The venturi mouth would probably be closer to 10-15 times the size of the Ducted Fan. Bruce I think you are right it would not look like the stacks on a dragster, but rather the elongated shape of a wind sock, it would have to be long to allow for the pressure needed to spin the fan. The venturi for this setup would look a lot like the old microwave horn antennas used for satellite systems years ago. The size and mass of the system is going to be the down side I think. Then again to make something that will work in wind anywhere from 3-5 mph up to 70mph is going to require a lot of work as well.
If you want to try an experiment take a hyperflow ducted fan unit with the brushed motor it come with and use that for a test, it is around 3 inches which is 1/12 the size mentioned my the OP. Set it up with a venturi so that it will produce 8 volts at 400ma load in a 3 mph wind (then increase the load so you are producing 10 amps) and that will give you the size and proportions needed for your inlet concentrator system. After you set it up to work in the 3 mph wind, try to use it in a 30 mph wind it will blow apart unless you have some sort of pressure bleed system, so you do not over speed you turbine/generator. The problem of wind power is that the wind is not constant or at a set speed and this is something you have to take into account, as well as storms when the sky is the limit. I have no doubt that something could be made to work it is just that it would be a massive, complex system , that need to be mounted on a tower high enough to clear obstacles and turbulence.
As I mentioned before I had though about something like this myself, but the logistics of making it work made me shy away from trying it.[sm=75_75.gif]