[gmbeltrami]

Hello,

I am trying to set a simple test case in openfoam to test buoyantSimpleFoam, but I was not able to properly set my simulation, causing divergence/weird flows with every option I tried, I was hoping someone could give me some new insights.

The case is a simple atmospheric flow through a 3D domain, from W (inlet) to E (outlet), or a flow along the x-axis.
My bottom boundary should be a no-slip boundary, my side boundary (N) has to be symmetric, but the other one (S) can be symmetric or a slip boundary. My top boundary (sky) would ideally be an open boundary (maybe something like the S boundary, or an outlet-ish boundary?).

When I try to run this, I get huge recirculations on the east boundary, with complete non-physical behavior.

I will leave my U and p_rgh files here, but you can also find the case here.

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.3.1                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    location    "0";
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
dimensions      [ 0 1 -1 0 0 0 0 ];

internalField   uniform (1.17 0 0);

boundaryField
{
    "(W)"
    {
	type            atmBoundaryLayerInletVelocity;
	flowDir         (1 0 0);
	zDir            (0 0 1);
	Uref            0.5;
    	Zref            10;
	z0              uniform 0.5;
	d               uniform 0.0;
	value           uniform (1.17 0 0);
    }

    "(E)"
    {
        type	        zeroGradient;
    }

    "(sky)"
    {
	type		slip;
	
    }

    "(N|S)"
    {
        type            symmetry;
    }

    bottom
    {
        type            noSlip;
    }


}

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  v2406                                 |
|   \\  /    A nd           | Website:  www.openfoam.com                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [1 -1 -2 0 0 0 0];

internalField   uniform 101325;

boundaryField
{
    "(bottom)"
    {
        type            fixedFluxExtrapolatedPressure;
    }

    "(W|sky)"
    {
	type		fixedFluxExtrapolatedPressure;
    }

    "(N|S)"
    {
        type            symmetry;
    }


    "(E)"
    {
	type 		fixedValue;
	value 		uniform 101325;
    }

   
}


// ************************************************************************* //

atmosphericFlow.gz

uc.png

[Alczem]

Hey,


Depending on the vertical size of your domain, the hydrostatic pressure could play a significant role at the vertical boundaries.


I assume you initialize the pressure p_rgh as constant in the domain? If you do, during the first steps, the density at high elevation will drop. The fixedValue at the East boundary will then see a lower pressure in the adjacent cells at the top of the domain, and thus add an inflow. The opposite happens at the bottom of the boundary (higher density in the domain than at the bottom of the boundary). At least that is how I interpret it, even though p_rgh supposedly gets rid of the rho*g*h part.


It always is a headache to run buoyantSimpleFoam on large domains and open boundaries. If you use the ESI version, I had some decent results using the FO hydrostaticPressure. It creates another field, ph_rgh, accounting for hydrostatic contribution, and gives you access to an additional pressure boundary condition called prhgTotalHydrostaticPressure which is way better at handling density differences due to elevation. It is tricky to set up and I never quite got the hang of it, but maybe you can try to figure it out.


To test out what I said, maybe try to run the case with a constant density in thermophysicalProperties and see if you get the same backflow at the outlet. If you do not, then this might be the reason