Compressible Flow Fundamentals Questions and Answers

Answer: C

All four follow from the isentropic relations at Mach one with a specific heat ratio of 1.4. The critical pressure ratio of 0.528 is the one most worth committing to memory, since it is the back-pressure fraction below which a converging nozzle chokes and its mass flow becomes fixed.

Answer: D

A shock is adiabatic, so the stagnation temperature is carried through unchanged. Everything else listed alters sharply: static pressure and temperature jump up, the Mach number drops below one, and stagnation pressure falls because entropy rises.

Answer: A

Static pressure falls, not rises, as a supersonic flow accelerates through the divergent section; the option inverts the physics. The other three are all correct and together make up the standard derivation of the area-velocity relation.

Answer: B

At a fixed temperature the pressure and density change together, leaving their ratio and therefore the speed of sound unaltered. The other three are correct: helium's low molar mass gives it a large gas constant and a high speed of sound, and the tropospheric lapse rate cools the air with altitude.

Answer: C

Accelerating an isentropic flow converts thermal energy into kinetic energy, so the static temperature falls. The stagnation values stay fixed while the static ones drop, so all three ratios increase, which is exactly the trend the isentropic tables show.

Answer: D

The Prandtl-Glauert factor tends to zero as Mach one is approached, so the predicted coefficient grows without limit. That is a failure of the linearisation, not a physical prediction. The rule may be used only below the critical Mach number, well short of 0.98.

Answer: A

Once choked, the mass flow depends only on the stagnation state and the throat area, and it is proportional to stagnation pressure at fixed stagnation temperature. Back pressure has no effect at all, and enlarging the exit changes the exit Mach number rather than the flow passing the throat.

Answer: B

Acceleration over the upper surface means some point reaches sonic speed before the free stream does, so a supersonic pocket appears with the free stream still subsonic, and it usually terminates in a shock. This mixed character is precisely what defines the transonic regime and makes it so difficult.

Answer: C

Stagnation pressure is destroyed by the entropy rise across a shock, typically losing tens of per cent for a strong one. The other three statements are all correct, and the contrast between the two stagnation quantities across a shock is one of the most heavily examined points in the subject.

Answer: D

A thicker section accelerates the flow over it more strongly, so it reaches sonic conditions at a lower free-stream Mach number and drag divergence arrives earlier. Thinning the section, sweeping the wing and adopting a supercritical shape all push the onset the other way.