The reason is that the
sweep mesh scheme can only be swept “from a single surface, to another single surface” or “from multiple surfaces, to a single surface”. It is not possible to sweep (in Coreform Cubit/Trelis) from one surface to multiple surfaces.
A surface-meshing scheme that you might be interested in exploring is the
triprimitive scheme. While that might sound like a triangular mesh generator, it actually meshes a topological triangle with quads and produces meshes quite similar to the
circle mesh scheme if the surface is a quadrant of a circle.
Another scheme you may be interested in is the
polyhedron scheme which is similar to
triprimitive but tends to place the extraordinary point (valence-3 node) closer to the center of the quadrant. Below is a comparison of these three schemes.
However these schemes don’t provide as much control over the mesh layout as
An alternative approach, that is more difficult to manage, is to use your initial approach, but not assign those elements to a block:
cylinder radius 1 height 4
webcut volume all with plane zplane offset 1.5
webcut volume all with plane zplane offset 1
surface 3 scheme circle
mesh surface 3
mesh vol all
block 1 vol 1 3
block 1 hex in vol 2 with x_coord<0
draw block 1
But notice that because there wasn’t any geometry to constrain the layout of the hex-mesh, that the cutout region isn’t flat. Thus my recommendation is to webcut the cylinder into quadrants and then use the
polyhedron mesh scheme as discussed above.