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general rule of thumb for bi-exponential meshing

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Old   February 7, 2013, 15:15
Default general rule of thumb for bi-exponential meshing
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I'm fairly new to this, so pardon if I say something stupid.

I'm creating a 3d mesh in gambit for a lid-driven cavity flow. The cavity is 1:1:3 with Re~ 10,000 and we'll be using RANS (probably Fluent, maybe openfoam) to run cases.

I created the geometry and mesh, but I have no idea what number to use for bi-exponential meshing. Another student mentioned using 1.05 in his research, but that seems too sparse in some parts in the middle. I tried 0.7, but I wanna make sure and have sufficient resolution near the walls. I honestly don't know even a typical range for this, and I'm only guessing on what's too coarse or fine, since I've never made my own grid before.

I realize there's no exact answer or whatever, I just want a general guide.
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Old   February 11, 2013, 03:44
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The most general rule of thumb is that the mesh has to be finer close to the walls. Consequently, only values greater than 1 make sense here.
Now the second constraint is that the growth rate should not be too high because of accuracy issues, most often values below 1.05 are considered ok.
The third (and most important) constraint is the height of the first at the wall. Depending on the type of wall modeling you use, the dimensionless wall distance Y+ has to be kept within a specific range. You might want to run a simulation, evaluate Y+, and change the mesh accordingly.

Now if you consider the mesh to be too coarse in the middle with the first cell height chosen to match Y+: choose a lower growth factor, because in general:
low growth rate -> high accuracy
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cavity flow, gambit, mesh and grid

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