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Why am I seeing supersonic flow when pressure is below 10psig

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Old   July 5, 2020, 13:38
Default Why am I seeing supersonic flow when pressure is below 10psig
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I have a simulation where I have a stagnation tank that I pressurize to 10 psi above atmospheric. This stagnation tank is effectively a long tube. At one end it is closed and at the other it is open to atmosphere by a nozzle. The nozzle is a Converging diverging nozzle thatís very simple. I Initialize the simulation with 10 psi Inside the tube. When I run the simulation I get supersonic flow at Mach 1.2 in the nozzle. Shouldnít this be inpossible since
My pressure isnít high enough? Or is there some weird transient effect that could be casing this.

This is meant to represent a quick nozzle opening.
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Old   July 5, 2020, 15:58
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Howdy:

Considering 1 atm = 14.7 psi, if the reservoir pressure is 10 + 14.7 psi and the flow is expanding to 1 atm, the pressure ratio is 0.595. This is pretty close to the sonic condition (0.571). Your Ma ~ 1.2 is due to the fact that your flow is not perfectly 1D, and there is a radial velocity profile. You can check the location of Ma=1 in your nozzle, which I suspect would be slightly downstream.

Gerry.
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Old   July 5, 2020, 20:58
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For real nozzles, the pressure ratio to achieve sonic conditions is around 1.2, much lower than the 1.8 that you get from 1D isentropic relations (or 0.8 vs 0.5 if you like to divide the other way). Meaning, it's well within the realm of possibility. =)
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Old   July 5, 2020, 22:00
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Thank you both for your posts

Gerry - I checked and my sonic flow is right at the throat. Certainly its not 1D as its a conical flow.

LuckyTran - I've never heard of this occurring at such a low pressure. Can you point me to any papers or text that show that this is possible?
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Old   July 20, 2020, 14:56
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Bumping this post. Does anyone have any links to papers or text books that can confirm that one can achieve Mach 1.3 flow from 10PSI, or something similar. Just an odd concept for me that i don't fully understand and would like to understand better
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