Mesh Density in Gaps
When using any turbulence model there should be at least five elements across inlet and outlet passages so that gradients can be properly resolved. Mesh Enhancement automatically ensures that this criteria is met.
Mesh Refinement
It is good practice to refine the mesh near openings so that the boundary conditions correctly influence the flow in the interior. Generally Automatic Mesh Sizing ensures this requirement is satisfied, but if not, the mesh should be adjusted.