GATE 2022 AE – Question 25

Correct answer: (B) the stagnation entropy changes; (D) the speed of sound changes

Explanation

Across an **oblique shock** in a calorically perfect gas (steady, adiabatic):

• **A. Stagnation enthalpy changes?** No. The flow is adiabatic with no work, so $h_0$ is the same on both sides. ✗
• **B. Stagnation entropy (entropy) changes?** Yes. A shock is irreversible; the entropy increases. ✓
• **C. Stagnation temperature changes?** No. For a calorically perfect gas $h_0=c_pT_0$, so $T_0$ is constant across the shock. ✗
• **D. The speed of sound changes?** Yes. The static temperature rises across the shock, and $a=\sqrt{\gamma RT}$ increases. ✓

(The stagnation pressure falls, which is the real loss caused by the shock.) Answer **B and D**.