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 recnice November 18, 2011 13:20

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

 daveatstyacht November 18, 2011 15:15

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

 recnice November 18, 2011 15:40

1 Attachment(s)
Hi Dave,

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

 daveatstyacht November 18, 2011 21:31

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

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