Experimental study has been conducted on the response of turbulent flow in a rectangular duct to a smooth-to-rough step change in surface roughness on its short-side walls. The distribution.; of the primary flow velocity, secondary flow velocities, turbulence intensities. and turbulent shear stresses been measured in seven cross-sections located from 2.0d(h) to 37.5d(h) downstream from the smooth-to-rough junction, and the relaxation process of this turbulent flow has been examined. It has been found that the relaxation process of this flow is considerably complex, but the flow characteristics except for the primary flow velocity in the core region as well as near the rough wall reach an almost equilibrium state. i.e., the fully developed rough-duct flow condition, within a distance of 37.5d(h) from the change in the surface roughness. (C) 2004 Elsevier Ltd. All rights reserved.