Fluids

Fluids

Residuals are stable and don’t decrease by iterations

    • Walaa
      Subscriber

      why the residuals may stay almost stable ? that will not make a convergence right ?


      where might the problem is ?

    • DrAmine
      Ansys Employee

      Please refer to Judging Convergence subchapter in the Fluent' User's Guide. At which level are the residuals stalling and what about imbalances and monitor points?

    • Walaa
      Subscriber


      I didn't made monitor points !!, and what is the imbalance point ?


      my residuals as shown fluctuated at the same range !

    • DrAmine
      Ansys Employee

      Okay I see. Imbalances are referring to scrutinize whether the overall continuity and fluxes are conserved and not and are accessible in a general form under Flux report.  Are you expecting reversal flow at your outlet?


      Increasing number of iterations per time step is not a good idea.


      Moreover the backflow quantities for turbulence seem to be causing convergence issues. Please summarize your case in order to provide assistance. I have a feeling that your case is not properly setted up

    • Keyur Kanade
      Ansys Employee

      Hi, 


      Just to add, please insert images of your set up as well as of mesh. This will help us to understand your problem more clearly. 


      Regards,


      Keyur

    • Rob
      Ansys Employee

      I'd also look at your turbulence boundary inputs, and (probably) increase the turbulent viscosity limiter value by a few orders of magnitude once you've confirmed the bc's are sensible. 


      In terms of the convergence, look at the flow velocity and cell size. Divide cell (edge) size by  velocity and then that number by 10. This is a good starting point for a time step. You really need to converge each time step to get a physically sensible result. 


      As an aside, are you using the University system? You may need to refine the mesh near the free surface and the student licence may struggle with the cell count. 

    • Raef.Kobeissi
      Subscriber

      your simulation looks like setup incorrectly or the mesh is in bad shape. 20,000 iteration / time step is an exaggeration. Can you please provide more information about your model? 

    • Walaa
      Subscriber

      Thanks for all of you for cooperating, i am uploading screenshots for all the steps now

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    • Walaa
      Subscriber

      i have changes things in different runs before, time scale factor =2, use transit type, reducing courant number and volume of fraction, but all these were in vain and the calculations still stabled as shown.


      i am aiming to try different scenarios to validate the program according to an experimental data,



      my case is an open channel has 2 inlets and one outlet to check the zone of separation after the confluence.


       

    • Walaa
      Subscriber

      i am sorry for all these things i have shared, but i am beginner using ANSYS and i need to finish my work, thanks a lot and appreciating your help 

    • DrAmine
      Ansys Employee

      Just read this post and as I won't be able to answer the next days here a comment: your atmospheric boundary is wrong. First of all it has to be a bit far from the free surface and besides that you do not need to use open channel here just assume zero gauge pressure with zero backflow volume fraction. Use oméga sst with damping and switch off surface tension at first. At the outlet do not fore the free surface just let it be from neighbouring cells as I assume you do not know much there. After you corrected all this use transient solver. 

    • Walaa
      Subscriber

      Thanks abenhadj, i will check that and let you know  

    • Rob
      Ansys Employee

      I'd check running with 4 and 8 cores, with about 2M cells 17 may actually be slower as the inter-core traffic may outweigh the benefit of more cores. 


      I think your domain is a cm or so across, what is the Reynolds Number? 

    • Walaa
      Subscriber

      Well rwoolhou, I don't get the first part of your comment, but my domain dimensions are as the following, and i didn't calculate Reynolds Number, my Froude number is constant at outlet and equals 0.37 and water level is constant at outlet and i  took it the same at the two inlets = 0.296 m


    • Dilini
      Subscriber

      I pretty sure you could have solved the issue by now. Just noticed something though. You have given the gravitational acceleration in the z direction and it should be in y I guess.

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