Vortex structure of turbulence over permeable walls

In order to understand the effect of the wall permeability on the turbulent vortex structure near porous walls, based on PIV experimental data, a probability density analysis of fluctuating velocities, statistical quadrant and quadrant-hole analyses of the Reynolds shear stress are performed. The investigated flow fields are turbulent channel flows whose bottom walls are made of porous media. The porous media used are three kinds of foamed ceramics which have almost the same porosity (∼0.8) but different permeability. From the discussions on those analyses, a conceptual scenario of the development of the vortex structure near a permeable wall is proposed for a moderate permeability Reynolds number case. It explains the reason why the near-wall long streaky structure tends to vanish near a porous wall with increasing wall permeability.     

Turbulent time and length scale measurements in high-velocity open channel flows

In high-velocity open channel flows, the measurements of air–water flow properties are complicated by the strong interactions between the flow turbulence and the entrained air. In the present study, an advanced signal processing of traditional single- and dual-tip conductivity probe signals is developed to provide further details on the air–water turbulent level, time and length scales. The technique is applied to turbulent open channel flows on a stepped chute conducted in a large-size facility with flow Reynolds numbers ranging from 3.8E+5 to 7.1E+5. The air water flow properties presented some basic characteristics that were qualitatively and quantitatively similar to previous skimming flow studies. Some self-similar relationships were observed systematically at both macroscopic and microscopic levels. These included the distributions of void fraction, bubble count rate, interfacial velocity and turbulence level at a macroscopic scale, and the auto- and cross-correlation functions at the microscopic level. New correlation analyses yielded a characterisation of the large eddies advecting the bubbles. Basic results included the integral turbulent length and time scales. The turbulent length scales characterised some measure of the size of large vortical structures advecting air bubbles in the skimming flows, and the data were closely related to the characteristic air–water depth Y ₉₀. In the spray region, present results highlighted the existence of an upper spray region for C > 0.95–0.97 in which the distributions of droplet chord sizes and integral advection scales presented some marked differences with the rest of the flow.     

Temporal Evolution of Vortical Structures in the Wall Region of Turbulent Channel Flow

Hairpin-like vortical structures that form in the wall region of turbulent channel flow are investigated. The analysis is performed by following a procedure in which the Navier-Stokes equations are first integrated by means of a computational code based on a mixed spectral-finite difference technique in the case of the flow in a plane channel. A DNS turbulent-flow database, representing the turbulent statistically steady state of the velocity field through 10 viscous time units, is computed and the vortex-detection method of the imaginary part of the complex eigenvalue pair of the velocity-gradient tensor is applied to the velocity field. As a result, hairpin-like vortical structures are educed. Flow visualizations are provided of the processes of evolution that characterize hairpin vortices in the wall region of turbulent channel flow. The relationship is investigated between vortex dynamics and 2nd- and 4th- quadrant events, showing that ejections and sweeps play a fundamental role in the way the morphological evolution of a hairpin vortex develops with time.     

The effect of a single vortex generator jet on the characteristics of a turbulent boundary layer

A flat plate experiment was performed in a water tunnel to determine the effects of a vortex generator jet on the characteristics of a turbulent boundary layer at various wall normal locations. The results show that the characteristic distributions of the turbulent fluctuation quantities are nearly unaffected by the induced vortex structures neither in the steady nor in the dynamic blowing case. The shear layer interaction between the turbulent main flow and the jet flow produces less turbulent fluctuations than it is expected from a turbulent free jet flow. Thus, the mixing process of this flow control strategy is based only on a large-scale momentum transport superimposed by the turbulent fluctuation quantities. This allows a separation of scales for physical interpretation and numerical simulations.     

Numerical study on the turbulent mixing of planar shock-accelerated triangular heavy gases interface

The interaction of a planar shock wave with a triangle-shaped sulfur hexafluoride (\(\mathrm{SF_6}\)) cylinder surrounded by air is numerically studied using a high resolution finite volume method with minimum dispersion and controllable dissipation reconstruction. The vortex dynamics of the Richtmyer–Meshkov instability and the turbulent mixing induced by the Kelvin–Helmholtz instability are discussed. A modified reconstruction model is proposed to predict the circulation for the shock triangular gas–cylinder interaction flow. Several typical stages leading the shock-driven inhomogeneity flow to turbulent mixing transition are demonstrated. Both the decoupled length scales and the broadened inertial range of the turbulent kinetic energy spectrum in late time manifest the turbulent mixing transition for the present case. The analysis of variable-density energy transfer indicates that the flow structures with high wavenumbers inside the Kelvin–Helmholtz vortices can gain energy from the mean flow in total. Consequently, small scale flow structures are generated therein by means of nonlinear interactions. Furthermore, the occasional “pairing” between a vortex and its neighboring vortex will trigger the merging process of vortices and, finally, create a large turbulent mixing zone.     

Meander of a fin trailing vortex and the origin of its turbulence

The low-frequency meander of a trailing vortex shed from a tapered fin installed on a wind tunnel wall has been studied using stereoscopic particle image velocimetry in the near-wake at Mach 0.8. Distributions of the instantaneous vortex position reveal that the meander amplitude increases with downstream distance and decreases with vortex strength, indicating meander is induced external to the vortex. Trends with downstream distance suggest meander begins on the fin surface, prior to vortex shedding. Mean vortex properties are unaltered when considered in the meandering reference frame, apparently because turbulent fluctuations in the vortex shape and strength dominate positional variations. Conversely, a large peak of artificial turbulent kinetic energy is found centered in the vortex core, which almost entirely disappears when corrected for meander, though some turbulence remains near the core radius. Turbulence originating at the wind tunnel wall was shown to contribute to vortex meander by energizing the incoming boundary layer using low-profile vortex generators and observing a substantial increase in the meander amplitude, while greater turbulent kinetic energy penetrates the vortex core. An explanatory mechanism has been hypothesized, in which the vortex initially forms at the apex of the swept leading edge of the fin where it is exposed to turbulent fluctuations within the wind tunnel wall boundary layer, introducing an instability into the incipient vortex core.     

逆向涡对超疏水壁面减阻影响的TRPIV实验研究

超疏水壁面由于具有减阻和自清洁功能而成为国内外减阻和海洋防污等研究领域的热点之一,而20世纪湍流中相干结构的发现为湍流的控制指出新的方向,尤其近壁区涡结构对摩擦阻力贡献很大.利用高时间分辨率粒子图像测速技术,研究了超疏水壁面(SH)以及亲水壁面(PH)湍流边界层中正负展向涡的空间分布特征,研究逆向涡对超疏水壁面近壁区流动结构的影响和超疏水壁面的减阻机理.首先利用空间多尺度局部平均涡量的概念提取壁湍流发卡涡展向涡头(顺向涡)和逆向涡,实现了准确识别涡心并排除小尺度涡的干扰;然后根据检测到的顺向涡和逆向涡流线分布图,发现逆向涡始终处于正向涡的上游和下方,并且对正向涡的进一步发展起抑制作用;最后对两种壁面边界层中逆向涡数量以及出现概率进行对比,发现具有减阻效果的超疏水壁面边界层中出现更多逆向涡.说明逆向涡可抑制上方顺向涡与壁面的强烈剪切,并使靠近壁面的流体加速,从而产生减阻效果;超疏水壁面中涡结构具有更大的β角,使其更好地阻碍了发卡涡头附近强烈的喷射和扫略;超疏水壁面逆向涡出现概率明显大于亲水壁面.这些结果表明:超疏水壁面表现出的减阻特性(Reδ≈13 500,减阻5.8%)与两板产生逆向涡的差异有关.     

Transition from vortex to wall driven turbulence production in the Taylor–Couette system with a rotating inner cylinder

Axisymmetrically stable turbulent Taylor vortices between two concentric cylinders are studied with respect to the transition from vortex to wall driven turbulent production. The outer cylinder is stationary and the inner cylinder rotates. A low Reynolds number turbulence model using the k‐ω formulation, facilitates an analysis of the velocity gradients in the Taylor–Couette flow. For a fixed inner radius, three radius ratios 0.734, 0.941 and 0.985 are employed to identify the Reynolds number range at which this transition occurs. At relatively low Reynolds numbers, turbulent production is shown to be dominated by the outflowing boundary of the Taylor vortex. As the Reynolds number increases, shear driven turbulence (due to the rotating cylinder) becomes the dominating factor. For relatively small gaps turbulent flow is shown to occur at Taylor numbers lower than previously reported. Copyright © 2002 John Wiley & Sons, Ltd.     

An experimental study of viscous vortex rings

The work focuses on the problem of stability and viscous decay of single vortex rings. A tentative classification scheme is proposed for vortex rings which is based on extensive hot-wire measurements of velocity in the ring core and wake, and flow visualization, viz. laminar, wavy, turbulence-producing, and turbulent. Prediction of vortex ring type is shown to be possible, at least approximately, based on the vortex ring Reynolds number alone. Linear growth rates of ring diameter with time are observed for all types of vortex rings, with different growth rates occurring for laminar and turbulent vortex rings. Data on the viscous decay of vortex rings are used to provide experimental confirmation of the accuracy of Saffman's equation for the velocity of propagation of a vortex ring.The work reported herein is supported through a grant of the Natural Sciences and Engineering Research Council of Canada. Special thanks are due to CAPES (Brazil) for the award of a scholarship to the senior author.     

基于Stereo-PIV技术的三维发卡涡结构定量测量研究

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.      

Perturbed vortical layers and shear sheltering

New theoretical results and physical interpretations are presented concerning the interactions between different types of velocity fields that are separated by thin interfacial layers, where there are dynamically significant variations of vorticity across the layers and, in some cases within them. It is shown how, in different types of complex engineering and environmental flow, the strengths of these interactions vary from the weakest kind of superposition to those where they determine the flow structure, for example by mutual exclusion of velocity fields from the other region across the interface, or by local resonance near the interface. We focus here on the excluding kinds of interactions between, on the one hand, elongated and compact regions containing vortical flows and large variations in velocity, and on the other hand various kinds of weak perturbation in the surrounding external flow region: rotational, irrotational; time-varying, steady; large, small; coplanar, non-coplanar; non-diffusive, diffusive. It is shown how all these kinds of external disturbances can be wholly, or partially, ‘blocked’ at the interface with the vortical region, so that beyond a certain sheltering distance into the interior of this region the fluctuations can be very small. For the special case of quasi-parallel co-planar external straining motions outside non-directional shear flows, weak sheltering occurs if the mean velocity of the shear flow increases – otherwise the perturbations are amplified. For non-parallel flows, the sheltering effect can be greater when the vorticity is distributed in thin vortex sheets. The mechanism whereby the vortical flow induces ‘blocking’ and ‘shear-sheltering’ effects can be quantitatively explained in terms of the small adjustments of the vorticity in the vortical layers, and in some cases by the change in impulse of these layers. If the vorticity in the outer part of the vortical region is weak, it can be ‘stripped away’ by the external disturbances until the remaining vorticity is strong enough to ‘block’ the disturbances and shelter the inner flow of the vortical region. The mechanisms presented here appear to explain on the one hand some aspects of the observed robustness of vortical structures and jet or plume like shear flows in turbulent and geophysical flows, and on the other hand the levels of external perturbation needed to erode or breakdown turbulent shear flows.     

The control of turbulent end-wall boundary layers using surface suction

An experimental evaluation of the effects of spatially-limited (i.e. localized) surface suction on a turbulent junction flow was performed using Particle Image Velocimetry (PIV). The results indicate that surface suction can (1) weaken both the instantaneous turbulent vortex and its associated surface interactions in the symmetry plane, (2) effectively eliminate the presence of the average turbulent necklace vortex in the symmetry plane, and (3) weaken the average downstream extensions of the vortex. It was also established that suction effectively reduces the low frequency component of the Reynolds-stress in both the symmetry plane and trailing-edge cross-stream planes, and stabilizes the behavior of the trailing vortex legs. Received: 18 May 1998/Accepted: 26 March 1999     

TRPIV investigation of space-time correlation in turbulent flows over flat and wavy walls

The present experimental work is devoted to in- vestigate a new space-time correlation model for the turbulent boundary layer over a flat and a wavy walls. A turbulent boundary layer flow at Reo = 2460 is measured by tomographic time-resolved particle image velocimetry （Tomo-TRPIV）. The space-time correlations of instantaneous streamwise fluctuation velocity are calculated at 3 different wall-normal locations in logarithmic layer. It is found that the scales of coherent structure increase with moving far away from the wall. The growth of scales is a manifestation of the growth of prevalent coherent structures in the turbulent boundary layer like hairpin vortex or hairpin packets when they lift up. The resulting contours of the space-time correlation exhibit elliptic-like shapes rather than straight lines. It is suggested that, instead of Taylor hypothesis, the elliptic model of the space-time correlation is valid for the wallbounded turbulent flow over either a flat wall or a wavy wall. The elliptic iso-correlation curves have a uniform preferred orientation whose slope is determined by the convection velocity. The convection velocity derived from the space-time correlation represents the velocity at which the large-scale eddies carry small-scale eddies. The sweep velocity rep- resents the distortions of the small-scale eddies and is intimately associated with the fluctuation velocity in the logarithmic layer of turbulent boundary layers. The nondimensionalized correlation curves confirm that the elliptic model is more proper for approximating the space-time correlation than Taylor hypothesis, because the latter can not embody the small-scale motions which have non-negligible distortions. A second flow over a wavy wall is also recorded using TRPIV. Due to the combined effect of shear layers and the adverse pressure gradient, the space-time correlation does not show an elliptic-like shape at some specific heights over the wavy wall, but in the outer region of the wavy wallbounded flow, the elliptic model remai     

Interaction of a normal shock wave with a compressible turbulent flow

A speckle photographic method, which is sensitive to changes of gradients in fluid density, is applied for analyzing a compressible turbulent air flow with density fluctuations. Spatial correlation coefficients, turbulent length scales, and energy spectra are determined under the assumption of homogeneous isotropic turbulence. The experiments are performed in a shock tube where the flow is passed through a turbulence grid. Measurements are taken before and after the turbulent regime interacts with the normal shock wave reflected from the tube's end wall. Amplification of the turbulence intensity by the shock interaction process is verified quantitatively and is shown to be restricted to the lower wave numbers in the spectrum.A version of this paper was presented at the 11th Symposium on Turbulence, University of Missouri-Rolla, Oct. 17–19, 1988.To Professor Dr.-Ing. Klaus Gersten on the occasion of his 60th birthday     

纵向涡对近壁湍流影响的激光测试

用激光测试方法详细地研究了单涡、上洗双涡、下洗双涡对近壁湍流的影响．在涡的下洗区，流体流速高，湍流度低，流体流向壁面；在涡的上洗区，流体流速低，湍流度高，流体远离壁面     

Bubble clustering and trapping in large vortices. Part 2: Time-dependent trapping conditions

In turbulent, periodically excited jets, interactions between bubbles and large coherent vortices are quantitatively studied. Simultaneous, two-phase PIV (particle image velocimetry) and photographic recordings were applied for tracking the large vortices and bubble structures and for investigating trapping phenomena. In order to quantify the interaction between bubbles and the large vortices that are formed in the shear layer, characteristic phase-averaged quantities were determined by PIV. The time-dependent vortex radius, the vorticity at the vortex centre and the time-dependent trapping conditions, obtained from the simulation of the vortex development, were tested against the experimental data.     

EXPERIMENTAL STUDY ON QUANTITATIVE MEASUREMENT OF THREE-DIMENSIONAL STRUCTURE OF HAIRPIN VORTEX BY STEREO-PIV 1)

发卡涡是湍流相干结构研究中最为关注的内容,实现发卡涡三维结构的定量测量并进行流体动力学分析,对深入研究湍流相干结构、实现湍流精准控制等具有重要意义.本研究通过对合成射流装置进行合理控制,使得层流边界层中产生了规则的人造发卡涡结构,进而用体视图像粒子测速仪(Stereo-PIV)锁相实验技术对发卡涡结构所在的三维空间流场进行了定量测量,并得到了一个完整周期内形成的发卡涡三维结构的空间流场. 结果发现,重构所得的三维发卡涡结构质量较高, 实验技术和方案具有可行性.发卡涡结构所在空间流场情况,符合目前人们对于发卡涡、高低速条带、喷射和扫掠事件的常规认识. 此外,对近壁二次流向涡、展向涡量集中区域的展向涡头和强剪切区域、与低速喷射流体相关的汇聚流动和发散流动等有了更细致的认识.同时, 也探讨了"基于二维脉动流场的相关特征去重构发卡涡三维流场"的可行性.为进一步定量探究发卡涡结构的形成演化、不同涡结构的融合及二次诱导等壁湍流相干结构问题提供思路.     

充分发展圆管湍流的实验研究

采用粒子数字图像测速（digital particle image velocimetry,DPIV)和定量流动显示技术（quantitative flow visualization,QFA)对充分发展的圆管湍流进行了研究。测量结果和直接数值模拟（direct numerical simulation,DNS)结果进行了比较，结果表明作者开发的DPIV技术取得了满意的精度。在此基础上对圆管湍流的动力学机理进行了研究，分析了上抛和下扫在湍流生成中的贡献以及流动显示结构内的脉动速度分布，测量结果显示在圆管湍流的近壁区存在横向强脉冲现象和流动显示所能观察到的结构为上抛占主导地位的结构。     

Spatial resolution and dissipation rate estimation in Taylor--Couette flow for tomographic PIV

This paper assesses the spatial resolution and accuracy of tomographic particle image velocimetry (PIV). In tomographic PIV the number of velocity vectors are of the order of the number of reconstructed particle images, and sometimes even exceeds this number when a high overlap fraction between adjacent interrogations is used. This raises the question of the actual spatial resolution of tomographic PIV in relation to the various flow scales. We use a Taylor--Couette flow of a fluid between two independently rotating cylinders and consider three flow regimes: laminar flow, Taylor vortex flow and fully turbulent flow. The laminar flow has no flow structures, and the measurement results are used to assess the measurement uncertainty and to validate the accuracy of the technique for measurements through the curved wall. In the Taylor vortex flow regime, the flow contains large-scale flow structures that are much larger than the size of the interrogation volumes and are fully resolved. The turbulent flow regime contains a range of flow scales. Measurements in the turbulent flow regime are carried out for a Reynolds number Re between 3,800 and 47,000. We use the measured torque on the cylinders to obtain an independent estimate of the energy dissipation rate and estimate of the Kolmogorov length scale. The data obtained by tomographic PIV are assessed by estimating the dissipation rate and comparing the result against the dissipation rate obtained from the measured torque. The turbulent flow data are evaluated for different sizes of the interrogation volumes and for different overlap ratios between adjacent interrogation locations. The results indicate that the turbulent flow measurements for the lowest Re could be (nearly) fully resolved. At the highest Re only a small fraction of the dissipation rate is resolved, still a reasonable estimate of the total dissipation rate could be obtained by means of using a sub-grid turbulence model. The resolution of tomographic PIV in these measurements is determined by the size of the interrogation volume. We propose a range of vector spacing for fully resolving the turbulent flow scales. It is noted that the use of a high overlap ratio, that is, 75?%, yields a substantial improvement for the estimation of the dissipation rate in comparison with data for 0 and 50?% overlap. This indicates that additional information on small-scale velocity gradients can be obtained by reducing the data spacing.     

Vortex reynolds number in turbulent boundary layers

The effects of vortex Reynolds number on the statistics of turbulence in a turbulent boundary layer have been investigated. Vortex Reynolds number is defined as the ratio of circulation around the vortex structure to the fluid viscosity. The vortex structure of the outer region was modeled and a full numerical simulation was then conducted using a high-order spectral method. A unit domain of the outer region of a turbulent boundary layer was assumed to be composed of essentially three elements: a wall, a Blasius mean shear, and an elliptic vortex inclined at 45° to the flow direction. The laminar base-flow Reynolds number is roughly in the same range as that of a turbulent boundary layer based on eddy viscosity, and the vortex-core diameter based on the boundary-layer thickness is nearly the same as the maximum mixing length in a turbulent boundary layer. The computational box size, namely, 500, 150, and 250 wall units in the streamwise, surface-normal, and spanwise directions, respectively, is approximately the same as the measured quasi-periodic spacings of the near-wall turbulence-producing events in a turbulent boundary layer. The effects of vortex Reynolds number and the signs of the circulation on the moments of turbulence were examined. The signs mimic the ejection and sweep types of organized motions of a turbulent boundary layer. A vortex Reynolds number of 200 describes the turbulence moments in the outer layer reasonably well.