槽流的亚临界转捩与局地湍流

在中低雷诺数时,处于线性稳定的槽道流若受到强扰动可发生亚临界转捩变为湍流。近年人们对该类转捩机理的研究取得重要进展。对于平板泊肃叶流,亚临界转捩之初是稀疏湍流态,其特征结构是远间隔的包含小尺度涡和高低速条带的大尺度的湍流带,可倾斜伸长。该阶段的湍流占比有上限但并非雷诺数的单值函数。随着雷诺数的增加转捩进入平衡局地湍流态,即存在统计定常态,其湍流占比是雷诺数单值函数,可由定向逾渗模型描述。进一步增加雷诺数,湍流带的分裂愈发频繁,最终流场会布满湍流带,在更高的雷诺数时变为均匀湍流。论文概述了为比拟亚临界转捩过程所提出的动力学模型,以及为定量表征管流、平板库艾特流和平板泊肃叶流转捩过程的相似性所提出的局地稳定性参数,并在最后对槽道流亚临界转捩研究的发展做了简要展望。

Subcritical transition in channel flows and localized turbulence

Triggered by strong disturbances,linearly stable channel flows may turn to be turbulent at moderate Reynolds numbers,and encouraging progresses have been made on the study of these subcritical transitions in the past two decades.For plane-Poiseuille flow,the initial stage of the transition is a sparse turbulent state,where large-scale oblique turbulent bands are composed of small-scale vortices and high and low speed streaks,separated by large laminar regions,and are able to extend obliquely.At this stage,the turbulence fraction has an upper bound,but is not a single-valued function of the Reynolds number.With the increase of the Reynolds number,the equilibrium localized turbulence state is reached,i.e.when the channel domain is large enough the flow is statistically steady and its turbulent fraction is a single-valued function of the Reynolds number and can be described by the Directed Percolation model.When the Reynolds number is increased further,band split occurs more frequently and the turbulent bands spread to the whole flow domain eventually,and the flow turns to be uniform turbulence at higher Reynolds numbers.The dynamic models proposed for the subcritical transitions are summarized,and the local stability parameters to quantitatively characterize the similarity among the transitions of pipe flow,plane-Couette flow,and plane-Poiseuille flow are surveyed.Finally,the development of the study on the subcritical transitions in channel flows is prospected.