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Voyage_gui October 22, 2010 09:17

Mesh generator with OpenFoam / Urban Design

I'd like to know what the best mesh generator to use with OpenFoam will be. We're planning to use this CFD solver for wind study (in LES) and I don't find any people using this software in Urban environnement. My CAD file are .stl .


eugene October 25, 2010 09:59

The built-in mesher in FOAM should be fine for your purposes: snappyHexMesh.

Voyage_gui October 25, 2010 10:12

Thks you for answer.
Are you using snappyHexMesh as mesh program ? I've just tried today the tutorial for the motorbike, it seems not easy for complex building such as stadium or large district. Could you define easily where you want to refine ?

val46 October 26, 2010 02:20

have you read the sHM section in the user guide? It provides good information about the meshing process.

To answer your question:
You can define refinement areas (called "refinementBox" in the snappyHexMeshDict of the motorbike tutorial)


Voyage_gui October 26, 2010 08:22

Hi Val46,
I haven't yet started with the user guide. I've just run the motorbike case and read (too) fast the snappyhexMeshDict. I'll spend more time with this tool.
Do you still think I can reasonably use sHM for industrial case ? Is it an efficient tool for complex geometries ?

val46 October 26, 2010 08:36

I'm not that experienced with OF (Just using it for 3 months now).
But I would say it should be a useful tool for your case.
Don't expect to get used to it within one week though.

Voyage_gui October 26, 2010 08:49

That's fast anwer, thanks a lot.
I'll insist a little bit more. Not sure to have enough patience nevertheless.
Are you working in Urban design using OF and sHM?

val46 October 26, 2010 09:23

No, I'm using sHM for cases similar to the motorbike tutorial.

Voyage_gui October 26, 2010 09:26

Ok, I appreciate your help.

colinB October 26, 2010 10:11

Hi Voyage_gui

I've been working with sHM for the past 2 month and I'm still not through every detail of the sHMD.
However in principal sHM should be capable dealing with the problems you have.
I'm using sHM for modeling problems with 2 or more axis of deformation e.g. ship hull surfaces. Again in general you can say sHM can deal with it, but you need a proper stl file.
As soon as you don't have a solid as stl you reach the limits of sHM.
Also if you have sharp edges they aren't modeled very good, not to say bad. For me this is of minor importance for I have to deal with smooth forms anyway.
Nevertheless you can determine refinement regions as mentioned above or you define different areas in your stl file which then will be imported to sHM and then separately can be refined, as you like.
I also know that other companies are using sHM for modeling also for highly complex structures e.g. p-brackets.
If you have some more questions on it don't hesitate to ask me I try to help as far as I can


Voyage_gui October 27, 2010 03:27

Hi ColinB,
Thanks a lot for your advice. I see that you are dealing too with complex geometry and that's a good news. Could i ask you from a stl file, how long does it take you to fully mesh let's say for a ship hull surfaces ? A couple hours , a day ?
Nevertheless you pointed out a really problematic issue for my stl file. In many of my cases, i have on building sharp edge (such as roof, ridge of building ...). According of your post, I should spend some time to smooth them with a CAD software ?

colinB October 27, 2010 07:39

Hi Guillaume

it is hard to tell how long the whole process takes. I get .iges files from the system
which are comparatively good and I manly have to fix some patches with salome.
After that I build a solid with salome and export it to a .stl file.
To get so far I would say it takes me 2-3h, but I also have to mention, that I'm still new to salome as well and don't have routine and the work is done by lots of tries and errors.

Then I go to sHM and basically just hand over the .stl file. Since we have standardized procedures I usually don't have to adjust any parameters in the sHMD and immediately can run sHM. The same for blockMesh, but earlier to create the background mesh.
It takes about 20-30 min for a 1.7M cell mesh to generate. (Assuming you don't have to change parameters in sHMD)
So in total it takes may be ~4h +- to generate a mesh including some breaks and interruptions ;)

Smoothening is actually done by sHM but the question is, do you like that?
compare here the user tutorial. the car geometry in the beginning has clear edges and
in the end it has a smooth surface. As I told you for me thats not an issue for I prefer smooth surfaces.

I hope I could help you
regards Colin

7islands October 27, 2010 11:57

Hi Guillaume,
Just to get you an idea, my colleague has done a superb job in meshing an urban area with sHM. In his presentation slides are slides 13-15 for geometry in STL and slides 18-20 for the meshes (about 15 million cells, ran sHM in 16 cpu parallel). The mesh in slide 18 may seem rugged but it's because the elevation was reproduced accurately from aerial laser scanned geographical data. They are staircased (not body-fitted, I heard that was because convergence was better), however in turn retain sharp edges.


Voyage_gui October 28, 2010 08:12

Thanks a lot ColinB and 7Islands

ColinB : half a day of work for meshing your case is the order of time that i would like to spend. I need now to spend some time on sHM.Thanks a lot for your answer.

7islands : I'm really impressed by your colleague's work. It's more or like what i'd like to achieve. I have several questions to ask him if you can transmit him (or by email?):

- from a clean CAD file (after post-processing of aerial laser scanned geographical data), how long did it take him to mesh the whole district ?

- Did it mesh with constant space interval in the whole domain ? Was it easy to refine closed to the building ?

- I've seen that he worked in RANS (RNG k-epsilon). Did it try to compute using LES (or DES) instead of RANS ?

- Does he have any papers regarding urban design ?

Thanks a lot for both of your answers

elvis October 29, 2010 02:46

Hi Guillaume

read this

what Eugene did not mention his former employer Icon also had an example of Terrain meshing in that presentation

kroetenechse October 29, 2010 05:25

Hi Guillaume,

canīt you get the CAD files as .stp-datas? For meshing, I use first netgen to mesh the surface, after that I use blender to work on the geometry, then give it to engrid to mesh the inner side. From engrid i can give it to OF as an OF case. Did hear something of this way?

hope, i could help you.

7islands October 29, 2010 12:20

Hi Guillaume,
I had a chat with him and he said he'll come to this thread to reply. However he's busy right now so in the meantime I'd give brief answers to your 2nd and 3rd points where I have info:


Originally Posted by Voyage_gui (Post 281172)
- Did it mesh with constant space interval in the whole domain ? Was it easy to refine closed to the building ?

The cell sizes of the meshes were 8m high above the ground and gradually refined to 1m around the buildings except for 25cm around the building of interest, which is shown as blued out volumes in the slides due to too dense mesh lines.


Originally Posted by Voyage_gui (Post 281172)
- I've seen that he worked in RANS (RNG k-epsilon). Did it try to compute using LES (or DES) instead of RANS ?

Yes he did. In slides 38 and 39 of his another presentation you can see the snapshots of the LES computation.


eugene November 2, 2010 04:56

Oh yes, I forgot about Chicago. However, the version of snappy that made that mesh is not freely available, so there is little point in pointing to it as an example.

Voyage_gui November 2, 2010 13:39

Hi everyone, sorry i was off for some time.

Elvin & Eugene : Thanks a lot for the presentation of your case in Chicago. It's exactly what i'd like to achieve with DES simulation. It is said page 8 that company Icon develop their in-house mesh-generator. How efficient this tool can be for the mesh cleaning ? How about the sharp edge, any problem with this tool? Is there some equivalient free tool that we heart about than we can save you time for the meshing process?

7islands : Thks for the LES presentation. Too bad i don't know japanese (by any chance same document in English?). Was he satisfied by the result of the DES simulation (any exp. measure to compare with) ?

Kroetenechse : Thks for sharing your methodology. Never heard about those CAD tools before. Are they free/easy to use ?


elvis November 3, 2010 16:48

i saw

maybe it is of use


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