Which statement about the convergent nozzle for subsonic flow is true?

Which statement about the convergent nozzle for subsonic flow is true?

• A. The velocity increases
• B. The velocity decreases
• C. The velocity remains constant
• D. The velocity fluctuates

Answer: (A)

In a converging nozzle with subsonic flow, the area the fluid must pass through gets smaller along the route. Because the mass flow rate must be the same at every cross-section, the velocity must rise as the area decreases (rising velocity converts pressure energy into kinetic energy). As the flow speeds up, the static pressure also drops. This acceleration continues through the converging section and is the defining behavior of subsonic flow in a converging nozzle. The other possibilities would imply the flow isn’t being compressed into a smaller area or is oscillating, which isn’t how steady subsonic flow through a converging nozzle behaves. If the flow reached sonic speed at the throat, choking could occur, but for general subsonic operation the velocity indeed increases through the converging section.

In a converging nozzle with subsonic flow, the area the fluid must pass through gets smaller along the route. Because the mass flow rate must be the same at every cross-section, the velocity must rise as the area decreases (rising velocity converts pressure energy into kinetic energy). As the flow speeds up, the static pressure also drops. This acceleration continues through the converging section and is the defining behavior of subsonic flow in a converging nozzle. The other possibilities would imply the flow isn’t being compressed into a smaller area or is oscillating, which isn’t how steady subsonic flow through a converging nozzle behaves. If the flow reached sonic speed at the throat, choking could occur, but for general subsonic operation the velocity indeed increases through the converging section.