Hi, I'm very new to Solidworks and have a question about obtaining the inside volume of a pipe I built so it can be modelled with CFD software.

I've designed a pretty basic pipe in Solidworks so now I just need the inverse of it, effectively.  I've searched around a lot and found solutions that sounded promising but none of them seem to work, unless I'm doing it wrong.

So far I've experimented most with using a cavity or a subtract combine, but neither method gave me what I wanted.  I think it's probably just because I don't understand the software very well, so I'd appreciate the most step-by-step answer possible (I had never even used CAD software last week, I'm an absolute beginner).

I'll attach a picture of one of the pipes I want the inverse of (thin pipe with larger inlet/outlet) and try to show you what happened when I do a subtract which I get the feeling is my most promising option.  I'll also attach the Solidworks files in case they help too.

First I made the pipe part file, then I opened a new part and made a cylinder that should just cover the total diameter and length of the built part.

I then did Insert>Part>Pipe Part.

I then did Insert>Features>Combine

Selected 'subtract' from the options, chose the new cylinder as the main part and subtracted the pipe.

I ended up with a solid cylinder just like the one I'd just made.  I don't see how that works - it shouldn't be solid if I subtracted my pipe from it, right?!

Thanks in advance everyone, I apologise for my ignorance!