[Gmsh] structured 3d mesh with unstructured center region

[Danny Lathouwers - TNW]

Hello,

We are trying to mesh a geometry which mainly consists of a three
dimensional rectangular lattice divided in 14x14x4 (x,y,z)-regions. For
this purpose I have used the extrusion-commands to generate a structured
mesh of hexahedron-8 elements. At the moment i'm trying to replace the 4
center (in x,y-plane) geometrical volumes by a circular 'beam'
(cilinder).

I have added a simplified version (6x6x1) of the model as an attachment.
Here prims are generated in the center region, but our fem-solving
software can't handle prisms at the moment.

Is it possible to generate a mesh for this geometry which only consists
of hexes (necessarily unstructured inside the center region), without
having to refine the mesh outside of the center area? This is important
for the time of computation. (so each side of an element (in the
x,y-plane) has a length equal to that of the side of the smalles
geometrical entity (8 cm in this case)).
So only 8 nodal points (in the x,y-plane) are available at the boundary
of the center region.

Or is there another nice way to handle this situation and create a
conforming mesh? A rough estimate of the cilinder is already
satisfactory in my case.
Or is the use of prisms the only really usable option?

Greetings and I'm curious to hear some good ideas,

(I would also like to know how I can prevent the two errors in the
creation of the 1D-mesh due to deleting the original elementary entities
in the center of the geometry. But this is not the most important
problem at the moment.)
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