In the expansion wave, what happens to static pressure?

• A. Decrease
• B. Increase
• C. No change
• D. Oscillate

Answer: (A)

In an expansion wave, the flow turns outward and accelerates, converting some of the flow’s energy into kinetic energy. Because the process is isentropic, the total (stagnation) properties stay the same, but static pressure falls as velocity increases. So the correct outcome is that static pressure decreases.

This matches what happens in a Prandtl-Meyer expansion: the flow expands smoothly, speeds up, and the static pressure drops. The other options would imply either compression, no change, or an oscillation, which don’t occur in a simple expansion wave.