# How to size a fan for open air application?

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.

• Can you add any more detail about what you're ultimately trying to accomplish? This is a pretty complex problem, and depends a lot on ambient conditions, position of the fan in the room (if it's close to a wall or floor) and what kind of fan it is. If we know what you are trying to do, we might be able to narrow the scope of your question to give you a better answer. Dec 2, 2015 at 18:35
• In addition to Trevor's comments, it also depends on whether the fan is being used outside or inside, & if inside, the size of the room matters & the roughness of the sides of the room.
– Fred
Dec 3, 2015 at 1:23

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%

• Going straight to CFD models may be a little much for a one-off design.
– hazzey
Dec 6, 2015 at 13:29