[kevin.pku]

Hello everyone,

I am a new guy in OpenFOAM. I did a 2D airfoil simulation by OpenFOAM. Unfortunately, the airfoil drag coefficient from the simulation is negative, this
is unphyiscal. I tried my best but still can not find the reason. It may be
caused by the mesh generation, case setup, or postprocessing setup. Could anyone help me? Thank you.

1. The mesh of the airfoil is a C-Mesh. In order to run the .msh file in OpenFOAM, I extrude the 2D airfoil mesh to 3D, in which I keep the boundary setup of OUTFLOW, WALL and FRONTANDBACK, but remove the INLET.

2. The setup of the initial velocity and pressure in the 0/U file is as follows:

dimensions [0 1 -1 0 0 0 0];

internalField uniform (6.894 1.216 0);

boundaryField
{
OUTFLOW
{
type freestream;
freestreamValue uniform (6.894 1.216 0);
}
FRONTANDBACK
{
type empty;
}

WALL
{
type noSlip;
}


}

3. The AoA is 10 deg, and the initial Re is about 1e+7.

4. The setup of function for calculating force coefficients in the system/controlDict file is as follows

functions
{

forceCoeffs
{
type forceCoeffs;
functionObjectLibs ("libforces.so");
patches (wall);
log true;
rho rhoInf;
rhoInf 1;
CofR (0 0 0);
liftDir (0 1 0);
dragDir (1 0 0);
pitchAxis (0 0 1);
magUInf 7;
lRef 1;
Aref 1;
writeControl timeStep;
writeInteval 100;
}
};


5. The results of Cm, Cd, Cl are as follows

Cm = 4.613587e-01
Cd = -1.192186e-01
Cl = 1.315706e+00

[yambanshee]

How large is your C diameter in relation to the chord length of the blade?
How 'converged' is your result?
What does your streamlines over the blade look like?