Fluids

Fluids

FLUENT – Issues with Boundary Conditions

    • manjuk
      Subscriber

       


      I'm trying to simulate a steady, 2D, internal, compressible flow through a channel. I have a pressure inlet [the flow is compressible so I can't use a velocity inlet] and a pressure outlet.


       


      For the pressure inlet, the Reference Frame is Absolute, the Gauge Total Pressure is 691485 Pa, the Supersonic/Initial Gauge Pressure is 0 Pa [simulating a Mach number of 2], the Direction Specification Method is Normal to Boundary, Prevent Reverse Flow is checked, and the Total Temperature is 518.67 K.


       


      For the pressure outlet, the Backflow Reference Frame is Absolute, the Gauge Pressure is 606174 Pa, the Pressure Profile Multiplier is 1, the Backflow Direction Specification Method is Normal to Boundary, the Backflow Pressure Specification is Static Pressure, the Average Pressure Specification Averaging Method is Weak, the Target Mass Flow Rate is 669.09 kg/s [the upper and lower limits of absolute pressure are 5000000 and 1 Pa respectively], and 518.67 K. I've also tried it without Target Mass Flow Rate checked, but then my residuals explode sooner.


       


      My problem is that when I try running the calculations, when I check the contours, the inlet boundary conditions seem to be completely ignored by FLUENT. For example, the pressure is way too high and the velocity is way too low. I believe this is due to the fact that my initial values are much closer to the outlet values, but this is because if they're too close to the inlet values [or even somewhere in the middle], then my residuals explode really quickly. They still explode even if the initial values are closer to the outlet values, but not until much later. How would you suggest I initialize this? Should I try patching different initial values at the inlet, outlet, and in the middle? I've never patched zones before, so I'm not sure how to do it.


       


      Any help would be greatly appreciated.


       

    • DrAmine
      Ansys Employee
      For supersonic flow you need to specify static pressure at inlet.
    • manjuk
      Subscriber
      How do I specify that with a pressure inlet? Is that what “Supersonic/Initial Gauge Pressure” is referencing? If so, I’ve specified it as 0 Pa.
    • DrAmine
      Ansys Employee

      Yes that is it. For supersonic flow you need to provide for 3D cases 5 out of 5 inputs. You need to calculate the static pressure corresponding to Ma=2 at that total pressure level.

    • manjuk
      Subscriber
      This is a 2D case, does that make a difference?

      I’ve specified both the gauge total pressure (691485 Pa) and the gauge static pressure (0 Pa), which corresponds to M = 2.
    • manjuk
      Subscriber

      I'm sorry for double-posting, but can someone please help me with this? I would really appreciate it.

    • nepomnyi
      Subscriber

      I have almost the same problem. Every time I run FLUENT it changes boundary conditions which I specify. Namely, after the simulation is done, I check boundary conditions and see that they are different.


      I am wondering if anybody can share an explanation please?


      Thank you very much in advance.


      Ivan

    • Rob
      Ansys Employee

      2d works in exactly the same way, we just don't worry about the other dimension (it's one metre for all the reports). 


       


      Ivan: bc's don't change themselves unless you're using profiles or UDFs. I assume you are clicking the OK button after making changes? 

    • nepomnyi
      Subscriber

      Thank you for trying to help rwoolhou.


      1) I apologize for the tone of my comment. I've just edited it to sound less emotional.


      2) I do use UDFs to apply time-dependent boundary conditions. I also use constant boundary conditions which I specify in FLUENT'S GUI interface without using UDFs. Sometimes I use only constant boundary conditions, other times I use constant boundary conditions at one inlet and time-dependent ones at the other inlet (I am doing multiphase flows, therefore I have several inlets). My boundary conditions appear to be changed after the simulation is done every time. I've verified it by different methods: looking at the inlet contours, plotting graphs at the inlets using FLUENT RESULTS tab, exporting results files.


      3) Every time I specify or change boundary conditions I click OK button.


      Ivan

    • Rob
      Ansys Employee

      No worries. 


      Check what you're comparing for boundary conditions. For example, if I set a velocity boundary of 1m/s but plot velocity on that bc with node values on I'll see a different result due to interpolation: Fluent is using 1m/s and plotting with node values off will show this. Similarly graphs, are you plotting a point value or area/mass weighted average? 

    • nepomnyi
      Subscriber

      Thanks a lot.


      I have started suspecting something like this, but was not sure. For the last question - I plotted an area averaged value.


      I'll dig into these details - thank you.


      Ivan

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