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Numerical study on evolution of subharmonic varicose low-speed streaks in turbulent channel flow

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Applied Mathematics and Mechanics(English Edition) 2016年第3期37卷 325-340页

作者： Jian LI Gang DONG Jianlei ZHANG National Key Laboratory of Transient Physics Nanjing University of Science and Technology

The evolution of two spanwise-aligned low-speed streaks in a wall turbulent flow, triggered by the instability of the subharmonic varicose （SV） mode, is studied by a direct numerical simulation （DNS） method in a small spatial-periodic channel. The results show that the SV low-speed streaks are self-sustained at the early stage, and then transform into subharmonic sinuous （SS） low-speed streaks. Initially, the streamwise vortex sheets are formed by shearing, and then evolve into zigzag vortex sheets due to the mutual induction. As the intensification of the SV low-speed streaks becomes prominent, the tilted streamwise vortex tubes and the V-like streamwise vortex tubes can be formed simultaneously by increasing ＋~. When the SV low-speed streaks break down, new zigzag streamwise vortices will be generated, thus giving birth to the next sustaining cycle of the SV low-speed streaks. When the second breakdown happens, new secondary V-like streamwise vortices instead of zigzag streamwise vortices will be generated. Because of the sweep motion of the fluid induced by the secondary V-like streamwise vortices, each decayed low-speed streak can be divided into two parts, and each part combines with the part of another streak, finally leading to the formation of SS low-speed streaks.

关键词： low-speed streak subharmonic varicose mode turbulent boundary layer,direct numerical simulation

SOME CHARACTERISTICS OF low-speed streakS UNDER SHEARED AIR-WATER INTERFACES

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Acta Mechanica Sinica 2001年第2期17卷 115-124页

作者：王双峰贾复 Institute of Mechanics Chinese Academy of SciencesBeijing 100080China

The characteristics of low-speed fluid streaks occurring under sheared air-water interfaces were examined by means of hydrogen bubble visualization technique. A critical shear condition under which the streaky structure first appears was determined to be u(tau) approximate to 0.19 cm/s. The mean spanwise streak spacing increases with distance from the water surface owing to merging and bursting processes, and a linear relationship describing variation of non-dimensional spacing +)over bar> versus y(+) was found essentially independent of shear stress on the interface. Values of +)over bar>, however, are remarkably smaller than their counterparts in the near-wall region of turbulent boundary layers. Though low-speed streaks occur randomly in time and space, the streak spacing exhibits a lognormal probability distribution behavior. A tentative explanation concerning the formation of streaky structure is suggested, and the fact that +)over bar> takes rather smaller values than that in wall turbulence is briefly discussed.

关键词： air-water interface low-speed streak turbulent coherent structure flow visualization

Numerical simulation on evolution of subharmonic low-speed streaks in minimal channel turbulent flow

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Applied Mathematics and Mechanics(English Edition) 2013年第9期34卷 1069-1082页

作者：李健董刚 State Key Laboratory of Transient Physics Nanjing University of Science and Technology

The evolution of low-speed streaks in the turbulent boundary layer of the minimum channel flow unit at a low Reynolds number is simulated by the direct numer- ical simulation （DNS） based on the standard Fourier-Chebyshev spectral method. The subharmonic sinuous （SS） mode for two spanwise-aligned low-speed streaks is excited by imposing the initial perturbations. The possibilities and the physical realities of the turbulent sustaining in the minimal channel unit are examined. Based on such a flow field environment, the evolution of the low-speed streaks during a cycle of turbulent sus- taining, including lift-up, oscillation, and breakdown, is investigated. The development of streamwise vortices and the dynamics of vortex structures are examined. The results show that the vortices generated from the same streak are staggered along the streamwise direction, while the vortices induced by different streaks tilt toward the normal direction due to the mutual induction effect. It is the spatial variations of the streamwise vortices that cause the lift-up of the streaks. By resolving the transport dynamics of enstrophy, the strength of the vortices is found to continuously grow in the logarithmic layer through the vortex stretching mechanism during the evolution of streaks. The enhancement of the vortices contributes to the spanwise oscillation and the following breakdown of the low-speed streaks.

关键词： low-speed streak streamwise vortice instability enstrophy directnumerical simulation (DNS)

Effect of surface roughness element on near wall turbulence with zero-pressure gradient

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Science China(Physics,Mechanics & Astronomy) 2015年第6期58卷 95-102页

作者： ZHANG Xin PAN Chong SHEN JunQi WANG JinJun School of Aeronautic Northwestern Polytechnical University Fluid Mechanics Key Laboratory of Ministry of Education Beijing University of Aeronautics and Astronautics

In the present paper,we present an investigation on the effect of roughness elements onto near-wall kinematics of a zeropressure-gradient turbulent boundary *** array of spanwisely-aligned cylindrical roughness elements was attached to the wall surface to regulate the near-wall low-speed streaky *** both qualitative visualization and quantitative measurement,we found that the regularization only occurs in the region below the height of the roughness *** analysis on the probability distribution of the streak spanwise spacing showed that the mean spanwise streak spacing is dominated by the roughness elements;however,the latter's effect is in competition with the intrinsic streak generation mechanisms of smooth wall *** the top of the roughness elements,local streamwise turbulent fluctuation intensity can be reduced by about 10%.We used POD analysis to depict such regularization effect in terms of near-wall structure *** further found that if the spanwise spacing of roughness elements increased to be larger than the mean streak spacing in the smooth wall turbulence,there is no streak-regularization effect in the buffer region,so that the near-wall streamwise turbulent fluctuation intensity doesn't reduce.

关键词： turbulent coherent structures low-speed streak flow control

Analysis of solid particle clusters in coherent structures of turbulent channel flow

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Science China(Technological Sciences) 2013年第10期56卷 2525-2530页

作者： LU Hao WANG Bing ZHANG HuiQiang QIN JianXiu WANG XiLin School of Aerospace Tsinghua University

A particle-laden turbulent channel flow is investigated to study particle clusters in large-scale turbulent coherent structures. The fluid phase is calculated by large eddy simulation and particles are tracked using Lagrangian trajectory method. The flow Reynolds number is 180 based on the friction velocity and half-width of the channel. The particle is lycopodium with St=0.93 which may well follow the fluid phase. The mean and fluctuating velocities of both two phases are obtained, which are in good agreement with previous data. The strongest accumulations of particles in low-speed streak structures are observed at y~=l 1.3. Moreover, once particles are captured in low-speed streaks, most of them will reside there for a long period. Particles clustered in low-speed streaks obtain smaller instantaneous wall-normal and spanwise velocities than those out of there, which induce a larger particle flux into low-speed streaks than that out of there. The study is important for understanding particle dispersion mechanisms in gas-particle turbulent channel flows.

关键词： particle cluster preferential concentration low-speed streak coherent structure large eddy simulation Lagrangiontrajectory method