If you got the book, please read 2.6 section (page 135). Here is my sumary of this section:
An airfoil in a wind tunnel (wingtips were butted against both sidewalls of wind tunnel). The control volume is 'abcdefghia', it has unit depth in the z direction (perpendicular to the page). Through some steps that you can read from pages, we got:
$$ Drag\space on \space the \space airfoil = -\unicode{x222F}_{ai+bh}(\rho \vec V.\vec {ds})u $$
In experinment, how can we know where the line bh is, how long it is..?
In the picture above, can we move line bh backward (to the right) ?
How big is the wind tunnel or how far from top to bottom walls of the wind tunnels (with given airfoil)is in oder to have the lines ab and hi ?
Do you feel this method is very inaccurate with a lot of negligibly and pitot rake thing (page 141) ?
@Peter: Thank you, nice to hear that !! . Actually, I thought I understood this section until I face the problem 2.2 at page 198 (by the end of chapter 2). I reviewed this section and a lot of questions arised. First, sorry because English is not my mother language.
1st question: I really have no idea where the line bh is (both in vertically and horizontally) and how is the length of bh ?.
2nd question: Suppose we already have the line bh located properly. My physical feel tell me that the line bh can move backward and the velocity gradinent $ u_2 $ = f(y) stay the same. But imagine in real life, if we move the line bh far enough, the velocity gradient $u_2$ will be uniform and equal the freestream velocity $u_1$ !!
About the third question, I think: we must have top and bottom walls of the wind tunnel that the pressure over them is equal to the free stream pressure, I mean: move them far from the airfoil in vertical direction so that the pressure over them is uniform and equal to freestream pressure.
4th question: I still had no idea because I never use my hands to do these kind of experiments, but my instinct tell me it will have a lot of errors. My word maybe too dumb but I'd like to learn what people do in real experiment.