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Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 19. 2017

I want to generate a mesh for an FSI problem. Fluid geometry is defined from an STL file. This is highly unstructured. Solid geometry is a box enclosing the fluid geometry. I have created the combined geometry in SALOME using "partition" method. I am able to generate a 3D tetrahedral mesh but the number of volume elements is very high. I want the mesh to coarsen as we move away from the fluid geometry. 

Please suggest me a way to create this type of mesh. I have uploaded the image of the fluid geometry. 

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Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 19. 2017
These are the links for my working files:
 
 
Currently generated mesh is very fine, but I want a coarse mesh when we move away from the fluid zone (shown as white in the image). Fluid zome may appear just as a line in the image but actually, it's a region with very small thickness. Because of this small thickness netgen takes this thickness as minimum element size and generates mesh using that. 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at December 19. 2017

Hi

If you use MEFISTO as 2D algorithm, try NETGEN 2D with Growth Rate = 0.7 instead.

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 20. 2017

I tried to use NETGEN 2D, but the option to increase the growth rate is disabled and fixed to 0.3

I have added a screenshot for that https://www.dropbox.com/s/kht6779yyu51d6q/netgen2D_growthrate.PNG?dl=0

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 21. 2017

Hi Michael,

To give you a better idea about my meshing problem, Please check the following document. 

https://www.dropbox.com/s/3na1ki6qvamvg4r/Meshing%20Problem.pdf?dl=0 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at December 21. 2017

Growth Rate gets modifiable if you set Fineness=Custom. Growth Rate =0.7 if Fineness=Very Coarse.

St.Michael

 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 21. 2017

I had tried fineness = very coarse, but that also doesn't help. And now I have realized that it would be better if I can generate mostly hexahedral cells. Can you please suggest some ways or some tutorial case to create a hexahedral mesh for complex geometries. 

https://www.dropbox.com/sh/03t8jxw9y7b7qao/AACcxGGPUAH6_xvJWG_aw2fUa?dl=0 
This folder contains complex2.hdf file which has final geometry which I want to mesh. Can you please check if it is possible to mesh it with mainly hexahedral cells. 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at December 21. 2017

Here is what I got. I give you more details later.

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 21. 2017

Thanks. This looks very close to what I wanted. Please provide me the hdf file of your case, so that I can learn the methods used to generate this type of mesh. 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at December 22. 2017

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 22. 2017

Thanks for the file but I am not able to open the mesh module for the given file, it is showing
" FATAL ERROR: No persistent file" (as shown in the attached image)
 I am using version 8.3.0 

Attachments

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 24. 2017

I think that is a version problem (screenshot attached). But I have downloaded the latest version from the website. Can you send me the mesh created by you in the version 8.3.0 or please send me the link to download the same Salome version which you are using. 

Or please explain me the methods used in creating the mesh. 

Attachments

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at December 25. 2017

 I uploaded a study created in 8.3.0:

https://www.dropbox.com/s/4dim8t12ptctmwu/complex2.hdf?dl=0

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 26. 2017

Thanks Michael, I understood the method of meshing by 3D extrusion. 

But in some cases, 3D meshing algorithm doesn't show 3D extrusion. Actually, I need to create conforming mesh around the given surface, the previous geometry I sent you was a solid inside a box. But actually, it's a surface inside a box.  

When I use the partition (box and the given surface), then while meshing the partition geometry 3D extrusion method is not shown. I need the mesh at the crack surface to be exactly similar to edges in the geometry. And around the crack faces mesh should be hexahedral. 

I have uploaded my case here https://www.dropbox.com/sh/3n4cuawh6o7itm0/AACYEKix8_fNmV4q-g2sRUcza?dl=0 

please check if it is possible to create a hexahedral mesh around the given crack surface. 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at December 26. 2017

You can compute the mesh only on a base side of your domain and then extrude it using Modification > Extrusion.

https://www.dropbox.com/s/y2tdrmkyfsux6m2/crackMesh.hdf?dl=0

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at December 27. 2017

Thank you very much. Creating mesh for base and extruding it works for me.

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at January 12. 2018

Hi Saint Michael,

Extruding the base mesh is good for creating 2D cases. But now I am facing problems for cases where the input geometry has layers in vertical directions. Please refer to the following picture, I have hidden the front and back faces of the box to show the interior.  https://www.dropbox.com/s/7yxku68r3a3uen6/Geometry.PNG?dl=0

I have tried creating sub-mesh for the crackFaces and then using extrusion 3D method for the main mesh. But this shows the following error:

" Invalid input mesh. More than 2 faces with not quadrangle elements:" 

Can you please suggest some other method to create a mesh so that the crack faces are retained in the final mesh. I have attached my case file, geometry in the following folder. 
https://www.dropbox.com/sh/cf6d6gtffq49lfu/AAAmS-J9xGwwVn5o_f8xxyZTa?dl=0

Thanks a lot in advance.

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at January 12. 2018

Hi

In this particular case you can create a hexahedral domain separately from the mesh of crack faces, assuring that nodes of the domain and corresponding nodes of the crack mesh coincide. And then join them into one mesh using Mesh > Build Compound.

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at January 12. 2018

How can I create a hexahedral domain separately from the mesh of crack faces?

Previously Saint Michael wrote:

Hi

In this particular case you can create a hexahedral domain separately from the mesh of crack faces, assuring that nodes of the domain and corresponding nodes of the crack mesh coincide. And then join them into one mesh using Mesh > Build Compound.

St.Michael

 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at January 12. 2018

Either create a box and mesh it with hexahedra, or create it using extrusion http://docs.salome-platform.org/latest/gui/SMESH/extrusion_page.html#extrusion_struct

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at January 12. 2018
Sorry I could not understand the part of creating a box around crack faces. I have already created a box.
 
And for the extrusion method do I need to mesh different planes of crackFace separately and extrude them separately?

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at January 12. 2018

Sorry I could not understand the part of creating a box around crack faces. I have already created a box.

Two problems with your box:

1) You partitioned it with the crack faces. Due to which you can't apply Hexahedron(ijk) algorithm to it. You could create a Compound instead of Partition.

2) You translated the box so that if meshed with hexahedra (at least with the density as in your frac3D study), its nodes won't coincide with nodes of the crack faces mesh.

Here is what I meant https://www.dropbox.com/s/j4v0qolxgedi2q3/frac3D.hdf?dl=0

St.Michael

 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at January 12. 2018

Thanks, I understood the problems with the box. This works for a simple case. 

But I may require meshing the cases where these crack faces may be aligned at random angle (other than 90 degrees). Actually, I don't need hexahedral mesh, I just want these crack faces to be retained as it is in the final mesh. Can you please suggest some general method to achieve this type of mesh. 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at January 12. 2018

A general case 3D mesher is NETGEN. I tried NETGEN-1d2d3d on your geometry and got a tetrahedral mesh with internal crack faces kept.

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at January 12. 2018

Yes, I also got that. But in that case crackFaces were also meshed and converted to triangles. Is there some way to keep the crackFaces as rectangle.

Previously Saint Michael wrote:

A general case 3D mesher is NETGEN. I tried NETGEN-1d2d3d on your geometry and got a tetrahedral mesh with internal crack faces kept.

St.Michael

 

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at January 12. 2018

A usual way is to create a sub-mesh on crack faces. I tried it. In this case the mesh is incorrect and internal faces are not taken into account. It seems to be a bug in NETGEN plug-in.

St.Michael

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Ashish Kumar at Monday 08:29

Is there any method other than NETGEN?

Re: Create non-uniform tetrahedral 3D meshes around the given surface

Posted by Saint Michael at Monday 14:03

There is a commercial mesher MG-Tetra.

St.Michael

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