Hi, I have a question about the mesh. In the mesh pane, the mesher sub-pane, some tolerances like small cell tolerance, small area tolerance, merge tolerance, intersection tolerance and dot product tolerance may needed to be defined. How to determine these tolerance parameters? Are they the smaller the better?
My impression is that by defining higher tolerances (numbers), for small cell tolerance at least, you will get rid of the larger smaller cells, that potentially can cause problems. But I am also a bit uncertain to when this removal actually happens - it looks like they’re still there after the meshing, but are “removed” when solvers starts.
Perhaps have a look at 3.8. SMS meshing workflow, cyclone, Discrete Element Model (DEM) — MFiX 21.3.2 documentation
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Thanks for your reply @kjetilbmoe. I also checked whether the tolerances may effect the grid quality. Then, I tested the mesh by running the toturials 3.4. Three Dimensional Single phase flow over a sphere — MFiX 21.3 documentation and 3.5. Three Dimensional Fluidized Bed — MFiX 21.3 documentation, with the default tolerances (small cell tolerance = 0.01 and small area tolerance = 0.01) and the smaller tolerances. The smaller tolerances are listed as follows, in toturial 3.4, small cell tolerance = 1e-3 and small area tolerance = 1e-6, and in toturial 3.5, small cell tolerance = 1e-5 and small area tolerance = 1e-10.
The generated grid aspect ratios are shown in the following illustrations.
Toturial 3.4
using the default tolerances
using smaller tolerances
Toturial 3.5
using the default tolerances
using smaller tolerances
It seems that the tolerances, such as small cell tolerance and small area tolerance, have slight or even negligible influence on the grid quality at least in these two cases.
Thank you for advicing me to read the toturial 3.8. In this toturial, some sentences are very meaningful, which help me to understand the function of the tolerance in MFIX. For example, “The default value of 0.01 means a cut cell is considered small if its volume is below 0.01 times the volume of the corresponding standard, uncut cell. These small cells will be removed from the computation. Having a Small cell tolerance set to 0.0 will keep all small cells and this may lead to numerical stiffness and loss of convergence.”
Now I have understood. Again, thanks.
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