Outflow of gas through the diaphragm effect
 

Hello,
I have a problem with modelling an ideal gas flow through the diaphragm in Ansys Fluent.

Process
I have P0 (constant) pressure on the inlet and P1 (variable) pressure on the outlet. See picture https://www.dropbox.com/s/oz96rt0arc...11.39.png?dl=0
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 https://www.dropbox.com/s/u1gkawi0s3...30.40.png?dl=0

Theory
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).

Problem
My problem is that velocity in Fluent increases in reverse ratio with P1 lke this https://www.dropbox.com/s/9azq829931...32.00.png?dl=0

Why? Help is very appreciated!