Question

Across an oblique shock wave in a calorifically perfect gas,

• A. the stagnation enthalpy changes
• B. the stagnation entropy changes
• C. the stagnation temperature changes
• D. the speed of sound changes

Hint:

Across any shock: entropy increases, speed of sound changes, total enthalpy remains constant, and stagnation temperature stays the same.

Answer 1

The Correct Option is B, D

Step 1: Understanding shock behavior.
A shock wave is an irreversible, compressive wave that causes sudden jumps in pressure, density, temperature, and velocity. Oblique shocks behave similarly to normal shocks, except the flow direction also changes. Step 2: Stagnation enthalpy.
Since shocks obey conservation of total energy, \[ h_0 = \text{constant} \] across the shock. Thus stagnation enthalpy does NOT change → (A) is false. Step 3: Stagnation temperature.
Stagnation temperature depends only on stagnation enthalpy (calorifically perfect gas). Since $h_0$ is constant, \[ T_0 = \text{constant} \] Therefore stagnation temperature does NOT change → (C) is false. Step 4: Stagnation entropy.
Shocks are highly irreversible processes. Entropy always increases across any real shock: \[ s_2>s_1. \] Thus stagnation entropy changes → (B) is correct. Step 5: Speed of sound.
The speed of sound is related to temperature: \[ a = \sqrt{\gamma R T}. \] Since static temperature increases across the shock, \[ a_2>a_1. \] So the speed of sound changes → (D) is correct.