Helllo, thank you for your reply. 

As you can see from the picture below (frontal view of the duct), the spots are on the leading edge of the duct. 

I'm not really concerned about the mesh refinement but more about the quality around the leading edge, which is a region characterised by high gradients. However, the pressure spots should not be there. 

I was wondering if there is the possibility of generating a mapped mesh in the watertight geometry workflow or if there is a way to have a better mesh in that area. As an example, here there is a picture of a mapped mesh obtained by ansys meshing. 

Unluckly, I'm not so experienced with this tool so that I'm not always able to get a mapped mesh and I lose control over the local refinements.