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Outflow of gas through the diaphragm effect

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Old   January 8, 2015, 12:24
Default Outflow of gas through the diaphragm effect
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Misha Radionov
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I have a problem with modelling an ideal gas flow through the diaphragm in Ansys Fluent.

I have P0 (constant) pressure on the inlet and P1 (variable) pressure on the outlet. See picture
When P0=P1: v=0 (gas velocity), then we decrease P1 and velocity must increase until P1 reaches critical value P1*=0,5P0, like this

It's happening becase gas velocity reaches local speed of sound and gas in P0 area can't feel P1 pressure. I think it comes from Saint Venant Equations (can't find actual source in English, sorry).

My problem is that velocity in Fluent increases in reverse ratio with P1 lke this

Why? Help is very appreciated!
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