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

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

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