"INTRODUCTION One-dimensional Flow of a Perfect Gas The application of one-dimensional flow analysis to real duct flow@ in which the fluid conditions vary across the duct@ is a good approximation provided that the distribution of properties across the duct does not vary markedly from one section to another. This requires approximately fully-developed flow and the conditions that the rate of change of duct area is small and that the radius of curvature of the duct centre-line is large compared with the cross-sectional dimensions. Most common gases@ including air@ may be considered to behave as perfect gases* provided that the liquefaction or the dissociation temperatures are not approached. One-dimensional Isentropic Flow Applications Although one-dimensional isentropic flow can only be realised exactly in the flow of a perfect gas with zero viscosity and thermal conductivity@ the relations that may be derived for it do find useful application in real gas flows. The isentropic flow relations are most commonly applied in the relation between ""static"" and ""total"" conditions at a point in a flow since these states can be related through an isentropic? deceleration. Use of these relations does not require the flow to be isentropic since they are applied at a point or@ with the assumption of one-dimensional flow@ across a single section. Provided that due account is taken of the non-isentropic changes in a real flow@ e.g. via the total or static pressure losses or the increase in total temperature due to heat addition@ the isentropic relations may be used to evaluate conditions at one station from known values at another. In real gas flows the change in entropy depends not only on the viscosity and thermal conductivity of the gas but also on the velocity gradient and temperature gradient in the flow. In flows at high Reynolds numbers these effects may often be considered to be confined to a thin boundary layer and the flow outside this analysed as isentropic. This is of particular application in external flows. In internal flows the rate of heat transfer is often small enough for the flow to be assumed adiabatic and if the Reynolds number is high enough for viscous effects to be small the flow may be closely approximated by isentropic flow. A particular example of this application is the analysis of flow from a large reservoir to a smaller duct working section (see Section 3.4). Mass Flow Functions From the one-dimensional flow equation of continuity for a perfect gas@ functions of the fluid mass flow rate@ pressure and temperature and the duct area may be derived that are each a function only of the Mach number and ratio of specific heat capacities. These are commonly known as the ""mass flow functions"". An example of the application of the mass flow functions is given in Example 2 in Section 5. * At very high temperatures@ while the perfect gas equation of state (p = ??T) remains a close approximation@ the assumption of constant specific heat capacities may not. Some flow functions for such a ""thermally-perfect@ calorically-imperfect"" gas are given in Item No. Aero. S.00.01.10. ? See Section 3.2."