Personally, I prefer to avoid calculations when the geometry will return the answer (see attached).
Less prone to errors and fewer clicks to boot!
...and a really big advantage - if you change the geometry (within reason, for example not adding another hole) the answer is automatically updated for you. If you change the geometry subtantially (like adding another feature rather than simply changing size) it is still much easier to get updated answer than doing hand calculations (even if the hand calculation is simply a subtraction porblem).
...another big advantage to using multi-body with the Sculpt rather than combining is the second body is available for other things like Cores or analysis.
...and I suspect that is not what the OP was after anyhow
http://forums.autodesk.com/t5/Autodesk-Inventor/How-to-fill-empty-volumes/m-p/3015056
cmbv, if you are interested in CFD you will need to become very keen on computers and when you post a question it helps if you include all of the information needed to get the solution you want. It is best to continue a discussion in a single thread until the solution is achieved.
Can you attach the real problem file here rather than the simplified representation?
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