Ansys Employee

Difficult to see the mesh, but I suspect the jump in cell size & aspect ratio (centre, enlarged image, section) isn't helping the solver. O-grids (butterfly meshes) are only really used for block structured solvers now, and given Fluent has been unstructured for years they're not always helpful. 

So, check the mesh quality (cell quality and mesh quality are linked but aren't quite the same thing) and whether you're resolving all the flow features.