For downstream pressure less than the choked pressure, which statements are valid?

• A. The velocity at the throat equals the speed of sound.
• B. The velocity and mass flow cannot be increased further by reducing downstream pressure.
• C. Both of the above.
• D. Neither of the above.

Answer: (C)

When downstream pressure is reduced below the critical value, the flow becomes choked and the throat reaches Mach 1. At Mach 1, the velocity equals the local speed of sound, so the throat velocity is sonic (the speed of sound depends on the throat temperature). In this choking regime, lowering downstream pressure further cannot increase the mass flow rate or the throat velocity, because the flow is limited by the upstream conditions and the throat area. Therefore, both statements are true.