The movement in Figure 4 is always from the left to the right in order to satisfy the second law of thermodynamics. The Fanno flow model is also used extensively with the Rayleigh flow model. These two models intersect at points on the enthalpy-entropy and Mach number-entropy diagrams, which is meaningful for many applications. However, the entropy values for each model are not equal at the sonic state. The change in entropy is 0 at "M" = 1 for each model, but the previous statement means the change in entropy from the same arbitrary point to the sonic point is different for the Fanno and Rayleigh flow models. If initial values of "s""i" and "M""i" are defined, a new equation for dimensionless entropy versus Mach number can be defined for each model. These equations are shown below for Fanno and Rayleigh flow, respectively. Figure 5 shows the Fanno and Rayleigh lines intersecting with each other for initial conditions of "s""i" = 0 and "M""i" = 3. The intersection points are calculated by equating the new dimensionless entropy equations with each other, resulting in the relation below. The intersection points occur at the given initial Mach number and its post-normal shock value. For Figure 5, these values are "M" = 3 and 0.4752, which can be found the normal shock tables listed in most compressible flow textbooks. A given flow with a constant duct area can switch between the Fanno and Rayleigh models at these points.