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[mesh manipulation] How to mesh the boundary layer during a mesh independance study

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Old   January 19, 2025, 08:36
Default How to mesh the boundary layer during a mesh independance study
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Hi,


I want to perform my first mesh independance study and i'm finding myself quite puzzeled as to how to mesh the boundary layer.


Right now i have a coarse mesh (~ 1.5M cells) and a target Y+ ~1 generated using cfmesh.


If i want to make my mesh finer, the boundary layer inflation zone will get thinner because of cfmesh. hence two questions:


1. Is simply making the mesh finer ok for such a study, even if it makes the boundary layer thinner or do i need to switch to snappyhexmesh to control the boundary layer thickness?


2. During a mesh independance study, is it good practice to adjust the boundary layer parameters to make the Y+ identical between all the meshes or shoud the finer meshes get a lower Y+?
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Old   January 20, 2025, 07:25
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Hey,


If I am not mistaken, cfMesh allows you to set the first layer thickness.


As for the y+, it really depends on what you are trying to solve in my opinion, and if you can afford the computation. Since you are already at y+ <= 1, I suppose you could just mesh at half the original cellsize, even the boundary layers, and see the consequences. I would be careful if the change in cellsize results in boundary conditions not being applicable anymore (for instance, wall laws becoming invalid for the resulting y+).


Regardless of the nature of your simulation, I would adjust the cellsize of the bulk flow first, and keeping the same y+. Once satisfied with the bulk flow, I would then adjust the y+ without modifying the cellsize.
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Old   January 20, 2025, 13:28
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Thanks for the reply. Looks like a plan. I'll try that and if i have problems i'll come back to this thread.



Quote:
Originally Posted by Alczem View Post
If I am not mistaken, cfMesh allows you to set the first layer thickness.

kind of...
- You can only specify maximum thickness. Cmesh is free to go as low as it wants
- The boundary layer inflation zone can only be as thick as one cell. (cfmesh simply subdivides the cells in contact to the surface of your object) so if you make the overall mesh smaller, your boundary layer inflation will also be thinner, which can be very bad.
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