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[snappyHexMesh] Can Snappyhexmesh work with Prism cells?

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Old   August 19, 2015, 06:16
Default Can Snappyhexmesh work with Prism cells?
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Vineet Bhola
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Hi All,
I have always worked with snappyhexmesh alongwith blockmesh containing only hexahedral cells. But one of the demand that has come up from a customer is if I could use Prism cells instead of hex cells. My concerns are:
1. I saw on OPENFOAM user guide that instead of hex keyword, if I use prism, I can have prismatic cells. But my concern is that then the box itself would be a prism. I want to know if it is possible to have a cuboid as box and prism cells inside.
2. Suppose I find a solution to the first concern, is it then possible to work with prism cells in Snappyhexmesh? What parameters do I need to change for that?
Thanks,
Vneet
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Old   August 19, 2015, 08:39
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Quick answers:
  1. blockMesh only supports hexahedral cells; the prism is simply a collapsed hexahedral shape.
  2. snappyHexMesh cannot handle cells other than hexahedral, at least not for the refinement and snapping steps. As the name states "snappy hex mesh". The resulting mesh might have cells that are not hexahedral, but that's because sHM is able to split and morph cells. But it has issues trying to handling the cells again in a second run, e.g. it can't handle the re-refinement of a refinement transition zone.
  3. Perhaps you're confusing your customer's request, because there is something called "prismatic boundary layer", which is mostly only applicable to tetrahedral meshes, where a layer of prismatic cells is added between the tetrahedral internal mesh and the surface mesh. In such a case, try using cfMesh instead of sHM.
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Old   August 20, 2015, 08:13
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Thanks for the clarification Bruno. You are right regarding prismatic boundary layer. My question now is,
  • for a complex shape which mesh style is better? Hex or tetra with prismatic boundary? can i meet the similar Efficiency of prismatic boundary if I just use "add layers" and "ncells snappy?
  • How easy/difficult is cfMesh compared to snappy? I had used a simplified geometry for multiregion volume and after working last couple of months with snappy i think I have a solution. Do you think it would be easier to extend snappy to the real geometry OR should I try with cfMesh. Given that I have 2 more months remaining for my internship. I know it is quite subjective but a rough comparison between cfMesh and snappy would be helpful
Thanks,
Vineet

-----

I mean "ncellsBetweenLevels"

Last edited by wyldckat; August 20, 2015 at 15:04. Reason: merged posts that were a few minutes apart
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Old   August 20, 2015, 15:10
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Quick answers:
  • tetrahedral mesh is something known to be hard to use with OpenFOAM. It's possible, but it's somewhat hard to calibrate "fvSchemes" and the "fvSolution" with the correct working parameters.
  • The "add layers" feature in snappyHexMesh will literally add layers, therefore it will compress the internal mesh, which sometimes results in difficulties in generating the mesh.
  • cfMesh is a lot easier to use than sHM. The difference is that the layers are generated on the existing cells, which sometimes has its downsides as well, but usually it's easier to configure.
  • Both cfMesh and sHM demand the same kind of STL configuration, i.e. explicit "solid" definitions for each surface that is meant to be preserved. Not doing so can result in sharp edges being smoothed out.
  • edit: "nCellsBetweenLevels" doesn't always work as intended: http://openfoamwiki.net/index.php/Sn...sBetweenLevels

Last edited by wyldckat; August 20, 2015 at 15:11. Reason: see "edit:"
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Old   August 25, 2015, 06:02
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sorry for the delay in responding. I thought of first using the mesh generated by snappy for solver and if the results are accepted by customer then if needed try cfMesh. It seems that he is more concerned with gradually increasing boundary layers and bigger cells in center than the prismatic boundary layer itself.
If I have to use cfMesh later and find any problems, I will continue the discussion. (or start another post)
btw, is there any way to track my own posts? I see an option of "new posts" OR "your blog" but not "your post"
Thanks
Vineet
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Old   August 26, 2015, 19:19
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Quick answers:
Quote:
Originally Posted by vineet_bhola View Post
If I have to use cfMesh later and find any problems, I will continue the discussion. (or start another post)
Search and use an existing thread that already addresses the question you have, or start a new thread, since the topic will be a bit different from this one.
Quote:
Originally Posted by vineet_bhola View Post
btw, is there any way to track my own posts? I see an option of "new posts" OR "your blog" but not "your post"
Click on your user-name vineet_bhola (or "User Panel" on the top left, then "Your Profile"), then tab "Statistics" and you should see the link "Find all posts by vineet_bhola"
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