question about grid convergence study(boundary layer)

dmschd153246 · May 8, 2023, 08:04

hi dear former
i have a small question about gird convergence study

If what I know is right in grid convergence study
Wall Y+ or initial cell spacing from wall for boundary layer resolution should be same for all levels of grid (i.e. Coarse, Medium, Fine, etc.).
(They offer only the first cell size)

so this is my question
Is there no need to match the total layer thickness or expansion ratio and the number of layers in coarse, medium, fine gird system?

I would really appreciate it if you could give me the answer.

neosuper · July 7, 2023, 15:27

Here is my view, I can not confirm my answer is totally right.

Suppose you are doing a RANS CFD.

The Wall Y+ configuration is closely related to your turbulence model. Then you set a suitable Y+ and set a good first wall distance, growth rate, BL layer numbers, etc.

Here comes your question, will this mesh configuration be independent of the mesh number? As a result, you run a grid convergence check process.

"Is there no need to match the total layer thickness or expansion ratio and the number of layers in coarse, medium, fine gird systems?"

The treatment that remains most mesh RATE constant to make the results more comparable. I think this treatment itself is not wrong.

In my view, the key point for the grid convergence check is to ensure that the "resolution" in the region I am interested in is enough. In other words, when I refine the mesh but the result shows less difference from my former mesh, it will be regarded as a grid convergence in plain view.

When we come back to your question, if your interest is in the BL regions, for instance, the wall flow separation, wall turbulence, etc., the "mesh RATE constant" treatment in BL regions is not right in my view. And the RATE should be changed to run the refinement process and the mesh convergence check.

If your interest is in the main flow region, the "mesh RATE constant" treatment in BL regions is not wrong and can simplify the process when you treat the wall region.

May 8, 2023, 08:04	question about grid convergence study(boundary layer)	#1
dmschd153246 New Member eunchong Join Date: Feb 2022 Posts: 8 Rep Power: 4	hi dear former i have a small question about gird convergence study If what I know is right in grid convergence study Wall Y+ or initial cell spacing from wall for boundary layer resolution should be same for all levels of grid (i.e. Coarse, Medium, Fine, etc.). (They offer only the first cell size) so this is my question Is there no need to match the total layer thickness or expansion ratio and the number of layers in coarse, medium, fine gird system? I would really appreciate it if you could give me the answer.

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July 7, 2023, 15:27		#2
neosuper New Member Dawn NEE Join Date: Jul 2023 Posts: 2 Rep Power: 0	Here is my view, I can not confirm my answer is totally right. Suppose you are doing a RANS CFD. The Wall Y+ configuration is closely related to your turbulence model. Then you set a suitable Y+ and set a good first wall distance, growth rate, BL layer numbers, etc. Here comes your question, will this mesh configuration be independent of the mesh number? As a result, you run a grid convergence check process. "Is there no need to match the total layer thickness or expansion ratio and the number of layers in coarse, medium, fine gird systems?" The treatment that remains most mesh RATE constant to make the results more comparable. I think this treatment itself is not wrong. In my view, the key point for the grid convergence check is to ensure that the "resolution" in the region I am interested in is enough. In other words, when I refine the mesh but the result shows less difference from my former mesh, it will be regarded as a grid convergence in plain view. When we come back to your question, if your interest is in the BL regions, for instance, the wall flow separation, wall turbulence, etc., the "mesh RATE constant" treatment in BL regions is not right in my view. And the RATE should be changed to run the refinement process and the mesh convergence check. If your interest is in the main flow region, the "mesh RATE constant" treatment in BL regions is not wrong and can simplify the process when you treat the wall region.