4.2 Defining physical surfaces and volumes

In order for us to use the mesh, we need to define the physical surfaces which correspond to the inflow, outflow and walls so that we can set appropriate boundary conditions. The numbering resulting from the extrusions in this case is not straightforward. In the graphical interface, select Geometry > Physical Groups > Add > Surface, and then hover over each of the surfaces which are shown by the dashed gray lines. The numbering will be revealed in the toolbar underneath the geometry as a ruled surface. In this case:

• Walls: surfaces 7, 8, 28, 29.

• Inflow: surface 16.

• Outflow: surface 24.

We also need to define the physical volumes, which can be done in a similar fashion. For this example, there is only one volume having ID 1. Adding these groups to the end of the .geo file is very straightforward:

1Physical Volume(0) = {1}; 
2Physical Surface(1)= {7,8,28,29}; 
3Physical Surface(2) = {16}; 
4Physical Surface(3) = {24};

Either choose the option File->Save Mesh or, assuming this is saved in a file named test.geo, run the command

gmsh -3 test.geo

which will produce the resulting MSH file test.msh. One can generate a high-order mesh by specifying the order on the command line, for example

gmsh -3 -order 6 test.geo

will generate a sixth-order mesh. Note that you will need to use a current version of Gmsh in order to do this, most likely from subversion.