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-   -   Bounded scheme gives unbounded solution (http://www.cfd-online.com/Forums/openfoam-solving/57926-bounded-scheme-gives-unbounded-solution.html)

su_junwei February 20, 2009 14:49

Hi everyone With almost
 
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

ehsanyasari October 10, 2011 04:34

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

ata October 11, 2011 06:34

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

hkhosravi November 18, 2011 03:35

Hi su junwei

did u try with smaller deltaT? it has a very important role.


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