# Problem obtaining right Cd

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 February 19, 2013, 17:19 Problem obtaining right Cd #1 New Member   Matt Join Date: Oct 2012 Posts: 3 Rep Power: 4 Hi guys, quite new to openFoam. I'v been running these 2 cases recently. A cylinder as well as a square cylinder. I decided on using a value of Re of 100,000 so as to compare with a book. Im using pisoFoam, k-e model and used initial values of k and e from the equations given here: http://en.wikipedia.org/wiki/Turbulence_kinetic_energy Currently running on a single core (dont know who to run in parallel just yet) Meshing for both are quite dense but i get values of Cd of 0.7 for cylinder (should 1.2) and Cd of 1.7 for sqaure cylinder (should be 2.1) Could these inaccurate values be a meshing problem? I did increase the mesh and after almost a 6hr simulation, i still got roughly the same Cd for the square cylinder. so im not sure exactly what my next step is. Control dic: application pisoFoam; startFrom startTime; startTime 0; stopAt endTime; endTime 100; deltaT 1; writeControl timeStep; writeInterval 5; purgeWrite 0; writeFormat ascii; writePrecision 6; writeCompression uncompressed; timeFormat general; timePrecision 6; runTimeModifiable yes; functions { forces { type forces; functionObjectLibs ("libforces.so"); //Lib to load patches (cylinder); // change to your patch name rhoName rhoInf; rhoInf 1.1614; //Reference density for fluid CofR (0 0 0); //Origin for moment calculations outputControl timeStep; outputInterval 1; } forceCoeffs { // rhoInf - reference density // CofR - Centre of rotation // dragDir - Direction of drag coefficient // liftDir - Direction of lift coefficient // pitchAxis - Pitching moment axis // magUinf - free stream velocity magnitude // lRef - reference length // Aref - reference area type forceCoeffs; functionObjectLibs ("libforces.so"); patches (cylinder); rhoName rhoInf; rhoInf 1.1614; CofR (0 0 0); liftDir (0 1 0); dragDir (1 0 0); pitchAxis (0 0 1); magUInf 1; lRef 1; // Aref 0.1; // outputControl timeStep; outputInterval 1; } } Any help would be much appreciated!

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