# Oscillating residuals and no convergence!

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 August 20, 2014, 08:35 Oscillating residuals and no convergence! #1 Senior Member   Srivathsan N Join Date: Jan 2013 Location: India Posts: 101 Rep Power: 13 Hi all, I am facing a problem while using buoyantBoussinesqSimpleFoam to solve a problem in natural convection. My residuals plot is attached herewith. There is huge oscillation in the residuals as the case runs. My last time step residuals are Code: ```DILUPBiCG: Solving for Ux, Initial residual = 1.13167e-09, Final residual = 1.13167e-09, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 3.7544e-10, Final residual = 3.7544e-10, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 0.000492656, Final residual = 4.17232e-06, No Iterations 1 DILUPBiCG: Solving for T, Initial residual = 9.9924e-06, Final residual = 9.9924e-06, No Iterations 0 DICPCG: Solving for p_rgh, Initial residual = 1.05994e-08, Final residual = 7.33015e-09, No Iterations 1 time step continuity errors : sum local = 3.02676e-11, global = 9.96442e-20, cumulative = -1.45381e-16``` However my fvSolution file reads as follows, Code: ```solvers { p_rgh { solver PCG; preconditioner DIC; tolerance 1e-08; relTol 0.01; } "(U|T|k|epsilon|R)" { solver PBiCG; preconditioner DILU; tolerance 1e-05; relTol 0.1; } } SIMPLE { nNonOrthogonalCorrectors 0; pRefCell 0; pRefValue 0; residualControl { p_rgh 1e-4; U 1e-5; T 1e-2; }``` . Q: Why hasn't the solution converged even though the SIMPLE tolerances have been met? Any help would be great! __________________ Regards, Srivaths

August 20, 2014, 08:37
#2
Senior Member

Srivathsan N
Join Date: Jan 2013
Location: India
Posts: 101
Rep Power: 13
Apologies.

The snapshot of the Residuals.
Attached Images
 Residuals.jpg (38.7 KB, 256 views)
__________________
Regards,

Srivaths

 August 20, 2014, 09:14 #3 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 27 Hi. Well, eventually the residuals look fine, after about 17000 iterations. The noise looks like numerical noise to me. Why do they jump like that in the beginning? Is there any sort of additional regulation (flux, ...) in you setup? __________________ The skeleton ran out of shampoo in the shower.

 August 20, 2014, 09:19 #4 Senior Member   Srivathsan N Join Date: Jan 2013 Location: India Posts: 101 Rep Power: 13 Hi, There is nothing in the setup that would cause that jump, so even I'm assuming that it has to be numerical. I'm just not able to pinpoint the reason which is why I posted my fvSolution file to see if there needs to be any change made. UPDATE: I have tried increasing the pseudo time step in the controlDict. There are no oscillation if I do that but the solution diverges after a few iterations. Again, have no idea why. I'm using OF 2.2.2, by the way __________________ Regards, Srivaths

 August 20, 2014, 09:25 #5 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 27 1) Did you try to start with a better initialization? Such as one made by potentialFoam? 2) If you increase the pseudo time step you will of course get divergence at some point. It's like increasing the under-relaxation. You need to decrease the time step if you encounter numerical problems. __________________ The skeleton ran out of shampoo in the shower.

 August 20, 2014, 09:33 #6 Senior Member   Srivathsan N Join Date: Jan 2013 Location: India Posts: 101 Rep Power: 13 1) Could you explain 'better initialisation'. Its primarily buoyancy driven flow without any initial velocity field. What should I expect the potentialFoam run to give me? 2) My bad. I'll correct that and re run. Sorry if the questions are too novice-llike __________________ Regards, Srivaths

 August 20, 2014, 09:37 #7 Senior Member     Philipp Join Date: Jun 2011 Location: Germany Posts: 1,297 Rep Power: 27 1) But you run for steady-state. If your initial values are closer to the final steady-state you can expect better convergence. There is no reason to initialize with velocity=0. For a complete buoyancy driven flow (without inlet and outlet) I don't know how to find a good initialization... rgd likes this. __________________ The skeleton ran out of shampoo in the shower.

 August 20, 2014, 09:39 #8 Senior Member   Srivathsan N Join Date: Jan 2013 Location: India Posts: 101 Rep Power: 13 Thank you for your inputs. I'll give a very small uniform velocity field initially and see if the solution readjusts and converges. Will get back with results. __________________ Regards, Srivaths