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Courant number explodes

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Old   December 24, 2015, 08:37
Default Courant number explodes
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Evangelos
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Hello Foamers!

i am trying to simulate the bifurcation that is depicted in the bifurcation.png file. There is one inlet in the right of the geometry and three outlets at the right.

I use icoFoam to solve the flow problem. the deltaT is 1e-4. The courant number at first is very small but is increased with a small step after each iteration. After 10000 iterations the courant number is getting too big so the simulation stops.

The mesh is ok after checkMesh.

I calculate the Dp in order to have the desired mean velocity in the pipe before the bifurcation and then i divide with density.

0/p File

object p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField uniform 0;

boundaryField
{
inlet
{
type fixedValue;
value uniform 0.012957446;
}

outlet_internal
{
type fixedValue;
value uniform 0;


}

outlet_externalbig
{
type fixedValue;
value uniform 0;

}

outlet_externalsmall
{
type fixedValue;
value uniform 0;

}

walls
{
type zeroGradient;
}

}



0/U File


object U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField uniform (0 0 0);

boundaryField
{
inlet
{
type pressureInletVelocity;
value uniform (0 0 0);
}

outlet_internal
{
type zeroGradient;
}

outlet_externalbig
{
type zeroGradient;
}

outlet_externalsmall
{
type zeroGradient;
}

walls
{
type fixedValue;
value uniform (0 0 0);
}


}


transportproperties

transportModel Newtonian;

nu nu [ 0 2 -1 0 0 0 0 ] 0.000002057;




Can anyone help me to solve the problem ? I stuck over 15 days
Attached Images
File Type: png Bifurcation.png (62.3 KB, 29 views)
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Old   December 24, 2015, 11:49
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Akshay Kumar
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Hey
Why are you running a transient simulation for this case?
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Old   December 24, 2015, 14:33
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Evangelos
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I know that Poiseuille flow is treated as steady state.

But i have developed a code based on transient solver and i need to evaluate the results of my case with experimental data
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Old   December 25, 2015, 05:06
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Akshay Kumar
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Ahh okay. Did you try running adjustable time stepping? Also, try pimplefoam solver : use higher outerCorrectors and relax the solution
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Old   December 25, 2015, 23:36
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Alberto Passalacqua
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If you know the desired velocity at the inlet, just impose that, rather than the pressure at the inlet (it seems you use that to find the pressure drop?).

Also, as a suggestion, I would not recommend icoFoam. You could use pimpleFoam, and have better control on the solution process (outer correctors, adaptive time-step, under-relaxation, ...).
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GeekoCFD - A free distribution based on openSUSE 64 bit with CFD tools, including OpenFOAM. Available as in both physical and virtual formats (current status: http://albertopassalacqua.com/?p=1541)
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To obtain more accurate answers, please specify the version of OpenFOAM you are using.
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