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In this image we see a normal shockwave in a pipe, the pressure builds at the inlet, and chokes at the throat as the fluid reaches mach 1. We call it choked because the pressure builds up and the pressure terminates in a shockwave, downstream the pressure drops. Along the wall we see as the pipe widens, the pressure drops at the wall, but raises at the center. The pressure at the center is caused by secondary shockwaves by the supersonic flow along the wall reflecting and adding to the drag caused by the primary shockwave. You can see in the first image the expansion fan is weaker and the reflecting compression waves are also weaker leading to a more uniform flow downstream.

The bottom image where the normal shock is strongest has the strongest compression waves from the expansion fans, creating in effect a constriction in a supersonic flow. In compressible/trans/super sonic flows constrictions raise pressure and slow down airflow. It is only after the bulk airflow passes this constriction that the overall pipe pressure decreases. Had the wall been constructed more carefully, one could largely avoid the compression waves while maintaining the expansion fans that further accelerate a choked flow. This would reduce losses substantially.

In the bottom image if you look closely you can see the recirculation bubble.

In this image we see a normal shockwave in a pipe, the pressure builds at the inlet, and chokes at the throat as the fluid reaches mach 1. We call it choked because the pressure builds up and the pressure terminates in a shockwave, downstream the pressure drops. Along the wall we see as the pipe widens, the pressure drops at the wall, but raises at the center. The pressure at the center is caused by secondary shockwaves by the supersonic flow along the wall reflecting and adding to the drag caused by the primary shockwave. You can see in the first image the expansion fan is weaker and the reflecting compression waves are also weaker leading to a more uniform flow downstream. The bottom image where the normal shock is strongest has the strongest compression waves from the expansion fans, creating in effect a constriction in a supersonic flow. In compressible/trans/super sonic flows constrictions raise pressure and slow down airflow. It is only after the bulk airflow passes this constriction that the overall pipe pressure decreases. Had the wall been constructed more carefully, one could largely avoid the compression waves while maintaining the expansion fans that further accelerate a choked flow. This would reduce losses substantially. In the bottom image if you look closely you can see the recirculation bubble.

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[–] • 0 pt

Nice. That is definitely something. Doesn’t matter what it looks like, liquid physics is weird stuff, just that the end sum of traction and drag is less.