Engrid modify shape of stl
 

Hi Engrid users,

I am modifying the edge lenght and cell grow factor of a stl in Engrid. My goal is to obtain more density of triangles in order to obtain more tetrahedral elements in volume mesh. But in some kinds of geometries, I see that Engrid modifies the shape of stl file containing the geometry a big problem for me because stl file contains the info of all geometry, it´s more than the surface mesh for me and if it changes, the geometry and the later CFD computations change), making even the crash of volume meshing (yellow triangles indicate inside part I guess).

How can I avoid this?I thought that when I changed surface mesh I was only introduing more triangles but never changing the original shape...What about smoothing?Or more Delaunay steps?

I attach some pictures for ilustrate the problem better.

Thanks for all your help!

Greetings James,

From the (little) experience I have with enGrid, the idea is that you have to properly separate STL surfaces into independent boundary codes, so that enGrid is more easily able to ascertain which surfaces should be preserved.
In other words, whenever you have an edge that has to be preserved, you need to keep each adjacent triangle in their respective boundary code.

The other detail is regarding the refinement options you use for the surface mesh, as explained here: https://github.com/enGits/engrid/wik...e-surface-mesh

Best regards,
Bruno

Hi Bruno,

Thanks for the reply.

The images attached in the previous post were taken from he wall of my geometry, so first of all I set boundary codes correctly (and I assure that they are well assigned), but with surface meshing this is the result, the stl changes (and all of this triangles belong to the same boundary code).

Any other idea? Maybe using "fix CAD geometry" option or "store geometry for surface meshing", but I am not sure how this tools work...

Oliver Gloth, please can you give some light over this issue? I guess you are the only one who can...:):)

Can I refine my surface mesh from .stl file without modifying the shape of my geometry? If not, how can I refine volume mesh without changing surface mesh?

Thanks in advance.

Hi James,

Can you provide a simple geometry example that we can use to create an example guide from it?

Best regards,
Bruno

Hi Bruno,

Sorry, I am not allowed to share geometries. My problem occurs when the geometry shows high sharp corners. The software remesh my stl file and breaks this corner in something like a shell, showing some triangles in green and some in yellow. I guess this make Engrid crash, because it is not able to understand wich triangles belongs to interior parts, and when I try the volume mesh it crashes ("Boundary meh is overlapping") and also my geometry has changed, which is not acceptable for the next CFD step.

Please help! S.O.S :D:D

Thanks Bruno for your interest. I am so sorry. I´d like to share geometry but I can´t!:(:(

Hi James,

We do not need a geometry that reveals the nature of your project. We only need a simple geometry that reflects the kind of situations you are having.

As for sharing such a simple example geometry, ask for permission, after fully explaining what it's for.

I ask this because as you can see from this tutorial: https://github.com/enGits/engrid/wik...TL-geometry%29 - sharp edges are handled just fine by enGrid, as long as you properly identify each major surface with a boundary code.

Best regards,
Bruno

Hello everyone,

how do you do this? I guess it is not possible in enGrid. Can I do this with Blender? Any hint is welcome!
Originally I thought I can preserve the edges by lowering the "edge angle to determine fixed vertices" in Tools/Configure enGrid/surface Meshing but this didn't work.

Kate

I'll try to answer my question myself now. According to https://github.com/enGits/engrid/wik...n-STL-geometry)
it should be possible to select a face by first pressing P and selecting Mesh/set boundary code afterwards. Somehow this didn't work for me.
I managed to do it with Blender instead:

1) Select a face in Edit mode
2) Select/Select Similar/Normal
3) Press P and rename the boundary code

Hi Kate,

Many thanks for sharing! I guess I was too late to at least tell you that in Blender you should be able to do that and any other number of manipulations to the geometry.

And sorry about the instructions on that wiki page being outdated; I handled the transfer from PDF to wiki format back in 2012, and only managed to do some minor updates to the document.
And since I don't have enough experience with Blender, I didn't manage to update any further the wiki page.

If you want to, you can login at Github, edit the wiki page and update the information provided in it.
If you cannot, then please let us know which Blender version you're using. That way I or anyone else can update the instructions on the wiki and indicate for which Blender version it is.

Best regards,
Bruno

Hi Bruno,

I would be glad to contribute, but as of now, I didn't find out how it is to be done with enGrid. I did it with Blender instead. Do I misunderstand you here? However, I am using Blender 2.74.

Best regards,

Kate

Hi Kate,

Most surface manipulations can only be done outside of enGrid. In enGrid is some capability to handle the "easy" situations, but complex surface mesh manipulation should be done in the applications used for this kind of manipulation.

OK, Blender 2.74. I'll try to make a note of this on the wiki in the next few days.

Best regards,
Bruno

Hi Kate,

I finally managed to look better into this... and now I've understood the confusion:

• You were referring to this wiki page: https://github.com/enGits/engrid/wik...TL-geometry%29
• But I was referring to this one: https://github.com/enGits/engrid/wik...and-enGrid-1.2

Which is why I was asking about the Blender version and the instructions were outdated... because the wiki page I was thinking about has instructions on how to use Blender... version 2.49b! Which was why I thought the instructions for Blender had changed...


OK, now that I've remembered about a few more details... the issue you had in enGrid would be more easily diagnosed if you had provided some images, to show the faces in question and the settings used in the dialogue box for defining a new boundary code.

But anyway, separating the surfaces in Blender is a lot easier and gives you the most control over the surface mesh :)


As for updating the wiki page... please have a look at the wiki page I mentioned, namely this one: https://github.com/enGits/engrid/wik...and-enGrid-1.2 - and please let me know if there was substantial changes in how to use Blender.
I don't suggest a full update to said wiki page, because it would take some considerable time and since the next generation of enGrid is in development, I guess updating the instructions for enGrid 1.4 wouldn't make much sense.

Best regards,
Bruno

Hello Bruno,

somehow I was referring to the wrong page. Sorry, my mistake.

The instructions for blender work with version 2.74 perfectly. I mentioned that on the tutorial and also added some instructions for installing the import and export scripta on OS X.

Best regards,

Kate