Discussion
In the figure shown below, the flow at the nozzle exit is ________.
Answer: A
The flow at the exit is overexpanded.
In an overexpanded nozzle the exit pressure is below the ambient pressure, so the jet must be compressed to match it. Oblique shock waves therefore stand at the exit lips, turning the flow inward, and the jet boundary narrows just downstream of the exit, as in the figure. The shocks reflect from the axis and the pattern repeats as a chain of diamonds.
An underexpanded jet (exit pressure above ambient) starts instead with expansion fans at the lips and a jet that widens. An ideally expanded jet leaves with no waves at all. Shock and expansion patterns of this kind exist only in supersonic flow, so the exit flow cannot be subsonic.
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