# Bounded scheme gives unbounded solution

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 February 20, 2009, 13:49 Hi everyone With almost #1 Senior Member     su junwei Join Date: Mar 2009 Location: Xi'an China Posts: 151 Rep Power: 20 Hi everyone With almost all the bounded scheme for alpha, twoPhaseEulerFoam always gives the negative values for alpha in my test cases. The numerical schemes I used are listed below FoamFile { version 2.0; format ascii; class dictionary; object fvSchemes; } // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * // ddtSchemes { default CrankNicholson 1; //ddt(alpha) boundedBackward 0.5; } gradSchemes { default Gauss linear; } divSchemes { default none; div(phia,Ua) Gauss MUSCL ; div(phib,Ub) Gauss MUSCL ; div(phib,k) Gauss Gamma 1; div(phib,epsilon) Gauss Gamma 1; div(phi,alpha) Gauss Gamma01 1; div(phir,alpha) Gauss Gamma01 1; div(phi,Theta) Gauss Gamma 1; div(Rca) Gauss linear; div(Rcb) Gauss linear; } laplacianSchemes { default none; laplacian(nuEffa,Ua) Gauss linear limited 1.0; laplacian(nuEffb,Ub) Gauss linear limited 1.0; laplacian((rho*(1|A(U))),p) Gauss linear limited 1.0; laplacian(alphaPpMag,alpha) Gauss linear limited 1.0; laplacian((alphak*nuEffb),k) Gauss linear limited 1.0; laplacian((alphaEps*nuEffb),epsilon) Gauss linear limited 1.0; } interpolationSchemes { default linear; } snGradSchemes { default corrected; } fluxRequired { default no; p; } A segment for residual is DILUPBiCG: Solving for alpha, Initial residual = 0.000124877, Final residual = 3.70067e-14, No Iterations 5 Dispersed phase volume fraction = 0.22513 Min(alpha) = -5.31042e-05 Max(alpha) = 1 DILUPBiCG: Solving for alpha, Initial residual = 0.000124877, Final residual = 3.70067e-14, No Iterations 5 Dispersed phase volume fraction = 0.22513 Min(alpha) = -5.31042e-05 Max(alpha) = 1 DILUPBiCG: Solving for alpha, Initial residual = 1.3635e-05, Final residual = 1.13321e-15, No Iterations 4 Dispersed phase volume fraction = 0.22513 Min(alpha) = -5.58751e-11 Max(alpha) = 1.00002 DILUPBiCG: Solving for alpha, Initial residual = 1.3635e-05, Final residual = 1.13321e-15, No Iterations 4 Sometimes the Max(alpha) > 1 I have used the low diffusion bounded scheme for both unsteady term and convection term. Why the negative alpha always appears. Any suggestion is appreciated. Regards, Junwei

 October 10, 2011, 04:34 #2 New Member   ehsan yasari Join Date: Mar 2009 Location: Gothenburg, Sweden Posts: 14 Rep Power: 17 Hi Junwei I have the similar problem with my simulation. Did you figure it out how to fix this problem? I was wondering if you could help me. Thanks, Ehsan

 October 11, 2011, 06:34 #3 Senior Member     ata kamyabi Join Date: Aug 2009 Location: Kerman Posts: 323 Rep Power: 17 Hi This is not an unusual situation. I think these schemes are bounded on a quadrangular and orthogonal mesh. But I think if you use a good mesh with good boundary conditions alpha remains almost bounded. As your results show upper bound of alpha (Max(alpha) = 1.00002 ) has problem not the min of it (Min(alpha) = -5.58751e-11). May be you can get better results with changing BC.s our may be using better initial conditions. Good luck Bets regards Ata

 November 18, 2011, 02:35 #4 New Member   Hamed Kh. Join Date: Dec 2010 Location: Tehran , Iran Posts: 18 Rep Power: 15 Hi su junwei did u try with smaller deltaT? it has a very important role.