Hi YF,
Currently Flow Design only simulates air flow. There is a request on the IdeaStation section of the message board about simulating water flow, so I would encourage you to add kudos & comments to that -- the development team monitors these requests constantly.
Flow Design has been optimized for external flow situations, including flow around buildings. See this help topic for more information. There is also a request pertaining to internal flow (it is written in Portugese, so you may need to use Google Translate or similar) that I would encourage you to kudo & comment.
Ryan