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The isentropic stagnation state is the state a flowing fluid would attain if it underwent a reversible adiabatic deceleration to zero velocity. There are both actual and the isentropic stagnation states for a typical gas or vapor. Sometimes it is advantageous to make a distinction between the actual and the isentropic stagnation states.
In fluid dynamics, an isentropic flow is a fluid flow that is both adiabatic and reversible. That is, no heat is added to the flow, and no energy transformations occur due to friction or dissipative effects. For an isentropic flow of a perfect gas, several relations can be derived to define the pressure, density and temperature along a streamline.
Since the process is isentropic, the stagnation properties (e.g. the total pressure and total temperature) remain constant across the fan. The theory was described by Theodor Meyer on his thesis dissertation in 1908, along with his advisor Ludwig Prandtl, who had already discussed the problem a year before. [2] [3]
For isentropic compression, ν ( M 2 ) = ν ( M 1 ) − θ {\displaystyle \nu (M_{2})=\nu (M_{1})-\theta \,} where, θ {\displaystyle \theta } is the absolute value of the angle through which the flow turns, M {\displaystyle M} is the flow Mach number and the suffixes "1" and "2" denote the initial and final conditions respectively.
Point 3 labels the transition from isentropic to Fanno flow. Points 4 and 5 give the pre- and post-shock wave conditions, and point E is the exit from the duct. Figure 4 The H-S diagram is depicted for the conditions of Figure 3. Entropy is constant for isentropic flow, so the conditions at point 1 move down vertically to point 3.
For this model, the duct area remains constant and no mass is added within the duct. Therefore, unlike Fanno flow , the stagnation temperature is a variable. The heat addition causes a decrease in stagnation pressure , which is known as the Rayleigh effect and is critical in the design of combustion systems.
Three assumptions will be made: the flow from behind the normal shock in the inlet is isentropic, or p t4 = p t2, the flow at the throat (point 4) is sonic such that M 4 = 1, and the pressures between the various point are related through normal shock relations, resulting in the following relation between inlet and throat pressures, [1]
This set of models is often referred to collectively as SGP4 due to the frequency of use of that model particularly with two-line element sets produced by NORAD and NASA. These models predict the effect of perturbations caused by the Earth’s shape, drag, radiation, and gravitation effects from other bodies such as the sun and moon.