Compressible Flow Fundamentals Questions and Answers

Answer: C

Both are true and the second explains the first. Since the body outruns its own pressure signals, those signals can never get ahead of it and are confined within a cone. The fluid outside that cone is the zone of silence, which is why a supersonic aircraft is heard only after it has passed.

Answer: D

Both statements are true and the second is the reason for the first. The normal component is smaller than the free-stream speed by the cosine of the sweep angle, so the section behaves as though flying more slowly and reaches sonic conditions later. This is why every high-subsonic transport has a swept wing.

Answer: A

The assertion is false while the reason correctly states the rule. The factor tends to zero as Mach one is approached, so the predicted coefficients become unbounded, which is physically impossible. The correction is a linearisation and fails once local supersonic flow appears, that is above the critical Mach number.

Answer: B

Both are true and the second is the reason for the first. The threshold is chosen by accepting a five per cent density variation as negligible, and working backwards through the isentropic relation puts that at roughly Mach 0.3. It is a convention, not a physical discontinuity in the flow.

Answer: C

Both statements are true and the second explains the first. Temperature falls steadily with altitude through the troposphere, and since the speed of sound follows its square root, the speed of sound falls with it. A jet holding a constant Mach number therefore has a lower true airspeed the higher it climbs.

Answer: D

The assertion is false and the reason is true. Adiabatic is not enough: the process must also be reversible. A shock wave is the standard counter-example, being adiabatic yet strongly irreversible, and it destroys stagnation pressure while leaving stagnation temperature untouched.

Answer: A

Both statements are true, but the second is not the reason for the first. The critical value marks where sonic flow first appears anywhere on the surface; drag does not rise sharply until the supersonic pocket has grown large enough to end in a strong shock, which happens a little later. Where the sonic point first appears is a separate matter.

Answer: B

Both are true and the second explains the first. Once the gas is fixed, the specific heat ratio is a constant, so the stagnation-to-static ratios and the area ratio all reduce to functions of Mach number. That is exactly what makes the familiar one-column isentropic tables possible.

Answer: C

The first two are correct: a shock is adiabatic, so total energy survives, but it is irreversible, so stagnation pressure does not. The third violates the second law outright. Entropy must rise across a shock, and it is that rise which forbids expansion shocks from existing at all.

Answer: D

The first two describe correct operation on the supersonic branch. The third is wrong: as the supersonic flow accelerates in the diverging duct its static pressure falls. Pressure would rise there only on the subsonic branch, or across a shock standing in the divergent section.