[Gimlas]

I am totally confused with the hydrostatic pressure calculations in openfoam. I've prepared a simulation of air flow through a vertical pipe (buoyantBoussinesqSimpleFoam). I've used following BC for pressures:

p

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.2.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 0;

boundaryField
{
    Wall
    {
        type            calculated;
        value           $internalField;
    }
    Inlet
    {
        type            calculated;
        value           $internalField;
    }
    Outlet
    {
        type            fixedMean;
        meanValue       0;
    value        uniform 0;
       
    }
}

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

p_rgh

/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.2.0                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 0;

boundaryField
{
    Wall
    {
        type            fixedFluxPressure;
        rho             rhok;
        value           $internalField;
    }

    Inlet
    {
        type            fixedFluxPressure;
        rho             rhok;
        value           $internalField;
    }
    Outlet
    {
        type            fixedMean;
        meanValue       0;
    value        uniform 0;
    }
}

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

p_rgh profile is identical to the results obtained from Ansys Fluent (please see attached image). However the p profile is wrong. Althouth uniform 0 value was set at the Outlet, the results show high undepressure. I've made my own calculations using calculator filter in paraView (see p_calculated image). The profile was obtained as follows:

p = p_rgh+(H-coordsY)*9.81*1.225
where H is the pipe height, 9.81 is gravity and 1.225 is air density

As you can see this profile is correct.

My qestion is: why the original p calculations went so wrong? Is there a mistake in the BC?

I've also tried to run calculations without p file in 0 folder. But in next iterations such file was created with incorect pressure field.

Please, help me