Expansion Waves Questions and Answers

Answer: B

The assertion is false and the reason is true. Turning away from itself is exactly what accelerates the flow: the corner produces an expansion, so the Mach number rises while pressure, temperature and density all fall. It is a concave corner that decelerates the flow, through a shock.

Answer: C

Both are true and the second explains the first. The recompression needed to match ambient pressure is achieved by shocks, and if the mismatch is large enough the shock moves inside the nozzle and the adverse gradient tears the boundary layer off the wall. The resulting side loads are a genuine structural hazard at start-up.

Answer: D

Both statements are true and the second is the reason for the first. Because each face of the section is flat and each corner is sharp, the flow over it is fully determined by exact relations rather than by any linearisation. The result is exact within the inviscid model, though it says nothing about viscosity.

Answer: A

Both are true and the second explains the first. Each successive compression wave sits in slower flow and lies at a larger angle, so the family converges and the waves eventually cross and merge. Expansion characteristics do the opposite and spread apart indefinitely.

Answer: B

The first two are correct: the flow accelerates and, because no entropy is generated, the stagnation pressure survives. The third is wrong. An expansion converts thermal energy into kinetic energy, so the static temperature falls along with the pressure and density.

Answer: C

The first two correctly locate the two edges of the fan. The third is false: because the flow has accelerated, the downstream Mach angle is the smaller, so the characteristics diverge and the fan opens out. That divergence is what distinguishes an expansion from a compression.

Answer: D

The first two state the rule correctly: a solid wall reflects in kind, a free boundary reverses the sense. The third breaks it. A shock reflecting from a wall produces another shock, since the deflection has to be cancelled by a turn in the same sense.

Answer: A

The first two describe the underexpanded case: the gas is still above ambient pressure and continues expanding outside the nozzle through fans at the lip. The third belongs to the overexpanded case, where the exit pressure is too low and shocks are needed to recompress the plume.

Answer: B

The first two are correct and are why the function is tabulated. The third is false: it approaches a finite limit of about 130.45 degrees for air, at which point the pressure has fallen to zero and no further expansion is possible.

Answer: C

The first two correctly describe the wave pattern. The third is false: unlike the subsonic case, a supersonic plate carries wave drag even with no viscosity, because the pressures on its forward- and rearward-facing surfaces no longer balance.