CFD Online Discussion Forums - Low-Re Boundary Conditions for K-omega SST (aerodynamics)

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Low-Re Boundary Conditions for K-omega SST (aerodynamics)

Hi all,
Noob here, having a simple question.
Let's take the Motorbike tutorial, that is included in OpenFOAM:

Code:

/opt/openfoam7/tutorials/incompressible/simpleFoam/motorBike

How would you modify the files in the '0' folder in order to fully resolve the boundary layer of (both) the motorbike and the floor (Low Reynolds, y+ <= 1)?
I have already seen this post, but it is quite old and I am not sure if it is still valid: https://www.cfd-online.com/Forums/op...m-1-5-dev.html
I will include for your convenience the code of the k, nut, omega, p, U.orig files.

k

Code:

#include        "include/initialConditions"



dimensions      [0 2 -2 0 0 0 0];



internalField   uniform $turbulentKE;



boundaryField

{

    //- Set patchGroups for constraint patches

    #includeEtc "caseDicts/setConstraintTypes"



    //- Define inlet conditions

    #include "include/fixedInlet"



    outlet

    {

        type            inletOutlet;

        inletValue      $internalField;

        value           $internalField;

    }



    lowerWall

    {

        type            kqRWallFunction;

        value           $internalField;

    }



    motorBikeGroup

    {

        type            kqRWallFunction;

        value           $internalField;

    }



    #include "include/frontBackUpperPatches"

}

nut

Code:

boundaryField

{

    //- Set patchGroups for constraint patches

    #includeEtc "caseDicts/setConstraintTypes"



    frontAndBack

    {

        type            calculated;

        value           uniform 0;

    }



    inlet

    {

        type            calculated;

        value           uniform 0;

    }



    outlet

    {

        type            calculated;

        value           uniform 0;

    }



    lowerWall

    {

        type            nutkWallFunction;

        value           uniform 0;

    }



    upperWall

    {

        type            calculated;

        value           uniform 0;

    }



    motorBikeGroup

    {

        type            nutkWallFunction;

        value           uniform 0;

    }

}

omega

Code:

#include        "include/initialConditions"



dimensions      [0 0 -1 0 0 0 0];



internalField   uniform $turbulentOmega;



boundaryField

{

    //- Set patchGroups for constraint patches

    #includeEtc "caseDicts/setConstraintTypes"



    #include "include/fixedInlet"



    outlet

    {

        type            inletOutlet;

        inletValue      $internalField;

        value           $internalField;

    }



    lowerWall

    {

        type            omegaWallFunction;

        value           $internalField;

    }



    motorBikeGroup

    {

        type            omegaWallFunction;

        value           $internalField;

    }



    #include "include/frontBackUpperPatches"

}

Code:

#include        "include/initialConditions"



dimensions      [0 2 -2 0 0 0 0];



internalField   uniform $pressure;



boundaryField

{

    //- Set patchGroups for constraint patches

    #includeEtc "caseDicts/setConstraintTypes"



    inlet

    {

        type            zeroGradient;

    }



    outlet

    {

        type            fixedValue;

        value           $internalField;

    }



    lowerWall

    {

        type            zeroGradient;

    }



    motorBikeGroup

    {

        type            zeroGradient;

    }



    #include "include/frontBackUpperPatches"

}

U.orig

Code:

#include        "include/initialConditions"



dimensions      [0 1 -1 0 0 0 0];



internalField   uniform $flowVelocity;



boundaryField

{

    //- Set patchGroups for constraint patches

    #includeEtc "caseDicts/setConstraintTypes"



    #include "include/fixedInlet"



    outlet

    {

        type            inletOutlet;

        inletValue      uniform (0 0 0);

        value           $internalField;

    }



    lowerWall

    {

        type            fixedValue;

        value           $internalField;

    }



    motorBikeGroup

    {

        type            noSlip;

    }



    #include "include/frontBackUpperPatches"

}

I assume that the boundary layer is already refined so that y+<1. If not then try to refine the boundary layer using the command:
refineWallLayer 'patchNmae' 0.5

Then

type omegaWallFunction;
value $internalField;

automatically switches to a low Re in case y+<1 and it resolve the boundary layer without using any wall function and switches to wall function when y+ is large.

No need to modify anything

Bassam, thanks for your answer.

Quote:

I assume that the boundary layer is already refined so that y+<1.

Yes, the average yplus is ~0.04 on the whole surface thanks to tiny layers.

Quote:

No need to modify anything

Do you have any source to back this up; perhaps an OpenFOAM page I didn't read?

For omega wall function I am sure! You can revise the theory behind this wall function which switch automatically.

However, the k and nut should be:
kLowReWallFunction
nutLowReWallFunction

I just realised that they were not a LowRe

Please revise page 27 from this report that support my claims:
A Thorough Description Of How Wall Functions Are Implemented In OpenFOAM - Chalmers

Thank you very much for your help and time, I will modify accordingly!

So did it work with you? I tried the approach in another case and compared the results to experiment which was satisfactory

Hi, right now I am trying to replicate in OpenFOAM a Simscale case that investigates the Ahmed body, using the boundary conditions that you suggested. I will get back to you as soon as possible and send you the outcomes when the solver will be done; it will take ~ 1 day because the mesh is pretty fine (18M cells and I am using a server with 8 cores only). Thank you again for your dedication ;)
https://www.simscale.com/projects/gs...ni/ahmed_body/

Wow 18M :eek: that definitely needs a cluster computer to handle the case. All the best in your simulations!