I am a bit confused by the definition of an isentropic process in the flow of an ideal gas. isentropic implies reversible & adiabatic. for a process to be reversible, there are no losses to friction (viscosity in this case), for a process to be adiabatic, there is no heat transfer with the surroundings. That being said, when a compressible gas flows through a nozzle, there is a temperature change. When this happens, a temperature gradient occurs. Where there is a temperature gradient, heat transfer occurs within the gas. I was always taught that, according to the 2nd principle of thermodynamics, a heat transfer due to a temperature gradient (from hot -> cold areas) is irreversible...which is directly contradicting the definition of a reversible & adiabatic flow... Hope this is a pertinent question... Thanks a lot for any help.