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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

Code:

```functions { forces { type forces; functionObjectLibs ("libforces.so"); //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; } 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 (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*r²/2         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 m²/s², which means you actually deal with p/rho (that's at least my assumption).
And so does of course patchIntegrate.

Cheers,
naval

edit: In the transportProperties dictionary rho is set to 1 kg/m³. Changing that to your reference density will also produce matching results.

 LVDH February 26, 2010 04:15

You are right.
Thanks.

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