I have an assembly that has multiple cavities and need to select which cavity should be analysed by FloXpress. Is this possible ?

Using the image shown (file attached as I don't think the image is visible in this post): The upper flange is blanked and what I want to be the inlet. The bottom angled flange is open. The bottom flat flange is blanked and what I want to be the outlet. The straight tube between the two blanking flanges is the volume I want to show balls flowing through.

The problem is the D-shaped cavity to the right of the centre of the tube. This cavity is a part of the central round 'drum' component and is selected by FloXpress automatically. If I add a hole to this cavity then FloXpress automatically chooses another cavity in the assembly (not visible in this cross section). Do I have to keep putting holes in automatically selected cavities until the one I want is the only one left, or am I missing something ? How does FloXpress choose which cavity to analyse when multiple cavities are present ?

Thanks.