Generating structured meshes (volumes)

To mesh volumes with a structured mesh, select the option Mesh->Structured->Volumes->Assign number of divisions to volume lines.
Select volumes 1 and 2 and press ESC.
A window appears in which to enter the number of divisions that the lines to be selected will have. Enter 6 and click Assign.
Select lines of both volumes parallel to the X and Z axes. GiD automatically selects all the lines in each volume parallel to these in order to create the structured mesh. Press ESC.
Another window appears in which to enter the number of divisions on the lines. Divide the lines parallel to the Y axis into 8 segments. Enter 8 and click Assign.
Select an edge of volume 1 or 2 parallel to the Y axis and press ESC . Again, the line-division window comes up. Since we have already finished the assignments, click Close.
For structured volumes, GiD generates tetrahedron meshes by default, but hexahedron structured meshes can also be assigned. Let's assign the element type that we wish to volume 1 and 2. Select Mesh->Element type->Tetrahedra, then select volume number 2 and press ESC. Select Mesh->Element type->Hexahedra, then select volume number 1 and press ESC.
Select Mesh->Generate mesh.
A window appears asking whether the previous mesh should be eliminated. Click Yes.
Another window comes up in which to enter the desired element size. Leave the default value unaltered and click OK . The result is the mesh shown.

Structured volume mesh of tetrahedra with the option Symmetrical structured tetrahedra not set.

GiD can obtain volume structured meshes made of hexahedra, tetrahedra or prisms. As can be seen in the figures, there are two kinds of tetrahedron structured mesh: the next one shown is obtained when the option Utilities->Preferences->Meshing->Structuration type->Symmetrical structured->tetrahedra is set. If this option is not set, the mesh presented in previous figure is produced (with fewer nodes).

Structured volume mesh of hexahedra an tetrahedra with the option Symmetrical structured tetrahedra set.

GiD user manual