The GATE Grind

GATE 2024 AE – Question 23

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

Consider a steady, isentropic, supersonic flow (Mach number $M > 1$) entering a Convergent-Divergent (CD) duct as shown in the figure. Which one of the following options correctly describes the flow at the throat?

A converging-diverging duct with a supersonic stream ($M > 1$) entering from the left and leaving to the right.
  1. Can only be supersonic
  2. Can only be sonic
  3. Can either be sonic or supersonic
  4. Can only be subsonic

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Show answer and explanation

Correct answer: (C) Can either be sonic or supersonic

Explanation

In the converging part the supersonic flow slows down as the area shrinks. In isentropic flow it cannot pass through $M = 1$ anywhere except at the minimum area. If the throat area equals the critical area the flow becomes sonic there, and if it is larger the flow is still supersonic. It can never be subsonic.