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 matdgonz February 19, 2013 17:19

Problem obtaining right Cd

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;
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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