I would like to know how far above and below a wing's surface an observer must look in order to see no change to the airflow. Not simply visually but mechanically, assuming a perfect scenario. I am not particularly adept at mathematics, so if there is a rule of thumb, or a general upper boundary, that would suffice for the figure I need.
For a visual representation, a wind tunnel with smoke trails. At some point distant from the top and bottom of the wing in the tunnel, all flow lines will be identical again, implying that wings have an area of effect. How far is this point from each respective surface?
Another way to ask would be to say how far apart must biplane wings be (in a perfect scenario) for them to have no effect on each other?