Understanding Reynolds Number

Hey guys,

I'm looking for a little help understanding Reynold's Number. I know that its the ratio of the inertial forces versus the viscous forces. With that being said, everywhere I look, the Reynold's Number is calculated for flow in a pipe or duct or across some sort of surface. Why can't the Reynold's Number be calculated for a random point? For example, if i was interested in the Reynold's Number of air after exiting an A/C duct how could one apply any of the general equations to that? The general equations all use some sort of characteristic length or diameter to make the calculations.
I assume air (since it is viscous) will have some effect on flow that is introduced into is (such as air exiting a duct), so I would assume there has to be a way to calculate the Reynold's Number for air (knowing the viscosity of the air and the velocity) that is not in any duct or tube or moving across any surface.

Any help understanding this would be appreciated.

Thanks.

 

I do realize that Reynold's Number is not a property of the fluid.. but is dependant on the fluids properties.
My confusion comes with determining the Reynold's number in an open environment such as a room or other open space where there are no defined boundaries where the flow exists.

So if I were to measure the velocity of the flow out of an A/C duct at a distance of 1 meter from the exit of the duct, would I consider 1 meter to be the characteristic length?

Thanks for helping.

 

"Yep I suppose you could, but the real question is why would you want to? What is intersting about the flow in free air after a duct?"

Well there is nothing interesting in particular. That was just an example I came up with to explain my question. It had just occured to me that there must be a way to measure flow in free air, but I couldn't seem to find anyplace that touched upon such an occurence.