不可压缩壁面剪切流动中层流-湍流界面处湍流生成和扩展机制研究

The near wall structure and self-sustained mechanism of fully developed wall-bounded shear flows have been extensively studied, whereas relevant studies of the laminar-turbulence interface are much fewer. This project will focus on the kinematic and dynamic characteristics of turbulent structures at the laminar-turbulent interface in pipe flow, with the objectives to understand the local transition to turbulence, i.e., the creation of turbulent fluctuations and vortical structures locally at the interface, and better understand the localization mechanism of turbulence at low Reynolds numbers and the spreading process of turbulence at higher Reynolds numbers via the motion of interfaces. Meanwhile, this project will also attempt to investigate the formation of turbulence cascade by studying the development of vortical structures through the interface. This project will offer insights to turbulence control, therefore, is of great interest for both fundamental research and applications. In this project, we will perform the research using direct numerical simulation (DNS) combined with a linearized Navier-Stokes equation model and Barkley pipe flow model. Further, based on the results obtained in pipe flow, we will also investigate the formation of turbulent spots and stripes and the spreading of turbulence via more complex motion of interfaces in channel flow.

完全发展的壁面剪切湍流的近壁面结构和自维持机理已经得到了大量的研究，而针对层流-湍流界面的相关定量研究还较少。本课题着重研究不可压缩管道流动中层流-湍流界面附近的流动结构及其运动学和动力学特性，旨在获得界面处局部转捩，即湍流结构和湍动能在界面处的生成和演化机制，深层次理解较低雷诺数下局部化湍流的时-空间歇性的形成和更高雷诺数下的湍流扩展机制。同时，该课题还将分析从湍流在界面处生成直到发展为统计平衡态湍流的过程中能量级串的建立过程。从应用角度该研究也将有助于高效湍流控制技术的开发，因而本课题有着重要的理论和现实意义。本课题将主要采用直接数值模拟结合一种线化NS方程模型和Barkley管流模型来定量分析上述问题。其次，在管流工作基础上，研究受壁面限制较弱的槽道中湍流斑和倾斜湍流带界面的形成及湍流的扩展机制。

很多情况下湍流仅在流动空间的局部区域中存在，在层流和湍流之间则会形成层流-湍流界面（或湍流锋面）。锋面处发生着层流和湍流区域间的动量、能量和物质交换，其动力学特性决定了湍流向层流区域的扩展方式。本项目主要研究管道和槽道流动中湍流锋面处的流动结构、湍流生成机制、锋面的运动学特性及其决定机制。相关结果阐明了槽道流动带状湍流结构锋面处的流动失稳机制和倾角形成机制，指出了槽道湍流锋面处横流线性失稳在湍流生成、湍流带自维持和湍流锋面运行速度中的决定性作用；完成了管道流动高雷诺数下湍流锋面的运动学特性及锋面处局部转捩过程分析，给出了迄今为止最大雷诺数范围内的锋面速度测量且首次揭示了锋面速度随雷诺数的幂次标度律，且提出了锋面速度的一个可能的决定机制；对管道流动各向异性滑移边界条件下的线性稳定性进行了理论和数值分析，指出了各向异性滑移对流动的致失稳效应。通过该项目的研究，管道和低雷诺数槽道流动中湍流向层流区域的扩展机制很大程度上得到了解释。