Rob
Forum Moderator

OK, boundaries are covered in the tutorials (click on Help in Fluent) and on the Learning section here (CFD/Fluids courses). That should cover the basics. 

In your case I assume your ducts have fans in them? Ie air is drawn into the duct and blown out of the other end. Do you need to model inside the duct? Or can you just set an inflow & outflow for the duct and ignore the middle bit? You images look a bit like tunnels, but at 0.5m across it's a lot smaller than I've modelled with ducts/jet fans present.