# Convergence problems

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 January 10, 2003, 10:30 Convergence problems #1 Olivier Macchion Guest   Posts: n/a Hello All ! I'm trying to compute the flow inside a bent pipe using the SST model in CFX 5.5.1. I managed to compute it without problems for a coarse mesh and the first prism at 0.0001 m fron the surface, but when I look at the y+ values they are around 100 whereas they should be between 2 and 5 to get an accurate solution. So I decided to reduce the size of the first prism and right now it is 0.5e-05. The solution converges well, but when it is near the MAX residual 1e-04, it begins to oscillate. And then it stops saying that the solution has converged. I have looked at the mesh again and it seens ok. Is there something to do in order to get a cleaner convergence ? Thank you very much. Regards, Olivier

 January 11, 2003, 15:11 Re: Convergence problems #2 Chris Guest   Posts: n/a Hello Olivier, oscillating residuals can have different reasons: -where are the maximal residuals? -check your grid quality in CFX-post (are the worst residuals in the region where the worst cells are?) -use a different timestep If that is all does not give you a hint, the most probable thing is, that the flow shows some instationary effects. Change to transient simulation with >=3 inner loops and you will be able to reduce the residuals further than with stationary effects. Greeetings, Chris

 January 21, 2003, 11:17 Re: Convergence problems #3 Olivier Macchion Guest   Posts: n/a Thank you very much Chris. Just another question : how can you know where the maximal residuals are ? Thank you very much. Olivier

 January 21, 2003, 15:12 Re: Convergence problems #4 Chris Guest   Posts: n/a Hello Olivier, there are two possibilities: - The location of the maximum values of the residuals is always included in the ".out"-file that is also displayed in the solver-monitor-window. You can view the location by creatin a point at the specified coordinates with CFX-post. - Set the expert variable "output eq residuals" to "T". Then you will be able to view the residuals in CFX-Post just like any other variable. Greetings, Chris

 January 22, 2003, 02:57 Re: Convergence problems #5 Olivier Macchion Guest   Posts: n/a Thank you Chris ! Greetings Olivier

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