autodesk autocad - the program allows modeling in 2d and 3d environment - but for the purpose of this excersice i would recommend using either autodesk Inventor or Solidworks

i think you should be able to ge a good running 3d model from a 2d set of prints with either of these programs ... but setting up a flow test would be alot more tricky

i can help out with the 3d models in Inventor if you need ... juss send me the 2d plans and ill build the models for you

DC2-PWR - for what you are doing - you would be better of ussing a program like Autodesk Alias