Diameter of aperture, or diameter of pipe?
 

I am using ANSYS to model turbulent flow through a pipe (2D), with a cavity in the middle. My problem, is that there is flow coming from an opening in the cavity, and to successfully model this problem I need to find out the turbulent intensity, which requires a diameter. The problem is, do i take the diameter of the aperture, or the diameter of the cavity itself? the cavity is about 0.6m wide, and the aperture is about 0.04m wide. the flow is flowing upward from the aperture toward the main pipe.
Thanks!!:)