Definition
An exhaust duct used on turbojet and turbofan engines whose internal cross-section first narrows (converges) and then widens (diverges). The converging section accelerates the exhaust gases to the speed of sound at the narrowest point (the throat), and the diverging section then accelerates the flow further to supersonic speed, producing greater thrust at high-power, high-speed operating conditions.
Plain English
A specially shaped exhaust pipe on a jet engine that gets narrower in the middle and wider at the end. This shape lets the hot gases speed up to faster than the speed of sound as they leave the engine, which gives the engine more thrust.
Context Anchor
Seen in turbine engine and jet engine discussions, especially when studying exhaust nozzle design and engine thrust.
Derivation
Convergent comes from the Latin convergere, 'to incline together' — the duct walls lean inward. Divergent comes from divergere, 'to incline apart' — the walls lean outward again. Picturing the two words side by side gives the shape of the duct: in, then out.
Why Pilots Care
Enables efficient supersonic exhaust acceleration and higher thrust in high-speed jet aircraft.
Grounding Statement
Picture hot exhaust gas being squeezed through a narrow point, then expanding through a flared outlet as it rushes out the back of the engine.
Intuition Check
Do not assume the widening section always slows the gas down. At the right pressure and speed, the widening part of a convergent-divergent exhaust duct can help the exhaust speed up even more.
Example Sentence 1
The afterburning turbojet used a convergent-divergent exhaust duct to accelerate the gases to supersonic speed and maximize thrust.
Example Sentence 2
During inspection the crew checked the convergent-divergent exhaust duct for cracks after high-Mach flight.