In supersonic airflow, static pressure behind an expansion wave will

• A. Decrease relative to the pressure ahead of the wave
• B. Increase relative to the pressure ahead of the wave
• C. Remain the same as the pressure ahead
• D. fluctuate

Answer: (A)

In supersonic flow, turning the flow away from its direction creates a Prandtl–Meyer expansion fan, which is an isentropic expansion. As the flow expands, it accelerates (Mach number increases) and the static pressure drops. This energy shifting from pressure to kinetic energy lowers the static pressure behind the expansion wave compared with the pressure ahead of it. So the static pressure behind the expansion wave decreases relative to the pressure ahead.