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sudden overflow of the courant number

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Old   July 3, 2013, 07:54
Default sudden overflow of the courant number
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Kai Zhang
Join Date: Jan 2013
Location: yokohama, Japan
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dear everyone
In my simulation with pimpleFoam, I firstly set the MaxCo at 0.5, it went smoothly. To slightly save the computation time I increased the MaxCo to 0.85, then bad things happened.
The first few hundreds iterations after the MaxCo change were fine, with the courant number varying a little below and above 0.85. But at a precise moment, a sudden increase in the courant number(from 0.85 to 3) occured, here is what it looks like.
Code:
Courant Number mean: 0.0554198 max: 0.834122
deltaT = 0.0055491
Time = 7.717

PIMPLE: iteration 1
DILUPBiCG:  Solving for Ux, Initial residual = 0.000262906, Final residual = 3.06265e-07, No Iterations 1
DILUPBiCG:  Solving for Uy, Initial residual = 0.00444772, Final residual = 5.03058e-06, No Iterations 1
DILUPBiCG:  Solving for Uz, Initial residual = 0.000323524, Final residual = 3.89366e-07, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.0589746, Final residual = 0.000576175, No Iterations 28
time step continuity errors : sum local = 1.20116e-09, global = 4.29182e-10, cumulative = 3.79531e-10
PIMPLE: iteration 2
DILUPBiCG:  Solving for Ux, Initial residual = 6.00503e-07, Final residual = 6.00503e-07, No Iterations 0
DILUPBiCG:  Solving for Uy, Initial residual = 1.21582e-05, Final residual = 1.39211e-07, No Iterations 1
DILUPBiCG:  Solving for Uz, Initial residual = 5.54895e-07, Final residual = 5.54895e-07, No Iterations 0
GAMG:  Solving for p, Initial residual = 0.189246, Final residual = 0.00174058, No Iterations 30
time step continuity errors : sum local = 4.25715e-09, global = 1.74154e-09, cumulative = 2.12107e-09
PIMPLE: iteration 3
DILUPBiCG:  Solving for Ux, Initial residual = 1.23274e-06, Final residual = 1.80222e-08, No Iterations 1
DILUPBiCG:  Solving for Uy, Initial residual = 4.09755e-05, Final residual = 1.73558e-08, No Iterations 2
DILUPBiCG:  Solving for Uz, Initial residual = 1.13289e-07, Final residual = 1.79303e-09, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.589279, Final residual = 9.8303e-10, No Iterations 412
time step continuity errors : sum local = 4.74361e-15, global = 1.39072e-15, cumulative = 2.12108e-09
ExecutionTime = 162003 s  ClockTime = 165697 s

forceCoeffs output:
    Cm    = -0.0148137
    Cd    = 1.20736
    Cl    = 0.31442
    Cl(f) = 0.142396
    Cl(r) = 0.172023

Calculating averages

Courant Number mean: 0.055421 max: 3.13703
deltaT = 0.00149738
Time = 7.71849

PIMPLE: iteration 1
DILUPBiCG:  Solving for Ux, Initial residual = 7.40663e-05, Final residual = 1.49712e-07, No Iterations 1
DILUPBiCG:  Solving for Uy, Initial residual = 0.00127949, Final residual = 2.6649e-06, No Iterations 1
DILUPBiCG:  Solving for Uz, Initial residual = 8.75115e-05, Final residual = 5.88664e-08, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.92039, Final residual = 0.00902539, No Iterations 25
time step continuity errors : sum local = 2.01598e-08, global = 5.18475e-09, cumulative = 7.30583e-09
PIMPLE: iteration 2
DILUPBiCG:  Solving for Ux, Initial residual = 8.33085e-06, Final residual = 8.33085e-06, No Iterations 0
DILUPBiCG:  Solving for Uy, Initial residual = 0.000220168, Final residual = 2.06308e-06, No Iterations 1
DILUPBiCG:  Solving for Uz, Initial residual = 5.25244e-07, Final residual = 5.25244e-07, No Iterations 0
GAMG:  Solving for p, Initial residual = 0.950336, Final residual = 0.00888994, No Iterations 26
time step continuity errors : sum local = 4.3503e-08, global = 1.43381e-08, cumulative = 2.1644e-08
PIMPLE: iteration 3
DILUPBiCG:  Solving for Ux, Initial residual = 1.29058e-05, Final residual = 1.3311e-08, No Iterations 2
DILUPBiCG:  Solving for Uy, Initial residual = 0.000512759, Final residual = 1.47315e-08, No Iterations 3
DILUPBiCG:  Solving for Uz, Initial residual = 7.73346e-07, Final residual = 8.57267e-09, No Iterations 1
GAMG:  Solving for p, Initial residual = 0.980802, Final residual = 9.85576e-10, No Iterations 546
time step continuity errors : sum local = 1.26467e-14, global = 3.30246e-15, cumulative = 2.1644e-08
ExecutionTime = 162067 s  ClockTime = 165761 s

forceCoeffs output:
    Cm    = -1.12254
    Cd    = 6.38589
    Cl    = 3.02313
    Cl(f) = 0.389023
    Cl(r) = 2.6341
This sudden change seemed to take place from nowhere
anyone knows why this is happening?
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