Question

Consider a steady, isentropic, supersonic flow (\(M > 1\)) entering a convergent-divergent (CD) duct as shown. Which option correctly describes the flow at the throat?

• A. Can only be supersonic
• B. Can only be sonic
• C. Can either be sonic or supersonic
• D. Can only be subsonic

Hint:

In any isentropic nozzle/duct, the minimum area is the \(A^*\) station and is always sonic. Supersonic or subsonic states occur only on sections with \(A > A^*\).

Answer 1

The Correct Option is B

Step 1: Area-Mach relation for isentropic quasi-1D flow.
\[ \frac{dA}{A}=(M^2-1)\frac{du}{u}. \] The minimum geometric area corresponds to the station where \(dA=0\), which requires \(M=1\) for an isentropic solution (otherwise \(du=0\) at a minimum area would be inconsistent with upstream and downstream states). Thus the station of minimum area is the sonic area \(A^{*}\). 

Step 2: Implication for a CD nozzle with supersonic inlet.
A steady, isentropic solution passing through a minimum area must have \(M=1\) there, no matter whether the upstream is subsonic or supersonic. Hence at the throat the flow is necessarily sonic. 

\[\boxed{\text{At the throat } M=1 \text{ (sonic only).}}\]