Question about number of elements in a mesh file

[gamma-spectrum]

Dear all,

I have Moltres (version 2025.09.02) within a MOOSE installation (version 2025.09.04) and I'm currently following the tutorial Introduction to the Moltres Input File Syntax in order to reproduce the Molten Salt Reactor Experiment (MSRE) results. My aim is to understand some of the properties of the mesh files.

File 2d_lattice_structured.msh has the mesh of a 2D simulation of MSRE and the number of mesh elements is 45340 (that number is in the next line to $Elements).

However, when I run the Moltres input file 2group.i and I study the information sent to the standar terminal output the number of elements (Elems) is different. I put here the whole Mesh block and the number in bold

Mesh:
  Parallel Type:           replicated
  Mesh Dimension:          2
  Spatial Dimension:       2
  Nodes:
    Total:                 44655
    Local:                 1441
    Min/Max/Avg:           1320/1462/1395
  Elems:
    Total:                 **43904**
    Local:                 1365
    Min/Max/Avg:           1346/1396/1372
  Num Subdomains:          2
  Num Partitions:          32
  Partitioner:             metis

As it can be seen, the number (45340) in file 2d_lattice_structured.msh is different to the number obtained from the Moltres output (43904). Which one is the correct number of elements in the mesh? Maybe each number defines a different quantity?

Kind regards,
Juan García

[smpark7]

That tutorial uses the mesh.e file found within the same directory.

[gamma-spectrum]

Dear Mr Park,

Thank you for your answer. I have checked for the tutorial case that the number of mesh elements (or cells) in the mesh file and in the Moltres simulation matches.

In my case, I have opened with Paraview the file mesh.e and checked the number of cells (2070, see the attached figure)

Once I run the file nts.i with Moltres I have checked that the number of elements in the mesh that Moltres loads is the same (2070). I put here a short extract of the information sent to the terminal standard output.

Mesh:
  Parallel Type:           replicated
  Mesh Dimension:          2
  Spatial Dimension:       2
  Nodes:                   2224
  Elems:                   **2070**
  Num Subdomains:          2

However, for the case that I presented previously the number of elements in the mesh and those loaded by Moltres seems not to match. Do you have some idea about why there two numbers do not match? Although this discrepancy is not critical from my work I'm curious about why these two numbers are not the same.

Kind regards,
Juan García

[smpark7]

It seems that gmsh counts element sides in sidesets as elements as well. If you add the number of elements and the number of element sides of sidesets, you get the gmsh value.