How does one define a volume for a volume goal?

Imagine the case of a simple pipe flow with a cylindrical internal flow volume.   Now suppose I am interested in a subset of that volume for some analysis goal...say a small cubic fluid space within that pipe.

This is what I've tried:

1. model the cube as a part in the assembly.

2. click on "Goals" -->"Insert Volume Goals..." and select the cube.

3.  click on "Input Data"-->"Component Control" to turn off the cube. (This makes it a fluid volume as far as the calculation is concerned?)

In any case, when I run my analysis, the cube still obstructs the flow as if it's still part of the solid.

I have also tried a second method whereby I modeled the whole project as a part rather than an assembly and created the cube volume as a solid body with the same screwy result.  If I suppress the cube then the flow returns to normal but then I can't use it to define a goal.

Am I missing something basic?