Vibrating Residual values
 

Hi,
I am new to Ansys and I'm trying to carry out the 2D CFD analysis of a NACA0012 airfoil. Initially I started with a mesh convergence study, and everything was going to plan (after a few initial teething problems), until I increased the number of nodes in the mesh above 65,000.

When I used 55,000 and 60,000 nodes, I got similar answers (Coefficient of lift=0.54). Although this was slightly lower than I expected (0.62), I was trying to increase the mesh one more time to ensure that it had converged, when I ran into this problem

When I used 70,000 nodes, for example, the residual values start to vibrate excessively after about 6000 iterations. Does anyone have an idea why this is the case, and could you please suggest a solution?

Thanks a lot for any help.
Aidan

(Note: my mesh is fairly basic, in that I did not use the "Split Block" option in ICEM Meshing to refine the mesh around the airfoil - each time I used this, I got much lower values of the Coefficient of Lift and I'm not sure why. Therefore, I used a very basic mesh, similar to the one shown at the end of this video, where the mesh size grew as I move away from the airfoil: https://www.youtube.com/watch?v=EknKVAJGEJ8