[broccolibadger]

Hi,

As has been the case lately with the buoyant solvers, results are looking alright but the residuals have a curious way of behaving that I can't seem to get my head around. It has to do with the p_rgh residual not stabilizing and decreasing, even though the flow is qualitatively/apparently converged.

Is there a mistake in my case setup or is this the way it's supposed to be?

The case is a simple heat source in a free stream 2D compressible laminar flow (no gravity), see attached image.

First I assumed the boundary conditions do not permit convergence though I haven't been able to find any combination that alleviates the residual behavior.

I've tried running it with PISO and PIMPLE with similar issues for the residual.

Files for boundary conditions, fvSchemes, fvSolution below:

p_rgh:

internalField   uniform $pAtm;

boundaryField
{


    inlet
    {
        type            freestreamPressure;
        freestreamValue uniform $pAtm;
    }

    outlet
    {
        type            freestreamPressure;
        freestreamValue uniform $pAtm;
    }

    walls
    {
        type            freestreamPressure;
        freestreamValue uniform $pAtm;
    }

}

U:

internalField   uniform (0.01 0 0);

boundaryField
{

    inlet
    {
        type            freestreamVelocity;
        freestreamValue uniform (0.01 0 0);
        value           $internalField;
    }
    
    outlet
    {
        type            freestreamVelocity;
        freestreamValue uniform (0.01 0 0);
        value           $internalField;
    }

    walls
    {
        type            freestreamVelocity;
        freestreamValue uniform (0.01 0 0);
        value           $internalField;
    }

}

T:

internalField   uniform 291.45;


boundaryField
{
    inlet
    {
        type            inletOutlet;
        inletValue      uniform 291.15;
        value           uniform 291.15;
    }
    outlet
    {
        type            inletOutlet;
        inletValue      uniform 291.15;
        value           uniform 291.15;
    }

    walls
    {
        type            inletOutlet;
        inletValue      uniform 291.15;
        value           uniform 291.15;
    }

}

fvSchemes:

ddtSchemes
{
    default         steadyState;
}

gradSchemes
{
    
    default         Gauss linear;
//    default         cellMDLimited Gauss linear 0.5;
//    default       cellMDLimited leastSquares 0.5;
}


divSchemes
{
    default         none;
    div(((rho*nuEff)*dev2(T(grad(U))))) Gauss linear;


    div(phi,U) Gauss vanLeerV;

//    div(phi,K)     Gauss limitedLinear 1;
//    div(phi,h)     Gauss limitedLinear 1;

    div(phi,K)     Gauss linearUpwind grad(K);
    div(phi,h)     Gauss linearUpwind grad(h);


//    div(phi,T)     Gauss limitedLinear 1;

    turbulence Gauss upwind;

//    turbulence Gauss limitedLinear 1;

    div(phi,k) $turbulence;
    div(phi,omega) $turbulence;
    div(phi,epsilon) $turbulence;

}


laplacianSchemes
{
    default          Gauss linear corrected;
}

interpolationSchemes
{
    default         linear;
}

snGradSchemes
{
    default           corrected;
}

wallDist 
{ 
    method exactDistance; 
}

fvSolution:

solvers
{
    cellDisplacement
    {
        solver          GAMG;
        smoother        GaussSeidel;
        tolerance       1e-7;
        relTol          0.001;
    }

    p_rgh
    {
        solver           PCG;
        preconditioner   DIC;
        tolerance        1e-08;
        relTol           0.01;
    };
    
    "(T|U|h|k|epsilon|omega)"
    {
        solver          PBiCGStab; //smoothSolver; PBiCGStab;
        //smoother        DILUGaussSeidel;
        preconditioner  DILU;
        tolerance       1e-08; // refine after some time to 1e-8
        relTol          0.001;
    }

}

SIMPLE
{
    nNonOrthogonalCorrectors 2;

    consistent yes;

    residualControl
    {
        p_rgh           1e-5;
        U               1e-5;
        h               1e-5;

        "(k|epsilon|omega)" 1e-4;
    }
}

relaxationFactors
{
    fields
    {
        rho             1;
        p_rgh           0.7; // 0.3;
    }

    equations
    {
        U               0.2;
        h               0.6;
        "(k|epsilon|omega|R)" 0.7;
    }
}

thermophysicalProperties:

thermoType
{
    type            heRhoThermo;
    mixture         pureMixture;
    transport       const;
    thermo          hConst;
    equationOfState perfectGas;
    specie          specie;
    energy          sensibleEnthalpy;
}

pRef    101325;

dpdt off;

mixture
{
    specie
    {
        molWeight       28.96;
    }
    thermodynamics
    {
        Cp              1006;
        Hf              0;
    }
    transport
    {
        mu              1.8e-05;
        Pr              0.71;
    }
}

Same behavior of residuals seem to be common on the forums.