A semi-structured mesh can be assigned to a volume, which has to be topologically prismatic. That is: there can be recognized two top surfaces, and a set of lateral four-sided surfaces connecting them. This kind of mesh is structured in only one direction of the volume and is unstructured in the other two directions. For example, in a prismatic volume, the meshing on the sides of the object can be structured, while the meshing on the top surfaces can be unstructured. The top surfaces must be topologically equal. This implies they must have the same number of contour lines, but they can have different shape.
Depending on the element type selected for each surfaces of the volume, the elements will be hexahedra, prisms or tetrahedra.
Note: When assigning divisions in the structured direction, GiD may need to reassign some number of divisions to make the structured mesh conformal with the surrounding ones; this will be done automatically. If it is impossible to create compatibility between surfaces, a message is displayed.
Set Master surface or Structured direction:
The structured direction of the prismatic volume can be set by using this option. If no structured direction is set, GiD will assign it automatically. This option makes sense in the case of volumes with multiple prismatic directions, so you can specify which one you want to use.
With semi-structured volumes (since they are prismatic) two top surfaces can be distinguished. They will have topologically identical meshes, so one of them is obtained from the other. The Master surface is the one that is meshed first, so its mesh will determine the topology of the slave mesh at the opposite end. By selecting Set -> Master surface one top surface can be forced to be the Master one, and the structured direction is then set automatically.
Trick: there is a variable: AlignSemiStructuredNodes (see Preferences) to force the prismatic nodes de be exactly aligned if possible.