[Truehnue]

Hello everybody,

I am a very new user of OpenFoam and currently I have some trouble with these boundary conditions of phase fractions.
Currently, I am looking into the alpha-file that can be found in the 0-folder of the example multiphase/twoPhaseEulerFoam/bubbleColumn.
There, the following boundary conditions are defined:

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform 0.5;
    }
    outlet
    {
        type            inletOutlet;
        phi             phi.air;
        inletValue      uniform 1;
        value           uniform 1;
    }
    walls
    {
        type            zeroGradient;
    }
}

My question now is how to interpret the inletOulet boundary condition at the patch "outlet". The documentation states that U and p are switched between fixedValue and zeroGradient depending on direction of U.

For this example, U is the velocity of air (since we are in the alpha.air-file), right?

But why should the direction of velocity of the air change? Initially, it moves up with a velocity of 0.1 m/s (defined in U.air) and afterwards, the air should escape at the top of the column as well.

And what does "U and p are switched between fixedValue and zeroGradient" mean?

I am really looking forward for you answers. Thank you :-)

Greetings,
Katharina

[nimasam]

Dear Katharina

this Bc works as:
1- for outflow similar zeroGradient
2- for inflow or in other word back flow like fixedValue

[nic92]

Dear Foamers,
I'm experiencing some convegence problems for a closed box initially uniformly filled with a two-phase mixture. I am using twoPhaseEulerFoam 3.0.x.
Since there isn't inlet and outlet mass flow rate, I set for velocity:

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

internalField uniform (0 0 0);

boundaryField

{

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

}

For pressure (p_rgh):
{

bodyWall
{
type fixedFluxPressure;
value $internalField;
}

}
Since there is not a patch with fixed pressure, I need to set it in pRefCell and pRefValue (fvSolution) the reference values. Even by doing so, the solver crashes after few time steps since the GAMG solver is not able to zero the residuals.

Courant Number mean: 0 max: 0
Max Ur Courant Number = 0
deltaT = 1.5e-06
Time = 1.5e-06

PIMPLE: iteration 1
MULES: Solving for alpha.air
alpha.air volume fraction = 0.2 Min(alpha.air) = 0.2 Max(alpha.air) = 0.2
MULES: Solving for alpha.air
alpha.air volume fraction = 0.2 Min(alpha.air) = 0.2 Max(alpha.air) = 0.2
MULES: Solving for alpha.air
alpha.air volume fraction = 0.2 Min(alpha.air) = 0.2 Max(alpha.air) = 0.2
Constructing momentum equations
min T.air 300
min T.water 350
GAMG: Solving for p_rgh, Initial residual = 1, Final residual = 0.000486295, No Iterations 1000
GAMG: Solving for p_rgh, Initial residual = 0.00374525, Final residual = 0.000509884, No Iterations 1000
PIMPLE: iteration 2
MULES: Solving for alpha.air
alpha.air volume fraction = 0.2 Min(alpha.air) = 0.2 Max(alpha.air) = 0.2
MULES: Solving for alpha.air
alpha.air volume fraction = 0.2 Min(alpha.air) = 0.2 Max(alpha.air) = 0.2
MULES: Solving for alpha.air
alpha.air volume fraction = 0.2 Min(alpha.air) = 0.2 Max(alpha.air) = 0.2
Constructing momentum equations
min T.air 225.521
min T.water 349.979
GAMG: Solving for p_rgh, Initial residual = 0.00374532, Final residual = 1.02015e+49, No Iterations 1000
GAMG: Solving for p_rgh, Initial residual = 1, Final residual = 9.65999e-07, No Iterations 54
PIMPLE: iteration 3
MULES: Solving for alpha.air
alpha.air volume fraction = 4.97724e+18 Min(alpha.air) = -1.10963e+35 Max(alpha.air) = 6.01367e+34
MULES: Solving for alpha.air
alpha.air volume fraction = 4.40872e+89 Min(alpha.air) = -9.57781e+105 Max(alpha.air) = 1.16947e+106
MULES: Solving for alpha.air
alpha.air volume fraction = -1.98169e+229 Min(alpha.air) = -8.59538e+246 Max(alpha.air) = 1.2115e+247
Constructing momentum equations
#0 Foam::error:rintStack(Foam::Ostream&) at ??:?
#1 Foam::sigFpe::sigHandler(int) at ??:?
#2 ? in "/lib/x86_64-linux-gnu/libc.so.6"
#3 Foam::tmp<Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh> > Foam::mag<Foam::Vector<double>, Foam::fvPatchField, Foam::volMesh>(Foam::tmp<Foam::GeometricField<Foam ::Vector<double>, Foam::fvPatchField, Foam::volMesh> > const&) at ??:?
#4 Foam::dragModels::segregated::K() const at ??:?
#5 Foam::BlendedInterfacialModel<Foam::dragModel>::K( ) const at ??:?
#6 Foam::twoPhaseSystem::Kd() const at ??:?
#7 ? at ??:?
#8 __libc_start_main in "/lib/x86_64-linux-gnu/libc.so.6"
#9 ? at ??:?
Floating point exception (core dumped)

I have also tried the PCG solver but the problem still remains (it is only postponed).

The strange aspect is that if I set a fixedValue for pressure in one the patch, I obtain a convergent solution. However, I would like to avoid this possibility since I am treating a strictly incompressible case and where I set a velocity I do not want to set a pressure.

I have attached the system directory.
Thanks for your feed-back.