# forceCoeffs.C

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 June 30, 2011, 12:58 forceCoeffs.C #1 New Member   Join Date: Apr 2011 Location: Magdeburg, Germany Posts: 23 Rep Power: 8 Sponsored Links 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

 July 1, 2011, 02:44 #2 Member     Norman Del Puppo Join Date: Mar 2009 Location: Hinwil, CH Posts: 57 Rep Power: 10 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 bhushanvelis likes this.

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