Question regarding converging-diverging nozzles

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To achieve sonic conditions in the throat of a converging-diverging nozzle, a critical pressure differential is necessary. If the downstream pressure is high and does not reach the critical pressure, the flow cannot be accelerated to sonic speeds. A higher upstream pressure is required to create the necessary motive force for choking to occur. Without this pressure differential, the flow will not reach the desired conditions. Therefore, achieving choking in a nozzle is contingent upon maintaining the appropriate pressure ratios.
Amaelle
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Good day!

I have a question regarding the converging-diverging nozzle (look at the image)

Normally, in order to expand the mass flow rate, we reduce the downstream pressure

my question is how can we reach the sonic condition in the throat when the downstream pressure is considerably high(does not reach the critical pressure?

many thanks in advance
Good day!
I have a question regarding the converging-diverging nozzle (look at the image)
Normally, in order to expand the mass flow rate, we reduce the downstream pressure
my question is how can we reach the sonic condition in the throat when the downstream pressure is considerably high(does not reach the critical pressure?
many thanks in advance
convergent divergent nozzle.png
 
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In short: you can't. If you don't have the required pressure differential, you don't have the appropriate motive force to accelerate the flow that much.
 
To be picky, if you have a considerable pressure downstream, you need a higher pressure upstream to achieve the choking condition. But the downstream pressure will always have to be the critical pressure (either by lowering downstream pressure or increasing upstream pressure) in order to get choking.
 

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