[pradyumnsingh]

Hi Foamers,

I am relatively new to openfoam. I am trying to calculate Turbulence Intensity using the Calculator filter . I am comparing ABL flow over a ridge from wind tunnel and CFD.
For CFD i am using K-Epsilon model and SimpleFoam. Currently I am using the following formula.

Turbulence Intensity = sqrt ( 2/3 * k) / sqrt ( U_X ^2 + U_Y^2 )

This TI is very different from my wind tunnel results.

Is this TI correct or is there something else that is used.

Here is my slice through the centerline of the ridge .

https://imgur.com/a/mD6co0I

[wyldckat]

Quick answer: This is how OpenFOAM uses the Turbulent Intensity value when defined for an inlet: https://github.com/OpenFOAM/OpenFOAM...arField.C#L138

Namely:

k  = 1.5*sqr(intensity_)*magSqr(Up)

So it does seem like it's the same as your expression...

You didn't give use the image for what you are getting with OpenFOAM...

It's possible that the problem with the results has to do with the inlet boundary conditions you are using with simpleFoam.

[pradyumnsingh]

Thanks for replying !

The Image that I posted is what I get from OpenFoam, its not from wind tunnel.
One update I have is that flow field and velocity contours are exactly the same as wind tunnel except for a bit smaller wake as expected from a RANS k-epsilon model. But for some reason, Turbulence Intensity is way off !
The inlet boundary condition for velocity is as following :

Uinlet          (8 0 0); // Use 10 if wind tunnel is at 10 

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

internalField   uniform (0 0 0);

boundaryField
{
    outlet
    {
        type            inletOutlet;
        inletValue      uniform (0 0 0);
        value           $internalField;
    }

    inlet
    {
        
    

    type            atmBoundaryLayerInletVelocity;
    #include        "include/ABLConditions" 
    value           $Uinlet;
    }

    ground
    {
    type            uniformFixedValue;
     uniformValue    (0 0 0);
     value           uniform (0 0 0);
    } 
    
    sides
    {
    type slip;
    }
    
    ridge
    { 
     type            uniformFixedValue;
     uniformValue    (0 0 0);
     value           uniform (0 0 0);
    }
    

}

ABL conditions are as following :

Uref 6.5;
 Zref 60;
 zDir (0 0 1);
 flowDir (1 0 0);
 z0 uniform 0.1;
 zGround uniform 0;

The velocity of 6.5 was observed at h= 60 from LDA in a wind tunnel with ABL flow so I am using that in OpenFoam as well.
I can post rest of my dictionaries if thats of any help. I have definitely lost a few hairs trying to fix this case. Any help would be much appreciated !

[wyldckat]

Quick question: The "k" field is the most worrying here, so that is the critical one we need to diagnose.

[pradyumnsingh]

Oh yes , k would be more important.

Here is my dictionary for K .......

I tried playing around with the values of kInlet, but it didnt really change much except unless a drastic change of value is made. I wish I could post my wind tunnel results, dont wanna get sued or something.

kInlet          0.63;   // approx k = 1.5*(I*U)^2 ; I = 0.1

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

internalField   uniform $kInlet;



boundaryField
{
    outlet
    {
        type            inletOutlet;
        inletValue           uniform $kInlet;
    
    }

    inlet
    {
        type            atmBoundaryLayerInletK;
        #include        "include/ABLConditions"
        value           $internalField;
    }

   

    ground
        {
        type            kqRWallFunction;
        value           uniform 0.0;
        } 

        sides
        {
            type slip;
         }

   ridge
    {
        type            kqRWallFunction;
        value           uniform 0.0;
        
    }

    top
    {
        type            slip;
    }


}

[wyldckat]

Sorry for the late reply.

The only thing that comes to mind is that the k-epsilon model might not be usable for your geometry. Perhaps only the models Realizable k-epsilon or k-omega SST will give you better results that best approximate the results you are getting.

I only thought of this after re-reading your first post, where you mentioned the k-epsilon model... it's possible that this model is too dissipative, hence giving fairly different "k" values, since it's dissipating more, so it re-balances things by changing the kinetic energy...

Or... wait... I forgot to check the k-epsilon model source code: https://github.com/OpenFOAM/OpenFOAM...Epsilon.C#L276 - notice that "rho" is used in the equation for "k", but in "simpleFoam" the "rho" field is set to "1.0", therefore it's somewhat natural that your "k" values might be off by a factor of 1.2 in comparison with the experimental wind tunnel...