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When Mach diamonds are formed, (in the over expanded case) the flow is firstly compacted by the atmosphere when it leaves the nozzle. This makes a cone-like shape, seen where the flow first exits the nozzle. Picture:

enter image description here

Now my confusion is here: Why is the outlined green part in the picture below not also compacted? What is different about that section of flow?

enter image description here

In other words, why is not all of the flow getting compacted into a shock diamond?

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You have drawn a triangle around a flow zone that is downstream of a shock.

The flow in the triangle has two properties

  1. It is parallel to the flow outside the jet flow at that point
  2. The pressure is equal to the ambient

I.e. the boundary between the triangle and the surrounding non-exhaust flow is a slip line.

The flow from the engine exits going straight. It goes through the shock to enter your triangle. That shock jumps the pressure to match ambient. That shock also turns the flow.

Nothing is unusual here for an under expanded nozzle flow.

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  • $\begingroup$ That is not something that I considered, thanks. Would this situation be much different with over-expanded flow? I incorrectly assumed the pictures were of an overexpanded case. $\endgroup$
    – Wyatt
    Commented Sep 14 at 4:36
  • $\begingroup$ under-expanded flow starts with a shock and necks in. over-expanded flow starts with an expansion fan and spreads out. perfectly expanded flow has no shock/fan features and stays straight. $\endgroup$
    – Rob McDonald
    Commented Sep 14 at 16:28
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    $\begingroup$ Ah okay. This other answer must've gotten that mixed up. It says over-expanded flow starts with a shock, and under-expanded starts with a expansion fan. (See the 2 pictures at the bottom). $\endgroup$
    – Wyatt
    Commented Sep 14 at 16:48
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    $\begingroup$ Maybe I have it backwards.... Lets think through it... The shock will increase the pressure. If you have over-expanded, then you've dropped the pressure below ambient -- and it will need a shock to get back up to ambient. Oops, looks like I got it mixed up. Everything else I said was right though. $\endgroup$
    – Rob McDonald
    Commented Sep 14 at 17:48

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