Question

The shape of a supersonic diffuser that slows down a supersonic flow to subsonic flow is

• A. converging
• B. diverging
• C. diverging–converging
• D. converging–diverging

Hint:

In supersonic flows, area–velocity relation reverses: convergence slows the flow and divergence accelerates it.

Answer 1

The Correct Option is D

In compressible flow, the behavior of gases changes significantly between subsonic and supersonic regimes.

Step 1: Behavior in subsonic flow.
A subsonic diffuser uses a diverging passage to slow down the flow because pressure increases when the area increases.

Step 2: Behavior in supersonic flow.
In contrast, for supersonic speeds, the relation is reversed: a converging passage causes the flow to decelerate. So to convert supersonic flow into subsonic, the diffuser must first converge.

Step 3: Shock and further diffusion.
After the converging part slows the flow to near sonic speed, a shock or pressure rise occurs, and a diverging shape helps complete the subsonic diffusion.
Thus, a converging–diverging (C–D) diffuser is used.

Final Answer: converging–diverging