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twoPhaseEulerFoam bubble column crashes due to problems at outlet region 

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March 30, 2015, 07:05 
twoPhaseEulerFoam bubble column crashes due to problems at outlet region

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Hello everyone,
I'm investigating a water air bubble column in a cuboid after Deen. When using more complex modells for the interface momentum transport coefficients I get problems at the outlet area (see pictures below). With SchillerNaumann for drag coefficient everything worked fine, while with IshiiZuber for drag coefficient the simulation crashes. This is my case setup: //========= 1. Mesh =============================// The cuboid measurements are 0.15 m x 0.15 m x 0.45 m. The inlet at the bottom is modelled as rectangular. The water surface is not included. I generated my mesh using blockMesh. It is equidistant. CheckMesh is okay. //========= 2. Boundary conditions ==================// I am using the following boundary condtitions: alpha.air Code:
internalField uniform 0; boundaryField { inlet { type fixedValue; value uniform 1; } outlet { type inletOutlet; phi phi.air; inletValue uniform 0; value uniform 0; } walls { type zeroGradient; } } Code:
internalField uniform (0 0 0); boundaryField { inlet { type fixedValue; value uniform (0 0 0.0784); } outlet { type zeroGradient; } walls { type fixedValue; value uniform (0 0 0); } } Code:
internalField uniform (0 0 0); boundaryField { inlet { type fixedValue; value uniform (0 0 0); } outlet { type slip; } walls { type fixedValue; value uniform (0 0 0); } } Code:
internalField uniform 100000; boundaryField { inlet { type zeroGradient; } outlet { type fixedValue; value $internalField; } walls { type zeroGradient; } } Code:
internalField uniform 1e4; boundaryField { inlet { type turbulentIntensityKineticEnergyInlet; intensity 0.05; value uniform 0; } outlet { type inletOutlet; phi phi.air; inletValue $internalField; value uniform 0; } walls { type zeroGradient; } } Code:
internalField uniform 1e5; boundaryField { inlet { type turbulentMixingLengthDissipationRateInlet; mixingLength 1e3; k k.air; value uniform 0; } outlet { type inletOutlet; phi phi.air; inletValue $internalField; value uniform 0; } walls { type zeroGradient; } } Code:
internalField uniform 1e5; boundaryField { inlet { type fixedValue; value $internalField; } outlet { type inletOutlet; phi phi.water; inletValue $internalField; value uniform 0; } walls { type zeroGradient; } } I'm assuming a constant air bubble diameter of 4 mm. I only consider drag, lift and virtual mass force. Drag is modelled after Ishii and Zuber as describes here: http://cfdonline.com/Forums/openfoa...eulerfoam.html Lift and virtual mass coefficents are both constant with a value of 0.5. //========= 4. Discretization =======================// Used methods in fvScheme Code:
ddtSchemes { default Euler; } gradSchemes { default Gauss linear; } divSchemes { // from bubble column tutorial default Gauss linear; "div\(phi,alpha.*\)" Gauss vanLeer; "div\(phir,alpha.*\)" Gauss vanLeer "div\(alphaPhi.*,U.*\)" Gauss limitedLinearV 1; "div\(phi.*,U.*\)" Gauss limitedLinearV 1; "div\(alphaPhi.*,.*rho.*\)" Gauss linear; "div\(alphaPhi.*,(he).*\)" Gauss limitedLinear 1; "div\(alphaPhi.*,(K.*p)\)" Gauss limitedLinear 1; "div\(alphaPhi.*,(kepsilon).*\)" Gauss limitedLinear 1; "div\(phim,(kepsilon)m\)" Gauss limitedLinear 1; "div\(\(\(alpha.*nuEff.*\)*dev2\(T\(grad\(U.*\)\)\)\)\)" Gauss linear; } laplacianSchemes { default Gauss linear uncorrected; } interpolationSchemes { default linear; } snGradSchemes { default uncorrected; } fluxRequired { default no; p ; } I only use GAMG as suggested by my professor. fvSolution: Code:
solvers { alpha.air { nAlphaCorr 1; nAlphaSubCycles 2; } p { solver GAMG; smoother DIC; nPreSweeps 0; nPostSweeps 2; nFinestSweeps 2; cacheAgglomeration true; nCellsInCoarsestLevel 160; agglomerator faceAreaPair; mergeLevels 2; tolerance 1e6; relTol 0.0001; } pFinal { $p; relTol 0; } "e.*" { solver GAMG; smoother DILU; nPreSweeps 0; nPostSweeps 2; nFinestSweeps 2; cacheAgglomeration true; nCellsInCoarsestLevel 160; agglomerator faceAreaPair; mergeLevels 2; tolerance 1e6; relTol 0.0001; } "(kepsilonTheta).*" { solver GAMG; smoother DILU; nPreSweeps 0; nPostSweeps 2; nFinestSweeps 2; cacheAgglomeration true; nCellsInCoarsestLevel 160; agglomerator faceAreaPair; mergeLevels 2; tolerance 1e6; relTol 0.0001; } } PIMPLE { nOuterCorrectors 1; nCorrectors 2; nNonOrthogonalCorrectors 0; residualControl { "(Ukepsilon)" { relTol 0.0001; tolerance 0.0001; } } turbOnFinalIterOnly off; } relaxationFactors { } //========= 6. Problem ===========================// After air reaches the outlet area it didn't seem to leave. alpha.air gets big (still <1) in the upper cells and the solution eventually crashes shortly after 7.6 s flow. The flow field for U.water shows there is water inflow. I first set up my case using SchillerNaumann for drag and everything worked fine. But using IshiiZuber instead breaks my simulation. I already tried different things that I found on the forum:
Regards, hester Last edited by hester; March 30, 2015 at 08:33. 

March 30, 2015, 08:47 

#2 
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Here are the source files of the used Ishii Zuber drag model.


March 30, 2015, 16:00 

#3 
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Mattia de\' Michieli Vitturi
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Hi Hester,
I am wondering why you are using a "slip" boundary condition at the outlet for the liquid velocity and not a "zeroGradient". Have you tested the zeroGradient? Ciao Mattia 

March 31, 2015, 06:47 

#4 
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Hello Mattia,
thank you for your answer. I tried zeroGradient for the liquid face at the outlet. I get no simulation problems but the results were wrong. I didn't observe the recirculation pattern in the liquid face. Also was the velocity compared with experimental results too low. So zeroGradient is not what I want here. I use slip because the the outlet boundary is where the water surface would be if I included it in my simulation. I don't want any water to leave the domain. Regards, hester 

May 18, 2016, 10:20 

#5  
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Quote:
were you able to resolve your problem at the outlet region? I think the slip BC is OK to model water surface but i recognized you did not post your alpha.water file. What did you put there for outlet? For the other approach, when i try to model water and air as two different parts in my domain via setFields i experience that even with a fine mesh the water/air interface (surface) becomes very fuzzy over time which i can not explain or resolve so far.. 

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bubble column, twophaseeulerfoam 
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