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LVDH February 5, 2010 05:50

Forces calulated through pressure
Hello my fellow Foamers,
I have to address something I find confusing:

I have found two ways to calculate forces.
The first one is with the help out of this forum. Someone suggested to hang this piece of code to the controlDict


type forces;
functionObjectLibs (""); //Lib to load
patches (wall); // change to your patch name
rhoName rhoInf;
rhoInf 1.205; //Reference density for fluid
CofR (4.5 1.5 2.125); //Origin for moment calculations
        outputControl  timeStep;
        outputInterval  50;
// 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 ("");
patches (wall);
rhoName rhoInf;
rhoInf 1.205;
CofR (2 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 1);
magUInf 50;
lRef 4.245; // sphere diameter
Aref 2.2555; //1/2 * projected area = pi*r/2

        outputControl  timeStep;
        outputInterval  50;


This does seem to work. I do get a folder containing the forces.

My second way of calculating the forces is by using the command patchIntegrate. If I use this command with the p field and the wall it should (thats what I thought) give me the forces resulting through pressuredifferences.

My problem is that the two methodes come to different solutions. If I run the simpleFoam/airfoil tutorial in timestep 500 I get these two solutions:

controlDict: (-97.6164 755.598 -6.01592e-14)
patchIntegrate: (-81.0095 627.052 -4.99247e-14)

The two solutions are not completly different, but still the question remains:
Why are they different?

naval February 25, 2010 07:27

Hi Andre,

Your forces differ all by the factor 1.205 - which is exactly the value of your reference density rhoInf.

If you check out the 2D-Airfoil tutorial you will notice, that the pressure dimension is set to m/s, which means you actually deal with p/rho (that's at least my assumption).
And so does of course patchIntegrate.


edit: In the transportProperties dictionary rho is set to 1 kg/m. Changing that to your reference density will also produce matching results.

LVDH February 26, 2010 04:15

You are right.

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