Fluid flow uniformity parameter

Hello

I've modeled something like a wind tunnel and because the fan has a hub at the center (with zero air flow), the flow profile at the end of the wind tunnel is not going to be uniform.
I am in need of an explicit quantitative parameter that can express the degree of uniformity of the flow at the end of the tunnel, i.e. one parameter that will define the unifromity of flow over the entire cross sectional area at the exit of the tunnel.

My fluid dynamics knowledge is inadequate, so I'd really appreciate any help that i get.

Thanks in advance.