CFD Online Discussion Forums - Strange values for Lift & Drag Coefficient

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Strange values for Lift & Drag Coefficient

Hi,

i'm trying to simulate the flow over a symmetric airfoil and computing the lift and drag coefficients. The Reynolds Number is 1e6 and i'm using the Spalart Allmaras turbulance modell. The angle of attack is 0

Here is the function to compute the coefficients:

Code:

functions

{

    forces

    {

        type            forceCoeffs;

        functionObjectLibs ( "libforces.so" );

        outputControl   timeStep;

        outputInterval  1;



        patches

        (

           Finne-Wall

        );



        pName       p;

        UName       U;

        log         true;

        rhoName     rhoInf;

        rhoInf      1;

        CofR        ( 0 0 0 );

        liftDir     (0 1 0);

        dragDir     (1 0 0);

        pitchAxis   ( 0 0 0 );

        magUInf     22.22;          // [m/s] 

        lRef        0.11;                // [m]

        Aref        0.00036385;     // [m^2]

And this is the solution :confused:

Code:

SIMPLE solution converged in 319 iterations



forceCoeffs output:

    Cd = 789148

    Cl = -12015.9

    Cm = 0

The lift coefficient has to be 0, because it is an symmetric airfoil. Does anyone has an idea, what could be wrong ?? :confused:

Denis

Denis,
Your Aref and LRef are likely the reason you are getting such huge lift and drag coefficients. It is not unusual for a very small lift coefficient at 0 degrees (your lift is 65 times smaller than the drag) due to not quite perfect symmetry of the mesh surface. It is pretty difficult to get it exactly 0 with most meshers.

Regards,
Dave

Hi Dave,

thank you for your reply!

Ok I understand that the lift-coefficient has not to be exactly zero.

The main problem is the extreme high value.
I attached a picture of the model. It looks like a NACA airfoil, but it is actually a fin. So I expected a lift coefficients between 0 and 4. So do you think that this is a "phyiscal" solution?

Denis

Denis,
As I said in my prior post, I think the issue is your Aref. You could manually calculate the coefficients from your force output: "type forces" rather than "type forcecoeff" to confirm that your simulation is producing physically the right results.
Dave