[giorgianig]

Dear all,

I am modeling a network of pipes, here is a simplified geometry that contains the issue:


I would like to mesh the three straight pipes with structured hexahedra and the Ts with unstructured thetrahedra. I have make a partition with 10 solids (3 Ts and 7 straight pipes) :


I start meshing using submeshes from the central T with Netgen 3D-2D-1D, applying boundary layers on walls, and propogating on pipes with 3D extrusion, everything seems to go smoothly

There is the boundary layer on the walls as wanted

However, when I try to export to OpenFoam (I use salomeToOpenFOAM.py because there are piramids in the mesh), the export fails

and some defaultPatches are generated in the mesh tree:

I think those defaultPatches are generating the export error, and I think they are generated because the mesh is not conformal. I visualized these patches, here is where they are located:

and here is the shape

So those patches are genereated at the interface pipe/T when the T mesh is generated AFTER the pipe mesh. Using a clip also it is clear the mesh is not conformal.

Before the T


After the T:


Does anyone know how to fix it and generate a valid mesh?
Thanks in advance

[Alczem]

Hey, me again


I tried to mesh several junctions the way you tried, and failed too, it seems impossible to create an extrusion with boundary layers between two junctions.


After seeing the pictures you attached, I think you should give cfMesh a try. It can generate nice boundary layers everywhere, and you can apply a nonuniform stretching to cells in the straight pipes to lower the cell count.

[giorgianig]

Originally Posted by Alczem

Hey, me again


I tried to mesh several junctions the way you tried, and failed too, it seems impossible to create an extrusion with boundary layers between two junctions.


After seeing the pictures you attached, I think you should give cfMesh a try. It can generate nice boundary layers everywhere, and you can apply a nonuniform stretching to cells in the straight pipes to lower the cell count.

Hey Alczem, thanks again
This problem is really giving me headhaches!
I'll try out cfMesh right away

[GerhardHolzinger]

You could try to separate your pipe geometry into two distinct entities: the pipe interior and the wall layer. In my experience, it is easier to generate matching meshes, when using tet and extrusion meshing, if you do not bother with boundary layers in the first step.


So, instead of meshing the T-sections using a tet meshing algorithm with a boundary layer hypothesis; you would mesh the interior of the T-section using tets, and then mesh the boundary layer region using an extrusion algorithm.


This increases the complexity of your geometry and the meshing workflow, but in my experience it is not possible to "inform" a boundary layer hypothesis from the outside.



I hope this makes somewhat sense.


Salome is very good at making meshes in a bottom-up approach. Hence, if you separate meshing of the interior and the creation of the boundary layers, you will be able to achieve nice meshes.

[giorgianig]

Originally Posted by GerhardHolzinger

You could try to separate your pipe geometry into two distinct entities: the pipe interior and the wall layer. In my experience, it is easier to generate matching meshes, when using tet and extrusion meshing, if you do not bother with boundary layers in the first step.


So, instead of meshing the T-sections using a tet meshing algorithm with a boundary layer hypothesis; you would mesh the interior of the T-section using tets, and then mesh the boundary layer region using an extrusion algorithm.


This increases the complexity of your geometry and the meshing workflow, but in my experience it is not possible to "inform" a boundary layer hypothesis from the outside.



I hope this makes somewhat sense.


Salome is very good at making meshes in a bottom-up approach. Hence, if you separate meshing of the interior and the creation of the boundary layers, you will be able to achieve nice meshes.

Thanks for your reply. It seems unnecessarily complicated. I don't even know how to extract the inner geometry (inside the boundary layer) at the intersection between the pipes.

I mean, this is a trivial geometry, it's unbelievable it cannot be done with a classic procedure...

[linnemann]

If you dont mind sharing the Geometry (BREP) I can give it a quick go

[giorgianig]

Originally Posted by linnemann

If you dont mind sharing the Geometry (BREP) I can give it a quick go

Sure, here it is.

https://www.dropbox.com/scl/fi/una1c...kpspxm472x52fd

[linnemann]

Hi


Here is the results after about 30min.
The trick is to mesh the Tee first, parallel project the inletFace of the Tee to the internalFace of the Tee.
Project each of the Mesh faces of the Tee to their own face from the geo.
Use the projected mesh to make an extrusion of the face into 3D along a 1D line Mesh.
Make a mesh compund of the Tee and the Vertical Mesh.
Copy that 2 times, copy the internal pipe 1 time and mirror the inlet mesh around the center.
Make a Compund mesh of all these meshes and that will automatically merge internal nodes/faces if they lie on top of each other.


I have provided a link to the Study and the resulting UNV file for you to look at.
checkMesh shows nice values.


https://drive.google.com/file/d/1Ddo...ew?usp=sharing

Checking geometry... 
    Overall domain bounding box (-300 -10 -10) (300 10 300) 
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1) 
    Mesh has 3 solution (non-empty) directions (1 1 1) 
    Boundary openness (-6.36013e-17 1.32379e-17 -3.5847e-17) OK. 
    Max cell openness = 3.46929e-16 OK. 
    Max aspect ratio = 42.7402 OK. 
    Minimum face area = 0.19949. Maximum face area = 31.8451.  Face area magnitudes OK. 
    Min volume = 0.141434. Max volume = 29.4749.  Total volume = 460177.  Cell volumes OK. 
    Mesh non-orthogonality Max: 60.3271 average: 8.61917 
    Non-orthogonality check OK. 
    Face pyramids OK. 
    Max skewness = 2.06016 OK. 
    Coupled point location match (average 0) OK.

[giorgianig]

Originally Posted by linnemann

Hi


Here is the results after about 30min.
The trick is to mesh the Tee first, parallel project the inletFace of the Tee to the internalFace of the Tee.
Project each of the Mesh faces of the Tee to their own face from the geo.
Use the projected mesh to make an extrusion of the face into 3D along a 1D line Mesh.
Make a mesh compund of the Tee and the Vertical Mesh.
Copy that 2 times, copy the internal pipe 1 time and mirror the inlet mesh around the center.
Make a Compund mesh of all these meshes and that will automatically merge internal nodes/faces if they lie on top of each other.


I have provided a link to the Study and the resulting UNV file for you to look at.
checkMesh shows nice values.


https://drive.google.com/file/d/1Ddo...ew?usp=sharing

Checking geometry... 
    Overall domain bounding box (-300 -10 -10) (300 10 300) 
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1) 
    Mesh has 3 solution (non-empty) directions (1 1 1) 
    Boundary openness (-6.36013e-17 1.32379e-17 -3.5847e-17) OK. 
    Max cell openness = 3.46929e-16 OK. 
    Max aspect ratio = 42.7402 OK. 
    Minimum face area = 0.19949. Maximum face area = 31.8451.  Face area magnitudes OK. 
    Min volume = 0.141434. Max volume = 29.4749.  Total volume = 460177.  Cell volumes OK. 
    Mesh non-orthogonality Max: 60.3271 average: 8.61917 
    Non-orthogonality check OK. 
    Face pyramids OK. 
    Max skewness = 2.06016 OK. 
    Coupled point location match (average 0) OK.

Dear linnemann,

Thank you for the explication and the link Indeed, your solution is very interesting, but it does not apply to my real case. In fact, this is a simplified geometry, and in your procedure you exploited the symmetry of it. In my real case I can't do that.



In my case, I cannot avoid to generate the mesh of a T imposing the mesh on one (or more) of the external faces connected to the pipes. In your procedure, you meshed one T (without constraints), and copied that mesh. This cannot be done in a real case where every T is different from the other.

However, seeing what you did, I think I understood some key ingredients I was missing in Salome. Maybe I can mesh a T imposing the mesh on the faces using the 1D projections as you did in this simplified case?

[giorgianig]

Originally Posted by linnemann

Hi


Here is the results after about 30min.
The trick is to mesh the Tee first, parallel project the inletFace of the Tee to the internalFace of the Tee.
Project each of the Mesh faces of the Tee to their own face from the geo.
Use the projected mesh to make an extrusion of the face into 3D along a 1D line Mesh.
Make a mesh compund of the Tee and the Vertical Mesh.
Copy that 2 times, copy the internal pipe 1 time and mirror the inlet mesh around the center.
Make a Compund mesh of all these meshes and that will automatically merge internal nodes/faces if they lie on top of each other.


I have provided a link to the Study and the resulting UNV file for you to look at.
checkMesh shows nice values.


https://drive.google.com/file/d/1Ddo...ew?usp=sharing

Checking geometry... 
    Overall domain bounding box (-300 -10 -10) (300 10 300) 
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1) 
    Mesh has 3 solution (non-empty) directions (1 1 1) 
    Boundary openness (-6.36013e-17 1.32379e-17 -3.5847e-17) OK. 
    Max cell openness = 3.46929e-16 OK. 
    Max aspect ratio = 42.7402 OK. 
    Minimum face area = 0.19949. Maximum face area = 31.8451.  Face area magnitudes OK. 
    Min volume = 0.141434. Max volume = 29.4749.  Total volume = 460177.  Cell volumes OK. 
    Mesh non-orthogonality Max: 60.3271 average: 8.61917 
    Non-orthogonality check OK. 
    Face pyramids OK. 
    Max skewness = 2.06016 OK. 
    Coupled point location match (average 0) OK.

There is something I don't understand in your procedure: when I display the mesh called InletMesh it shows a 3D mesh, but when I try to edit I can only see a 2D algorithm, as you meshed only the face on the base:



How is it done exactly this 3d extrusion?

Thanks in advance

[linnemann]

Sure, attached are the steps.


Regarding your real case.
You can do the same with "Extrusion along path" and the projecting the faces where they are in-line.
The face meshes need to match up in order for the merging of nodes on the compound mesh.
So its doable, just takes a bit of time.

[giorgianig]

Originally Posted by linnemann

Sure, attached are the steps.


Regarding your real case.
You can do the same with "Extrusion along path" and the projecting the faces where they are in-line.
The face meshes need to match up in order for the merging of nodes on the compound mesh.
So its doable, just takes a bit of time.

Well, I am not sure. Every time I try, it fails. The problem resides on the fact that the Netgen 3D algorithms seems incompatible with the imposition of a face mesh. I am able to propagate the mesh from an unstructured to a structured (3d extrusion) one, but not the other way around.

[giorgianig]

Originally Posted by linnemann

Sure, attached are the steps.


Regarding your real case.
You can do the same with "Extrusion along path" and the projecting the faces where they are in-line.
The face meshes need to match up in order for the merging of nodes on the compound mesh.
So its doable, just takes a bit of time.

See this example: I am extruding a mesh in a pipe, I get this



Now I need to mesh the next connected part (this cone):



I projet the 2D mesh from the pipe to the upper face of the cone



this goes well:



Now I want to mesh the cone with an unstructured mesh, keeping the mesh of the face on the top. Here I am not sure what algorithm to use.. If I use Netgen 3D-2D-1D, the final mesh looks like this



See what I mean? The mesh of the top face has changed! Why is it doing that is beyond my comprehension.

If I just mesh using Net 3D-2D-1D WITHOUT projecting the face mesh before, the mesh on the top face looks kind of OK, but the nodes are not matching (which is understandable since the two meshes are completely independent)

[linnemann]

Hi


You are correct, doing it this way for you case breaks the face mesh when it is projected when it has boundary layers. Without BL it works fine.


There is a way to do it using the "Extrusion 3D" meshing strategy.


Here is a link to the case and the checkMesh output.


https://drive.google.com/file/d/1De3...ew?usp=sharing

Checking geometry... 
    Overall domain bounding box (0 -49.9013 -49.6057) (700 49.9013 50) 
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1) 
    Mesh has 3 solution (non-empty) directions (1 1 1) 
    Boundary openness (7.93909e-17 3.36127e-18 -1.20437e-17) OK. 
    Max cell openness = 7.47279e-16 OK. 
    Max aspect ratio = 46.4566 OK. 
    Minimum face area = 1.33659. Maximum face area = 316.204.  Face area magnitudes OK. 
    Min volume = 8.42164. Max volume = 973.656.  Total volume = 3.36768e+06.  Cell volumes OK. 
    Mesh non-orthogonality Max: 19.7951 average: 7.36319 
    Non-orthogonality check OK. 
    Face pyramids OK. 
    Max skewness = 0.315532 OK. 
    Coupled point location match (average 0) OK. 

Mesh OK.

[giorgianig]

Originally Posted by linnemann

Hi


You are correct, doing it this way for you case breaks the face mesh when it is projected when it has boundary layers. Without BL it works fine.


There is a way to do it using the "Extrusion 3D" meshing strategy.


Here is a link to the case and the checkMesh output.


https://drive.google.com/file/d/1De3...ew?usp=sharing

Checking geometry... 
    Overall domain bounding box (0 -49.9013 -49.6057) (700 49.9013 50) 
    Mesh has 3 geometric (non-empty/wedge) directions (1 1 1) 
    Mesh has 3 solution (non-empty) directions (1 1 1) 
    Boundary openness (7.93909e-17 3.36127e-18 -1.20437e-17) OK. 
    Max cell openness = 7.47279e-16 OK. 
    Max aspect ratio = 46.4566 OK. 
    Minimum face area = 1.33659. Maximum face area = 316.204.  Face area magnitudes OK. 
    Min volume = 8.42164. Max volume = 973.656.  Total volume = 3.36768e+06.  Cell volumes OK. 
    Mesh non-orthogonality Max: 19.7951 average: 7.36319 
    Non-orthogonality check OK. 
    Face pyramids OK. 
    Max skewness = 0.315532 OK. 
    Coupled point location match (average 0) OK. 

Mesh OK.

Yes of course, but that was just an example. What I need is an unstructured mesh. What do you do if you have a T instead of a cone?

[linnemann]

Originally Posted by giorgianig

Yes of course, but that was just an example. What I need is an unstructured mesh. What do you do if you have a T instead of a cone?

I see your problem now. If I make a Tee after the cone, the project face trick does not work anymore as the BL will cause issues in that instance.

So the best approach IMO is either to make the connection between the parts with AMI and live with the small interpolation errors introduced.
It can be done in Salome, but it will be tedious and error prone.

Or, you do as previously suggested and use cfMesh and live with the higher cellcount. This would be my prefered method.
You will loose some time in solving, but you will gain it back manyfold by not having to deal with the mesh.

Do you want to share the complex shape in post #9? I can give it a quick cfMesh go.

[giorgianig]

Originally Posted by linnemann

I see your problem now. If I make a Tee after the cone, the project face trick does not work anymore as the BL will cause issues in that instance.

So the best approach IMO is either to make the connection between the parts with AMI and live with the small interpolation errors introduced.
It can be done in Salome, but it will be tedious and error prone.

Or, you do as previously suggested and use cfMesh and live with the higher cellcount. This would be my prefered method.
You will loose some time in solving, but you will gain it back manyfold by not having to deal with the mesh.

Do you want to share the complex shape in post #9? I can give it a quick cfMesh go.

Thank you Niels. Yes, here is the geometry:

https://www.dropbox.com/scl/fi/b7zpl...i9k426iwzt83gf

I have already tried cfMesh. Without anisotropic refinement, it produces a good mesh with about 500K elements. I believe this case could be done with a good resolution with less then 200K. Unfortunately, I think there is a bug in the anisotropic refinement in cfmesh. I posted the issue on a simplified case here:

Anisotropic refinement on box not working

and here

Anisotropic refinement fails

If you could take a look to it and tell me what you think it would be great.

Your help is really appreciated.
Giorgio

[linnemann]

Originally Posted by giorgianig

Thank you Niels. Yes, here is the geometry:

https://www.dropbox.com/scl/fi/b7zpl...i9k426iwzt83gf

I have already tried cfMesh. Without anisotropic refinement, it produces a good mesh with about 500K elements. I believe this case could be done with a good resolution with less then 200K. Unfortunately, I think there is a bug in the anisotropic refinement in cfmesh. I posted the issue on a simplified case here:

Anisotropic refinement on box not working

and here

Anisotropic refinement fails

If you could take a look to it and tell me what you think it would be great.

Your help is really appreciated.
Giorgio

I honestly dont understand why you would want to deal with anisotropic elements here. I have 186k cells in 19s using a 6core virtual machine.
Its more hasle than its worth. I mean 19s for meshing!!


Regarding Paid vs Open Source, please keep in mind that there is no such thing as free beer.

I have used open source professionally since 2010.

The companies I have worked for/with have contributed back with either knowledge sharing or sponsoring to implement the features missing.

The cost for implementing something in an open source tool is a one-time cost vs. a yearly subscription/license fee for commercial.

[giorgianig]

Originally Posted by linnemann

I honestly dont understand why you would want to deal with anisotropic elements here. I have 186k cells in 19s using a 6core virtual machine.
Its more hasle than its worth. I mean 19s for meshing!!


Regarding Paid vs Open Source, please keep in mind that there is no such thing as free beer.

I have used open source professionally since 2010.

The companies I have worked for/with have contributed back with either knowledge sharing or sponsoring to implement the features missing.

The cost for implementing something in an open source tool is a one-time cost vs. a yearly subscription/license fee for commercial.

Hello Niels, could you share the meshDict file please? I think I remember with my options I got 500K elements.

Well it works, no doubt, I can use it. Nonetheless, the bug in anisotropic meshing is there. I think stretching the elements along the pipe would be useful, but I don't care discussing about it.

I am not trying to drink beer for free here . Just inquiring opportunities.
Obviously, if we open this activity with free softwares, my work will feed back the software I would use.

[linnemann]

Sure, here you go.


To prepare the geometry I scaled the geometry to mm first then export to stl as in the attached image.


Ran these commands.

cd STL
./renameSTL.sh
cd ..
surfaceToFMS STL/joined.stl
surfaceFeatureEdges -angle 34 STL/joined.fms STL/joined2.fms
cartesianMesh

The "renameSTL.sh" is just a little utility to rename the boundary to the stl filename and join into one file called "joined.stl".

Makes more sense when you have many STL files.

This is just my normal workflow.