The velocity of subsonic air as it flows through a convergent nozzle does which of the following?

• A. Remains constant
• B. Increases
• C. Decreases
• D. Increases then decreases

Answer: (B)

In subsonic flow, reducing the cross‑sectional area forces the fluid to speed up to conserve mass. As a gas moves through a converging nozzle whose area continuously decreases, the same mass flow rate must pass through smaller and smaller passages, so the velocity rises. This acceleration comes with a drop in static pressure and a rise in kinetic energy, until the flow potentially reaches sonic speed at the throat; beyond that point, a pure convergent nozzle cannot accelerate subsonic flow any further. Therefore, the velocity increases as the gas flows through a convergent nozzle.