My practical understanding: With URANS your eddy viscosity represents the total viscosity effect of the eddies - ie refining the mesh more shouldn't have any effect on nut, you should reach a point where the mesh is refined enough and you get a converged solution. In this case, nut will be high and suppress the formation of any eddies For LES, the eddy viscosity only represents the sub grid contribution - the more you refine your mesh, the lower nut will go until you reach DNS What allows the hybrid methods to work is building in some sort of switch or blending factor to drop nut way down in regions where you meet the right conditions for LES, which allows appropriate turbulent eddies to form