Plotting Nusselt's number and Flow in a rectangular Channel
 

I am simulating flow in a Solar Chimney Ventilation system; a solar chimney attached to a room, and I'm interested in Plotting Nusselt's of the Solar chimney wall as a function of the velocity flow (Flow rate) in the room, noting that there are various values of velocity at different points in the room. How do I find the average velocity for the room to do this plot?