It is possible to obtain physical picture of the flow through a normal shock by employing some of the ideas of Fanno line and Rayleigh line Flows. Flow through a normal shock must satisfy Eqs (41.1), (41.2a), (41.3), (41.5), (38.8) and (41.6).
Since all the condition of state "1" are known, there is no difficulty in locating state "1" on T-s diagram. In order to draw a Fanno line curve through state "1", we require a locus of mathematical states that satisfy Eqs (41.1), (41.3), (41.5), (38.8) and (41.6). The Fanno line curve does not satisfy Eq. (41.2a).
While Rayleigh line curve through state "1" gives a locus of mathematical states that satisfy Eqs (41.1), (41.2a), (41.5), (38.8) and (41.6). The Rayleigh line does not satisfy Eq. (41.3). Both the curves on a same T-s diagram are shown in Fig. 41.5.
We know normal shock should satisfy all the six equations stated above. At the same time, for a given state" 1", the end state "2" of the normal shock must lie on both the Fanno line and Rayleigh line passing through state "1." Hence, the intersection of the two lines at state "2" represents the conditions downstream from the shock.
In Fig. 41.5, the flow through the shock is indicated as transition from state "1" to state "2". This is also consistent with directional principle indicated by the second law of thermodynamics, i.e. s2>s1.
From Fig. 41.5, it is also evident that the flow through a normal shock signifies a change of speed from supersonic to subsonic. Normal shock is possible only in a flow which is initially supersonic.
