[Yann]

Originally Posted by DCanas

And how do I use the fieldMinMax function?

You can check tutorials to find examples using this function. For instance this command will point to every tutorial using this function.

grep -r "fieldMinMax" $FOAM_TUTORIALS/tutorials/

If you are not familiar with function objects, I suggest to read the user guide and go through some tutorials to get a grasp of OpenFOAM basics.
(For instance: https://doc.openfoam.com/2306/tools/...ction-objects/)

Originally Posted by DCanas

I used SnappyHexMesh, how do I improve the cell quality?

The bad cell is only an idea. You need to search for the source of your problem first (is it physical or not? If not, what causes the issue?)
Then you can decide what you need to do.

A good thing to do would be to run checkMesh to see if there are low quality cells in your mesh. Something like:

checkMesh -constant -writeSets vtk

The output will give you mesh statistics and quality. If there are errors on some cells, it will write it in a postProcessing directory in vtk format. You can then open these files in paraView to check where these cells are located in your mesh.

Cheers,
Yann