The supersonic engine air intake is designed to slow down the supersonic external flow to a subsonic speed that can be tolerated by the compressor. Even if an aircraft is flying at supersonic speed, jet engine turbines require subsonic airflow at the entrance to the compressor. To perform this work, different kinds of air intake are utilized. The higher the Mach number, the more complex they are.
To reduce the supersonic airflow, the air intake features what is called flow or shock diffuser. Basically, it is the air duct in the jet engine that widens steadily in the direction of flow to slow down the speed through expansion and increase the pressure. At subsonic speed, the diffuser cross-section area increases in the direction of flow; at supersonic speed, however, the area diminishes. For aircraft traveling at the top speed of Mach 1.5 a normal shock diffuser is usually enough to efficiently reduce the supersonic airflow to the speed required by the compressor. This simple type diffuser does not require any sophisticated mechanism for adjusting the flow. The normal shock is attached to the inlet lip at the design point of the diffuser as maximum pressure recovery is steadily reached.
The deceleration (reduction) of flow and build-up of pressure is obtained in two steps; 1) the supersonic flow is suddenly decelerated through a normal shock to subsonic velocity with an accompany abrupt increase in static pressure; 2) in the diffuser divergent duct, where the flow is still faster than would be acceptable to the compressor, deceleration of flow still continues, with pressure increasing further.
A major disadvantage of a normal shock diffuser is a fast reduction in efficiency as Mach number increases. The reason is that normal shock sustains large total pressure losses, even if the shock remains attached to the intake lip. An intake configuration able to provide such a staged supersonic decelaration will need either a wedged or a cone-shaped body suitably set within a subsonic diffuser.
The F-16 fighter aircraft jet engine has a fixed-geometry intake. It means it has no movable parts. It was designed so, in order to cut back on costs. Located in a forward position under the fuselage, it has also a normal shock diffuser. This is so, because the aircraft was designed to be extremely maneuverable, being capable to operate at high angles of attack. Compared to other aircraft, the F-16 features a short duct, which minimizes flow distortion and contributes to the fighter light weight.
Diagram of normal shock diffuser operation
