# Ahmed body simulation

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 May 30, 2012, 12:18 Ahmed body simulation #1 New Member   alessandro Join Date: Oct 2010 Posts: 28 Rep Power: 7 Here the continuation of the thread about meshing of Ahmed body

 May 30, 2012, 12:23 #2 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 Mesh Quality Do you think that the turbulence intensity has such a huge impact on drag?

 May 30, 2012, 12:28 #3 New Member   alessandro Join Date: Oct 2010 Posts: 28 Rep Power: 7 I don't know how much is relevant but with default 10% cd was about 0.35. With 0.25% 0.289.

May 30, 2012, 12:33
#4
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alessandro
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Quote:
 Originally Posted by scipy Did you read any of the Ahmed body papers? Usual turbulence intensity of the free stream airflow in a windtunnel is around 0.25 %, so me using 1 % is already a bit higher and more "real world"-like, but 2.5 is unnecessary. You (and most others) seem to confuse a no slip wall and a no shear wall. No slip wall is what I've used for the ground and the ahmed body (since that's what they are, stationary walls with shear stress or 0 slip). However, for the side and top wall - if you are going after ultimate accuracy and recreating exact wind tunnel conditions, then those walls should also have a boundary layer (same as the ground and the Ahmed body itself), but since most of the time they're far enough away to only affect the solution a little bit (if left as no slip), they can be given no shear stress boundary conditions so their viscosity effects will be disregarded. Since this is mathematically the same as a symmetry BC for Fluent, I used that for simplicity. /edit, seems you have read about turbulence intensity
[/QUOTE] Sorry, 2.5% was a typing mistake

About no-slip wall and no-shear wall, I knew the the difference, I was just wondering if using one instead of the other could affect the solution. But if it's for simplicity now I try with symmetry

May 30, 2012, 13:50
#5
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alessandro
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Simulation completed
Here settings used:
-Coupled with pseudo-transient, first order on momentum and turbulence kinetic energy and dissipation rate for 25 iterations and then switch to second order
- explicit relaxation factors to default except for Pressure and momentum at 0.4 and turbulent viscosity to 0.95.
-Automatic time-step method with Timescale Factor=10
-bcs with 1% turbulence intensity and symmetry also on top and lateral surfaces
-fmg initialization with default settings

Convergence in 168 iterations and 55 minutes ( about 20 seconds per iteration)
Cd=0.27444
Cl=4.2129e-02
(unfortunatly i forgot to hard copy cl and cd histories)
maximum residuals at 10^-5
Attached Images
 ahmed_far_fmg_pseudo.jpg (47.1 KB, 89 views)

 May 30, 2012, 14:07 #6 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 did you compare to experimental values? How much error is there? What is the slant angle in current simulation and are you going to try the URANS? These results are taken with mesh3? Try to refine mesh to 4 million and see the results.

 May 30, 2012, 14:53 #7 New Member   alessandro Join Date: Oct 2010 Posts: 28 Rep Power: 7 I've found different experimental values for cd, from 0.25 to 0.295. For the LSTM case i found two values: 0.279 and 0.285. The ercoftac case 9.4 on which I rely doesn't report cd but only velocity profiles and pressure coefficients in some locations along the body. I have to compare those values but it will take some time. For cl I found a value of 0.004. I used 35° configuration. I don't think I will perform a URANS simulation, I'll rather try different turbulence models to match velocity profiles as much as possibile.

 May 30, 2012, 14:58 #8 New Member   alessandro Join Date: Oct 2010 Posts: 28 Rep Power: 7 Results are taken on mesh 3. Do you think wake zone should be refined? And at the beginning of the slanted surface? That point should be critical for flow separation

 May 30, 2012, 15:01 #9 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 http://css.engineering.uiowa.edu/~me...medcarrodi.pdf I think mesh is similar to used by Rodi in above paper. However it is always necessary to check the mesh independence.

 May 30, 2012, 16:16 #10 New Member   alessandro Join Date: Oct 2010 Posts: 28 Rep Power: 7 I checked yplus on veicle surface and it's between 0.05 and 1.05. Is is necessary to have such low values? Shouldn't it be sufficient between 1 and 5?

 May 30, 2012, 18:27 #11 Senior Member     Alex Pasic Join Date: Aug 2011 Location: Croatia Posts: 184 Rep Power: 8 The most recent paper I could find that listed Cd and Cl for the Ahmed body was "Experiments and numerical simulations on the aerodynamics of the Ahmed body" by W. Meile, G. Brenn, A. Reppenhagen, B. Lechner, A. Fuchs in which they reported the experimental Cd of 0.279 and Cl of 0.004 for the 35° slant. This means the agreement of Cd with the experimental value was within 0.95 % and Cl was unfortunately an order of magnitude higher at 0.044 (11x higher). However, the same paper lists their numerical figures at 0.276 for Cd and 0.013 for Cl which is again 3.25x higher than experiment.

May 30, 2012, 22:21
#12
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Sijal
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Quote:
 I checked yplus on veicle surface and it's between 0.05 and 1.05. Is is necessary to have such low values? Shouldn't it be sufficient between 1 and 5?
Yes, you are correct. Y+ between 1 and 5 is sufficient.

 May 31, 2012, 10:48 #13 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 few updates are available here NEW : Tutorial on road vehicle alenglaro likes this.

 May 31, 2012, 17:15 #14 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 I have used "Higher order term relaxation" technique to accelerate the convergence.

December 14, 2012, 12:44
#15
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Li Huang
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Quote:
 Originally Posted by Far few updates are available here NEW : Tutorial on road vehicle
Hi Far,

Thanks for the tutorial. But i am having trouble to create a full car body mesh when i mirror your blocking. Basically what i did was, first, mirror the geometry and blocking, and set associations for some edges and vertices. Everywhere else looks good, except the part showing in the attached pic. The problem is i have some mesh lines converged to some points. But i have checked the Pre-mesh params for that edge and parallel ones to make sure proper params. How can I fix this part of mesh?
Attached Images
 full body.jpg (63.8 KB, 54 views)

December 14, 2012, 13:04
#16
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Li Huang
Join Date: Jan 2011
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Quote:
 Originally Posted by lihuang Hi Far, Thanks for the tutorial. But i am having trouble to create a full car body mesh when i mirror your blocking. Basically what i did was, first, mirror the geometry and blocking, and set associations for some edges and vertices. Everywhere else looks good, except the part showing in the attached pic. The problem is i have some mesh lines converged to some points. But i have checked the Pre-mesh params for that edge and parallel ones to make sure proper params. How can I fix this part of mesh? Thanks for your help!
Hi Far,
The problem was just solved by remove association of that edge in problem. Don't know why it happened tho.
Thanks again!

 December 14, 2012, 13:06 #17 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,285 Blog Entries: 6 Rep Power: 43 .................

 March 9, 2016, 05:22 Ahmed body #18 Member   AdOo Join Date: Mar 2016 Location: Bordeaux Posts: 91 Rep Power: 2 Hi all, Thank you for your help Far. In fact I'm new in CFD and I have to work on the Ahmed body. I've seen your files ".tin" and ".blk" . What do they concern exactly ? I mean, is there one for the geometry and one for the mesh ? And do you know if it's possible to import and to work with it on OpenFOAM ? Thanks a lot, Adrien

 March 9, 2016, 05:29 #19 Senior Member     Alex Pasic Join Date: Aug 2011 Location: Croatia Posts: 184 Rep Power: 8 .tin is the ICEM CFD geometry file and .blk is the ICEM CFD hexa blocking file. You need to have ICEM CFD installed to open either of these files, then you need to generate a pre-mesh, convert it to an unstructured mesh and export in a file format compatible with OpenFOAM. Far has a series of video tutorials on YouTube that cover the whole subject of the Ahmed body in ICEM CFD hexa. (https://www.youtube.com/watch?v=2baEaHAI-08)

 March 9, 2016, 05:58 #20 Member   AdOo Join Date: Mar 2016 Location: Bordeaux Posts: 91 Rep Power: 2 Scipy thank you for these clear et quick explanations ! The problem is that I can't use ICEM... I only have OpenFOAM to my disposition. As you said, I would need to export and unstructured mesh because ".tin" and ".blk" where for the blocking and for the geometry on ICEM. Does it mean that even if someone would send me the correct extension of the mesh for openfoam, I would be able to change his mesh ? I mean to work on the mesh, I would have to work on the ".blk" equivalent for openfoam ?

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