[morfeus80]

Hallo,
I'm going to validate OpenFOAM with differents turbolence models for calculation of aerodynamic coefficients of formula cars. Before starting to work with a complete car, I want validate a model of bluff body with car shape, somethin as the Ahmed model. Unfortunately I don't have aerodynamic coefficients of models like this! Someone has a geometry and the relative experimental lift and drag coefficients to make this comparison?
Furthermore because I have experience of this kind of simulation with another commercial sotware but not with OpenFLOW, if someone is working with car aerodynamics, can give me information about the turbolence model is using please?

[mighelone]

Here http://cfd.mace.manchester.ac.uk/ercoftac/ you can find the experimental data relatives to Ahmed body and other test cases.

Registration is required to download data!

Michele

[morfeus80]

Thanks, that website is very interesting, but I didn't find what I need because there are pressure data and velocity fields, but nothing about lift and drag. Instead these are the parameters I need to validate my model!

[bastil]

Hi Mattia,

may I ask what software you intend to use for meshing?

[morfeus80]

In the past I used Ansa and Tgrid, now I have to decide. I thinked to can use the openFoam preprocessor, but it doesn't look very developed...

[bastil]

The OpenFoam preprocessor "blockMesh" is not suited for handling theses complex geometries. I think if you want to use open source you can try to use one of the tet-meshers out there and use polyDualMesh afterwards to convert to polyhedral. However, you nee more or less clean CAD data (geometries) or a clean surface mesh for that, do you have that? Hex-dominant meshes are better than polys and I am still seeking some mesher for that. Tell me if you have one. I am also seeking for some surface mesh repair without havin ggeometry.

Regards

[morfeus80]

I have the complete geometry of the car, but I need the software to make the surface and volume mesh. According to your opinion what are the best open source software to make mashes on very complex geometries?

[gdbaldw]

Regarding mesher. I needed a quad surface with hex grid, first cell less than 1000 aspect ratio and acceptable skew, first cell at y+ of about 2 to 5. I patched together a script for Blender to specify just this kind of geometry, which exports to Calculix for building a grid, then exports to OpenFOAM. Works great, and I posted the script elsewhere in this forum. In my opinion, it would be an ideal free and open source tool for this problem. I used it for aero analysis of a fuselage. You'd need to be proficient in Blender from blender.org, and also learn Calculix, neither are insurmountable. If interested, I'd suggest finding an online tutorial for "Blender subsurf" and mastering this modelling technique (using quads only), then download Calculix and learn how to generate and export a simple geometry to OpenFOAM using the Calculix tutorial. After that, you are good to go. Download the Blender script I had posted and start generating 3-D compound/convex hex grids in a matter of hours or a few days depending on the level of complexity. Ahmed would take less than a day, but you'd need to invest many days in learning the technique.

By the way, there is another thread in this forum regarding attempts to model Ahmed. As I recall from the other thread there already exists OpenFOAM models of Ahmed. I'd be interested in lift/drag results you generate.

Doug

[bastil]

Mattia,

is the geometry clean or not (holes, overlappings, small faces,...)?
If your geometry is more or less clean you could try using netgen or salome to create surface and volume meshes (tets). Afterwards use dualPolyMesh to convert to poly if desired.

Concerning the blender method I think it has the drawback that you can not import existing CAD data to blender, can you? Nevertheless, I do not think that this will give you meshes for really complex car geometries with all features and maybe underhood. How compplex is your geometry?

[gdbaldw]

True, the Blender method is best when new geometry is created. Ahmed could be quickly created in Blender. In theory it can also work with STL, since Blender can import STL format. I haven't needed to take that step.

Doug

[lucchini]

Dear Mattia,

are you working with OpenFOAM for car aerodynamics in Italy? Could you possibly contact me by e-mail ( tommaso . lucchini @ polimi . it)?

Thanks a lot, bye

Tommaso

[bastil]

I Think blender in combination with STL is possible if the stl is more or less clean. However stl from CAD packages normally are not. I hav a lot of dirty stl files I want to mesh with open source and so far I have no solution for this with accepable efford.

Regards

[gdbaldw]

BastiL,

If you are up to doing some modest Python coding, Blender with STL could work. I haven't the need, else I'd look into it. For my needs, I coded a Python script to create a Calculix input file from multiple subsurf manifolds extending from the body to the far field. In order to adapt this script for STL, the first subsurf manifold would closely follow the STL geometry, and become the template for projecting the STL faces onto each subsequent manifold. Then, in Calculix, each STL triangle would be subdivided into three quads before generating a hex grid for export to OpenFOAM. The only significant additional coding needed is to map STL vertices to the nearest manifold subsurf face.

Doug

[gdbaldw]

BastiL,

By the way, Blender has a new feature that could help with cleaning bad STL. While in edit mode, you can Select "Non-Manifold" edges, which I have found selects only those edges that are not connected to exactly two faces.

Doug

[morfeus80]

BastiL,
my geometry is very complex and I'll have to clean it.

[luca_g]

Ciao Mattia,

I can suggest you the following links (among many others you will find on the web) to papers related to computations on the Ahmed body. They all include pictures taken from the original Ahmed's SAE paper (which I don't have) reporting the experimental drag figures. No measure of downforce however, as far as I know, and it is anyway very low.

http://tmdb.ws.tn.tudelft.nl/worksho...B2000_NNFM.pdf

http://www.edpsciences.org/articlesp.../gillieron.pdf

http://www.imft.fr/GDR2502/journee24...Guilmineau.pdf

About the experimental drag figure I'm quite sure they are either "windtunnel corrected" values or the measures where done in a large tunnel with very low blockage and at 60m/s. On the contrary, the subsequent measurements (see first paper) where done on a small tunnel with 4% blockage at 40m/s and no drag figures are reported.

So it seems to me that to come closer to the experimental drag a large computational domain must be used, whereas a confined test section should be more appropriate to compare to the velocity profiles of the second experiment.

Luca

[morfeus80]

Luca,
Thanks for the advice, I'll start to work at this and as soon as I'll have some results, I'll give you.

Ciao!

[bastil]

Mattia,

are there some news concerning mesh tools? I am srill seeking for well suited open source tools and would be happy about hints.

BastiL

[morfeus80]

BastiL,
I have read this week the topic about Mesh3D, it's open-source and looks interesting. But I hadn't time to test it so far.
In this days I'm working with a mesh I alredy had an I'm testing several turbulence models and solvers. As soon as I'll reach good results, I'll start to work on the pre an post processors.

[bastil]

Mattia,

i have seen that, too. Looks interesting but has two drawbacks:
* Windows only
*32 Bit only

I Think this is not the way to go for large aerodynamic meshes.
I am also interested in your solver experiences. Is it possible to share them? What solvers? SimpleFoam?