Fluids

Fluids

Calculation of scaled residuals

    • trippleD
      Subscriber

      Hello,


      Ansys is calculating the scaled residuals in the following way:



      a_p an a_nb are the center coefficient and the influence coefficient for the neighboring cells. What is the meaning of these coefficients?


       


      Thanks a lot for your help,


      trippled

    • DrAmine
      Ansys Employee

      Check some text books like: 


      Numerical Heat Transfer and Fluid Flow (Hemisphere Series on Computational Methods in Mechanics and Thermal science) 

    • trippleD
      Subscriber

      Okay, i understand now that the coefficients come from the discretization. For judging convergence, the scaled residuals may not be appropriate, like in the follwoing situations:




      • If you make a good initial guess of the flow field, the initial continuity residual may be very small leading to a large scaled residual for the continuity equation. In such a situation it is useful to examine the unscaled residual and compare it with an appropriate scale, such as the mass flow rate at the inlet.



      • For some equations, such as for turbulence quantities, a poor initial guess may result in high scale factors. In such cases, scaled residuals will start low, increase as nonlinear sources build up, and eventually decrease. It is therefore good practice to judge convergence not just from the value of the residual itself, but from its behavior. You should ensure that the residual continues to decrease (or remain low) for several iterations (say 50 or more) before concluding that the solution has converged.



      I don"t understand the situations. If i make a good initial guess, i get a small resiudal, but why does it introduce a large scaled residual?

    • DrAmine
      Ansys Employee
      Because of the scaling.
    • trippleD
      Subscriber

      The denominator is the scaling factor? As a result, it is smaller than the numeator?

    • DrAmine
      Ansys Employee

      check differences between local and global scaling in the help for continuity in the documentation.

    • trippleD
      Subscriber

      I'm glad that you help me. But i didn"t ask the question just for fun. I have already take a look in the documentation, but it isn't clear for me

    • DrAmine
      Ansys Employee

      For continuity residual you are referring to the wrong formula. 

    • trippleD
      Subscriber

      Okay, I found the right formula:



      What is mentioned with "rate of mass creation"? I have a closed system without any production of mass.

    • DrAmine
      Ansys Employee

      If the documentation does not go into the details of that term then do not expect that I would describe in much details here on this public community. What I can tel here is only to create a mass in a volume you either have a source/sink or provide the mass through fluxes over cell boundaries.


      You can see it as a sort of control volume imbalance.

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