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-   -   forceCoeffs.C (http://www.cfd-online.com/Forums/openfoam-solving/90105-forcecoeffs-c.html)

Nico A. June 30, 2011 12:58

forceCoeffs.C
 
Hello everyone,

I computed the flow around a sphere and also calculated the lift and drag forces/ coefficients. At present I got to high values for Cd (0.61) at Re = 1*10⁴, due to the k-epsilon turbulence model I guess.
But anyway I checked the code for the calculation in the 'forceCoeffs.C' file. Most of it is clear to me, but I did not underśtand a few lines:
Code:

void Foam::forceCoeffs::write()
00110 {
00111    if (active_)
00112    {
00113        // Create the forces file if not already created
00114        makeFile();
00115
00116        forcesMoments fm = forces::calcForcesMoment();
00117
00118        scalar pDyn = 0.5*rhoRef_*magUInf_*magUInf_;
00119
00120        vector totForce = fm.first().first() + fm.first().second();
00121        vector totMoment = fm.second().first() + fm.second().second();

00122
00123        scalar liftForce = totForce & liftDir_;
00124        scalar dragForce = totForce & dragDir_;
00125        scalar pitchMoment = totMoment & pitchAxis_;
00126
00127        scalar Cl = liftForce/(Aref_*pDyn);
00128        scalar Cd = dragForce/(Aref_*pDyn);
00129        scalar Cm = pitchMoment/(Aref_*lRef_*pDyn);

I am not used to the C-language that much, but I want to know what is mathematically done in the red marked lines. Thanks for your advices!

Best regards Nico

norman1981 July 1, 2011 02:44

Hi Nico,

fm contains the computed forces and moments data (see forces.C). fm.first() contains forces data, fm.second() contains moments data. Forces and moments are stored in pressure and viscous contributions, so that:
fm.first().first() --> normal force
fm.first().second() --> tangential force
fm.second().first() --> moment due to the normal force
fm.second().second() --> moment due to the tangential force

The scope of the two lines you reported is to sum these contributions in order to get the total force and total moment.

Regards

Norman


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