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squanto773 March 7, 2012 02:27

Post Processing Drag Coefficient
Hi all

I used OpenFoam (simpleFoam with RAS turbulence model komegaSST) to calculate the drag coefficient of a car.
For calculating the drag coefficient I am using the tool and execut it after the simulation by using:


execFlowFunctionObjects -latestTime
That this works I added


    #include "forceCoeffs"

to system/controlDict



    type        forceCoeffs;
    functionObjectLibs ( "" );
    outputControl timeStep;
    outputInterval 1;

    patches    ( "car" );
    pName      p;
    UName      U;
    rhoName    rhoInf;      // Indicates incompressible
    log        true;
    rhoInf      1;          // Redundant for incompressible
    liftDir    (0 0 1);
    dragDir    (-1 0 0);
    CofR        (0.72 0 0);  // Axle midpoint on ground
    pitchAxis  (1 0 0);
    magUInf    27.8;
    lRef        1.0;        // Wheelbase length
    Aref        0.94293;        // Estimated

To dictionary forceCoeffs.

Since the car is in air I changed rhoInf to 1.188. However this did not have any effect to the drag coefficient. Actually changing rhoInf to any value you'd like does not have any effect on the drag coefficient. However from my point of view this should directly influence the drag coeff.

Is this a bug or did I miss something?

BTW: I am using 2.0.1


squanto773 March 7, 2012 09:43

Ah, I think I got it:
For incompressible calculations the pressure and the viscous forces are already divided by the density because kinetic viscosity is used.
see here

Therefore the density should be set to one because no further normalization is required for the calculation of Cd.
Thus also to display the correct pressure I have to multiply it by the density.. correct?

Could anyone comment on this to ensure that I am on the right track?

Thanks and regards


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