Courant-number explodes after a lon while (icoFoam)
 

Hi,
I am new with openFoam and I am trying to simulate a turbine-cascade with icoFoam.

0/p

dimensions [0 2 -2 0 0 0 0];

internalField uniform 5057.9;

boundaryField
{




PERIODIC1
{
type cyclic;
}
periodic_sh
{
type cyclic;
}

BLADE
{
type zeroGradient;
}
HUB
{
type zeroGradient;
}
INLET
{
type zeroGradient;

}
OUTLET
{
type advective;
phi phi;
value uniform 5057.9;
}
TIP
{
type zeroGradient;
}


}

0/u

dimensions [0 1 -1 0 0 0 0];

internalField uniform (30 30 0);

boundaryField
{



PERIODIC1
{
type cyclic;
}
periodic_sh
{
type cyclic;
}
BLADE
{
type fixedValue;
value uniform (0 0 0);
}
HUB
{
type zeroGradient;
}
INLET
{
type fixedValue;
value uniform (36.885 32.064 0);
}
OUTLET
{
type advective;
phi phi;
}
TIP
{
type zeroGradient;
}

}

controlDict

application icoFoam;

startFrom startTime;

startTime 0.0001;

stopAt endTime;

endTime 0.01;

deltaT 0.00000004;


adjustTimeStep yes; // Or no

writeControl adjustableRunTime;


writeInterval 0.000005;

purgeWrite 20;

writeFormat ascii;

writePrecision 9;

writeCompression off;

timeFormat general;

timePrecision 9;

runTimeModifiable yes;

The first itterations are very good, but everytime at the same time in my case time=0.001 the Courant-number explodes, after a day of simulation.

Can someone help pls?

The best way I've found to debug these errors is to write the output at every time step (use purgeWrite if required) and analyse the 'results'. This should tell you if the courant number is caused by physically high velocities or not.

Additionally I would check the residuals (pyFoam).

You will see if any variable is exploding.

Here are the last iterations:


Courant Number mean: 0.00135108479 max: 0.325747809
DILUPBiCG: Solving for Ux, Initial residual = 0.000560602417, Final residual = 4.03296652e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.00015680267, Final residual = 4.9929129e-09, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.000872529694, Final residual = 1.2753294e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0333809278, Final residual = 0.00163371246, No Iterations 99
DICPCG: Solving for p, Initial residual = 0.00664221968, Final residual = 0.000316368199, No Iterations 191
DICPCG: Solving for p, Initial residual = 0.00265865953, Final residual = 0.000131461652, No Iterations 202
time step continuity errors : sum local = 5.62127726e-11, global = 4.85646331e-13, cumulative = 7.05936807e-10
DICPCG: Solving for p, Initial residual = 0.00172637241, Final residual = 8.14285543e-05, No Iterations 191
DICPCG: Solving for p, Initial residual = 0.000829504218, Final residual = 3.89580035e-05, No Iterations 207
DICPCG: Solving for p, Initial residual = 0.000601586018, Final residual = 2.96378233e-05, No Iterations 197
time step continuity errors : sum local = 1.26902051e-11, global = -9.71962381e-14, cumulative = 7.05839611e-10
ExecutionTime = 313815.55 s ClockTime = 316731 s

Time = 0.000130464

Courant Number mean: 0.00135142615 max: 0.365206875
DILUPBiCG: Solving for Ux, Initial residual = 0.000624335245, Final residual = 4.49943786e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000173011857, Final residual = 8.25574465e-09, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.000923101222, Final residual = 3.27248077e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0343213889, Final residual = 0.00166055835, No Iterations 193
DICPCG: Solving for p, Initial residual = 0.00707040719, Final residual = 0.000339425603, No Iterations 100
DICPCG: Solving for p, Initial residual = 0.00245047902, Final residual = 0.000120873377, No Iterations 200
time step continuity errors : sum local = 5.52798683e-11, global = 2.32304936e-14, cumulative = 7.05862841e-10
DICPCG: Solving for p, Initial residual = 0.00169495729, Final residual = 8.09930343e-05, No Iterations 101
DICPCG: Solving for p, Initial residual = 0.000876530509, Final residual = 4.18015271e-05, No Iterations 202
DICPCG: Solving for p, Initial residual = 0.000634705028, Final residual = 3.14877683e-05, No Iterations 102
time step continuity errors : sum local = 1.44976153e-11, global = 1.18459615e-12, cumulative = 7.07047437e-10
ExecutionTime = 313844.61 s ClockTime = 316760 s

Time = 0.000130474

Courant Number mean: 0.00135182934 max: 0.413264518
DILUPBiCG: Solving for Ux, Initial residual = 0.000704629111, Final residual = 4.70605141e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.0001940312, Final residual = 1.57881622e-08, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.000985879925, Final residual = 1.43045728e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0325267583, Final residual = 0.00158705736, No Iterations 196
DICPCG: Solving for p, Initial residual = 0.00757949401, Final residual = 0.000378785221, No Iterations 108
DICPCG: Solving for p, Initial residual = 0.00280037641, Final residual = 0.000138433166, No Iterations 140
time step continuity errors : sum local = 6.85748798e-11, global = -5.09917634e-12, cumulative = 7.01948261e-10
DICPCG: Solving for p, Initial residual = 0.00193374674, Final residual = 9.46097161e-05, No Iterations 102
DICPCG: Solving for p, Initial residual = 0.00103330869, Final residual = 5.06354596e-05, No Iterations 202
DICPCG: Solving for p, Initial residual = 0.000753501164, Final residual = 3.73602746e-05, No Iterations 107
time step continuity errors : sum local = 1.86450563e-11, global = 1.84520793e-12, cumulative = 7.03793469e-10
ExecutionTime = 313872.26 s ClockTime = 316787 s

Time = 0.000130484

Courant Number mean: 0.00135231156 max: 0.472290254
DILUPBiCG: Solving for Ux, Initial residual = 0.000808944526, Final residual = 4.92679153e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000220969293, Final residual = 2.50528429e-08, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.00106574013, Final residual = 1.5683862e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0322361377, Final residual = 0.00152126195, No Iterations 198
DICPCG: Solving for p, Initial residual = 0.00820591951, Final residual = 0.000407452729, No Iterations 111
DICPCG: Solving for p, Initial residual = 0.00314755933, Final residual = 0.000155705809, No Iterations 139
time step continuity errors : sum local = 8.46944469e-11, global = -6.48447628e-12, cumulative = 6.97308993e-10
DICPCG: Solving for p, Initial residual = 0.0021942357, Final residual = 0.000101660495, No Iterations 103
DICPCG: Solving for p, Initial residual = 0.00118636758, Final residual = 5.76171022e-05, No Iterations 202
DICPCG: Solving for p, Initial residual = 0.000863136193, Final residual = 4.25000504e-05, No Iterations 106
time step continuity errors : sum local = 2.33183295e-11, global = 2.26961477e-12, cumulative = 6.99578608e-10
ExecutionTime = 313900.13 s ClockTime = 316815 s

Time = 0.000130494

Courant Number mean: 0.00135289441 max: 0.545196474
DILUPBiCG: Solving for Ux, Initial residual = 0.000941724687, Final residual = 5.49792424e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000255768375, Final residual = 3.89239595e-08, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.00116950018, Final residual = 2.21858032e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0337740886, Final residual = 0.00168285777, No Iterations 198
DICPCG: Solving for p, Initial residual = 0.00916882678, Final residual = 0.000445192936, No Iterations 112
DICPCG: Solving for p, Initial residual = 0.00356657676, Final residual = 0.000174221061, No Iterations 140
time step continuity errors : sum local = 1.05861353e-10, global = -8.43363183e-12, cumulative = 6.91144976e-10
DICPCG: Solving for p, Initial residual = 0.00248973046, Final residual = 0.000117714068, No Iterations 103
DICPCG: Solving for p, Initial residual = 0.00135836735, Final residual = 6.70189108e-05, No Iterations 202
DICPCG: Solving for p, Initial residual = 0.000989233567, Final residual = 4.88439898e-05, No Iterations 106
time step continuity errors : sum local = 2.99826103e-11, global = 2.88323813e-12, cumulative = 6.94028214e-10
ExecutionTime = 313928.19 s ClockTime = 316844 s

Time = 0.000130504

Courant Number mean: 0.00135360692 max: 0.635173148
DILUPBiCG: Solving for Ux, Initial residual = 0.00111241783, Final residual = 6.75475301e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000301108451, Final residual = 5.68671287e-08, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.00130073141, Final residual = 3.66935785e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0349797822, Final residual = 0.00170827006, No Iterations 200
DICPCG: Solving for p, Initial residual = 0.00970473193, Final residual = 0.000463780216, No Iterations 112
DICPCG: Solving for p, Initial residual = 0.0037789092, Final residual = 0.000183773907, No Iterations 140
time step continuity errors : sum local = 1.26279482e-10, global = -1.0214584e-11, cumulative = 6.8381363e-10
DICPCG: Solving for p, Initial residual = 0.00268332084, Final residual = 0.000132464647, No Iterations 102
DICPCG: Solving for p, Initial residual = 0.00145297631, Final residual = 7.13565785e-05, No Iterations 202
DICPCG: Solving for p, Initial residual = 0.00105976862, Final residual = 5.2285137e-05, No Iterations 106
time step continuity errors : sum local = 3.63288792e-11, global = 3.45070742e-12, cumulative = 6.87264337e-10
ExecutionTime = 313963.62 s ClockTime = 316880 s

Time = 0.000130514

Courant Number mean: 0.00135448235 max: 0.744505284
DILUPBiCG: Solving for Ux, Initial residual = 0.00132165459, Final residual = 8.89449741e-08, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000357101994, Final residual = 8.05188377e-08, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.00145279054, Final residual = 6.88082683e-06, No Iterations 1
DICPCG: Solving for p, Initial residual = 0.0371526458, Final residual = 0.0018550487, No Iterations 200
DICPCG: Solving for p, Initial residual = 0.00989807886, Final residual = 0.000481242641, No Iterations 110
DICPCG: Solving for p, Initial residual = 0.00380425511, Final residual = 0.000190083233, No Iterations 139
time step continuity errors : sum local = 1.4780888e-10, global = -1.14090625e-11, cumulative = 6.75855275e-10
DICPCG: Solving for p, Initial residual = 0.0027571914, Final residual = 0.000137054566, No Iterations 101
DICPCG: Solving for p, Initial residual = 0.00147465605, Final residual = 7.02733199e-05, No Iterations 202
DICPCG: Solving for p, Initial residual = 0.00107328549, Final residual = 5.31538962e-05, No Iterations 106
time step continuity errors : sum local = 4.18094633e-11, global = 4.06678558e-12, cumulative = 6.7992206e-10
ExecutionTime = 313991.65 s ClockTime = 316908 s

Time = 0.000130524

Courant Number mean: 0.00135554523 max: 0.870939992
DILUPBiCG: Solving for Ux, Initial residual = 0.00156394584, Final residual = 1.14334821e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000421740246, Final residual = 1.02036997e-07, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.00161100897, Final residual = 3.99879114e-07, No Iterations 2
DICPCG: Solving for p, Initial residual = 0.0384808651, Final residual = 0.00179635437, No Iterations 196
DICPCG: Solving for p, Initial residual = 0.00919366901, Final residual = 0.000445499412, No Iterations 101
DICPCG: Solving for p, Initial residual = 0.00331579402, Final residual = 0.000163210372, No Iterations 140
time step continuity errors : sum local = 1.40648799e-10, global = -9.43821672e-12, cumulative = 6.70483844e-10
DICPCG: Solving for p, Initial residual = 0.002405895, Final residual = 0.000117504712, No Iterations 100
DICPCG: Solving for p, Initial residual = 0.00124074764, Final residual = 5.87632053e-05, No Iterations 201
DICPCG: Solving for p, Initial residual = 0.000903733201, Final residual = 4.4399401e-05, No Iterations 106
time step continuity errors : sum local = 3.86305268e-11, global = 3.74852639e-12, cumulative = 6.7423237e-10
ExecutionTime = 314019.16 s ClockTime = 316936 s

Time = 0.000130534

Courant Number mean: 0.00135679454 max: 0.998997938
DILUPBiCG: Solving for Ux, Initial residual = 0.00179656253, Final residual = 1.25723893e-07, No Iterations 1
DILUPBiCG: Solving for Uy, Initial residual = 0.000481528948, Final residual = 7.05450051e-08, No Iterations 1
DILUPBiCG: Solving for Uz, Initial residual = 0.00172289179, Final residual = 1.35426585e-07, No Iterations 2
DICPCG: Solving for p, Initial residual = 0.0414029784, Final residual = 0.00200185089, No Iterations 31
DICPCG: Solving for p, Initial residual = 0.0083390347, Final residual = 0.000379814079, No Iterations 191
DICPCG: Solving for p, Initial residual = 0.00253898182, Final residual = 0.000115853871, No Iterations 203
time step continuity errors : sum local = 1.03571708e-10, global = -1.46157115e-12, cumulative = 6.72770799e-10
DICPCG: Solving for p, Initial residual = 0.00160164772, Final residual = 7.44660037e-05, No Iterations 193
DICPCG: Solving for p, Initial residual = 0.000650163566, Final residual = 3.19850235e-05, No Iterations 203
DICPCG: Solving for p, Initial residual = 0.000390252354, Final residual = 1.85219341e-05, No Iterations 200
time step continuity errors : sum local = 1.65702356e-11, global = 3.69269402e-14, cumulative = 6.72807726e-10
ExecutionTime = 314051.67 s ClockTime = 316969 s

I started the simulation with a program which controls and configure the courant-number (C<0.2), but the solution is also bad after a while. The pressure and the velocity value explodes at the trailing-edge e.g. pressure=9e+156 and velocity=10000 m/s ???

Did you do this?
You are using a fixed dt now ?

Sorry, I meant actually write the data and analyse it with a post-processing tool. This will tell you whether the issue is physical (i.e. fixed time step too large, mesh too large etc.) or not and you can go from there.

I do see now however that your run time is fairly large so you may want to wait for someone who knows a bit more than me to identify your issue without re-running.

I think the Problem was, that i got velocity over 1000 m/s at the outlet cause of a eddy....

I start the simulation with simpleFoam and use the solution for icoFoam.
Now it works fine.

Thanks