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

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

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

Hi su junwei

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