Subsonic air through a divergent duct will experience

• A. Constant density, increase in pressure and a decrease in velocity
• B. Increase in density and velocity
• C. Decrease in pressure and increase in velocity
• D. Decrease in density and pressure

Answer: (A)

The main idea is how subsonic flow responds to a widening cross-section. For subsonic speeds, mass flow must be conserved as the area increases, so if the density is effectively constant (a common approximation at low Mach numbers), the velocity must decrease to keep the same mass flow rate. With the flow slowing down, the static pressure rises to satisfy energy and momentum considerations. Because the changes in density are small at subsonic, we treat density as roughly constant.

So, subsonic air moving through a divergent duct slows down, static pressure increases, and density remains essentially the same.