create fully developed turbulent flow within the duct 12 on the downstream side of the fan 10 thus minimizing drag this is accomplished because the center zone 30 of the center body 22 is of a larger diameter than the central fan structure 16 thus forcing the fluid to pass through a generally annular space of reduced cross section between this center zone 30 and the wall of the duct 12 the velocity of the fluid is thus increased considering 1 the overall length of the fan 10 and noise abatement apparatus 14 in combination 2 the diameter of the duct 12 3 the velocity imparted to the fluid by the fan blades 18 and 4 the reynolds number of the local flow the increased velocity adjacent the center zone 30 is sufficient to create fully developed turbulent flow as the fluid moves along the inwardly tapered terminal zone 32 of the center body 22 the cross sectional area of the flow path is increased again toward the full cross sectional duct 12 and the flow is thus diffused the terminal zone 32 provides a blunt downstream end 38 on the center body 22 this blunt end 38 is generally to be preferred to a continuous taper leading to a point although.