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[cfMesh] Reasonable value for maxCellSize in cfMesh (and other parameters)

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Old   June 8, 2022, 06:06
Default Reasonable value for maxCellSize in cfMesh (and other parameters)
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Ruiyan Chen
Join Date: Jul 2016
Location: Hangzhou, China
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I've been using cfMesh for several months (flow past very complex terrains, mesh needs to be refined near the ground), and am satisfied with the output mesh most of the time. In general, cfMesh is very easy to configure, very robust, and efficient. However, I do realize the following several strange behavior.

1. The resulted largest cell size I get is smaller than the maxCellSize value I set, as expected, but sometimes the gap is quite large. e.g., I set maxCellSize to 3000, but the resulted max cell size turns out to be 750. I guess it has to do with the overall geometry and other parameters (e.g., I set the cell size near the ground to a small value, within the geometry the cell size gradually increases, but could never reach 3000). I haven't found a clear mechanism to follow though, so the maxCellSize has to be adjusted and tested several times before I am able to model a new case.

2. In some cases, if I set maxCellSize to a smaller value (while keeping other parameters the same), I end up getting less cells. e.g., I set maxCellSize to 600, I get 40 million cells, but when I set it to 1000 I get 70 million cells. This is not what one would expect right?

3. I use localRefinement quite often, and cellSize is used to control the cell size near the ground. cfMesh guarantees that the cell size near the ground is smaller than the cellSize value, which is great, but the resulted cell size can be quite small so I end up having many cells. e.g., I set cellSize near the ground to 30, but the resulted cell size near the ground turns out to be around 15. This causes the boundary layers to shrink as well.

My gut feeling is that the parameters in cfMesh are connnected very tightly, and they affect each other in a complex way. I understand that meshing is inevitably a trial and error process, and is case dependent, but are there any general rules I should follow first?

Thanks in advance.

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