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July 28, 2010, 19:16
#81
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Louis Gagnon
Join Date: Mar 2009
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You should find this information for 60 m/s in the article
Quote:
 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

 August 2, 2010, 09:14 #82 Senior Member   stephane sanchi Join Date: Mar 2009 Posts: 300 Rep Power: 9 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.

 August 2, 2010, 15:19 #83 Senior Member     Louis Gagnon Join Date: Mar 2009 Location: Québec, QC, Canada Posts: 208 Rep Power: 9 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

 August 3, 2010, 05:35 #84 Senior Member   stephane sanchi Join Date: Mar 2009 Posts: 300 Rep Power: 9 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.

 August 4, 2010, 03:57 #85 Senior Member   Tom Fahner Join Date: Mar 2009 Location: Delft, Netherlands Posts: 226 Rep Power: 10 Hello Stephane, You can acquire a copy via this link: http://papers.sae.org/840300 Tom

 September 22, 2010, 05:50 #86 Senior Member   Andrea Pasquali Join Date: Sep 2009 Location: Germany Posts: 142 Rep Power: 7 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 __________________ Andrea Pasquali

 September 22, 2010, 09:09 #87 Senior Member     Louis Gagnon Join Date: Mar 2009 Location: Québec, QC, Canada Posts: 208 Rep Power: 9 Hello Andrea, usually in CFD it is done without the legs, but some authors do differently. Best regards, -Louis

September 22, 2010, 10:19
#88
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Andrea Pasquali
Join Date: Sep 2009
Location: Germany
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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
Quote:
 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
Quote:
 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
__________________
Andrea Pasquali

 September 22, 2010, 11:53 #89 Senior Member     Louis Gagnon Join Date: Mar 2009 Location: Québec, QC, Canada Posts: 208 Rep Power: 9 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

September 22, 2010, 14:52
#90
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Andrea Pasquali
Join Date: Sep 2009
Location: Germany
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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!

Quote:
 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
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Andrea Pasquali

September 23, 2010, 11:59
#91
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Louis Gagnon
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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.

Quote:
 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

 September 24, 2010, 05:49 #92 Senior Member   stephane sanchi Join Date: Mar 2009 Posts: 300 Rep Power: 9 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.

 September 24, 2010, 08:06 #93 Senior Member   Andrea Pasquali Join Date: Sep 2009 Location: Germany Posts: 142 Rep Power: 7 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 __________________ Andrea Pasquali

 September 24, 2010, 08:44 #94 Senior Member   stephane sanchi Join Date: Mar 2009 Posts: 300 Rep Power: 9 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.

September 24, 2010, 08:55
#95
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Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
Quote:
 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

 October 4, 2010, 08:34 #96 Senior Member   Andrea Pasquali Join Date: Sep 2009 Location: Germany Posts: 142 Rep Power: 7 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

October 4, 2010, 11:21
#97
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Louis Gagnon
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Pretty neat results. What is the % contribution of pressure and friction on the drag coefficient?

-Louis

Quote:
 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

 October 5, 2010, 10:59 #98 Senior Member   Andrea Pasquali Join Date: Sep 2009 Location: Germany Posts: 142 Rep Power: 7 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 __________________ Andrea Pasquali

 October 5, 2010, 18:35 #99 Senior Member     Louis Gagnon Join Date: Mar 2009 Location: Québec, QC, Canada Posts: 208 Rep Power: 9 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 ;-)

 June 8, 2011, 04:20 #100 New Member   farahidayu Join Date: Aug 2010 Posts: 16 Rep Power: 7 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.

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