Fluids

Fluids

Improving the CFD Case Imbalances

    • arv2995
      Subscriber

      I have created a CFD Steady State Case and have run the case for 3000 iterations.  


      After the case was fully run, I checked the .res file in CFX Solver Manager to check the imbalances of the variables. I want to achieve imbalances below 10, if possible below 5. 


      But for a few variables, the imbalances are as high as 80(Mainly the mass imbalance).

      My question was, what are the methods I can use to achieve better imbalances in the next simulation for the same case? ( I want to keep the entire mesh constant).


      Looking forward to your reply.

    • RK
      Ansys Employee
      Hello,
      Firstly, in terms of mass balance, what is it in terms of percentage? Keep it within 1% for accurate results.
      There could be multiple reasons for the imbalance - improper setting of boundary conditions, solution methods, mesh quality. Depending on the physics involved, set up the simulation appropriately. You can also reduce the URFs to stabilize the solution. Refer to best practices in the Fluent user manual.
    • arv2995
      Subscriber
      It is ranging from -80 to 80 percent for mass balance.

      What do you mean by solution methods?
      Also, the same case was solved for a higher speed of the shaft in the model with good imbalance values. Why is it that changing the shaft speed is causing the solution to be bad?
    • DrAmine
      Ansys Employee
      Perhaps a more aggressive time scale in the beginning (say 0.1/angular speed) and smaller towards the end.
      As we do not know what do you have for models activated, it is difficult to provide any suggestions.
    • arv2995
      Subscriber
      I tried giving an aggressively small timescale in the beginning. I gave (1 / shaft speed), but the case got diverged due to high Mach Number.
      Then I tried with (10 / shaft speed), and it solved but with bad imbalances.

      Do you suggest I should try , (0.1 / shaft speed ) now after interpolating the solutions?
    • DrAmine
      Ansys Employee
      Without knowing more details I cannot suggest anything: perhaps there is an issue with the setup. Please refer to the CFX Reference Guide for Best Practices and Check the CFX Standard Tutorials if you can find similar definition file /description.
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