Don't settle for Computational Fluid Dynamics (CFD) software that uses (abuses) facets (e.g., STL) as geometry. If your original geometry is analytic (CAD) then mesh it directly without the conversion to facets. Otherwise it's like you have to create two surface meshes. What's wrong with that? Plenty!

Blocky Structures Are Not The NormDartmouth clock tower undergoing renovation aided by blocky scaffolding

If blocky (Minecraft) shapes ruled the world, then CFD with facets would be easy. Alas, not to be, as the world prefers curves.

To find out more see:

• CFD Prefers NURBS over STL
• A Perspective on Cartesian Meshing
• Geometry Exchange