# Are subsonic nozzle fully expanded?

this is about converging subsonic nozzles. Are they fully expanded? meaning exit pressure=ambient pressure?

What about diverging subsonic nozzle? Are they fully expanded? what would happen if the exit pressure was greater than ambient? shock formations?

• Fully expanding nozzles in the subsonic regime... wouldn't that mean exit velocity = 0? This would be great for a turbo-prop engine but not for a turbofan/turbojet engine. Thrust on turbofan engines is often measured as an engine pressure ratio (EPR). During takeoff this ratio can reach 1.5 or 1.6, which means the exit pressure is 1.6x greater than the intake pressure. Shocks only form when the air exits at super-sonic speeds.
– Jan
Commented Oct 27, 2019 at 14:49
• Yes, to convert the built up pressure and utilize its full potental, they are typically fully expanded. Commented Nov 19, 2021 at 11:34
• In some cases it might be good to overexpand your exhaust, to keep it behind the engine Commented Nov 19, 2021 at 11:35
• @Jan The static pressure would be 0, and the dynamic pressure (velocity) would be maxed out. Static pressure + Dynamic pressure = Total pressure. After combustion Static pressure would be high and dynamic pressure would be low. eg. 10 Sp + 2 Dp = 12 Tp Moving down the converging duct, the static pressure will gradually fall, and the dynamic will rise. About halfway. 6 Sp + 6 Dp = 12 Tp At the end of the converging duct (if we have full expansion, all static pressure will have been converted to dynamic pressure - velocity). 0 Sp + 12 Dp = 12 Tp Commented Nov 19, 2021 at 11:44