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-   -   getting wrong value of drag (http://www.cfd-online.com/Forums/openfoam-solving/120072-getting-wrong-value-drag.html)

shash June 29, 2013 20:08

getting wrong value of drag
 
Hi openfoamers,

I am simulating a 3d flow around a sphere enclosed in a large cube, but problem is that i am getting my drag =1.04 instead of expt. drag =0.66 for reynolds no. of 300, parameters: fluid:air with rho=1.2, nu=1e-04, dia. of sphere=1m, velocity at inlet=0.03m/s ; Can someone please suggest what i am doing wrong.

my controldict where drag lib parameters given :
Quote:

application icoFoam;

startFrom startTime;

startTime 0;

stopAt endTime;

endTime 50;

deltaT 0.005;

writeControl timeStep;

writeInterval 200;

purgeWrite 0;

writeFormat ascii;

writePrecision 6;

writeCompression off;

timeFormat general;

timePrecision 6;

runTimeModifiable true;


// ************************************************** *********************** //

functions
{

forces
{
type forceCoeffs;
functionObjectLibs ("libforces.so");
enabled true;

outputControl timeStep;
outputInterval 1;

patches (sphere);
rhoName rhoInf;
rhoInf 1.2;
log true;

CofR (0 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 0);

magUInf 0.03;
lRef 1;
Aref 0.785;
}
}

//************************************************** ***************************

immortality June 29, 2013 20:45

what does vortices look like?is there any?

shash June 30, 2013 16:28

Hi Ehsan,
you pointed out correctly, i donot know why i am not getting the double threaded vortices for Re =300 thou with similar settings i have got as expected karman vortex street in 2d case. Got any ideas whats wrong when i am using 3d geometry??

immortality June 30, 2013 16:43

Hi Shiv
are you using a turbulent model?which?how-Re or low-Re?

shash June 30, 2013 17:22

no, i am using icofoam solver so its laminar flow.

Ehsan, i noted something that when i use [nu=1e-04, U=0.03, Re=300] i am not getting vortices though when i use [nu=0.01, U=3, Re=300] i am getting vortices , both the cases have same Re why variation in results :confused:

immortality July 1, 2013 11:26

its unsteady if wakes there are.maybe deltaT isn't low enough.


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