Fluids

Fluids

How does Outlet Pressure changes effect the flow regime in a supersonic Flow?

    • CleverBoy
      Subscriber

      Hello everyone,

      Iam working on a supersonic nozzle as a school project and Iam trying different BCs and different pressure values for both inlet and outlet.


      For Pressure Inlet - Pressure Outlet BCs, I specify 3 different outlet pressure values and at each one of them I get a flow seperation, shock-waves, at different locations etc.


      All resources say that for a supersonic flow, outlet pressure does not concidered and will not be taken into even if you specify one. Then why do I keep getting different results for each different value? What's the reason and mathematical explanation for this phenomena?


      Thanks.

    • Karthik R
      Administrator
      Hello:
      Please have a look at this course and see if this addresses your questions.
      ConvergingÔÇôDiverging Nozzle | Ansys Innovation Courses
      There are also some simulation examples associated with this.
      Internal Compressible Examples | Ansys Innovation Courses
      Is this what you are looking for?
      Karthik
    • CleverBoy
      Subscriber
      Dear Karthik Thanks for the reply. I know everything about the CD nozzle theory. What I was wondering about is related to ANSYS Fluent.
      On this website: https://www.afs.enea.it/project/neptunius/docs/fluent/html/ug/node244.htm
      Under the Defining Static Pressure section it says : "To set the static pressure at the pressure outlet boundary, enter the appropriate value forGauge Pressurein thePressure Outletdialog box. This value will be used for subsonic flow only."
      This manual basically says any value for pressure outlet will not be used if the flow is supersonic. How can this be? How does Fluent calculate the exit pressure in this case? Every time I change the value of gauge pressure at the outlet I get a different result. Shouldn't I get the same result even if I change the outlet pressure? Are manuals wrong in this case?
    • Karthik R
      Administrator
      Hello:
      Should the flow become locally supersonic, the specified pressure will no longer be used; pressure will be extrapolated from the flow in the interior. All other flow quantities are extrapolated from the interior.
      Please have a look at the link below to understand this better.
      7.4.9. Pressure Outlet Boundary Conditions (ansys.com)
      Karthik
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