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"For simplicity, let us assume the flow turns around a corner downstream of the throat, where
the Mach number is M_{0} > 1, to an angle θ_{1}, and let us indicate just three of an infinite number of waves making up the "expansion fan". Since the centerline is an axis of symmetry, the
waves must reflect from it while canceling the flow deviation, so that the flow downstream of
the reflection is axial. When the flow passes through these reflected waves it is deflected
toward the axis. If the wall turns to match this deflection where the reflected wave hits it, the
wave is just cancelled. The net result is that the flow arrives at the exit with an axial
direction and a uniform Mach number."
Can anyone explain what this passage means? Isn't the centerline just something we draw and thus how can it reflect the waves? And how does the reflected waves cancel out the flow deviation?
Source: https://ocw.mit.edu/courses/aeronauticsandastronautics/1650introductiontopropulsionsystemsspring2012/lecturenotes/MIT16_50S12_lec8.pdf
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