Assume that you have an axial fan placed in open air with no duct. How can I go about calculating the air speed at a distance x from the fan?
The fan is about 1 m above the ground in an open space. I want to use the fan to generate power from a wind turbine, so I need to know what the air speed will be at the turbine.
I think that an Inlet/Outlet disk in free air is probably general enough that some smart dead person solved it theoretically. However, mortals use OpenFOAM or other CFD software to solve problems like these ;-).
For your application, that amount of work is probably not justified. Basic rule is that a larger diameter fan will allow you to have a larger separation distance between the two (for visual appeal) and still maintain reasonable efficiency. By reasonable I mean 10% of available wind energy as opposed to 1% or less. I assume you are turning a wind turbine for demonstration purposes; or we have an entirely different discussion on our hands ;-)
For a shot in the dark, I would estimate that with a wind turbine and fan of equal diameter, you would have the following "wind coupling" efficiency (not considering blade design, turbine rpm, or motor/generator efficiency).
Close as possible: 50%
1 Diameter away: 10%
2 Diameters away: 2%
3 Diameters away: 1%
