What occurs to the airflow through a nozzle when it transitions from subsonic to supersonic conditions?

When airflow transitions through a nozzle from subsonic to supersonic conditions, the air velocity increases. This phenomenon is largely attributed to the design of the nozzle, which typically has a converging section followed by a diverging section, known as a converging-diverging nozzle or Supersonic nozzle.

In subsonic flow, as the air approaches the narrowest portion of the nozzle (the throat), its velocity begins to increase as the cross-sectional area decreases, according to the continuity equation. At this point, as the pressure drops, the airflow accelerates, leading to an increase in velocity. When the airflow reaches the throat at the speed of sound (Mach 1), transitioning into a supersonic state occurs as the airflow moves into the diverging section of the nozzle. Here, the nozzle's geometry causes the flow to continue accelerating beyond the speed of sound.

In summary, this increase in velocity is essential for supersonic flight and is a result of the physics governing compressible flow, where as flow transitions from subsonic to supersonic, the kinetic energy of the air increases, resulting in higher velocities.