不可压壁湍流中基本相干结构

On elementary coherent structures in incompressible wall-bounded turbulence

条带和流向涡作为壁湍流中的两类基本相干结构,对于理解壁湍流的生成演化具有重要意义。本文首先回顾了充分发展不可压壁湍流中,这两类基本相干结构从近壁区到远壁区、从低雷诺数到高雷诺数的发现、发展历程,并着重点论述其自相似和自维持的两个核心特征。然后,进一步讨论基于湍流自维持过程发现的一系列精确相干态,从N-S方程不变解的角度精确地描述了条带和流向涡构成的自维持单元。精确相干态同时具有自维持和自相似特征,且维度更低,构成了充分发展壁湍流的骨架,为采用动力系统理论研究壁湍流奠定了基础。近些年,精确相干态在解释亚临界转捩方面也获得了较大成功,为转捩的研究提供了新的视角,有望从动力系统角度关联湍流与转捩两个经典问题。

As the two elementary structures in wall-bounded turbulence,streaks and streamwise vortices are of great importance in understanding the evolution of turbulence.In this paper,we first review the discovery and evolution of these coherent structures ranging from the near-wall region to the outer region and from low to high Reynolds numbers in fully-developed incompressible wall-bounded turbulence.A particular emphasis is placed on the two key characteristics of the structures,i.e.,the self-similarity and the self-sustaining process(SSP).Then,a series of newly identified exact coherent states(ECSs)based on the SSP of turbulence is discussed.As the invariant solution of the N-S equations,the ECS gives an exact description of the coherent structure pair composed of streaks and streamwise vortices.The ECS has the two key characteristics,i.e.,the self-similarity and the SSP,but with much lower dimension,thus it forms the skeleton of fully-developed turbulence and paves the way for the study of wall-bounded turbulence using the dynamical system theory.In recent years,the ECS study has also achieved a great success in explaining the subcritical transition by providing new views of this process.It is promising that the ECS may unify the two classical problems of turbulence and transition.