Is there a way in solidworks to create a part from the "void" or "negative" space in a part?  For example, if I where to extrude a square into a block and then cut a circular hole through that block, is there a way to have solidwork automatically recognize the cylindrical space within the block and make that a separate part?

Ultimately, I'd like to import that new part (of the air space) into fluent to run a CFD analysis.

Thanks-