I don't remember if FloXpress reports the internal volume of a cavity. Certainly it could, and it's not difficult to use, once you get the hang of it.
Or you could subtract a copy of the part from a non-hollow solid with the same exterior geometry, yielding a new solid with the volume of the original object's interior.
Or... I think there are some plastic- molding oriented commands that can generate or report internal volumes.
I think by negative space you mean the volume of a cavity. Using a CFD code to find a volume like this is overkill. If it is a pipe, cap both ends with a planar surface and knit to form solid.
All solids in SolidWorks are knit together from surfaces. A solid has two properties that set it aside from surfaces. The surfaces in a solid form a "water tight" volume and all surface normals point out from the inside of the volume.
The knit command has an option to form a solid if the surfaces included in the selection list comprise an enclosed, "water tight" volume.
