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[snappyHexMesh] On refinementRegions

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Old   October 13, 2022, 02:16
Smile On refinementRegions
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Good morning,

I have defined a refinement region in my snappyHexMesh dictionary. The cells in the transition area are shown in the screenshot.
I have got 2 questions.
  • How do I smooth this transition? At the current stage, I have got that the transition happens in the width of only one cell. How do I make the transition happens in the width of 5 cells? I thought it was the nCellsBetweenLayer keyword, but it made no difference.
  • Is it normal that every other node on the first column of refined cells is not shared with any other cell of the transition area? I do have a feeling that that node should not be "standing on its own" there and that I should see a cell edge starting on that node and connecting it to another node of the transition area. I have a feeling that I should see something like the second screenshot. How do I make that happen?

Thanks a lot, and have a great day from Francesco.
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File Type: png RefReg2.PNG (71.6 KB, 10 views)
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Old   October 14, 2022, 01:42
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There are a few things to note here:
First of all, the "transition" your are refering to (highlighted by your red lines) is not actually there. This is a representation issue in Paraview. There are multiple disussions here regarding that, for example: How to visualize polyhedron mesh in paraview

The entry nCellsBetweenLevels only comes into play, if the relative level between two regions differs by more than one. The reason for this is - as you saw already - that a relative jump from 1 to another level happens by just splitting the cell. It creates these "hanging nodes", which are not problematic for OpenFOAM, e.g. ANSYS CFX does not support that.
Now, if you have a more than 1 level jump, lets say your surrounding mesh is at level 0 (blockMesh) and your refinement region is at 2. It will make nCellsBetweenLevels of cells of level 1 in between, so it only jumps by max. one level per cell.
Here's an example with nCellsBetweenLevels = 2
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ncellsbetweenlayer, openfoam, refinement regions, refinementregion, snappy hex mesh

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