Exit pressure of a choked nozzle in Simscape

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Davide on 26 Apr 2022
Answered: Yifeng Tang on 22 Jun 2022
Hi, I'm trying to model a simple convergent nozzle in Simscape.
The pressure at the exit of such nozzle (P_e) can assume two values:
  • P_e equal to the ambient pressure if the flow is subsonic (Mach<1).
  • P_e equal to approximately 0.528*P0 if the flow is sonic (Mach=1). (P0 is the total pressure, P0=P + 0.5*density*velocity^2)
How can I impose these conditions and switch between them ?
I have used the reservoir block (I'm working in the gas domain ) to set the ambient pressure of the subsonic case but I don't know how to impose the total pressure for the sonic case since it is not known a-priori (it depends on the head loss in the piping system).
Thank you.

Answers (1)

Yifeng Tang
Yifeng Tang on 22 Jun 2022
Sounds like you are just trying to set an outlet boundary reservior with pressure of atm or 0.528*P0. Not sure if this will work, but may worth a try:
Use a reservoir at 1 atm, and conncet a controlled pressure source before that. The input to the pressure source can be the output of a look-up table or some function that will transit from 0 to 0.528*P0 rather quickly. You need a way to sense and calculate P0 of course, which means you will need to make quantities like rho and vel available by modifying some of the existing blocks (or maybe you are already writing your own custom blocks).





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