Discussion
In a closed-circuit supersonic wind tunnel, the convergent-divergent (C-D) nozzle and test section are followed by a C-D diffuser to swallow the starting shock. Here, we should have the
Answer: B
The diffuser throat must be larger than the nozzle throat, with the shock located downstream of the diffuser throat.
During starting, a normal shock passes through the test section at the test Mach number. The total pressure loss across it means the diffuser throat must pass the same mass flow at a lower stagnation pressure, so it needs a larger area: \(A_{t2}/A_{t1} = p_{01}/p_{02} \gt 1\). A diffuser throat the same size as the nozzle throat would choke and the shock could not be swallowed.
Once swallowed, the shock passes through the diffuser throat and settles in the diverging part just downstream of it, where it is weak and its position is stable. A shock held exactly at the throat is unstable: a small disturbance pushes it upstream and the tunnel unstarts. That rules out the two 'just at the diffuser throat' options.
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