To mesh volumes with a semi-structured mesh, select the option Mesh->Semi-structured->Volumes->Assign number of divisions in structured direction.
A window appears. Enter 8 and click Assign.
Select volume 3 and press ESC. As volume 3 is prismatic in one direction only (i.e. parallel to Y axis) GiD will automatically detect this fact and will select it to be the direction in which the semi-structured volume mesh is structured.
Another window appears in which to enter the number of divisions in the direction of the structure. In this case we do not want to select any more volumes, so click Close.
Select Mesh->Generate mesh.
A window appears asking whether the previous mesh should be eliminated. Click Yes.
Another window appears in which to enter the desired element size. Leave the default value unaltered and click OK. The result is the mesh shown.

Semi-structured volume mesh of tetrahedra.

As can be seen, volume 3 has been meshed with tetrahedra. Semi-structured volumes can be meshed with tetrahedra, prisms or hexahedra, if the compatibility with the element types of adjacent entities allows it.

In the following steps a hexahedron mesh is produced.

Select Mesh->Element type->Hexahedra.
Select volumes 2 and 3 and press ESC.
Select Mesh->Generate mesh.
A window opens asking whether the previous mesh should be eliminated. Click Yes.
Another window appears in which to enter the desired element size. Leave the default value unaltered and click OK. The result is the mesh shown in next figure.

Semi-structured volume mesh of hexahedra.

In case of volume number 3 there is only one direction in which it can possibly be structured (i.e. in the direction of the prism). If the volume is prismatic in more than one direction, there are two ways to choose between them: selecting one top surface (Mesh->Semi-structured->Volumes->Set master surface) or the direction of the structure (Mesh->Semi-structured->Volumes->Set structured direction). The following example explains this procedure.

Select the option Mesh->Semi-structured->Volumes->Assign number of divisions in structured direction
A window opens in which to enter the number of divisions in the structured direction (prismatic). Enter 6.
Select volume 1 and press ESC.
Another window appears, click Close.
Select Mesh->Semi-structured->Volumes->Set structured direction.
Select one line parallel to the X axis of volume number 1 (for example line number 11) and press ESC.
Select Mesh->Unstructured->Assign entities->Surfaces and select surfaces 1 and 6 and press ESC.
Select Mesh->Generate mesh.
A window opens asking whether the previous mesh should be eliminated. Click Yes.
Another window appears in which to enter the desired element size. Leave the default value unaltered and click OK. It can be appreciated in the following figure that the mesh in volume 1 is semi-structured, extruded along the x direction, now with an unstructured quadrilateral mesh on the surfaces 1 and 6.

Semi-structured volume mesh of hexahedra also in volume 1.

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