How to create internal flow volume with infinitesimally small cross section?

Hi, it seems like these air ducts I exported out of Revit have an infinitesimally small cross section. As such, the cross section at the opening where you would normally select the boundary faces for internal flow volume are not selectable. How would you go about creating an internal flow volume in a case like this so there can be an air region representing the volume in the duct ?

Only selecting the seed face would give you this message, but since there are seemingly very small cross sections, the boundaries are not selectable in the CAD editor: “The Seed face face 5337 @ Solid 71 defines an infinite flow region. Please select Boundary faces to cover openings and inspect your models for gaps. There should be no gaps between the bodies enclosing the flow volume. If your CAD has no holes, try also an External flow volume operation with a fitted flow box.”

Hi there!

The problem is that your duct is not hollow - the model fills the flow conduct.

Instead, your model for the solids should only contain the duct walls, leaving an empty space in the inside. Such space will be filled with the flow region operation.