Relaxation of turbulent flow and heat transfer in rectangular ducts with a step change in surface roughness

点击翻译按钮获取中文摘要

In turbulent flow through a duct with a rough-to-smooth step change of the surface roughness, the flow near the wall responds to this roughness change promptly. In the core region, however, the response of the flow is delayed relative to the vicinity of the wall, and thus the whole flow field attains to the new equilibrium state after a some distance downstream from the rough-to-smooth junction. This process of the streamwise variation of the field is called as relaxation process. In this study, the relaxation processes of turbulent flow and heat transfer in a rectangular channel with a rough-to-smooth step change of the surface roughness have been investigated experimentally.As the first step, we have made clear the characteristics of the flow and temperature fields for a fully developed turbulent flow and heat transfer in a rectangular duct with rough walls, which correspond to the initial states at the rough-to-smooth step change of the surface roughness. Based on the detailed data, the mechanism of the turbulent transport process of momentum and heat has been clarified. Then, the relaxation process of the flow field in a rectangular duct with a rough-to-smooth step change of the surface roughness has been investigated. The streamwise variations of the mean velocity and the turbulent stress fields have been made clear. In ptarticular, it has been found that turbulence intensities once overshoot the level for the rough duct, then undershoot that for the smooth duct, and finally attain to the new equilibrium state. Next, the relaxation process of the temperature field in the same flow system has been investigated. From a comparison of the streamwise variations of the turbulent shear stresses and turbulent heat fluxes, we have elucidated that the temperature field attains to the new equilibrium state after a shorter distance from the rough-to-smooth junction than the flow field.