[Isaiah_HZ]

Hi foamers,

I am a fresh guy in openfoam. Recently, i started with flow past cylinder. And when I set the parameters of k and omega for the inlet and wall surface, some confusions came into my mind. why we need to set wall surface omega and k? They should be calculated by openfoam. Also, for the pressure boundary condition, near wall, I set zerogradient by following the tutorials, but for stagnation point, there is a nonzero gradient near wall surface. I do not understand, can you guys help me explain? Thank you a lot.

[geth03]

can you share your 0-dict files?

[Isaiah_HZ]

Originally Posted by geth03

can you share your 0-dict files?

Thanks for your reply!
The following is my 0-dict files

PHP Code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      k;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
// k = 3/2*(Uref * I)^2 = 0.002159
// I = 0.16*Re^(-1/8)= 0.03794
dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0.002159;

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           0.002159;
    }
    outlet
    {
        type            zeroGradient;
    }
    topwall
    {
        type            symmetryPlane;
    }
    bottomwall
    {
        type            symmetryPlane;
    }
    cylinder
    {
        type            fixedValue;
        value           uniform 0.0;
    }
    front
    {
        type            empty;
    }

    back
    {
        type            empty;
    }
}


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


PHP Code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    location    "0";
    object      nut;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -1 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type            calculated;
        value           uniform 0;
    }
    outlet
    {
        type            calculated;
        value           uniform 0;
    }
    topwall
    {
        type            symmetryPlane; 
        value           uniform 0;
    }
    bottomwall
    {
        type            symmetryPlane; 
        value           uniform 0;
    }
    cylinder
    {
        type            calculated; 
        value           uniform 0;
    }
    front
    {
        type            empty;
    }

    back
    {
        type            empty;
    }
}


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


PHP Code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      omega;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //


dimensions      [0 0 -1 0 0 0 0];

internalField   uniform 11765;

boundaryField
{
    inlet
    {
        type            fixedValue;
        value           uniform 11765;
    }

    outlet
    {
        type            zeroGradient;

    }
    topwall
    {
        type            symmetryPlane; 
    }
    bottomwall
    {
        type            symmetryPlane; 
    }
    cylinder
    {
        type            fixedValue;
        value           uniform 238884;
    }
    front
    {
        type            empty;
    }

    back
    {
        type            empty;
    }
}


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


PHP Code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volScalarField;
    object      p;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 2 -2 0 0 0 0];

internalField   uniform 0;

boundaryField
{
    inlet
    {
        type           zeroGradient;
    }

    outlet
    {
        type           fixedValue;
    value           uniform 0;
    }
    
    topwall
    {
    type               symmetryPlane;
    }

    bottomwall
    {
    type           symmetryPlane;
    }
    
    cylinder
    {
    type           zeroGradient;
    }
    front
    {
        type            empty;
    }

    back
    {
        type            empty;
    }
}

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


PHP Code:

/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       volVectorField;
    object      U;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

dimensions      [0 1 -1 0 0 0 0];

internalField   uniform (0 0 0);

boundaryField
{
     inlet
    {
        type            fixedValue;
        value           uniform (1 0 0);
    }

    outlet
    {
        type            zeroGradient;
    }
    
    topwall
    {
    type               symmetryPlane;
    }

    bottomwall
    {
    type               symmetryPlane;
    }
    
    cylinder
    {
    type               noSlip;
    }

    front
    {
        type            empty;
    }

    back
    {
        type            empty;
    }

}


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


[geth03]

at walls you can use wall function from the turbulence models:

for k for example you can use:

wallName
{
type kqRWallFunction;
value $internalField
}

there are also wall functions available for epsilon or nut ...

check out this document:
http://www.tfd.chalmers.se/~hani/kur...nfoamFinal.pdf

[Flowkersma]

Hi Hui,

Just like for the other transport equations, you will need to specify boundary conditions for the k and omega. If you read the original papers by Menter et al, they also discuss about the boundary conditions.

Best, Mikko

[Isaiah_HZ]

Originally Posted by geth03

at walls you can use wall function from the turbulence models:

for k for example you can use:

wallName
{
type kqRWallFunction;
value $internalField
}

there are also wall functions available for epsilon or nut ...

check out this document:
http://www.tfd.chalmers.se/~hani/kur...nfoamFinal.pdf

thank you Geth!
But for wall functions, we still need to specify a value for wall surface. Can you explain why the fixed valued is used, can we calculated it (I mean openfoam calculated it by some equations)?

[Isaiah_HZ]

Originally Posted by Flowkersma

Hi Hui,

Just like for the other transport equations, you will need to specify boundary conditions for the k and omega. If you read the original papers by Menter et al, they also discuss about the boundary conditions.

Best, Mikko

Mikko, thank you for your help!
Could you send the link or website to download this paper?

best,
Hui

[Flowkersma]

Hi Hui,

For instance, this paper describes the boundary conditions. If you are looking which model exactly OpenFOAM uses then have a look at the comments in the kOmegaSST header file .

[geth03]

Originally Posted by Isaiah_HZ

thank you Geth!
But for wall functions, we still need to specify a value for wall surface. Can you explain why the fixed valued is used, can we calculated it (I mean openfoam calculated it by some equations)?

i just use it to start the simulation, in the beginning the initial values are not correct, but given a really small time step for the start of the simulation, the values will be corrected.

for my case, i did conduct some test, to see how dependent my results would be with regards to initial values of turbulence properties.
the result is, that the turbulence values will adjust themselves to correct values, if the time step initially is small enough.