9 Replies Latest reply on Mar 27, 2014 12:52 PM by Jared Conway

    Setting up boundaries

    Enda Roche
    Enda Roche Mar 25, 2014 7:36 AM

      Hi guys,

      Im a student studying mechanical engineering, and am currently doing a final year project on flow analysis of air through an impulse turbine. I have never used CFD before up until five weeks ago, I've done alot of research on the software and am kinda getting up to speed.

      I'm running the study as an internal flow, with the impulse turbine situated in a pipe. I'm running into difficulty when it comes to setting up the boundaries on my study. I've set the inlet boundary to a volume flow rate of 1.27m3/s. The boundary condition for the outlet says it has to be set at a pressure value (static). How do i avoid setting it to a pressure value, as one of my goals in running the study is analysing the change in pressure of the air as it enters the nozzle and the pressure as it exits the diffuser. If i have the oultet boundary set at amospheric pressure, is that not interfering with the calculations. I have a rotating region set about the blades also.

       

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        • Re: Setting up boundaries
          Robert D.
          Robert D. Mar 25, 2014 8:10 AM (in response to Enda Roche)

          Good morning, If you set the outlet boundary to atmospheric pressure to run the analysis then when it is done with the CFD study it will show the change in pressure relative to the atmosphere. In essence it is my understanding that it is saying the hole is open when you leave it at atmospheric. If your using floXpress then your limited to what you can actually view as results. Let me know if there is anything else. Thanks

            • Re: Setting up boundaries
              Enda Roche
              Enda Roche Mar 25, 2014 3:23 PM (in response to Robert D.)

              How come when i run the study and i set a surface goal for average pressure on the hub before the air enters the turbine, the pressure value is very high while the study is running. Should the pressure before entry be close to enviromental pressure seen as there is nothing happening to it before it enters the turbine. The values im getting are very high.

                • Re: Setting up boundaries
                  Jared Conway
                  Jared Conway Mar 26, 2014 3:24 AM (in response to Enda Roche)

                  your problem is relatively complicated

                  before you start getting into rotating regions...etc i would go through some of the tutorials and get a good understanding of how the software works and work up to your problem

                  here's what i suggest

                  1. pipe only. your options for bcs are pressure pressure which will drive the flow rate. or flow rate inlet and pressure outlet. this will tell you teh pressure at the inlet. these are the only 2 recommended combinations. you need to work within that constraint.

                  2. run the pipe with your impeller fixed. take a look at what happens. almost like it gets frozen.

                  3. then you can start playing with rotating regions. but go through the technical references. i do not think that your problem is a good fit for how flow simulation handles rotating reference frames. if i was going to take on your project from a consulting perspective, i would probably run it by the develoeprs first. your instructor should be able to help you with that through your reseller.

                    • Re: Setting up boundaries
                      Enda Roche
                      Enda Roche Mar 26, 2014 5:12 PM (in response to Jared Conway)

                      Thanks Jared for your response,

                      Im set up my inlet boundary condition to an inlet flow volume of 1.269m3/s and my outlet boundary condition to static pressure. The rotating region is surrounding the impulse blades and hub via a cylindrical extrusion of a new part around the hub and blade assembly. I have a surface goal set up on a point on the hub at either side of the impulse also to measure the pressure drop across the rotor.

                      I ran a test just now and warnings showed up saying i exceeded the mach number. How can this be possible when I have only set a velocity of 5.98m/s in the z direction. How can the Mach number be supersonic.

                      It also mentions that my inlet boundary conditions are conflicting with my supersonic speeds.

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