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forces on a hydrofoil

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Old   March 26, 2010, 08:57
Default forces on a hydrofoil
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I included a subroutine in the controlDict file to evaluate the lift and drag coefficients on a blade section of a marine propeller:

Code:
functions
(
forces
{
type forces;
functionObjectLibs ("libforces.so"); // Lib to load
patches (wall); // change to your patch name
rhoName rhoInf;
rhoInf 1025; // Reference density for fluid
CofR (0 0 0); // Origin for moment calculations
}
forceCoeffs
{
type forceCoeffs;
functionObjectLibs ("libforces.so");
patches (wall); // change to your patch name
rhoName rhoInf;
rhoInf 1025;
CofR (0 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 0);
magUInf 7.30;
lRef 0.305;
Aref 0,00306;
}
);
I set the hydrofoil chord as lRef and the wetted surface as Aref (or is Aref its projection on the chord?) and this is the output:

Code:
[0] 
[0] 
[0] keyword outputControl is undefined in dictionary "::functions::forces"
[0] 
[0] file: ::functions::forces from line 56 to line 60.
[0] 
[0]     From function dictionary::lookupEntry(const word&, bool, bool) const
[0]     in file db/dictionary/dictionary.C at line 388.
[0] 
FOAM parallel run exiting
[0] 
[1] 
[1] 
[1] keyword outputControl is undefined in dictionary "::functions::forces"
[1] 
[1] file: ::functions::forces from line 56 to line 60.
[1] 
[1]     From function dictionary::lookupEntry(const word&, bool, bool) const
[1]     in file db/dictionary/dictionary.C at line 388.
[1] 
FOAM parallel run exiting
[1]
What's the problem, please?

Last edited by vaina74; March 26, 2010 at 15:11.
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Old   March 28, 2010, 16:09
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Hi,
you missed one keyword! See here: force function not working in OF 1.6
Cheers,

maddalena
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Old   March 29, 2010, 12:49
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Thanks, Maddalena. First I merged all the threads about lift and drag, then I found the nacaAirfoil tutorial (in compressible/sonicFoam/ras). I included in my controlDict:
Code:
functions
{
    forces
    {
        type        forces;
        functionObjectLibs ( "libforces.so" );  // lib to load
        outputControl timeStep;
        outputInterval 1;
        patches
        (
            wall  // change to your patch name
        );
        // name of fields
        pName       p;
        UName       U;
        log         true; // dump to file
        rhoInf      1025;
        CofR        ( 0 0 0 );
    }
    forcesCoeffs
    {
        type        forceCoeffs;
        functionObjectLibs ( "libforces.so" );  // lib to load
        outputControl timeStep;
        outputInterval 1;
        patches
        (
            wall  // change to your patch name
        );
        // name of fields
        pName       p;
        UName       U;
        log         true; // dump to file
        rhoInf      1025;
        CofR        ( 0 0 0 );
        liftDir     ( 0 1 0 );
        dragDir     ( 1 0 0 );
        pitchAxis   ( 0 0 0 );
        magUInf     7.30;
        lRef        0.305;
        Aref        0.001525;
    }
}
I'm still in doubt: is Aref the projection of the area on the chord or the whole wetted surface?
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Old   March 29, 2010, 16:10
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Ehi Vaina,
Quote:
Is Aref the projection of the area on the chord or the whole wetted surface?
Well, I do not know if something changes with hydrofoils. In any case, the definition of aerodynamic force coefficient for an airfoil (2d) is c_F = F/(0.5 * rhoInf * v_inf^2 *c), thus I would set Aref = 1. What do you think?
Cheers,
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Old   March 30, 2010, 06:19
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I know the lift (or drag) equation is:

C_L= \frac{L}{\frac{1}{2}{\rho}v^2A_{ref}}

where A_{ref} is planform area. In the section lift coefficient (based on the concept of an infinite wing of non-varying cross-section), the lift is defined per unit span of the wing:

c_l= \frac{L}{\frac{1}{2}{\rho}v^2c}

In OpenFOAM all geometries are generated in 3 dimensions and my hydrofoil span is 0.05 mm. So I think A_{ref} is equal to c\cdot span.
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Old   March 30, 2010, 07:30
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Perfect!
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