Airfoil boundary conditions
 

Hello everybody,

I know that the issue of boundary conditions has been posed quite often.
Indeed, in a previous thread I read that generally pressure and pressure far field can be used in case of compressible subsonic flow.

In my case though, a NACA0015 airfoil with flap deflected by 10°, when I apply far field pressure conditions on outlet the Cd always comes out with a negative value.
Nonetheless, this does not occur in case of pressure outlet where the Cd results are good.
(The inlet conditions are pressure far field in both cases).

Could you give me any explanation?