Definition
Convergent-divergent ducts are engine air passages that first narrow (converge) and then widen (diverge) along the flow path. In a turbine engine inlet or exhaust, this shape is used to accelerate airflow through the sonic point at the narrowest section (the throat) and then continue accelerating it to supersonic speed in the diverging section, or to manage the pressure and velocity of high-energy gas flow.
Plain English
An engine duct that gets narrower, reaches its smallest point in the middle, then gets wider again. This shape is used to push air or exhaust gas to very high speeds, including faster than the speed of sound.
Context Anchor
Seen in turbine engine maintenance, especially when inspecting the airflow path between the compressor and the fuel-burning area of the engine.
Derivation
Convergent comes from Latin convergere, meaning to incline together — the walls move toward each other. Divergent comes from Latin divergere, meaning to incline apart — the walls move away from each other. The duct does both in sequence.
Why Pilots Care
On high-performance and supersonic-capable aircraft, the CD duct is what allows the engine to keep producing efficient thrust at very high speeds. Maintenance technicians need to recognize the shape and purpose so they understand why certain inlets and exhaust nozzles are built and inspected the way they are.
Analogy
Think of CD ducts like hallways for compressed air inside the engine. They do not create the air pressure; they guide that pressurized air where it needs to go.
Grounding Statement
Picture a tube that pinches in the middle like an hourglass and then opens back up — air squeezes through the pinch and then expands and accelerates as it exits.
Intuition Check
CD does not mean a compact disc here; it means compressor discharge. These ducts carry compressed air, not electricity, fuel, or oil.
Example Sentence 1
The technician inspected the convergent-divergent ducts on the exhaust nozzle for cracks and warping caused by high gas temperatures.
Example Sentence 2
CD ducts allow exhaust gases to reach supersonic speed in the diverging section.