Contour Problem
 

Hi everyone!
I have to plot contours of stream function. My mesh area is divided by a circular interface boundary. The meshing circle turn around his axis. (my problem is about a pitching airfoil)
The contours are very strange, because I obtain circular ones in the circle, and straight one over the other piece of domain. Why this? I explain also that the plotting of the vectors doesn't make any problems. Near the interface, for example, the vectors in the inner and the outer side of the circular interface boundary have about the same behaviour.
Thanks in advance.