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

表面粗糙度阶跃变化的矩形管道中湍流和传热的弛豫

基本信息

批准号：
11650220
负责人：
HIROTA Masafumi
金额：
$ 2.3万
依托单位：
Nagoya University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-11650220/
关键词：
Complex turbulent flow Heat transfer Relaxation process Surface roughness Secondary flow Velocity field Temperature field Simultaneous measurement

项目摘要

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.

在湍流中，通过管道的表面粗糙度从粗糙到光滑的阶跃变化，近壁流动响应这种粗糙度的变化迅速。然而，在核心区域中，流动的响应相对于壁附近延迟，因此，在从粗糙到光滑的接合部下游一段距离之后，整个流场达到新的平衡状态。这种场的流向变化过程称为弛豫过程。本文通过实验研究了表面粗糙度阶跃变化的矩形通道内湍流流动与换热的松弛过程，首先明确了粗糙壁面矩形通道内充分发展湍流流动与换热的流场和温度场特征，其对应于表面粗糙度从粗糙到光滑阶跃变化时的初始状态。在详细资料的基础上，阐明了动量和热量的湍流输运过程的机制。然后，研究了表面粗糙度阶跃变化的矩形通道内流场的松弛过程。得到了平均速度场和湍流应力场的流向变化规律。特别是在粗糙管道中，湍流强度先超过光滑管道的水平，然后又低于光滑管道的水平，最后达到新的平衡状态。其次，研究了同一流动系统中温度场的弛豫过程。从湍流切应力和湍流热通量的流向变化的比较，我们已经阐明，温度场达到新的平衡状态后，一个较短的距离从粗糙到光滑的交界处比流场。