Convergent Divergent inlet duct

A form of duct, or passage, used for the inlet air to supersonic airplane engines. The forward section of the duct is convergent (it becomes smaller in the direction of the airflow). The air entering the duct at a supersonic speed is slowed to a speed of Mach one by a shock wave in the narrowest part of the duct. Beyond this point, the area of the duct increases-becomes divergent. The air, now traveling slower than the speed of sound, is further slowed, and its pressure increases.