# time step continuity error

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 June 11, 2010, 03:51 time step continuity error #1 Member   Join Date: Apr 2010 Posts: 53 Rep Power: 7 Hi at all, i am doing some simulation of a flow through a pistol valve using the simpleFoam-solver containing k-w-SST turbulence model. unfortunately my calculations often crash or when they run from start- to endtime, solution are diverging. maybe you have an idea, what can be improved to avoid such errors?! timeStep is calculated with CFL-No. <1. examples: 1) here the velocity component U_z increases. Code: ```Time = 0.74498 DILUPBiCG: Solving for Ux, Initial residual = 2.50978e-05, Final residual = 8.49602e-07, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 2.54388e-05, Final residual = 2.2754e-06, No Iterations 2 DILUPBiCG: Solving for Uz, Initial residual = 8.85837e-06, Final residual = 1.02878e+13, No Iterations 1001 DICPCG: Solving for p, Initial residual = 1, Final residual = 0.00995054, No Iterations 108 DICPCG: Solving for p, Initial residual = 0.113132, Final residual = 0.00109658, No Iterations 32 DICPCG: Solving for p, Initial residual = 0.018198, Final residual = 0.00017058, No Iterations 98 time step continuity errors : sum local = 3.2839e+07, global = -124559, cumulative = -124377 63 additional processes aborted (not shown)``` 2) here just the cumulative time step error is big. Code: ```Time = 0.36633 DILUPBiCG: Solving for Ux, Initial residual = 0.0126915, Final residual = 0.000657026, No Iterations 2 DILUPBiCG: Solving for Uy, Initial residual = 0.00954027, Final residual = 0.000545504, No Iterations 2 DILUPBiCG: Solving for Uz, Initial residual = 0.00808909, Final residual = 0.000426527, No Iterations 2 DICPCG: Solving for p, Initial residual = 7.40292e-07, Final residual = 9.93734e-08, No Iterations 45 DICPCG: Solving for p, Initial residual = 5.64208e-07, Final residual = 9.98093e-08, No Iterations 4 DICPCG: Solving for p, Initial residual = 1.38451e-07, Final residual = 9.08471e-08, No Iterations 1 time step continuity errors : sum local = 11.0104, global = -0.956176, cumulative = 62676.9 63 additional processes aborted (not shown)``` And what can in general be said about the duration the user should define (startTime /endTime)? Is there any relation between this defined time and the physical time which the fluid needs to flow trough the geometry? or is most priority that the variable we are looking for, is converged?

June 11, 2010, 04:05
#2
Senior Member

Stefan Herbert
Join Date: Dec 2009
Posts: 129
Rep Power: 8
Hi bephi,

simpleFoam is a solver for steady state problems. Therefore the time step has no effect on solution, convergence, etc. The first step to improve stability is to reduce relaxation factors.

Quote:
 or is most priority that the variable we are looking for, is converged
Yes of course. You should only trust your solution if its converged.

Regards,
Stefan

 June 11, 2010, 04:25 #3 Member   Join Date: Apr 2010 Posts: 53 Rep Power: 7 Hi thanks for your answer. Which URF should be decreased in your opinion? Regards! Philipp

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