importing obj: no volumes available

I'm importing an obj file, but I can't see any "volumes". I only see "faces" and "groups".
As a result, I can't apply substances to them, and also the simulation seems to ignore the 3d model.

I notice that it's not a problem of my particular 3d model, but also the other obj files in your download section.

As an example here you will see that no "volumes" are created when importing one of obj file:

Is there a way to "promote" faces to create a volume?
I can create groups, but still this doesn't work.

Is there any other way to influence the export of my 3d model to have also "volumes"?

I'm exporting my 3d model from 3dsmax. Is there a different procedure to export it?
Is it better to use a different file format that doesn't give this problem?

Can you please advice?
Thanks for your support

OBJ doesn't support volumes

The Alias/Wavefront (.obj) format only supports facets as vertices, edges, and faces. Once you import your faces into Caedium you need to use the Volumes->Stitch tool on the Geometry Tool Palette with the Topology Only property enabled to create a volume.

If your faceted faces do not form a water tight flow volume then you'll need to first modify them either in 3ds Max or Caedium. Your flow volume needs to be a single volume that models the air around or through your geometry, i.e., the inverse of your original object.

Facets (as in .obj) are not a good format for representing geometry with curvature. If you have the option I'd suggest creating your geometry in a CAD system (analytic geometry) and use STEP (.stp) to transfer your model into Caedium.