Element min and max size not working properly

I am meshing a simple geometry, but either there is something that doesn’t work the way it should or I don’t unterstand how it should work.

I created a mesh using the algorithm “Extrusion 3D”, then created a submesh using the bottom face with “NETGEN 1D-2D” algorithm and another submesh using the vertical edges with “Wire discretization”.
When defining the bottom face submesh, I defined “Max. size” as 10 and “Min. size” as 2.

But after computation, I check the “Length 2D”, and there are some elements with edges longer than 10.

So I wander, is that a bug or something?
If not, what is the actual meaning of “Max. size” and “Min. size”?

Test_elem_size.hdf (243,9 KB)


Not 100% sure, but try the following : separate de netgen 2d-1d into the two algos separetly, nedgen 2d + netgen 1d (or wire discretization) and define maximum and minimum dices in the two hypothesis. And re run the mesh.
Best regards
I can confirm that this is a solution for your issue.
I change netgen 2D-1D to netgen 2D and added two different 1D submeshes for the circle edge and external edges to give different sizes (2 and 10) and fixed also the netgen 2D max an min sizes

Test_elem_sizeModified.hdf (438,8 KB)

Thnks @franco.ota for your answer.
I opened the file you uploaded, but when I checked the size of the elements, there are some smaller than 2 and some larger than 10

My bad, I am sorry i was wrong, the issue is that you are using a automatic mesh generation tool, I am not sure that is possible to fixed them directly, they are really similar but not perfectly equal to expected… maybe if you increase the number of optimization over netgen 2D it will improve but it will do it randomly (as it looks to increase the quality of the mesh…)

As I understand, the elements created by the automatic mesher should have sizes between 2 and 10.
And I think it is odd that there are elements with size outside that range.
Even if I use a “Very fine” mesh there are sizes outide of the range

I’m not sure I’m getting your question completely, but I’ve noticed in the past similar aspects of the algorithm. I came to the conclusion that the algorithm is sort of imposing those constraints “softly”.

There is an insight in the user guide that states:

NETGEN algorithm does not strictly follow the input parameters. The actual mesh can be more or less dense than required. There are several factors in it:

  • NETGEN does not actually use Number of Segments parameter for discretization of edge. This parameter is used only to define the local element size (size at the given point), so local sizes of close edges influence each other.
  • NETGEN additionally restricts the element size according to edge curvature.
  • The local size of segments influences the size of close triangles.
  • The order of elements and their size in the 1D mesh generated by NETGEN differ from those in the 1D mesh generated by Regular_1D algorithm, which results in different 2D and 3D meshes at the same 1D input parameters.