Creating waterproof STL using snappyHexMesh or salome
 

Dear all,

the last few years I experienced the necessity of a good surface triangulation while using snappyHexMesh. Most people are using common CAD software packages and export single surfaces as STL files. These STL files have two big problems, if they represent a closed volume:

• If the whole model is exported, the volume is closed, but the triangulation is very bad
• If you export the model in several steps (several STL's), the connected edges share not the same points (never be waterproof; see image below)
• Crazy never ending triangles that go to infinity small areas (image below)

Attachment 47017

Both (and especially the second one) influences the mesh generation in a very bad way. The good thing is that in some case it does not matter ;). The bad thing is, that you could really get unexpected results (I made a lot of tests with my collegue Dr. Alexander Vakhrushev). Some unexpected results were:

• regionRefinement refined cells that were far away from the region (inside mode); it seems that it propagates through the never ending triangle of the surface
• snapping problems
• due to snap problems the layer generation was influenced too
• other unexpeced behavior with refinement, snapping and zones
• Finally it could happend that snappyHexMesh can not recognize which fluid domain should be used (no cells will be removed, this happen due to gaps)

To avoid this I highly recommend to use a) a nice triangulated surface for each patch and b) a water proofed STL if it represent a closed surface (that means the shared edges have to be identical for each STL). For a) + b) you could use blender and/or salome. Both software packages offer a lot of features but for engineers I think salome is the first choice. This would be the best solution in my opinion because you end up with the exact surface, nice triangulation and a waterproofed STL. This is not always a simple task but could be handled.

Not water proofed STL have a further bad behavior in the point of feature edges. STL's (or region STL) that share share a curvatured edge will end up with two feature edges due to the fact that the STL's share not the same points (and therefore not the same edge). After creating the feature edges, you will extract both feature edges. Now ask yourself, which line should be used? (Here one comment, please check your featureEdge mesh using paraview and correct it if its wrong with blender).

• Since I use well triangulated STL's that share the same points at the interface, I always end up with the expected results while using snappyHexMesh and more over, the result is better.

If you do not want to build the closed nice triangulated STL using Salome or Blender, you can use snappyHexMesh instead. The result is not as accurate as the triangulation using Salome but finally you will end up with a waterproofed STL. To do that, you just have to use your non-closed STL and mesh it with snappyHexMesh as accurate as you can (note, that here you can get really problems). The important thing is not the mesh quality - it is the surface accuracy (all edges should be correct projected etc.). Maybe you also can unset all quality criteria for that procedure because the internal mesh is not needed. If you have the snapped mesh, you just have to use the following command:
The nice thing is, that all regions that you build before will stay in the new STL. The bad thing is, that the snapping could fail in some parts and therefore the new STL stores the failed surface. The good thing is, that you will end up with a 100% waterproofed STL and you can refine the surfaces and gaps as much as you want now without moving into a danger zone (cell size is small enough that snappyHexMesh will realize the gaps). One example is shown above.

Attachment 47016

The procedure is as follow (short form summarized and clear):

• Using non-closed STL and mesh it as good as possible (castellated + snap)
• Extract the surface with the command above
• Use the new STL for the further proceeding

Keep in mind that the STL you will never be as accurate as you will make it with salome but you will definitly get a waterproofed STL.


My personal suggestions are:

• using Salome and Blender for triangulation of the surface
• always check the featureEdge mesh that you extract

If you make these steps, you will see that meshing gets easier. Some examples of triangulated surfaces can be found in my tutorials.


If you have to mesh a lot of different designs, I would prefer the generation of the STL using snappyHexMesh. You also can implement this way into a optimization loop.
So good luck and keep foaming, hope this will help somebody.

So far freecad 0.16 seems to generate "waterproof " stl

Non -closed
 

Hi Tobi,

Can you tell me how to create a closed stl file when we combine individual surfaces (stl file) manually after labeling them ?

I always get "surface is not closed" when i do a surface check for the combined stl file but it does not happen for the stl file of the part. Can you tell me what i am doing wrong ?? I have attached the files

PS: I used salome for creating the solid which i exploded into individual surfaces.

Dear Vignesh,

if you have several surfaces in Salome you are not allowed to export these surfaces in the geometry module. You have to mesh the surfaces in the mesh module to make a homogeneous discretization on the lines. If you export these STL files then you end up with a waterproofed STL. That is the way I prefer for all STL's.

To check it, just load your exported STL into paraview. You will see that the contact lines will not share the same triangle points.

Dear Tobi,

Thanks for the quick reply. I tried as you suggested, still i get "surfaces is not closed"
:confused:. By the way when i view the combined stl file in paraview the contact lines share same triangle points !!

Hi,

if you check our STL it is not closed. Each surface has different points on the interface. The interface does not share the points of both surfaces. See the picture (for you it should be obvious). Check out my STL's in my tutorials and you see that the interface (the line that is shared by both) has the same points on both surfaces.

Hi Tobi,

I understood the mistake that i made during meshing in salome. Thanks for pointing it out.

Now its working :D

Hey Tobias,

I tried to reproduce your workflow:

-stl generation in salome
-created groups
-created edges of the groups
-change to mesh mode
-triangulated the geometry by meshing
-made mesh groups of the patches

BUT... when I combine the patches in in big file, I still get unconnected surfaces...
What point do I miss?
I want the stl file as input for cfMesh!

Thanks a lot for your advice!

Best regards,

Sebastian
At this point, I

Hi Sebastian,

Did you keep the edge length same for all faces (groups) while meshing ?

Hey Vignesh,

Jap, I did not change any settings for the meshing!

How is your workflo9w? Which geometry parts do you use?
I am not sure if I have to use the whole geometry to mesh (how do i capture feature edges) or if a mesh the grouped geometry parts on their own with the same settings?

Just from viewing the stl in paraview, it was fine. The triangles in the areas where the patches touch each looked quite nice...

Thanks again...

Best regards,

Sebastian

Hi Sebastian,

My workflow in salome is

• Generate the cad geometry
• Explode them to faces, group them if needed
• Mesh each face/group with same mesh size
• export each face/group individually in STL format
• Then modify each exported stl file (solid -> solid outlet, endsolid -> endsolid outlet) and combine them into one single file
• perform surfaceCheck to check whether the combined stl file is closed.

So far i am working on V-groove. When you are meshing each surface make sure you take the same mesh size (do it manually), by default salome may take values which might not be the same for all faces.


Hope this helps you

Thanks a lot for your fast replay!
I will give it a try on my geometry and report the result...

All good (for the moment :D )
Thank you so much!

Hi vigneshTG,
I'm using meshLab to convert text file to stl format. After that, I've used surfaceCheck to check my geometry. End of surfaceCheck, surface is not closed errors are shown no matter how I try to fix with admesh or meshLab. How should I fix that problem? Please advice me!! thanks

Then use salome to create your STL.
If you have single STL's it will not work.

Dear Tobi,
Thanks for your reply and suggest.
Can I use salome even my terrain is topography?

Dear Tobi,

I was having problems for days and now that actually hits the spot. I will try Salome right quick, thanks.

Utkan

The salome user interface is a little clunky, and for anything complex or repetitive you will need the script interface. This script interface can also help in understanding the work flow needed. In the attached salome python script I have created the stl files and UNV file for the snappyhexmesh tutorial for Chtmultiregion.
In this script you can see:

• the creation of the geometry of the regions
• the partitioning of the background box
• creation of patches and wall groups
• the creation of the region meshes
• creation of the stl files

I do not agree with your statement. I am using Salomes GUI for really high complex geometries and till now I could do anything without the python interface. Of course you can get a lot of information out of the python script and it allows to use it for optimization but the necessity of python for complex geometry - no, I cannot agree.

Sent from my HTC One mini using CFD Online Forum mobile app

Everything is relative... my current work involves over 700 basic shapes