Hello,

In general, the mesh is determined by starting with a rough mesh, and then refining until the result does not depend anymore on the mesh you're using.

Meaning that refining the mesh does not change the result more than the precision that is sufficient to you.

More precisely, the larger mesh you can use will depend on the specific problem you're treating. For instance, if you deal with phase field, the mesh has to be smaller than the interface. In flows, the mesh has to be such that there is no instability. So it is hard to answer with no knowledge of the kind of problem you're treating.

Also, note that you can set different meshes for the different domains if necessary. There is also an option for adaptive meshes in Comsol that are sometimes worth to use.

There is no a "minimum" a "maximum" element size for a mesh. What you have to do is to construct a set of meshes for which your numerical results do not vary with them (at least in an acceptable range). I follow the ERCOFTAC Best Practice Guidelines, which provide good results not only for CFD, but also for transport theory and MHD problems (we verified that).

In brief, you construct 3 different meshes and you evaluate some variables - split in local and global ones - on those meshes, then you evaluate a parameter called Grid Convergence Index which gives you the reliability of the more refined mesh with respect to the reference one or of the reference one with respect to the less defined mesh.

Take a look to these:

Article Experiments on the MHD Effect on the Drainage of a LiPb Chan...

Article A novel approach to the study of magnetohydrodynamic effect ...

you can find a section which briefly describes the GCI method.

Regards

When creating a model in COMSOL Multiphysics, the mesh always defaults to a Physics-controlled mesh with a Normal element size. We can easily use this, or any of the other predefined parameter sets, as a starting point when modifying individual parameters. The predefined element sizes are simply sets of parameters, five parameters to be exact, that are available for modification:

Maximum element size — limits how big each mesh element can be (lower value => finer mesh)

Minimum element size — limits how small each mesh element can be (lower value => finer mesh)

Maximum element growth rate — limits the size difference of two adjacent mesh elements (lower value => finer mesh)

Curvature factor — limits how big a mesh element can be along a curved boundary (lower value => finer mesh)

Resolution of narrow regions — controls the number of layers of mesh elements in narrow regions (higher value => finer mesh)

Adjusting one or more of these parameters lets you build a custom mesh, specific to your needs. In the case of the above geometry, the Normal element size parameter set is satisfactory for the bulk of the geometry, but results in too large elements for the fillets and the bevel around the hole.

https://www.comsol.com/blogs/size-parameters-free-tetrahedral-meshing-comsol-multiphysics/