[Gmsh] Almost structured mesh - Why aren't they structured?

[Paolo Tricerri]

Dear Gmsh users,

I have a small question about how GMSH extrudes a meshed surface. The
script I have attached to the email, creates a cylindrical tube
extruding one of the two flat borders in the z direction. The mesh of
the extruded border is structured and symmetric w.r.t the line x=y
(the straight line of 45 degrees). When I extrude the mesh, the two
curved surfaces I get are almost structured. There are some elements
(placed in random position?) which make the mesh non completely
structured.

Why this? I would need a structured mesh and not an almost structured
mesh. Is that possible to have the two curved surfaces structured?

Many thanks for any suggestion,

Paolo
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