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Since there is an increase in area, therefore we call this an isentropic expansion. If a supersonic flow is turned abruptly and the flow area decreases, the flow is irreversible due to the generation of shock waves. The isentropic relations are no longer valid and the flow is governed by the oblique or normal shock relations.
For an isentropic flow of a perfect gas, several relations can be derived to define the pressure, density and temperature along a streamline. Note that energy can be exchanged with the flow in an isentropic transformation, as long as it doesn't happen as heat exchange. An example of such an exchange would be an isentropic expansion or ...
Note that before and after the shock the isentropic relations are valid and connect static and total quantities. That means, p t o t a l ≠ p s t a t i c + p d y n a m i c {\displaystyle p_{total}\neq p_{static}+p_{dynamic}} (comes from Bernoulli, assumes incompressible flow) because the flow is for Mach numbers greater than unity always ...
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]
See relations between specific heats for the derivation of the thermodynamic relations between the heat capacities. The above definition is the approach used to develop rigorous expressions from equations of state (such as Peng–Robinson ), which match experimental values so closely that there is little need to develop a database of ratios or ...
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.
A schematic diagram of a shock wave situation with the density , velocity , and temperature indicated for each region.. The Rankine–Hugoniot conditions, also referred to as Rankine–Hugoniot jump conditions or Rankine–Hugoniot relations, describe the relationship between the states on both sides of a shock wave or a combustion wave (deflagration or detonation) in a one-dimensional flow in ...