[jabowery]

Under what conditions would it be "topologically" questionable to have both surface and volume elements in a 3D mesh?

I get this:
"warning: mesh with topological dimension 2 lower than space dimension 3"
from sfepy regarding a simple gmsh-generated EHD mesh consisting of a truncated cone suspended in air. The EHD is to be performed only on the air volume, driven by a charge emitter at the top surface of the truncated cone and a charge sink on its base -- both surfaces -- with air flow around the side of the cone, also a surface. A gmsh surface-only rendering:


The regions:


The cone volume:


The air volume: