Hello , I have a rather odd question. I am trying to find a phenomenon that occurs in nozzles but am quite unable to name the phenomenon per se. The vague description of the phenomenon I remember is that when a supersonic flow occurs in a nozzle (or superheated steam), there is heat produced due to the friction generated at the nozzle walls. This causes the enthalpy drop to be lesser than expected, hence an issue with nozzle efficiency arises. I am not looking for the difference in isentropic and adiabatic expansion in nozzles. I remember a vague name for this phenomenon (metaheating) but a search does not come up with anything.