Fluids

Fluids

Global vs Local Ranges

    • RCamp
      Subscriber

      Hi everyone, I am fairly new to ANSYS and have been trying to run a Fluent simulation that goes something like this: 


      Nitrogen gas is being pushed through a cylindrical column with a frustum bottom. My inlet boundary is a velocity inlet where my outlet is a pressure outlet. My wall boundary is no slip and has a constant temperature flux.  Additionally, the nitrogen is assumed to be an ideal gas and I have the source term of gravity on. I am running off of a pressure based solver and have the energy as well as the K-eplison model on.


      My issue is in my post-processing. When I am looking at the velocity streamlines my velocity values are not making a lot of sense. My initial value for velocity is 1 m/s and in the global range its stating my min is 0m/s and in the local range its much less than 1 m/s. Can someone please explain to me the difference between the global and local range values and why this would be happening?


       


      Thanks! 


       


      Global Range StreamlinesLocal Range Streamlines


       


       


       

    • mcoderon
      Ansys Employee

      Hi, a nice detailed description of Global and Local ranges is reported in the help guide, see below:



      Regarding the results, you need first to verify that the solution is properly converged before assuming that the results are accurate. It would be a good idea to compare your solution with some experimental results, if available. 


       


      Best Regards,


      Marco

    • RCamp
      Subscriber
      Thank you for your response. I agree that the results need to be compared to experimental data. However I am still a bit confused about interpreting the data in hand. From those definitions would it be fair to say that when a global velocity value range is displaying the velocity to equal zero, then the velocity is either A) essentially constant in that region or B) most of the velocity cell values in that region is zero so it will display zero in that region?

      I might be misunderstanding completely so thank you for your patience ahead of time.

      Best
      -RCamp
    • Keyur Kanade
      Ansys Employee

      Hello,


      As given in help, global range will be taken for all objects. That means if you have more than one cell zone then global range will be calculated considering all cell zones. When you see velocity zero, essentially your solution does not look converged. Can you please first run it further and get converged results and let us know. 


      Regards,


      Keyur

    • RCamp
      Subscriber

      Okay, the residuals I am running look to be converging to me (after a rough start) so I'll attach them below. Any advice on additional monitors run?



       


       

    • Karthik R
      Administrator

      Hello,


      You might want to create additional monitors related to average outlet temperature and average inlet pressure. Hope it helps.


      Best,


      Karthik

    • DrAmine
      Ansys Employee

      In R19.2 you can use the report definition based monitor and residual monitor at the same time to judge convergence: Solver will then stop if both are fulfilled. 

    • RCamp
      Subscriber

      So after running the additonal monitors (shown below) I am still getting global value mins of 0m/s. Any thoughts? Could it be how I am setting up my geometry or something along that line?



      Scaled residual



      velocity outlet



      temperature inlet



      pressure inlet



      velocity inlet


      global range


      local range


       

    • klu
      Ansys Employee

      Hi,


      Can you narrow down the range to for example 0 - 2 m/s? It might be just visualization problem because it seems to me at least 2m/s will be colored by blue from the colormap . Also it might help to double check if you can plot velocity contours at different y positions using local ranges. Thanks!

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