This Demonstration models compressible flow of air through a reversible nozzle/diffuser. The inlet pressure , exit velocity , and exit temperature are calculated from mass and energy balances. Because the system is reversible, the entropy change is zero. The air exits as a free jet at atmospheric pressure (). Set the outlet diameter and inlet velocity with sliders. When , the device acts as a nozzle, with the inlet pressure higher than the exit pressure () in order to provide the energy needed to accelerate the air through the nozzle. When , the device operates as a diffuser, and the exit pressure is higher than the inlet pressure (). Select "plots" to plot the outlet velocity or outlet temperature as a function of outlet diameter for compressible air flow (blue) and incompressible air flow (green). Air is assumed to behave as an ideal gas and all flow is subsonic.