The GATE Grind

GATE 2021 AE – Question 24

Aerodynamics · Compressible flows: isentropic flow, normal and oblique shocks, Prandtl-Meyer flow, nozzles and diffusers · 1 mark · Multiple select

Across an oblique shock, which statements hold?

source diagram and its labels are reproduced in the attached image.
  1. Total pressure decreases.
  2. Mach number based on velocity tangential to the shock decreases.
  3. Total temperature remains constant.
  4. Mach number based on velocity tangential to the shock remains the same and that based on velocity normal to the shock decreases.

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Correct answer: (A) Total pressure decreases.; (B) Mach number based on velocity tangential to the shock decreases.; (C) Total temperature remains constant.

Explanation

Split the velocity ahead of an **oblique shock** into a component **normal** to the shock and a component **tangential** to it.

Check the statements:
- **A. The total pressure decreases.** True. A shock is irreversible and the stagnation pressure drops. ✓
- **B. The Mach number based on the tangential velocity decreases.** True. The tangential velocity is the same, but the temperature (and so the speed of sound) rises, so $M_t=V_t/a$ falls. ✓
- **C. The total temperature stays constant.** True. The flow is adiabatic. ✓
- **D. The tangential Mach number remains the same.** False, for the reason in B.

Answer **A, B and C**.