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pipolaki November 29, 2012 06:28

Problem when I try to merge hexa & prism
Good morning,

I want to build an hybrid mesh and I need to merge the nodes at the interface. The geometry composed with :
- an external environement in hexa ;
- a swirl injector in tetra + its boundary layer in prism.

I can merge the two volumes if the swirl injector is purelly made with tetra.
But when I try to merge the two volumes with the swirl injector composed with tetra and prism, I obtain the message "unable to remove prims node xxxx" and then ICEM is blocked.

Do you think if it's possible to merge hexa with prism ?


diamondx November 29, 2012 12:13

that might not be possible. because prism is the result of height that you defined. hexa too. merging mesh only works between hexa and tetra (and not hexa/hexa), so it can be free
to change tetra and adapt it to hexa. In your case, you are asking him to change prism height... That is again based of how i understand the mecanism of prism and merging.
Someone can correct me if i'm wrong...

pipolaki November 29, 2012 15:31

Before reading the answer of diamondx (thank you by the way), I found a "solution" to my problem.
Step 1 : I create the mesh of the swirl injector only with tetra.
Step 2 : I smooth it in order to have a correct quality.
Step 3 : I create the mesh of the external area with hexa.
Step 4 : I merge the two meshes.
Step 5 : I create the layer of prism in the swirl injector.

By this way, I obtained a layer of prism inside my tetra mesh which is correctly connected to the hexa mesh. I guess this is well known by the experienced users but if needed, I can develop this method and I could show you some images next monday.

Now, the problem is that, even if my initial tetra mesh is quiet correct, the prism layer creates a lots of bad elements I didn't have before (quality < 0.2, Min angle < 9...) Even with different smooth operations, it's impossible to obtain the quality I've had before adding the prism layer. It's very frustrating because I'm feeling I'm really close from the definitive mesh...
I tried a first series of smoothing operations with prism freezed.
Then I released the constraint on the prism and continued with the smoothing but I still have very bad elements. It looks like the smoothing operations were useless. It's strange.

Do you have any advice regarding the smooth process of such mesh in such geometry ?

BrolY November 30, 2012 03:48

Quality of 0.01 for prism is good enough.
Check other threads about it in this forum.

pipolaki November 30, 2012 06:03


Originally Posted by BrolY (Post 394973)
Quality of 0.01 for prism is good enough.
Check other threads about it in this forum.

In fact, the problem is not the quality of the prism, it's the quality of some tetra elements.

PSYMN November 30, 2012 11:49

Correct, you can only merge meshes if one side is all tetra... It needs the flexibility of the tetra mesh make it work. Prisms are too restrictive. We are working on ways to improve this situation.

Prism quality looks worse than it is... No worries there either.

As for the poor tetras, run the smoother with the prisms and hexas frozen... see what it can do. It should fix most tetras. The tetras it can't fix are those that are stuck between close layers of prisms...

In that case, you may want to free up the prisms to be smoothed, but greatly reduce the "upto quality" number. So instead of trying to smooth just the tetras with an upto of 0.6, turn on prism smoothing and set the upto down to 0.1...

If the problem is just that your prism layers get too close together and there isn't room for the smoother to get any improvement, you could try regenerating the prisms with auto-reduction (an advanced option) on, or adjust the height or number of layers in that area.

Best regards,


pipolaki December 1, 2012 06:24

Thanks for your answer Simon.
I will try your solution next Tuesday and I will inform you about the results asap.
Have a good week end.

pipolaki December 4, 2012 09:44

I followed the advices of Simon and here are the results for the quality of my mesh. It helps me to improve it. I have to mention that I also modified node by node some elements (Edit Mesh -> Move Node) in order to obtain these histogram. What's your opinion ?
I think the worst parameter is the MIN ANGLE because I still have a lots of element between 9 and 18...

On these quality histograms I mentionned the results with or without the surface elements. As you can see, some surface elements have a bad influence on some histograms but I think that surface element are irrelevant in the analysis of the mesh quality of a 3D mesh. Am I wrong ?

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