question about general momentum theory
 

Hello everyone.
First, I am really sorry because my question is general fluid dynamics.
I have studied the general momentum theory for wind turbine (Glauert, 1935..etc.)
However, I cannot understand a bernoulli's equation across the disk. Please see the next link

https://www.e-education.psu.edu/aersp583/node/515

In figure 2a-9, Bernoulli's equations can be used before and after the disk, respectively (upstream and downstream of the disk in Fig. 2a-9).
However, Eq. a.28 shows the Bernoulli's equation for the region between the before and after the disk. Many books and thesis stated this equation in the view point of rotating reference frame, but I can not understand. The energy is conserved before and after the disk?

My English writing is poor. Sorry.