The role of small scale coherent structures in turbulent drag reduction

With a new visualization technique, a strong suppression of small scale coherent structures (filaments) in turbulence is observed when a drag reducing polymer is added. Simultaneously, the rate of formation of large scale structures (eddies) out of these filaments decreases, which may be an important observation in the explanation of turbulent drag reduction.     

A mechanism for excitation of coherent structures in wall region of a turbulent boundary layer

Introduction Themechanismforthegenerationofcoherentstructuresinthewallregionofaturbulent boundarylayerhasalwaysbeeninconcernandinvestigated.AccordingtoTsujimotoand Miyake[1],thecharacteristicsofturbulenceinthewallregionweremainlydeterminedbythe generationandevolutionofcoherentstructures,notbythesmall_scaleturbulence.However, excitationsfromregionofy >60werefoundtobenecessary,otherwisethewallregionwould degeneratetolaminarflow.Therefore,theinvestigationofthemechanismthathowcoherent structuresi…     

TRPIV measurement of turbulent coherent structures in a drag-reducing flow bypolymers

Fully developed turbulentflow fields with and without polymer solution at the same Reynolds number were measured by time-resolved particle image velocimetry (TRPIV) in a water channel toinvestigate the mechanism of drag-reducingsolution from theview of coherent structures manipulation. The streamwise mean velocity and Reynolds stress profiles in thesolution werecompared with those in water. After adding the polymer solution, the Reynolds stress in the near-wall area decreases significantly. Theresult relates tightly to the decease of the coherent structures' bursting. The spatial topology of coherentstructures duringbursts has been extracted by the new mu-level criterion based on locally averaged velocitystructure function.The effect of polymers onturbulent coherentstructures mainly reflects in the intensity, not in the shape. In the solution, it is by suppressing thecoherent structuresthat the wall friction isreduced.     

剪切湍流大尺度相干结构的模式研究

发展了一种计算剪切湍流大尺度相干结构的新模式．该模式的基础是认为大尺度相干结构为湍流场中流体脉动能量增长最快的那部分，且包含大部分的湍流脉动能量．在此基础上。通过对湍流相干能量方程的推演。建立了描述大尺度相干结构的特征控制方程，并应用Ｃｈｅｂｙｓｈｅｖ多项式方法求得湍流相干能量的最大增长率在波数空间的分布，从而获得对应的大尺度相干结构．应用该模式研究了槽流和一自然对流中的大尺度相干结构，得到的近壁区流动结构与实验现象十分接近．     

Effects of free‐stream turbulence on large‐scale coherent structures of separated boundary layer transition

It has been well established that large‐scale structures, usually called coherent structures, exist in many transitional and turbulent flows. The topology and range of scales of those large‐scale structures vary from flow to flow such as counter‐rotating vortices in wake flows, streaks and hairpin vortices in turbulent boundary layer. There has been relatively little study of large‐scale structures in separated and reattached transitional flows. Large‐eddy simulation (LES) is employed in the current study to investigate a separated boundary layer transition under 2% free‐stream turbulence on a flat plate with a blunt leading edge. The Reynolds number based on the inlet free stream velocity and the plate thickness is 6500. A dynamic subgrid‐scale model is employed to compute the subgrid‐scale stresses more accurately in the current transitional flow case. Flow visualization has shown that the Kelvin–Helmholtz rolls, which have been so clearly visible under no free‐stream turbulence (NFST) are not as apparent in the present study. The Lambda‐shaped vortical structures which can be clearly seen in the NFST case can hardly be identified in the free‐stream turbulence (FST) case. Generally speaking, the effects of free‐stream turbulence have led to an early breakdown of the boundary layer, and hence increased the randomization in the vortical structures, degraded the spanwise coherence of those large‐scale structures. Copyright © 2005 John Wiley & Sons, Ltd.     

Tomographic PIV investigation of coherent structures in a turbulent boundary layer flow

Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional coherent structures in the logarithmic region of the turbulent boundary layer in a water tunnel.The Reynolds number based on momentum thickness is Reθ = 2 460.The instantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low speed fluid,which is flanked on either side by highspeed ones.Statistical support for the existence of hairpins is given by conditional averaged eddy within an increasing spanwise width as the distance from the wall increases,and the main vortex characteristic in different wall-normal regions can be reflected by comparing the proportion of ejection and its contribution to Reynolds stress with that of sweep event.The pre-multiplied power spectra and two-point correlations indicate the presence of large-scale motions in the boundary layer,which are consistent with what have been termed very large scale motions(VLSMs).The three dimen-sional spatial correlations of three components of velocity further indicate that the elongated low-speed and highspeed regions will be accompanied by a counter-rotating roll modes,as the statistical imprint of hairpin packet structures,all of which together make up the characteristic of coherent structures in the logarithmic region of the turbulent boundary layer(TBL).     

An experimental study on turbulent coherent structures near a sheared air-water interface

The turbulence structures near a sheared air-water interface were experimentally investigated with the hydrogen bubble visualization technique. Surface shear was imposed by an airflow over the water flow which was kept free from surface waves. Results show that the wind shear has the main influence on coherent structures under air-water interfaces. Low- and high- speed streaks form in the region close to the interface as a result of the imposed shear stress. When a certain airflow velocity is reached, “turbulent spots” appear randomly at low-speed streaks with some characteristics of hairpin vortices. At even higher shear rates, the flow near the interface is dominated primarily by intermittent bursting events. The coherent structures observed near sheared air-water interfaces show qualitative similarities with those occurring in near-wall turbulence. However, a few distinctive phenomena were also observed, including the fluctuating thickness of the instantaneous boundary layer and vertical vortices in bursting processes, which appear to be associated with the characteristics of air-water interfaces. The project supported by the National Natural Science Foundation of China (Grant No.19672070)     

An experimental study of interacting coherent structures in the turbulent near wake behind a pair of square cylinders

An experimental investigation (LDA technique) of coherent structures in the turbulent plane near wake behind a pair of square cylinders in side-by-side arrangement is presented with particular emphasis put on the use of 2D invariant structural measures.     

Multi-scale coherent structures in turbulent boundary layer detected by locally averaged velocity structure functions

Introduction Inearlyperiodofstudyofturbulentflow,itwasdeemedrandomanddisorderedmotionsof fluidparticles,sothecharacteristicsofturbulencewerestudiedbystatisticsaveragemethod. Kolmogorov[1]analyzedtherelativemotionoffluidparticlesinfullydeveloped(Reynolds numbertrendstoinfinity)isotropicandhomogeneousturbulentflow,onbasisofrandomfield theory,andpresentedtheconceptofstructurefunctions,whichdescribedtherelativevelocityof twofluidparticlesindistanceofl,toresearchthelawbetweenthemulti_orderstatistic…     

An experimental study of the large scale coherent structures in a forced free shear layer

The dynamic characteristics of the large scale coherent structures in a forced free shear layer are experimentally studied by means of flow visualization. The quantitative measurements are acquired by the use of a LDV. It is shown that the development of the coherent structures can be greatly influenced by upstream artificial perturbations and as a result the mixing in the layer can be controlled. Like vortex merging, vortex splitting is also a common evolution pattern in the development of the coherent structures.     

A kind of fast changing coherent structure in a turbulent boundary layer

Coherent structures of a turbulent boundary layer were investigated by hydrogen bubble method. A kind of fast changing structure was observed. That is a spot in which all the hydrogen bubbles vanish much faster than in other regions. This investigation verified that dark-spot is formed by a strong sweep from outer layer. Inside a dark-spot the local instantaneous flow speed might be four times of its neighboring high-speed streaks. Comparing with the low/high speed streaks, both dark-spot and the vortical structures around it are changing very fast. Around dark-spot intensive shear layers are formed and indications of the generation of small-scale structures could be observed. The project supported by the National Natural Science Foundation of China (No. 19672004) and the National Climbing Project     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE(II)──A PHYSICAL MODEL OF COHERENT STRUCTURE FOR THE ROUGH BOUNDARY LAYER

In this paper, the differences of turbulent coherent structure between the smooth and rough boundary layers are analysed. Based on the discussing the transient properties from the smooth wall to the rough wall, the physical model of coherent structure for the rough boundary layer are established. The width of slowly-moving turbulent spot and the bursting time are obtained, which are in agreement with experimental results.The project supported by the National Nature Science Foundation of China     

The dynamics of coherent structures in a flat-plate boundary layer

From the data of a direct numerical simulation the three-dimensional coherent structures of an incompressible, spatially evolving flat-plate boundary layer have been calculated using the POD method. By Galerkin projection of the Navier-Stokes equations onto the corresponding system of eigenfunctions then a low-dimensional model of the flow in the form of a system of ODE's has been derived. In a region of the boundary layer just beyond the spike stages of transition this system displays deterministic chaos that has been quantified by determining its Lyapunov exponents.     

Some new results of the observation on the coherent structures of turbulent boundary layer in flow with adverse pressure gradient

By a suitable manipulation of hydrogen bubble generation, some new results were obtained: (1) The long-streaks are generated along the interfaces between low and high-speed streaks. The long-streaks are generally stretching and are moving faster than its neighboring high-speed streaks. The hydrogen bubbles in long-streaks have longer life. (2) The stream-wise vortices are also generated along the interfaces. The project is supported by the National Natural Science Foundation of China.     

Numerical simulation of vortical and coherent structures in compressible jet flows

The compressible flows of plane free jets and jets of the intake-stroke of a rectangular piston-engine model are investigated by numerical simulations. The observed vortical structures appear to be the well-known coherent structures of turbulent shear layers. The simulated structures are compared to experimental data by means of density fields and turbulent statistics taken from different authors. The computed flow depends on physical as well as on numerical parameters. The good agreement with the experimental data is obtained by direct simulation without any turbulence model.     

Instability waves in the wall region of a turbulent boundary layer on a flat plate

Coherent structures and the bursting phenomena in the wall region of a turbulent boundary layer play a very important role in determining the characteristics of the boundary layer. Yet the nature and the origin of the coherent structures are unclear until now. In this paper, nonlinear stability calculations for the wall region of a turbulent boundary layer have been made. It was found that there do exist instability waves which may be responsible for the coherent structures. The project is supported by the National Natural Science Foundation of China.     

Lagrangian model on the turbulent motion of small solid particle in turbulent boundary layer flows

.Intr0ductionSurfaceerosionofmaterialbysolid-particleimpactisanimportantprobleminmultiphaseflowindustriaIdevicesandthecharacteristicsoftheparticIe'smotioninaturbulentboundarylayerflowisthebaseofthestudyofthematerialsurfaceerosion.Manycalculationmodelshave…     

Experimental research on complex eddy viscosity modeling of multi-scale coherent structures in wall turbulence

在湍流相干结构动力学方程中,非相干结构成分对相干结构贡献的雷诺应力的模型为涡黏性 模型,即涡黏性系数乘以相干结构平均速度变形率的形式. 基于非相干结构成分对相干结构贡 献的雷诺应力与相干结构速度变形率之间存在相位差的事实,在理论上提出了非相干结构成 分对相干结构贡献的雷诺应力复涡黏性模型的假设. 应用热线测速技术,在低速风洞中对湍 流边界层非相干结构成分对相干结构贡献的雷诺应力与相干结构法向速度变形率之间的相位 关系进行了实验测量. 通过分析湍流相干结构猝发过程中非相干结构成分对相干结构贡献的 雷诺应力与相干结构速度变形率之间的相位关系,研究了相干结构雷诺应力分量与流向速度 法向梯度之间的相位差沿湍流边界层法向的变化规律,肯定了湍流相干结构复涡黏性系数模 型的合理性.     

The structures of turbulent boundary layer in the vicinity of separation

Large coherent structures of turbulent boundary layer in the vicinity of separation were observed in a water channel by the hydrogen bubble method. Motion pictures of the de views were taken. The features of the instantaneous velocity profiles, the large transverse and streamwise vortices were discussed.