

nu [0 2 -1 0 0 0 0] 1.5e-05;


I launch the simpleFoam, but in the end my coefficient are SO WRONG! and i don't know why!

I said the should be Cl=-0.4,-0.5 and Cd=0.010,0.014, but mine values are about Cl=-0.83 and Cd=0.027.

Anyone has any suggestion, please??

Have you tried other turbulence model or numerical scheme? kOmegaSST should be better model.

Hello.
I think Your Aref value may be wrong. Is Your 2d mesh 1000mm in z direction? As I remember Aref should be equal to lRef multiply by depth of Your mesh.

How about Your y+ values?

Thanks man, what i said was wrong, you were right, i used a Mesh about 2000mm because i didn't considered both ways, plus and less!!
thank you!!!

/*------------i'm dope-------------------------*\
Hi,
Yes my mesh is depht 1 m, i had used

transformPoints -scale '(1 1 1.33662678)'

to convert my initial depth into 1m.

Also yes, Aref is equal to Iref multiply by depth, so in this case i think it's right, and Iref and Aref are the same because my depth is 1m.

my value of y+ are about between 50 and 120. that's why i used a high RE model.

any other suggestion?
/*------------------------------------------------------------------*\

i had try to use different divSchemes, only 2°grades order :

bounded Gauss linear Upwind
bounded Gauss linear

ddtSchemes i only used
default steadystate

but now that i think about it maybe should use a Euler o CrackNikolson?

gradSchemes only used
Gauss linear

and because i had low orthogonality (<10)
i used laplacianSchemes and snGradSchemes orthogonal

thanks, i'm gonna try a kOmega SST then.

Hello again.
How Your simulation is going?
I just finished my naca airfoil calculations and my forces output isn't right for both - kEpsilon and kOmegaSST. I'm using sonicFoam because my case is compressible and getting wired results with rhoPimpleFoam. I'm getting acceptable (for now) values of lift (CL = 0.37 while there should be CL = 0.3) but drag values are way to high (CD=0.014 while there should be CD=0.004)

I think if Your case is incompressible then rho should be set to 1 not 1.2.
Check for examples:
tutorials\incompressible\simpleFoam\motorBike\syst em\forceCoeffs
tutorials\incompressible\pisoFoam\les\motorBike\mo torBike\system\forceCoeffs

Isn't Your liftDir and dragDir set for angle of attack +4degree?

You may also want to try using low Reynold number wall function for k and nut. A few layers of thin boundary mesh may also help.

Are your result include the wall shear stress as well? For airfoil, slender body, wall shear stress should have contributed a portion of the forces.

I understood my silly error, my friend.

I was Meshing a deltaZ equals to 2, because the mesh move from -1 to +1 in Z.
I was not considering that and i used deltaZ equals to 1 for my calculation of Aref, and in this way obviously i had Coeffs who were the double they should be. now I fixed it and the values are in the ranges, but i would like to improve my residuals, how can i do now?