[louisgag]

You should find this information for 60 m/s in the article

Ahmed, S.L., Ramm, G.: Some salient features of the averaged ground vehicle wake. SAE technical paper 840300 (1984)

and also in http://www.amcaonline.org.ar/ojs/ind...icle/view/1018

cheers,

-Louis

[openfoam_user]

Hi Louis,

Thanks for the link.

So Cd=0.23 for the ahmed body with 12.5 deg. slant angle with
Cp/Cf = 76/24

Do you know the value for the Lift Coefficient (and Cp/Cf) ?

Regards,

Stephane.

[louisgag]

Hi Stephane,

I don't have such information but you can find a breakdown of the drag contribution to different parts of the body in the article by Ahmed et al.

Best regards,

-Louis

[openfoam_user]

Hi,

Can someone provide me a PDF copy of the following paper ?

Ahmed, S.L., Ramm, G.: Some salient features of the averaged ground vehicle wake. SAE technical paper 840300 (1984)

Regards,

Stephane.

[tomf]

Hello Stephane,

You can acquire a copy via this link:

http://papers.sae.org/840300

Tom

[andrea.pasquali]

Hi everyone,
I have a question, where is done the force calculation?
1) All body (body + legs) with Aref = Aref_body + Aref_legs ?
2) Only body (without legs) with Aref = Aref_body ?
3) All body (body + legs) with Aref = Aref_body ?

Thanks

Andrea

[louisgag]

Hello Andrea,

usually in CFD it is done without the legs, but some authors do differently.

Best regards,


-Louis

[andrea.pasquali]

Hi Louis,
I have Ahmed Body 25° slant. From Graz Paper Workshop in Milan, experimental data gives Cd = 0.299.
My best result is Cd = 0.327 calculated on body only, with:
half model 1.8 Ml cells,
kOmegaSST turbulence model,
velocity: U (40 0 0) for inlet and floor, slip BC for side and roof, inletOutlet for outlet
pressure: fixedValue 0 for outlet, slip BC for side and roof, zeroGradient for inlet
k: kqRWallFunction for "walls" patch (ahmed and floor), fixedValue for inlet, inletOutlet for outlet, slip for side and roof
omega: kqRWallFunction for "walls" patch (ahmed and floor), fixedValue for inlet, inletOutlet for outlet, slip for side and roof
nut: nutWallFunction for "walls" patch (ahmed and floor), calculated for the others
fvSchemes

ddtSchemes
{
default steadyState;
}

gradSchemes
{
default Gauss linear;
grad(p) Gauss linear;
grad(U) cellLimited Gauss linear 1;
}

divSchemes
{
default none;
div(phi,U) Gauss linearUpwindV Gauss linear;
div(phi,k) Gauss upwind;
div(phi,omega) Gauss upwind;
div((nuEff*dev(grad(U).T()))) Gauss linear;
}

laplacianSchemes
{
default Gauss linear corrected;
}

interpolationSchemes
{
default linear;
interpolate(U) linear;
}

snGradSchemes
{
default corrected;
}

fluxRequired
{
default no;
p;
}

fvSolution

solvers
{
p
{
solver GAMG;
tolerance 1e-7;
relTol 0.1;
smoother GaussSeidel;
nPreSweeps 0;
nPostSweeps 2;
cacheAgglomeration on;
agglomerator faceAreaPair;
nCellsInCoarsestLevel 150; //np=3
mergeLevels 1;
};

U
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-8;
relTol 0.1;
nSweeps 1;
};

k
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-8;
relTol 0.1;
nSweeps 1;
};

omega
{
solver smoothSolver;
smoother GaussSeidel;
tolerance 1e-8;
relTol 0.1;
nSweeps 1;
};
}

SIMPLE
{
nNonOrthogonalCorrectors 0; //simpleFoam
// nNonOrthogonalCorrectors 10; //potentialFoam
}

relaxationFactors
{
p 0.3;
U 0.7;
k 0.7;
omega 0.7;
}

I'd like to try new discr schemes. Do you think I can improve my Cd trying different discr schemes?

Thanks

Andrea

[louisgag]

Hi Andrea,


you might find interesting info in my paper: http://web.student.chalmers.se/group...nPaperOFW5.pdf

As for the discretization schemes, I am no expert so I can't help you... I know changing numerical diffusion can sometimes help, also trying to model the full domain is a good idea. And, finally, small differences in results are hard to eleminate.

Does the flow look good? (compared to your experiment)
Is the ratio of contributions of pressure and friction drag accurately modeled?

Good luck with your work,


-Louis

PS: I will be spending November through Feb. at Polimi

[andrea.pasquali]

Hello Louis,
I attended at 5th Workshop in Gotheborg and I followed your presentation!
I'm interesting also to try different walls models.
For your Ahmed you have 9 m/s and you use LowRe in the end, right?
I have 40 m/s with Re 3 Ml, could also I try the LowRe? Which wall function you suggest me?
However I will try a lot of cases!

PS: I will be spending November through Feb. at Polimi

That's good! I am in Turin, it's very close to Milan! We could plan something!


Andrea

[louisgag]

Actually, for the Ahmed body, I used 40 m/s and wallFunctions. I tested the LowRe but was not entirely satisfied (wrong repartition of pressure and friction drag). I believe this is because my y+ at the walls of the body was too high for LowRe.

I suggest you try LowRe if you can afford it with small y+ at the wall (~1). Otherwise, if your y+ goes between 1 and 30, you can also try the nutSpallartAllmarasWallFunction (OF 1.6.x) which automatically switches between LowRe and WallFunctions.

That's good! I am in Turin, it's very close to Milan! We could plan something!

Sure! I will definitely be looking for people to snowboard and/or sail with!

cheers,

-Louis

[openfoam_user]

Hi Andrea,

Ahmed Body 25° slant.

One of my result is Cd = 0.300772 and Cl = 0.301195

calculated on body only (without legs)
fullmodel 4 M cells
kOmegaSST turbulence model

Regards,
Stephane.

[andrea.pasquali]

Hi Stephane,
good result for Cd, for the Cl the experimental is 0.345, right?! My last one is Cd 0.318 Cl 0.351 with half model 1.8 Ml cells KOmegaSST turbulence model.
Now I'm trying different nutWallFunctions.
Then I will try different turbulence models and also full model.
Which is your schemes setup and BC setup?

regards

[openfoam_user]

Hi Andrea,

You are right my Cl is low (experimental value = 0.345).

Floor, roof and sides are slip BC. No velocity on floor. My boundary conditions are the same except for omega (omegaWallFunction).

Differences with you are :

fvSchemes:
Gauss SFCD for div(phi,*)

fvSolution :
PBiCG solver for U, K, omega

As starting point I have used the 'best' setting of TU Graz.

For mesh generation I have used snappyHexMesh with snapEdge.

I have to improve the mesh (add layers) and try other settings. Unfortunately now I am too busy with other stuff.

Nevertheless let me know your improvements !

Regards,

Stephane.

[andrea.pasquali]

omega: kqRWallFunction for "walls" patch (ahmed and floor), fixedValue for inlet, inletOutlet for outlet, slip for side and roof

Sorry my error in copy/paste. I use "omegaWallFunction" too for omega BC.
Of course I'll update my result report!

Regards

[andrea.pasquali]

My last result Ahmed 25 slant (with half model hexa mesh)
Cd 0.3006 (Exp 0.299)
Cl 0.361 (Exp 0.345)

Andrea

[louisgag]

Pretty neat results. What is the % contribution of pressure and friction on the drag coefficient?

-Louis

Originally Posted by andrea.pasquali

My last result Ahmed 25 slant (with half model hexa mesh)
Cd 0.3006 (Exp 0.299)
Cl 0.361 (Exp 0.345)

Andrea

[andrea.pasquali]

Hi,
I continued my case until residuals converge well, now I have
Cd 0.299886
Cl 0.360877
I have 82% Drag in Pressure and 18% in Friction

Is there in OpenFOAM a function to calculate Cp?
Are there car geometries (no like Ahmed Body) available to test and compare Cd, Cl with experimental data?

Andrea

[louisgag]

I went back to Ahmed (1984) orignial results and yours seem quite good.
Although Ahmed is slightly different ( Uinf = 60 m/s, differtent wind tunnel,...)


Cd=0.29 with Cp=0.23 from pressure
or, 79% pressure and 21% friction


see you in Milan ;-)

[farahiam]

Hi Stefane,
I am currently working on my external aerodynamics model on car where I obtained about 8 drag count (diff = 0.008) difference compared to actual case. However I used different fvSchemes and fvSolution as you've suggested.
May I see both of these 2 files of yours for comparison?

Regards.