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You would need to use of the tools in HyperMesh CFD to construct additional surfaces, so that you have a closed volume for fluid.
Push or Pull Surfaces (altair.com)
Extend Surfaces (altair.com)
Repair Surfaces (altair.com)
Simplify Region (altair.com)
This might help:
HMCFD-T: 1000 Basic Geometry Editing (altair.com)
If you upload you CAD, it might be possible for someone to create a short video to show the process.