In the choked-flow equation for gases, what does P0 denote?

• A. Ambient atmospheric pressure
• B. Downstream pressure
• C. Upstream pressure
• D. Initial density

Answer: (C)

P0 is the pressure driving the flow from the inlet reservoir into the nozzle, i.e., the upstream (often called stagnation) pressure that sets the energy available to push the gas toward the throat. In choked flow, the mass flow rate becomes controlled by this upstream pressure once the throat reaches Mach 1, while the downstream pressure influences whether choking occurs but does not define P0 itself. This symbol is not the ambient atmospheric pressure, not the pressure on the exit side after the throat, and not a density. The upstream pressure reflects the conditions in the reservoir supplying the gas before expansion through the nozzle.