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Boundary Conditions of Phase Fractions (alphafile) 

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January 22, 2015, 08:09 
Boundary Conditions of Phase Fractions (alphafile)

#1 
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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 alphafile that can be found in the 0folder of the example multiphase/twoPhaseEulerFoam/bubbleColumn. There, the following boundary conditions are defined: Code:
boundaryField { inlet { type fixedValue; value uniform 0.5; } outlet { type inletOutlet; phi phi.air; inletValue uniform 1; value uniform 1; } walls { type zeroGradient; } } For this example, U is the velocity of air (since we are in the alpha.airfile), 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 

February 8, 2015, 05:22 

#2 
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Nima Samkhaniani
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Dear Katharina
this Bc works as: 1 for outflow similar zeroGradient 2 for inflow or in other word back flow like fixedValue
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December 2, 2016, 12:27 
b.c. for closed block uniformly filled with a twophase mixture

#3 
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Nicoḷ Scapin
Join Date: Apr 2016
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Dear Foamers,
I'm experiencing some convegence problems for a closed box initially uniformly filled with a twophase 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 = 1.5e06 Time = 1.5e06 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 = 9.65999e07, 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_64linuxgnu/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_64linuxgnu/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 feedback. 

Tags 
alpha, boundaries, inlet, outlet 
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