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p_rgh B.C for compressibleInterFoam

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Old   October 20, 2015, 12:18
Default p_rgh B.C for compressibleInterFoam
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Rohith
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Hi,

Can somebody clarify why should the p_rgh (Dynamic pressure) must be set to 1e5. I am trying to simulate a wetting process using compressibleInterFoam. The simulation is very unstable when the following B.C. have been used

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

internalField   uniform 1e5;


upperWall      
    {
         type            fixedFluxPressure;
          value		 uniform 1e5;
    }

    leftWall     
    {
       type            fixedFluxPressure;
       value		 uniform 1e5;
    }

    rightWall     
    {
       type            fixedFluxPressure;
      value		 uniform 1e5;
    }

    lowerWall    
    {
         type            fixedFluxPressure;
          value		 uniform 1e5;
    }

    frontAndBack    
    {
        type            empty;
    }
I have change the p_rgh B.C on the walls to 0 Pa and pMin the same as 1e5. My geometry has Atmospheric Conditions. By using p and pMin 1e5, the material properties of the Fluids remain constant. Am i modelling some non- physicality by considering the p_rgh to 0. My fluid is static at the beginning of the simulation i.e, the corresponding B.C of U are uniform (0 0 0) for all Walls.

Thanks in Advance,

Best Regards,
Rohith
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Old   October 26, 2015, 09:37
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Fabian Roesler
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Hi Rohith

Hope you are fine. When changing the pressure boundary to 0 Pa with a minimum pressure of 1e5 Pa, the solver will correct the interior pressure to the allowed minimum of 1e5 Pa in every cell. Thus, all cells have a corrected pressure, not matching the real conditions in the domain. Your results are wrong.
Actually I am surprised, that the solver gives you values at all. With a 0 Pa absolute pressure, the ideal gas law should give you a division by zero.

Cheers

Fabian
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Old   October 26, 2015, 09:40
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Hi Fabian,

I am doing fine. Thank you for the reply. But my question was on p_rgh instead of p. I set p to 1e5 but p_rgh to 0 in some cases. Does it make some sense? was my question.

VG,
Rohith
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Old   October 26, 2015, 09:49
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Fabian Roesler
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To clarify a little bit more: You're using a pressure of 1e5 Pa in every cell to calculate your thermophysical properties with. That's the reason why your properties are constant and the solver is more stable.
When choosing values for pMin/pMax, you have to decide where you want to bound your thermophysical properties. For example chose 1e4 Pa and 2e5 Pa for a simulation running at standard pressure 1e5 Pa with only slight pressure changes.

Cheers

Fabian
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Old   October 26, 2015, 09:50
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You define p_rgh and p is type calulated.
p_rgh 0 should not work.
However, sometimes p_rgh field is calculated in the first time step from the p field.
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Old   October 26, 2015, 09:55
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For a detailled Explaination let me show what i have as

Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.x                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 1e5;

boundaryField
{
    upperWall      
    {
        type            calculated;
        value           $internalField;
    }

    leftWall     
    {
        type            calculated;
        value           $internalField;
    }

    rightWall     
    {
        type            calculated;
        value           $internalField;
    }

    lowerWall    
    {
        type            calculated;
        value           $internalField;
    }

    frontAndBack    
    {
        type            empty;
    }
}

// ************************************************************************* //
Code:
/*--------------------------------*- C++ -*----------------------------------*\
| =========                 |                                                 |
| \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox           |
|  \\    /   O peration     | Version:  2.1.x                                 |
|   \\  /    A nd           | Web:      www.OpenFOAM.org                      |
|    \\/     M anipulation  |                                                 |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p_rgh;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

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

internalField   uniform 0;


upperWall      
    {
         type            fixedFluxPressure;
          value		 uniform 0;
    }

    leftWall     
    {
       type            fixedFluxPressure;
       value		 uniform 0;
    }

    rightWall     
    {
       type            fixedFluxPressure;
      value		 uniform 0;
    }

    lowerWall    
    {
         type            fixedFluxPressure;
          value		 uniform 0;
    }

    frontAndBack    
    {
        type            empty;
    }
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Old   October 26, 2015, 10:07
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Fabian Roesler
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No, this is not possible, as your pMin will correct the field everywhere. Thus, your thermophysical properties are wrong.
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Old   October 26, 2015, 10:13
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Rohith
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Thank you!! This is what was expected. Have a nice day
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