thanks, but its more to do with axial fans which are very predictable from a design standpoint, where the centrifugal fan designs are not as much...
i know there are efficiencies lost with various designs, sizes, speeds, etc, but there must be a way to calculate ideal air flow based on a prototype design without having to build it
If you want a cheap way of estimating the performance of a given fan design, go download the performance curves of a geometrically similar model from a quality manufacturer.
What you are asking for is specialist knowledge beyond a normal engineering curriculum or experience typically acquired in this discipline.
To Chris' point, CFD is a lot cheaper than prototype trial and error. If you are serious about building fans, then it is not a big outlay. If this is for one fan, then buy one. If you simply must build your own, then copy someone elses and good luck.