# Strange values for Lift & Drag Coefficient

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 November 18, 2011, 12:20 Strange values for Lift & Drag Coefficient #1 New Member   Denis Join Date: Jul 2011 Posts: 8 Rep Power: 15 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 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 ?? Denis

 November 18, 2011, 14:15 #2 Senior Member   Dave Join Date: Jul 2010 Posts: 100 Rep Power: 16 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

November 18, 2011, 14:40
#3
New Member

Denis
Join Date: Jul 2011
Posts: 8
Rep Power: 15
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
Attached Files
 meshing_fin.pdf (39.8 KB, 40 views)

 November 18, 2011, 20:31 #4 Senior Member   Dave Join Date: Jul 2010 Posts: 100 Rep Power: 16 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