Flow of compressible fluids in conduits

When a compressible fluid, ie a gas, flows from a region of high pressure to one of low pressure it expands and its density decreases. It is necessary to take this variation of density into account in compressible flow calculations. In a pipe of constant cross-sectional area, the falling density requires that the fluid accelerate to maintain the same mass flow rate. Consequently, the fluid s kinetic energy increases. 

It is found convenient to base compressible flow calculations on an energy balance per unit mass of fluid and to work in terms of the fluid s specific volume V rather than the density p. The specific volume is the volume per unit mass of fluid and is simply the reciprocal of the density  

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