Relaxation of Turbulent Flow in a Rectangular Duct with a Step Change in Surface Roughness.

Abstract

The response of a turbulent flow in a rectangular cross-sectioned duct to a rough-to-smooth step change in the surface roughness of the short-side walls has been investigated experimentally. The distributions of the primary and secondary flow velocities, turbulence intensities, and turbulent shear stresses have been measured by hot-wire anemometers in seven cross-sections set up in the smooth-duct part. In the core region of the duct cross-section, large secondary currents are maintained over a relatively long distance after the rough-to-smooth junction, and they cause large velocity gradient in the primary flow and large turbulent shear stress there. The streamwise changes of the turbulence intensities are rather complex. In the core region they once overshoot the values in the rough duct just after the rough-to-smooth junction, then undershoot those in the smooth duct, and finally approach an equilibrium state, while near the short-side wall they are decreased quite rapidly and monotonously in the streamwise direction.