why does drag coefficients turn to negative
 

I am working on a 2d wings using ANSYS FLUENT 13.0. The fluid is water. The convergence history looks ok for Residual, Cd and Cl, however, the drag coefficients are very close to 0 and turn to negative at last.

Does anyone know what's the problem.

Thank you.

The problem is solved by entering the force vectors: the lift coefficient (-sin, cos), the drag coefficient (cos, sin).

The negative drag coefficient again in a smaller angle of attack.

Why?

why u are doing calculating lift and drag coefficient manually. u can report the drag and lift coefficient in fluent even u can set as monitor. plz also check the reference area that is used for calculating drag and lift coefficient.