Hi @ProjectAvenir finally we have some results to share with you.
Since transient flow simulations, with high velocity, take too much time to complete so its good practice to save on mesh. We made a symmetric mesh with acceptable mesh size for this simulation. Here is the mesh inside image
For the simulation boundary conditions have to be robust, since compressible flow simulations are very sensitive to boundary conditions. For this simulation we used transonic (0.7 mach) inlet, and wave trasmissive pressure at the outlet, to let shock wave propagate.
Here are some of the results. As you can see even though the inlet is transonic, but when flow hits the inlet of the nozzle, it gets sonic, making the flow supper sonic in the nozzle, as the nozzle is diverging cavity. And at the very end of the nozzle there is shock wave causing the flow velocity to drop to subsonic, which completely agrees with the physics of compressible flows
Here is pressure profile in the domain
Here is the link to this project. You can change the parameters according to your problem. If you have any further question, feel free to ask. Have a nice day.