壁流动中的转捩

TRANSITION IN WALL-BOUNDED FLOWS

通过对近120篇壁流动的实验结果和理论分析的主要文献进行归纳提炼,分析对比了这些实验结果,并总结了相关理论方面的进展,发现尽管实验中所使用的初始扰动条件不同,但所发现的流动结构几乎是完全一样的,其中,最基本最重要的流动结构是:在边界层和管流中被称为类孤子相干结构(SCS)的三维非线性涡包、A涡、二次涡环和涡环链,近期的实验中发现,这些结构形成和转捩的动力学过程包括以下内容:(1)A涡和二次涡环间持续的相互作用过程,该过程决定了涡环链的产生方式,即总是从壁面区域周期性地形成,再进入边界层的外层。(2)高频涡的生成,这是理解转捩和湍流边界层(以及其他流动)发展的关键问题之一,尽管已有一些解释,但是二次涡环的实验发现将提供一个特别清晰的解释。(3)在所有湍流猝发中SCS所起的关键作用,这一点是低雷诺数湍流边界层中湍流产生的关键机制,与猝发直接相关联的是低速条带,针对壁流动情况,SCS的动力学过程更清晰地解释了低速条带的形成机制及其与流动结构的关系,实验中观察到的SCS和二次涡环,不仅能使我们重温壁面流动转捩中的经典故事,同时还为建立壁面流动转捩的普适动力学过程开辟了一条新的途径。

In this paper, we present direct comparisons of experimental results on transition in wall-bounded flows obtained by flow visualizations, hot-film measurement, and particle image velocimetry, along with a brief mention of relevant theoretical progresses, based on a critical review of about i20 selected publications. Despite somewhat different initial disturbance conditions used in experiments, the flow structures were found to be practically the same. The following observed flow structures are considered to be of fundamental importance in understanding transitional wall-bounded flows： the three-dimensional nonlinear wave packets called solitonlike coherent structures （SCS） in boundary layer and pipe flows, the - vortex, the secondary vortex loops, and the chain of ring vortices. The dynamic processes of the formation of these structures and transition as newly discovered by recent experiments include the following： （1） The sequential interaction processes between the-vortex and the secondary vortex loops, which control the manner by which the chain of ring vortices is periodically introduced from the wall region into the outer region of the boundary layer. （2） The generation of high-frequency vortices, which is one of the key issues for understanding both transitional and developed turbulent boundary layers （as well as other flows）, of which several explanations have been proposed but a particularly clear interpretation can be provided by the experimental discovery of secondary vortex loops. The ignorance of secondary vortex loops would make the dynamic processes and flow structures in a transitional boundary layer inconsistent with previous discoveries. （3） The dominant role of SCSs in all turbulent bursting, which is considered as the key mechanism of turbulent production in a low Reynolds number turbulent boundary layer. Of direct relevance to bursting is the low-speed streaks, whose formation mechanism and link to the flow structures in wall-bounded flows can be answered more clearly than before in terms of the SCS dynamics. The observed SCSs and secondary vortex loops not only enable revisiting the classic story of wall-bounded flow transition, but also open a new avenue to reconstruct the possible universal scenario for wall-bounded flow transition.