# blockMesh-how to use prism

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 August 29, 2013, 08:34 blockMesh-how to use prism #1 Member   xuhe-openfoam Join Date: Aug 2013 Location: DaLian，china Posts: 82 Rep Power: 5 Hi, I am a beginner. I know how to use hex in blockMeshDict because user guide has told us. for example: hex (0 1 2 3 4 5 6 7) (20 20 1) simpleGrading (1 1 1) But I wonder how to use prism in blockMeshDict, as user guide doesn't tell us! I don't know where to find how to use "prism" . Could you help me? Thank you!

 September 1, 2013, 17:17 #2 Super Moderator   Bruno Santos Join Date: Mar 2009 Location: Lisbon, Portugal Posts: 9,644 Blog Entries: 39 Rep Power: 99 Greetings bieshuxuhe, It's explained in the section "5.3.3 Creating blocks with fewer than 8 vertices": http://www.openfoam.org/docs/user/blockMesh.php edit: I see you've gotten the answer here: Prism Mesh with OpenFoam Best regards, Bruno __________________ OpenFOAM: FAQ | Getting started Forum: How to get help, to post code/output and forum guide What am I doing/planning: blog/wiki Read this before sending me PM Last edited by wyldckat; September 1, 2013 at 17:19. Reason: see "edit:"

March 31, 2014, 08:25
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Quote:
 Originally Posted by wyldckat Greetings bieshuxuhe, It's explained in the section "5.3.3 Creating blocks with fewer than 8 vertices": http://www.openfoam.org/docs/user/blockMesh.php edit: I see you've gotten the answer here: Prism Mesh with OpenFoam Best regards, Bruno
Dear Burno,

Is there any way to make uniform cell distribution in prism?

Like this (Uniform prism cells with blockMesh)

Thanks

Regards
CFDUser_

Last edited by wyldckat; August 11, 2014 at 10:36. Reason: fixed broken link

April 1, 2014, 15:16
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Quote:
 Originally Posted by CFDUser_ Is there any way to make uniform cell distribution in prism? Like this (http://www.cfd-online.com/Forums/ope...blockmesh.html)
Quick answer: AFAIK, it's not possible to do it in an easy way. You'll have to build the prism manually with several blocks.

April 2, 2014, 00:36
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Quote:
 Originally Posted by wyldckat Quick answer: AFAIK, it's not possible to do it in an easy way. You'll have to build the prism manually with several blocks.
Can you explain little more?

April 5, 2014, 18:57
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Bruno Santos
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Quote:
 Originally Posted by CFDUser_ Can you explain little more?
It's simple... well, sort-of simple. The triangular prism feature in blockMesh is actually a squished quadrangular prism, on one of the sides. Therefore, the only way to create with blockMesh such a prism which is shown in the post, namely this:

Is to build it in parts, namely by using:
• + 2 smaller quadrangular prisms
• + 4 triangular prisms
The other possibility is to use splitMesh, which means that you first need to create a normal mesh and then split it diagonally.

I suggest that you try using SwiftBlock instead of creating the "blockMeshDict" manually: http://openfoamwiki.net/index.php/Contrib/SwiftBlock

Best regards,
Bruno
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April 10, 2014, 01:46
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Quote:
 Originally Posted by wyldckat It's simple... well, sort-of simple. The triangular prism feature in blockMesh is actually a squished quadrangular prism, on one of the sides. Therefore, the only way to create with blockMesh such a prism which is shown in the post, namely this: Is to build it in parts, namely by using: 1 large quadrangular prism + 2 smaller quadrangular prisms + 4 triangular prisms The other possibility is to use splitMesh, which means that you first need to create a normal mesh and then split it diagonally. I suggest that you try using SwiftBlock instead of creating the "blockMeshDict" manually: http://openfoamwiki.net/index.php/Contrib/SwiftBlock Best regards, Bruno

Dear Bruno,

I am not looking for 1big hex+2small hex and 4 triangular blocks. I want grading as shown in the figure. I tried all the ways but getting squeezed cells to one corner of the triangle.
If you dont mind can you post blockMeshDict file n series of operations i have to follow. Sorry for struggling but i dont want to do it in blender because i have to try different triangle angles for my problem. for that i can write script file for generating it automatically. all i need now is how to do it in blockmesh.

Thankyou,

Regards
CFDUser_

 April 10, 2014, 03:15 #8 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,419 Rep Power: 25 Hi, do you REALLY need prisms? As the geometry you've shown can be easily meshed with hexagons.

April 10, 2014, 03:36
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Quote:
 Originally Posted by alexeym Hi, do you REALLY need prisms? As the geometry you've shown can be easily meshed with hexagons.
Hi Alexeym,

Yes, i need prisms as shown in the attached figure (all four sides in 3D) and i have to play with different theta.

Last edited by CFDUser_; April 13, 2014 at 07:03.

April 10, 2014, 03:51
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Quote:
 Originally Posted by alexeym Hi, do you REALLY need prisms? As the geometry you've shown can be easily meshed with hexagons.
Hi Alexeym again,

problem is grading. One can make prisms using blockMesh, but grading will be something like in the attached fig.

i want to make grading like in the below fig

Regards
CFDUser_
Attached Images
 untitled.JPG (6.7 KB, 9 views)

 April 10, 2014, 03:59 #11 Senior Member   Alexey Matveichev Join Date: Aug 2011 Location: Nancy, France Posts: 1,419 Rep Power: 25 Still not sure if I get you correctly. You need to construct a mesh which consists of a cuboid block and a prismatic block to simulate something. (You can create fully hexagonal mesh for the geometry you've attached) OR You need a mesh with the prismatic cells at the edges to check how good OpenFOAM at dealing with this type of meshes (AFAIK blockMesh can't automatically build the mesh you've shown in your figures. Well, that's more-or-less what wyldckat said already).

April 10, 2014, 04:09
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Quote:
 Originally Posted by alexeym Still not sure if I get you correctly. You need to construct a mesh which consists of a cuboid block and a prismatic block to simulate something. (You can create fully hexagonal mesh for the geometry you've attached) OR You need a mesh with the prismatic cells at the edges to check how good OpenFOAM at dealing with this type of meshes (AFAIK blockMesh can't automatically build the mesh you've shown in your figures. Well, that's more-or-less what wyldckat said already).
1st one. i need cuboids and prisms to define my geometry. Defining them in blockmeshDict is pretty straight forward. using simpleGrading i need somthing like in the attached pic.

blacklines define geometry and red lines define grading.

Thankyou

Last edited by CFDUser_; April 13, 2014 at 06:59.

April 10, 2014, 04:26
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Alexey Matveichev
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Well,

see my suggestion about splitting your mesh into hexagonal blocks in the attached picture. Arrows along the sides show grading direction (hope I get right what you need).

In general you'll need to define separate patches for top cuboid surface (patch-c in the picture) and prism bottom surface (patch-c in the picture). And then use mergePatchPairs or stitchMesh to merge these patches (so there's no obstacle between prism and cuboid).
Attached Images
 blocks.jpg (18.0 KB, 36 views)

April 10, 2014, 04:38
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Quote:
 Originally Posted by alexeym Well, see my suggestion about splitting your mesh into hexagonal blocks in the attached picture. Arrows along the sides show grading direction (hope I get right what you need). In general you'll need to define separate patches for top cuboid surface (patch-c in the picture) and prism bottom surface (patch-c in the picture). And then use mergePatchPairs or stitchMesh to merge these patches (so there's no obstacle between prism and cuboid).
Hi Alexeym,

I tried this already, but the problem is im getting the skewed cells at point P as shown in the attached fig. to avoid that i need triangular cells. thats what i mean by different kind of grading in prisms.
Anyway thanks for the help. Thanks a lot for your time.

Regards
CRDUser_

Last edited by CFDUser_; April 13, 2014 at 07:02.

 August 7, 2014, 03:17 #15 Member   Gareth Join Date: Jun 2010 Posts: 32 Rep Power: 8 Hi Foamers So when i read through this thread, you guys keep using the keyword hex In the user documentation there is a keyword prisim. Am i incorrect in assuming the difference between them is hex (0 1 2 3 4 5 6 7) ( )... prism (0 1 2 3 4 5) ( )... If this is the case can i have a prism and hex block in the same mesh? /* I have removed the prism entry from my file, it works now. But still not sure how to use prism as an option*/ Last edited by bullmut; August 7, 2014 at 08:16. Reason: Fixed my problem with a work around

 August 11, 2014, 10:41 #16 Super Moderator   Bruno Santos Join Date: Mar 2009 Location: Lisbon, Portugal Posts: 9,644 Blog Entries: 39 Rep Power: 99 Greetings bullmut, According to the documentation: http://www.openfoam.org/docs/user/blockMesh.php - currently only the "hex" blocks are supported. If you want a prism (a "wedge", according to the documentation), you'll have to repeat 2 vertex indexes, as explained in the subsection "Creating blocks with fewer than 8 vertices". The possibility to use other names was left open for other keywords so that other new blocks could be implemented, but I guess no one ever funded such an effort, possibly because configuring "blockMeshDict" files can be pretty hard to do, specially if done manually. Best regards, Bruno __________________ OpenFOAM: FAQ | Getting started Forum: How to get help, to post code/output and forum guide What am I doing/planning: blog/wiki Read this before sending me PM

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