Large eddy simulation of turbulent statistical and transport properties in stably stratified flows

Three dimensional large eddy simulation （LES） is performed in the investigation of stably stratified turbulence with a sharp thermal interface. Main results are focused on the turbulent characteristic scale, statistical properties, transport properties, and temporal and spatial evolution of the scalar field. Results show that the buoyancy scale increases first, and then goes to a certain constant value. The stronger the mean shear, the larger the buoyancy scale. The overturning scale increases with the flow, and the mean shear improves the overturning scale. The flatness factor of temperature departs from the Gaussian distribution in a fairly large region, and its statistical properties are clearly different from those of the velocity fluctuations in strong stratified cases. Turbulent mixing starts from small scale motions, and then extends to large scale motions.     

Orthogonal wavelet analysis of counter gradient transport phenomena in turbulent asymmetric channel flow

In this paper four families of orthogonal wavelets are applied to analyze the turbulent counter gradient transport phenomena in fully developed asymmetric channel flows. The results show that: (1) In the instance of counter gradient transport, the principal scale of the coherent structure is responsible for the strong local counter gradient transport; (2) Counter gradient transport phenomena have a strong effect on the intermittency of turbulence; (3) Non-Gaussian part of the principal coherent structure is essential for counter gradient transport phenomena.The project supported by the National Natural Science Foundation of China (10272071, 10472063)     

Visualization and analysis of longitudinal vortices at curved walls of 2D laminar and turbulent channel flows

The 3D structure of longitudinal vortices at the curved walls of both laminar and turbulent channel flows are visualized by the hydrogen bubble technique. Together with the conditional sampling of the turbulent characteristics at the wall processed by the VITA method the dynamics of the near-wall structure is discussed. The project is supported by National Natural Science Foundation of China.     

Effects of turbulent coherent structure on plume diffusion in atmosphere boundary layer

The simulation tests of plume diffusion have been carried out in an open water channel, in which a Laser Doppler Velocitymeter, optical concentration measurement and flow visualization technology have been used. Through experiments and theoretical analyses, the effects of environmental wind field, surface roughness and discharge condition of elevated source on plume diffusion have been investigated. The plume diffusion mechanism has been elucidated reasonably, based on the turbulent coherent structure theory.     

The effect of compliant coatings on coherent structure in turbulent boundary layers

The velocity profile, turbulence intensity profile, streaky structure and bursting frequency in turbulent boundary layers over a flat plate with compliant coatings were investigated by Laser Doppler Anemometry and conditional sampling techniques. This experiment led to the conclusions that in boundary layer flows on a compliant wall, as compared with that on a rigid wall, the log law region was extended further away from the wall, and that the maximum value of each turbulence intensity profile in the near wall region was reduced and the bursting frequency obviously decreased with the compliant coatings. One point worthy of notice was that the above results were very much like those of polymer drag reduction experiments. The project is supported by the National Natural Science Foundation of China.     

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     

Temporal large-eddy simulation of unstratified and stably stratified turbulent channel flows

Recently, Pruett et al. [Pruett, C.D., Gatski, T.B., Grosch, C.E., Thacker, W.D., 2003. The temporally filtered Navier–Stokes equations: properties of the residual stress. Phys. Fluids 15, 2127–2140] proposed an approach to large-eddy simulation (LES) based on time-domain filtering; their approach was termed temporal large-eddy simulation or TLES. In a continuation of their work, Pruett and collaborators tested their methodology by successfully performing TLES of unstratified turbulent channel flow up to Reynolds number of 590 (based on channel half-height and friction velocity) [Pruett, C.D., Thomas, B.C., Grosch, C.E., Gatski, T.B., 2006. A temporal approximate deconvolution model for LES. Phys. Fluids 18, 028104, 4p]. Here, we carefully analyze the TLES methodology in order to understand the role of its key components and in the process compare TLES to more traditional approaches of spatial LES. Furthermore, we extend the methodology to stably stratified turbulent channel flow.     

TURBULENT COHERENT STRUCTURE AND DYNAMIC SYSTEM

The main difficulties in the study of turbulence via dynamic system lie in how to relatecontinuum systems of infinite dimension with dynamic system in low dimension space andhow to depict its special structure.In this paper,we’ll give a comprehensive review onvarious methods to describe complex systems in low dimension space and new approaches tothe resolution of turbulence problems.     

Reynolds stress and vorticity in turbulent wall flows

Experimental measurements in a boundary layer and a large-eddy simulation of plane channel flow have been used to study the dynamics of vorticity and mass transport in the nearwall region. It was found that Reynolds stress generation occurs in the vicinity of quasi-streamwise vortices, and that smoke particles tend to be ejected from the wall near these vortical structures.     

槽道湍流近壁结构的DPIV观测实验

采用DPIV系统(由两台CCD相机组成)对槽道湍流进行速度场时间历程的观测实验,通 过对大量测量结果的综合分析,取得了槽道湍流近壁结构的空间结构及其时间演化过程特征 的结果,可以揭示上扫下掠、湍流瞬时速度型等现象与大尺度涡演化的物理关系,解释若干 湍流大尺度结构的特征机理,还表明DPIV系统提供了一种定量观测湍流的时空结构特征的手 段.     

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     

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE (III)──A STATISTICAL AND DYNAMICAL MODEL OF COHERENT STRUCTURE AND ITS HEAT TRANSFER MECHANISM

Following Tsai & Ma^[1] and Tsai & Liu^[2], a statistical and dynamical near-wall turbulent coherent structural model with separate consideration of two different portions: locally generated and upstream-transported large eddies has been established. With this model, heat transfer in a fully developed open channel in the absence of pressure gradient is numerically simulated. Database of fluctuations of velocity and temperature has also been set. Numerical analysis shows the existence of high-low temperature streak caused by near-wall coherent structure and its swing in the lateral direction. Numerical results are in accordance with the computations and experimental results of other researchers.     

Antisymmetric quadrupole mode of coherent structures in wall-bounded turbulence

Experiments were conducted in a water tunnel by tomographic time-resolved particle image velocimetry (Tomo-TRPIV). The Reynolds number Re_θ is 2460 on the base of momentum thickness. According to the physical mechanism of the stretch and compression of multi-scale vortex structures in the wall-bounded turbulence, the topological characteristics of turbulence statistics in logarithmic layer were illustrated by local-averaged velocity structure function. During coherent structures bursting, results reveal that the topological structures of velocity gradients, velocity strain rates and vorticities behave as antisymmetric quadrupole modes. A three-layer antisymmetric quadrupole vortex packet confirms that there is a tight relationship between the outer layer and the near-wall layer.     

Spatiotemporal representation of the dynamic modes in turbulent cavity flows

Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) were used to extract the coherent structures in turbulent cavity flows. The spatiotemporal representation of the modes was achieved by performing the circular convolution of a change of basis on the data sequence, wherein the transformation function was extracted from the POD or DMD. The spatiotemporal representation of the modes provided significant insight into the evolutionary behavior of the structures. Self-sustained oscillations arise in turbulent cavity flows due to unsteady separation at the leading edge. The turbulent cavity flow at Re_D = 12,000 and a length to depth ratio L/D = 2 was analyzed. The dynamic modes extracted from the data clarified the presence of self-sustained oscillations. The spatiotemporal representation of the POD and DMD modes that caused self-sustained oscillations revealed the prevalent dynamics and evolutionary behavior of the coherent structures from their formation at the leading edge to their impingement at the trailing edge. A local minimum in the mode amplitude representing the energy contributions to the flow was observed upon the impingement of coherent structure at the trailing edge. The modal energy associated with the periodic formation of organized coherent structures followed by their dissipation upon impingement revealed the oscillatory behavior over time.     

A theoretical model of the coherent structure in the wall region of a turbulent boundary layer

In this paper,the formation of the coherent structures in the wall region of aturbulent boundary layer was studied,using the nonlinear theory of the hydrodynamicstability.The spanwise and streamwise wavelengths of the most amplified unstablewave obtained by this study were found in good agreement with the experiments,whichmakes the distinct feature of this study in the present paper,as the basis of thestability analysis,a more rational velocity profile has been used,which is different fromthat of the turbulent mean flow.And also,the new nonlinear theory was used.Theresult is useful in understanding of the quasi-periodicity of the coherent structure in theturbulent boundary layer.     

Dynamical characteristics of coherent structure in the outer region of turbulence boundary layer

In the present paper the coherent structures in the outer region of turbulent boundary layer were investigated experimentally and analytically. From the observation of the flow field over smooth wall, rough wall and sand wave wall, it was found that the direct effect of wall on the flow structure can reachy ⁺¹≈100, and both lateral and vertical vortices exist in the outer region, but the coherent structures in the outer region are mainly the formation, development and decay of the large-scale lateral vortices. By experimental and dynamical analysis, some influence factors and their relations associated with the dynamical process of lateral vortices were deduced. The project supported by the National Natural Science Foundation of China     

An analysis of energy transport processes in the presence of coherent structures

The paper deals with the influence exterted by the presence of coherent structures upon the energy transfer processes in turbulent flow. The round free jet has been stimulated by the acoustic field with the frequency corresponding to the shedding of column-mode vortices (the so called helical structures). The results of hot-wire measurements enabled to calculate the particular terms of coherent energy transport equations that in turn made it possible to trace the energy transfer processes realized in the presence of helical-mode structures. The results obtained suggest that the organized motion modifies substantially the energy transfer processes in the flow considered.     

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…     

Three-dimensional evolution of vortices and early features of coherent structure in the turbulent wake behind a 2-D circular cylinder

3-D evolution of Kármán vortex filaments and vortex filaments in braid regions in the turbulent wake of a 2-D circular cylinder is investigated numerically based on inviscid vortex dynamics by analyzing the response of the initially 2-D spanwise vortex filaments to periodic spanwise disturbance of varying magnitude, wavelength and initial phase angles. Our results reveal a kind of 3-D vortex system in the wake which consists of large scale horseshoe-shaped vortices and small scale γ-shaped vortex filaments as well as vortex loops. The mechanism and the dynamic process about the generation of streamwise vortical structure and the 3-D coherent structure are reported. currently published in the Chinese Edition of Acta Mechanica Sinica, Vol.25, No.3, 1993 The project supported by National Natural Science Foundation of China and the National Basic Research Project “Nonlinear Science”     

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.