CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Car aerodynamics

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree2Likes

Reply
 
LinkBack Thread Tools Display Modes
Old   July 28, 2010, 19:16
Default
  #81
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to louisgag
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
louisgag is offline   Reply With Quote

Old   August 2, 2010, 09:14
Default
  #82
Senior Member
 
stephane sanchi
Join Date: Mar 2009
Posts: 300
Rep Power: 9
openfoam_user is on a distinguished road
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.
openfoam_user is offline   Reply With Quote

Old   August 2, 2010, 15:19
Default
  #83
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to 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
louisgag is offline   Reply With Quote

Old   August 3, 2010, 05:35
Default
  #84
Senior Member
 
stephane sanchi
Join Date: Mar 2009
Posts: 300
Rep Power: 9
openfoam_user is on a distinguished road
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.
openfoam_user is offline   Reply With Quote

Old   August 4, 2010, 03:57
Default
  #85
Senior Member
 
Tom Fahner
Join Date: Mar 2009
Location: Delft, Netherlands
Posts: 226
Rep Power: 10
tomf is on a distinguished road
Send a message via MSN to tomf Send a message via Skype™ to tomf
Hello Stephane,

You can acquire a copy via this link:

http://papers.sae.org/840300

Tom
tomf is offline   Reply With Quote

Old   September 22, 2010, 05:50
Default
  #86
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
andrea.pasquali is offline   Reply With Quote

Old   September 22, 2010, 09:09
Default
  #87
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to louisgag
Hello Andrea,

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

Best regards,


-Louis
louisgag is offline   Reply With Quote

Old   September 22, 2010, 10:19
Default
  #88
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
andrea.pasquali is offline   Reply With Quote

Old   September 22, 2010, 11:53
Default
  #89
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to 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
louisgag is offline   Reply With Quote

Old   September 22, 2010, 14:52
Default
  #90
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
__________________
Andrea Pasquali
andrea.pasquali is offline   Reply With Quote

Old   September 23, 2010, 11:59
Default
  #91
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to 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.


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
louisgag is offline   Reply With Quote

Old   September 24, 2010, 05:49
Default
  #92
Senior Member
 
stephane sanchi
Join Date: Mar 2009
Posts: 300
Rep Power: 9
openfoam_user is on a distinguished road
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.
openfoam_user is offline   Reply With Quote

Old   September 24, 2010, 08:06
Default
  #93
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
andrea.pasquali is offline   Reply With Quote

Old   September 24, 2010, 08:44
Default
  #94
Senior Member
 
stephane sanchi
Join Date: Mar 2009
Posts: 300
Rep Power: 9
openfoam_user is on a distinguished road
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.
openfoam_user is offline   Reply With Quote

Old   September 24, 2010, 08:55
Default
  #95
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
andrea.pasquali is offline   Reply With Quote

Old   October 4, 2010, 08:34
Default
  #96
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
andrea.pasquali is offline   Reply With Quote

Old   October 4, 2010, 11:21
Default
  #97
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to louisgag
Pretty neat results. What is the % contribution of pressure and friction on the drag coefficient?

-Louis


Quote:
Originally Posted by andrea.pasquali View Post
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 is offline   Reply With Quote

Old   October 5, 2010, 10:59
Default
  #98
Senior Member
 
Andrea Pasquali
Join Date: Sep 2009
Location: Germany
Posts: 142
Rep Power: 7
andrea.pasquali is on a distinguished road
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
andrea.pasquali is offline   Reply With Quote

Old   October 5, 2010, 18:35
Default
  #99
Senior Member
 
louisgag's Avatar
 
Louis Gagnon
Join Date: Mar 2009
Location: Québec, QC, Canada
Posts: 208
Rep Power: 9
louisgag is on a distinguished road
Send a message via ICQ to 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 ;-)
louisgag is offline   Reply With Quote

Old   June 8, 2011, 04:20
Default
  #100
New Member
 
farahidayu
Join Date: Aug 2010
Posts: 16
Rep Power: 7
farahiam is on a distinguished road
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.
farahiam is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Aerodynamics vengi FLUENT 5 October 25, 2011 10:43
Aerodynamics Bonny Jacob Zachariah Phoenics 3 February 10, 2009 05:43
CFD in aerodynamics Ujjwal Bhaskar FLUENT 1 December 26, 2007 11:29
Use of Pro-Am in aerodynamics Javidan Ahmad CD-adapco 8 December 3, 2004 00:27
unsteady aerodynamics R.KRISHNAMURTHY Main CFD Forum 1 December 6, 2000 02:17


All times are GMT -4. The time now is 15:49.