Liu Jinsheng, Xiao Yue, Li Mogeng, Tao Jianjun, Xu Shengjin
AML, School of Aerospace Engineering, Tsinghua University, Beijing 100084, China.
CAPT-HEDPS and SKLTCS, Department of Mechanics and Engineering Science, College of Engineering, Peking University, Beijing 100871, China.
Entropy (Basel). 2020 Dec 11;22(12):1399. doi: 10.3390/e22121399.
The intermittent distribution of localized turbulent structures is a key feature of the subcritical transitions in channel flows, which are studied in this paper with a wind channel and theoretical modeling. Entrance disturbances are introduced by small beads, and localized turbulent patches can be triggered at low Reynolds numbers (). High turbulence intensity represents strong ability of perturbation spread, and a maximum turbulence intensity is found for every test case as ≥ 950, where the turbulence fraction increases abruptly with . Skewness can reflect the velocity defects of localized turbulent patches and is revealed to become negative when is as low as about 660. It is shown that the third-order moments of the midplane streamwise velocities have minima, while the corresponding forth-order moments have maxima during the transition. These kinematic extremes and different variation scenarios of the friction coefficient during the transition are explained with an intermittent structure model, where the robust localized turbulent structure is simplified as a turbulence unit, a structure whose statistical properties are only weak functions of the Reynolds number.
局部湍流结构的间歇性分布是槽道流亚临界转变的一个关键特征，本文通过风洞实验和理论建模对其进行研究。通过小珠子引入入口扰动，在低雷诺数（）下可以触发局部湍流斑块。高湍流强度代表扰动传播能力强，每个测试案例中当≥950时都能发现最大湍流强度，此时湍流分数随急剧增加。偏度可以反映局部湍流斑块的速度缺陷，并且当低至约660时显示为负。结果表明，在转变过程中，中平面流向速度的三阶矩有最小值，而相应的四阶矩有最大值。利用间歇性结构模型解释了这些运动学极值以及转变过程中摩擦系数的不同变化情况，其中稳健的局部湍流结构被简化为一个湍流单元，该结构的统计特性仅是雷诺数的弱函数。
