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Weird result by k omega SST

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Old   May 23, 2018, 03:06
Default Weird result by k omega SST
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bingjr
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Dear Openfoam users,

I am doing the simulation of a diffuser by k omega SST model and I use simplefoam as the solver. We have tried many times with a few different settings. However, none of the results make sense,even there were irregular and strange flow profile (kind of like vortex) in some of the results. y+ is less than 1 here.

Could you please have a look at my settings and give me some advice? Thanks.

Here are my settings in each file. Note that the solver and k omega SST may not be replaced.

Boundary condition

frontAndBack empty
bottom symmetry
top wall
inlet patch
outlet patch

Initial condition

U

The velocity at the inlet is fully-developed turbulent flow.
Code:
inlet 
fixedValue 
value           nonuniform List<vector>

 frontAndBack
    {
        type            empty;
    }
    top
    {
        type            noSlip;
bottom
    {
        type            symmetry;
    }
    outlet
    {
        type            zeroGradient;
    }
}

P


Code:
internalField   uniform 0;

boundaryField
{
    frontAndBack
    {
        type            empty;
    }
    top
    {
        type            zeroGradient;
    }
    inlet
    {
        type            zeroGradient;
    }
    bottom
    {
        type            symmetry;
    }
    outlet
    {
        type            fixedValue;
        value           uniform 0;
    }
}
omega
Code:
internalField   uniform (we calculated it);

boundaryField
{
    frontAndBack
    {
        type            empty;
    }
    top
    {
        type            omegaWallFunction;
        value           uniform (a very large value);
    }
    inlet
    {
        type            fixedValue;
        value           uniform (we calculated it);
    }
    bottom
    {
        type            symmetry;
    }
    outlet
    {
        type            zeroGradient;
    }
}
nut
Code:
internalField   uniform 0;

boundaryField
{
    frontAndBack
    {
        type            empty;
    }
    top
    {
        type            nutUSpaldingWallFunction;
        value           uniform 0;
    }
    inlet
    {
        type            calculated;
        value           uniform 0;
    }
    bottom
    {
        type            symmetry;
    }
    outlet
    {
        type            calculated;
        value           uniform 0;
    }
}
k
Code:
internalField   uniform (we calculated it);

boundaryField
{
    frontAndBack
    {
        type            empty;
    }
    top
    {
        type            fixedValue;
        value           uniform (a very small value);
    }
    inlet
    {
        type            fixedValue;
        value           uniform (we calculated it );
    }
    bottom
    {
        type            symmetry;
    }
    outlet
    {
        type            zeroGradient;
    }
}
FvShceme
Code:
ddtSchemes
{
    default         steadyState;
}

gradSchemes
{
    default         Gauss linear;
}

divSchemes
{
    default       	 none;
    div(phi,U)     	 bounded Gauss linearUpwind grad(U);
    div(phi,k)     	 bounded Gauss upwind;          
    div(phi,omega)   bounded Gauss upwind;      
    div((nuEff*dev2(T(grad(U))))) Gauss linear;
}

laplacianSchemes
{
    default         Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default         corrected;
}

wallDist
{
    method meshWave;
}
Note: we also tried linearUpwind in k and omega, but it diverged sometimes. Even though the residual is right, the result did not make sense.

Fvsolution
Code:
solvers
{
    p
    {
        solver          GAMG;
        tolerance       1e-06;
        relTol          0.01;
        smoother        GaussSeidel;
    }

    "(U|k|omega)"
    {
        solver          smoothSolver;
	smoother	GaussSeidel;
        tolerance       1e-06;
        relTol          0.01;
    }
}

SIMPLE
{
    nNonOrthogonalCorrectors 0;
    consistent      yes;
}

relaxationFactors
{
    fields
    {
	p		0.1;
    }
    equations
    {
        U               0.1;
        k               0.1;
        omega           0.1;
    }
}
Note: we also tried various relaxation factors.


We are looking forward to your reply. Thanks!

Last edited by bingo641; May 23, 2018 at 07:04.
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Old   May 23, 2018, 09:15
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Jan
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Hi,


not sure if this improves your results, but I suggest using
Code:
kqRWallFunction
for your wall k-BC.Also your relaxation values are quite low, especially for U, k and omega. Are you sure you are looking at a fully converged solution?


Best regards,
Jan
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Old   May 23, 2018, 09:21
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bingjr
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Hi Jan,
Thanks for your reply! Actually we tried several times with different relaxation factors. This is because sometimes the residual value changed greatly in value.

In addition, since our y+ is expected to be less than 1, is it suitable to use this wall function?
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Old   May 27, 2018, 23:05
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Quote:
Originally Posted by bingo641 View Post
Hi Jan,
Thanks for your reply! Actually we tried several times with different relaxation factors. This is because sometimes the residual value changed greatly in value.

In addition, since our y+ is expected to be less than 1, is it suitable to use this wall function?
I would also like to understand this better too. What should the boundary conditions be for omega, k, and nut be if we're attempting to resolve the BL (y+~1)?

Last edited by bentkj; May 29, 2018 at 20:36.
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