From Sutton chapter 3 he uses this figure to describe the ratios of pressure//area//temperature as functions of Mach Number.

enter image description here

I know that we are bringing the fluid to an adiabatic "stop" through expansion which explains the drops in pressure and temperature. However, what I am having trouble understanding is why once we hit the throat at M=1, why the pressure and temperature continue to decrease.

Here's the relation for the area ratio:

enter image description here

Does this have to do with the stagnation conditions or? This is my first time dealing with compressible flow as my thermodynamics class did not cover this material.

  • $\begingroup$ a nozzle doesn't start expanding the flow until it has reached Mach 1, if it tries, it will un-start $\endgroup$ – JCRM Feb 2 '19 at 21:14

It is simply the thermodynamic properties of compressible gasses, that they accelerate in an expanding passage once they are flowing supersonically.

See also Why do rocket nozzles open near the end?

There is also this similar question on physics stack exchange, but the answer also boils down to, because that's the way it works.

  • 2
    $\begingroup$ A good rule of thumb for moving from study of incompressible flow to study of compressible flow is that once you hit Mach 1, everything works backwards from what you're used to. $\endgroup$ – Tristan Feb 4 '19 at 16:40

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