Converging-diverging, often referred to as a converging-diverging nozzle, is a type of fluid flow device that changes the flow area to control the speed and pressure of a fluid. In this design, the flow first converges, or narrows, which increases the fluid's velocity and decreases its pressure. This is followed by a diverging section, where the flow area expands, allowing the fluid to slow down and increase in pressure. This principle is commonly used in various applications, including jet engines and rocket propulsion systems. By efficiently managing the flow of gases, converging-diverging nozzles help optimize performance and thrust, making them essential in aerospace engineering.