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-   -   Forces (in controlDict) Giving Results 1/10 of Experimental Comparison (https://www.cfd-online.com/Forums/openfoam-post-processing/93460-forces-controldict-giving-results-1-10-experimental-comparison.html)

dancfd October 16, 2011 22:30
Forces (in controlDict) Giving Results 1/10 of Experimental Comparison
 
Hello all,

I am running a simulation on a NACA 0012 airfoil, and after confirming that my simulation yields very similar Cp plots to the experimental results, I am now trying to match Cl to the experimental data. The case in question is at 13 deg alpha, and while I should be getting a Cl of ~1.34, the forceCoeffs.dat file shows 0.134. The Cd is also coming out negative...

The forces sub-dict of controlDict is as follows:

Code:
forces
    {
        type        forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        outputControl outputTime;
        outputInterval 500;
        patches        (wing);
        pName      p;
        UName      U;
        rhoName            rhoInf;
        log        true;
        rhoInf      1.177;
        CofR        ( 0.25 0 0 );//Center of rotation for moment calcs
        liftDir    ( 0 1 0 );
        dragDir    ( 1 0 0 );
        pitchAxis  ( 0 0 1 );
        magUInf    104.15;
        lRef        1;
        Aref        1;
    }
Note that my Uinf is (104.15 0 0). I would appreciate any advice anyone could offer.

Thanks,
Dan

dancfd October 23, 2011 22:16
Follow-Up
 
1 Attachment(s)
I will add some additional information:
Code:
                                                                                      Cd                  Cl                Cm
Results from OF:                                                              -0.0284      0.134      0.000628
Results from OF rotated to account for alpha:                      0.00247  0.1369    0.000628
Results from manually integrating Cp with spreadsheet:        0.01280        1.167      -.00417
Results from Ref (NASA TM 4074 Table IV):                      0.01994    1.3939        0.0226
I have also attached a graph overlaying my CFD results with those from NASA TM 100526.

The graph of the Cp results suggest that the data should be accurate. However, integrating the data yields low results for Cd and Cl, while Cm is way off the mark. Is it just a coincidence that the OF Cl data is 10% of the experimental value?

Has anyone successfully integrated OF Cp results to yield Cl, Cd and/or Cm that are similar to experimental data?

I would appreciate any assistance that is offered. Regards,

Dan

fippo_dk November 14, 2011 07:03
The Aref is chordLength*width, so check that and run again your simulations, you may get better values after that.
Check also your Re number, it should be the same as in the transportProperties file.

dancfd November 14, 2011 19:11
Thanks, there was a problem with the setup that fixed Cl. Cd and Cm have a greater error margin, though.

Dan


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