GATE 2021 AE – Question 19
The shape of an ideal diffuser that decelerates a supersonic stream to a subsonic stream is
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Correct answer: (D) converging-diverging
Explanation
A **supersonic diffuser** must slow a supersonic stream down to a subsonic one, ideally without shocks.
**Area-velocity relation:** $\dfrac{dA}{A}=(M^2-1)\dfrac{dV}{V}$.
- **Supersonic part** ($M>1$): to decelerate ($dV<0$) we need $dA<0$, so the duct must **converge**. The Mach number falls toward 1 and reaches $M=1$ at the minimum area (the throat).
- **Subsonic part** ($M<1$): to decelerate further ($dV<0$) we need $dA>0$, so the duct must **diverge**.
The shape is **converging then diverging**, i.e. a converging-diverging duct (option D).