The pressure of subsonic air as it flows through a convergent nozzle does what?

• A. Increases.
• B. Remains constant.
• C. Doubles.
• D. Decreases.

Answer: (D)

As subsonic air flows through a converging nozzle, the area decreases and the air must speed up. To accelerate, some of the pressure energy is converted into kinetic energy, so the static pressure drops. This pressure decrease continues along the convergent section, until the throat is reached. In a purely convergent nozzle, you can’t sustain subsonic flow past the throat, and the key point is that the static pressure decreases as the flow accelerates.