Why are my Non-Design Spaces reduced?

If Non-Design Spaces are created with the direct Non-Design Space method, they are not automatically applied with Interfaces. The definition of Interfaces provides the information to the solver where clean intersections need to be in the geometry. By choosing cells with the direct Non-Design Space method and not applying boundary conditions or interfaces the intersection will not be a clean intersection in these areas.

If a functional surface is desired, an interface needs to be applied.