6.2 Elementary vs. physical entities

If only elementary geometrical entities are defined (or if the Mesh.SaveAll option is set; see Mesh options list), the grid produced by the mesh module will be saved “as is”. That is, all the elements in the grid will be saved using the identification number of the elementary entities they discretize as their elementary region number (and 0 as their physical region number(14); File formats). This can sometimes be inconvenient:

• mesh elements cannot be duplicated;
• the orientation of the mesh elements (the ordering of their nodes) is determined entirely by the orientation of their “parent” elementary entities, and cannot be modified;
• elements belonging to different elementary entities cannot be linked as being part of a larger group having a physical or mathematical meaning (like `Left wing', `Metallic part', `Dirichlet boundary condition', …).

To remedy these problems, the geometry module (see section Geometry module) introduces the notion of “physical” entities (also called “physical groups”). The purpose of physical entities is to assemble elementary entities into larger, possibly overlapping groups, and to control the orientation of the elements in these groups. The introduction of physical entities in large models usually greatly facilitates the manipulation of the model (e.g., using `Tools->Visibility' in the GUI) and the interfacing with external solvers.

In the MSH file format (see section File formats), if physical entities are defined, the output mesh only contains those elements that belong to physical entities. Other file formats each treat physical entities in slightly different ways, depending on their capability to define groups.

In all cases, Gmsh reindexes the mesh vertices and elements so that they are numbered in a continuous sequence in the output files. Note that the numbers displayed on screen after mesh generation thus usually differ from the ones saved in the mesh files. To check the actual numbers saved in the output file just load the file back using `File->Open'.