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-   -   SonicFoam - Forces, viscous force too big? (http://www.cfd-online.com/Forums/openfoam-solving/107027-sonicfoam-forces-viscous-force-too-big.html)

Kryo September 15, 2012 12:27

SonicFoam - Forces, viscous force too big?
 
Im just running a simulation of a rocket flying with Ma=1.7 through air. I use sonicFoam for this, and everything looks quite well when i view it with paraview. (compression, expansion and so on).

But then I try to calculate forces and coefficients and imo I get too high drag forces. Drag is directed to the bottom, therefore negative Z-axis is the drag.
Have a look at the output:

Code:

forces output:
    forces(pressure, viscous)((9.821624 11.33696 -398.8834) (0.6874167 -0.04157073 -1979.892))
    moment(pressure, viscous)((8.025379 -7.056551 0.2301483) (-0.1572154 -0.5736852 -0.04660123))

forceCoeffs output:
    Cd = 1.790804
    Cl = 0.008503463
    Cm = 0.003948905
    Cl(f) = 0.0003028264
    Cl(r) = 0.008200637

I expected the Cd value to be between 0.5 and 1 but not that high! Im not sure yet, but is it possible that the viscous force is too big? its 5x bigger than the pressure force.

The following settings might be connected to this, but as far as I know they are set correctly for my case :/

Code:

forces
    {
        type            forces;
        functionObjectLibs ( "libforces.so" );
        outputControl  timeStep;
        outputInterval  1;

        patches
        (
            rocketbody
        );

        pName      p;
        UName      U;
        rhoName    rho;
        log        true;
        rhoInf      1.16;
        CofR        ( 0 0 0 );
        liftDir    ( 0 1 0 );
        dragDir    ( 0 0 -.1 );
        pitchAxis  ( 1 0 0 );
        magUInf    600;
        lRef        1.500;
        Aref        0.00636172512351933130788685285114;
    }
forceCoeffs
    {
        type            forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        outputControl  timeStep;
        outputInterval  1;

        patches
        (
            rocketbody
        );

        pName      p;
        UName      U;
        rhoName    rho;
        log        true;
        rhoInf      1.16;
        CofR        ( 0 0 0 );
        liftDir    ( 0 1 0 );
        dragDir    ( 0 0 -1 );
        pitchAxis  ( 1 0 0 );
        magUInf    600;
        lRef        1.500;
        Aref        0.00636172512351933130788685285114;

this is in my controldict. As far as I see it, its all correct :O


thermophysicalProperties:
Code:

FoamFile
{
    version    2.0;
    format      ascii;
    class      dictionary;
    location    "constant";
    object      thermophysicalProperties;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

thermoType      ePsiThermo<pureMixture<constTransport<specieThermo<eConstThermo<perfectGas>>>>>;

mixture
{
    specie
    {
        nMoles          1;
        molWeight      28.9;
    }
    thermodynamics
    {
        Cv              717.5;
        Hf              0;
    }
    transport
    {
        mu              14.74e-06; mu=eta/rho ; eta= 17.1e-6, rho=1.16
        Pr              0.7;
    }
}

can anyone find a mistake? or does somebody know of another setting that might interfer here?

EDIT: Ok I found out, viscous force is bigger than the pressure force for rockets, but still, its far too high in total. What could be connected to: I am using LaunderSharmaKE as RAS turbulence model.

Kryo September 15, 2012 14:33

sorry for double post, but I found the reason for the high values: Random k-epsilon- boundaries for the turbulence model...

Any idea what to use here? are there any formulas to calculate the correct k and epsilon boundaries?


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