Similarly, the lower stream tubes expand and the flow slows down.
2.
Conservation of mass says that the flow speed must increase as the stream tube area decreases.
3.
The picture shows that the upper stream tubes constrict as they flow up and around the airfoil.
4.
The volume flux through a stream tube, bounded by a surface of constant " ? ", equals " 2? ? ", as before.
5.
Using the concept of stream tube, the maximal achievable extraction of wind power by a wind turbine is 59 % of the total theoretical wind power ( see : Betz'law ).
6.
Now let us consider post-rotor : immediately after interacting with the rotor, the fluid velocity is still v _ { \ infty } ( 1-a ), but the pressure has dropped to a value P _ { D-}; far downstream, pressure of the fluid has reached equilibrium with the atmosphere; this has been accomplished in the natural and dynamically slow process of decreasing the velocity of flow in the stream tube in order to maintain dynamic equilibrium ( i . e . P \ rightarrow P _ { \ infty } far downstream.
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