CFD Online Discussion Forums - forces on a hydrofoil

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- - forces on a hydrofoil (https://www.cfd-online.com/Forums/openfoam-solving/74202-forces-hydrofoil.html)

forces on a hydrofoil

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?

Hi,
you missed one keyword! See here: http://www.cfd-online.com/Forums/ope...ing-1-6-a.html
Cheers,

maddalena

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?

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,
maddalena

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 $c\cdot span$ .