# transient case diverged all of a sudden after already convergence

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 November 11, 2012, 11:39 transient case diverged all of a sudden after already convergence #1 Member   Cong Join Date: Nov 2012 Posts: 79 Rep Power: 6 Hi everyone, I am running an internal flow in a duct. Firstly I ran the steady simulation (3500 iterations ) using simpleFoam and everything is OK. Then I started running transient simulation using transient SIMPLE method. In the first 0.9958 s, everything is also OK. However, when the 0.99585 time step starts, the pressure calculation gets suddenly divergence. I tried to modify convergence tolerance control and other linear system solvers and ran from a previous time step, but the divergence exactly recurred at 0.99585 s. Although the pressure field diverged, the velocity field not. It is really strange. The log is as follows. I hope someone could give me a hint. Thank you everyone in advance. Courant Number mean: 0.0002247080246 max: 7.268116999 Time = 3500.99585 Max Inner Iteration: 40 ConvergenceCriterion: 9e-08 -- Inner loop: 0 --------------------------------- smoothSolver: Solving for Ux, Initial residual = 3.722149168e-06, Final residual = 4.894892661e-10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.756301862e-05, Final residual = 1.469450516e-09, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 1.01839746e-05, Final residual = 1.132826601e-09, No Iterations 1 GAMG: Solving for p, Initial residual = 4.942934403e-06, Final residual = 4.204448761e-08, No Iterations 7 time step continuity errors : sum local = 1.081550112e-13, global = 9.481328711e-15, cumulative = -2.379358346e-11 smoothSolver: Solving for nuTilda, Initial residual = 9.707065056e-06, Final residual = 2.616585371e-09, No Iterations 1 -> maxResidual(U,p) 4.942934403e-06 -- Inner loop: 1 --------------------------------- smoothSolver: Solving for Ux, Initial residual = 1.117159902e-06, Final residual = 1.819146731e-10, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 5.270313626e-06, Final residual = 4.919923245e-10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 3.05667921e-06, Final residual = 4.055659424e-10, No Iterations 1 GAMG: Solving for p, Initial residual = 4.934697876e-05, Final residual = 1.730930522e-07, No Iterations 4 time step continuity errors : sum local = 4.452616788e-13, global = -1.255549445e-13, cumulative = -2.39191384e-11 smoothSolver: Solving for nuTilda, Initial residual = 2.915621506e-06, Final residual = 9.895764204e-10, No Iterations 1 -> maxResidual(U,p) 4.934697876e-05 -- Inner loop: 2 --------------------------------- smoothSolver: Solving for Ux, Initial residual = 3.502918942e-07, Final residual = 7.495885771e-11, No Iterations 1 smoothSolver: Solving for Uy, Initial residual = 1.675695577e-06, Final residual = 1.964754382e-10, No Iterations 1 smoothSolver: Solving for Uz, Initial residual = 9.391553624e-07, Final residual = 1.568032312e-10, No Iterations 1 GAMG: Solving for p, Initial residual = 0.3252932271, Final residual = 0.000827214952, No Iterations 3 time step continuity errors : sum local = 2.924368657e-09, global = -1.922521341e-10, cumulative = -2.161712725e-10 smoothSolver: Solving for nuTilda, Initial residual = 1.225521515e-06, Final residual = 4.562680108e-10, No Iterations 1 -> maxResidual(U,p) 0.3252932271

 November 11, 2012, 18:31 #2 Senior Member     Vieri Abolaffio Join Date: Jul 2010 Location: Always on the move. Posts: 308 Rep Power: 9 your courant number is too large, try using a timestep equal to 1/10 of what you are actually using __________________ http://www.leadingedge.it/ Naval architecture and CFD consultancy

 November 12, 2012, 07:55 #3 Member   Cong Join Date: Nov 2012 Posts: 79 Rep Power: 6 Hi, Thank you for your reply. I know that CFL < 1 is a common recommendation for transient calculation, but I am also told that for URANS, CFL between 1 and 10 can be also acceptable. Is it true ? And even if CFL is too big, but why for the first 0.9958 s does it work 'well' ?

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