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#81 | |
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You should find this information for 60 m/s in the article
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cheers, -Louis |
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#82 |
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stephane sanchi
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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. |
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#83 |
Senior Member
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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 |
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#84 |
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stephane sanchi
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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. |
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#85 |
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#86 |
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Andrea Pasquali
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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
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#87 |
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Hello Andrea,
usually in CFD it is done without the legs, but some authors do differently. Best regards, -Louis |
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#88 | ||
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Andrea Pasquali
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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:
Quote:
Thanks Andrea
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#89 |
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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 |
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#90 | |
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Andrea Pasquali
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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:
Andrea
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#91 | |
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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:
cheers, -Louis |
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#92 |
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stephane sanchi
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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. |
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#93 |
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Andrea Pasquali
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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
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#94 |
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stephane sanchi
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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. |
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#95 | |
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Andrea Pasquali
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Quote:
Of course I'll update my result report! Regards
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#96 |
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Andrea Pasquali
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My last result Ahmed 25 slant (with half model hexa mesh)
Cd 0.3006 (Exp 0.299) Cl 0.361 (Exp 0.345) Andrea
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#97 |
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#98 |
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Andrea Pasquali
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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
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#99 |
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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 ;-) |
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#100 |
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farahidayu
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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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