Spatial structure of a jet flow at a river mouth

The present work concentrates on the latest data measured in the Jordan river flow in lake Kinneret. Spectral characteristics of fluctuating velocity components have been obtained and processed. The three-dimensional structure of turbulence along the flow has been described. The main features of the jet flow turbulence in the river mouth are: a) The supply of turbulent energy changes due to different mechanisms along the flow. b) The structure of turbulence formed in the river decays rapidly along the flow, and c) In the sand area and beyond it, a significant generation of turbulent energy occurs. Quantitative estimations of the above effects were carried out.     

Large-Eddy Simulation for the Mechanism of Pollutant Removal from a Two-Dimensional Street Canyon

Large-eddy simulation (LES) is conducted to investigate the mechanism of pollutant removal from a two-dimensional street canyon with a building-height to street-width (aspect) ratio of 1. A pollutant is released as a ground-level line source at the centre of the canyon floor. The mean velocities, turbulent fluctuations, and mean pollutant concentration estimated by LES are in good agreement with those obtained by wind-tunnel experiments. Pollutant removal from the canyon is mainly determined by turbulent motions, except in the adjacent area to the windward wall. The turbulent motions are composed of small vortices and small-scale coherent structures of low-momentum fluid generated close to the plane of the roof. Although both small vortices and small-scale coherent structures affect pollutant removal, the pollutant is largely emitted from the canyon by ejection of low-momentum fluid when the small-scale coherent structures appear just above the canyon where the pollutant is retained. Large-scale coherent structures also develop above the canyon, but they do not always affect pollutant removal.     

The detection and measurement of turbulent structures in the atmospheric surface layer

Turbulence data from the planetary boundary layer (PBL) indicate the presence of deterministic turbulent structures. These structures often show up as asymmetric ramp patterns in measurements of the turbulent fluctuations of a scalar quantity in the atmospheric surface layer (ASL). The sign of the slope of the sharp upstream edge of such a triangular pattern depends on the thermal stability conditions of the ASL.The turbulent structures in the ASL have been tracked by a detection method which searches for rapid and strong fluctuations in a signal — the VITA (variable interval time averaging) technique. This detection method has previously been employed in laboratory boundary layers. The VITA detection method performs well in the ASL and reveals the presence of vertically coherent turbulent structures, which look similar to those in laboratory shear flows. At the moment that a sharp temperature interface appears, the horizontal alongwind velocity shows a sharp increase, along with a sudden decrease of vertical velocity, independent of the thermal stability conditions of the ASL. The fluctuating static pressure reveals a maximum at that moment. The vertical turbulent transports show a twin-peak character around the time that the sharp jumps in the temperature and the velocity signals appear.     

强风天气下边界层结构特征

近地层观测的强风运动表明,叠加在平均流动之上的脉动通常有两种,一种是随机的湍流脉动,还有一种具有相干结构的阵风扰动。分析表明,上层强风的剪切运动产生阵风,并向下传递能量,对近地层的通量传输起到重要作用。本文利用北京325 m气象塔、位于海拔1257 m的妙峰山测风塔和位于海拔1688 m的灵山测风塔的资料,分析了强风天气下,边界层上层出现阵风并向下传递的过程,进一步证实无论在近地层还是边界层上层,强风期间,叠加在平均流动上除了高频湍流脉动之外,还有周期为1~10分钟的阵风,即相干结构。阵风峰期有下沉运动,阵风谷期有上升运动。这些相干结构在边界层上层产生,向下运动和传播过程中受到平均气流梯度的切变作用和地面摩擦,破碎为湍流结构。边界层上层的阵风和湍流产生的动量通量向下传递,使得强风期间,边界层中阵风和湍流对通量具有同样的输送能力,对边界层中沙尘、污染物等气溶胶的传输具有重要作用。本研究为模式中进行通量输送参数化方案的修正提供了观测和理论依据。     

Coherent motions in windbreak flow

Experimental data on windbreak flows have been analysed, using several statistical methods. Our analysis demonstrates that coherent structures exist in the mixing region of a porous windbreak (50% porosity) flow, and are largely responsible for the momentum transport that re-accelerates the leeward flow. A comparison between windbreak flow and laboratory turbulent mixing layers suggests that the dominant structures in these two flows are similar. Some previous numerical and experimental results are interpreted in light of the coherent structures and the self-similar property in the mixing region of a windbreak flow.     

河湖沙冲淤和三角洲发育的长期数值模拟研究

本文利用一个二维的水动力学和泥沙冲淤动力学模式,对河床演变和三角洲的发育过程进行数值模拟研究。动力模式包括水流方程、悬沙输送方程和底床变化（含推移质运动）方程。数值积分格式采用ＩＡＰ平方守恒格式和分解算法,使计算既省时又长期稳定。对一个理想的河湖区域进行了长达５０年的数值积分。模拟结果肯定了曾庆存等以前得到的主要结论,并得到一些新的结论。研究表明：泥沙首先在河流入口处沉积,并逐渐向下游推进和向河槽两边延伸;随着泥沙的沉积,底床逐渐升高,在河槽中形成了一个沙岛;在沙岛上游,原河槽中沉积的泥沙又被冲刷,重新出现深河道;数值积分５０年后,人口处已形成一个巨大的冲积扇（水下三角洲）,并不断向下游扩展,三角洲（包括沙岛）的前缘十分陡峭;由于泥沙的沉积,河床抬升,水深变小,水流速度场在整个堆积扇上明显增大,并在三角洲前缘出现明显的绕岛环流和河流分叉现象。模拟结果还表明,在漫滩区泥沙只淤不冲,而在河槽中既有淤积又有冲刷。此外,由于泥沙的淤积,使得湖型水库的水流入口处（库尾）水位明显抬高（即所谓“翘尾巴”现象）。这些结果在定性上与有关湖泊型水库淤积的观测事实是相当符合的。     

Coherent Structures at a Forest Edge: Properties, Coupling and Impact of Secondary Circulations

Little is known about the influence of coherent structures on the exchange process, mainly in the case of forest edges. Thus, in the framework of the ExchanGE processes in mountainous Regions (EGER) project, measurements of atmospheric turbulence were taken at different heights between a forest and an adjacent clear cutting using sonic anemometers and high-frequency optical gas analyzers. From these turbulence data, dominant coherent structures were extracted using an already existing wavelet methodology, which was developed for homogeneous forest canopies. The aim of this study is to highlight differences in properties of coherent structures between a forest and a clear cutting. Distinct features of coherent exchange at the forest edge are presented and a careful investigation of vertical and horizontal coupling by coherent structures around the surface heterogeneity is made. Within the forest, coherent structures are less frequent but possess larger time scales, indicating that only the largest coherent motions can penetrate through the forest canopy. At the forest edge, there is no crown layer that can hinder the vertical exchange of coherent structures, because these exhibit similar time scales at all heights. In contradiction to that, no improved vertical coupling was detected at the forest edge. This is mainly because the structures captured by the applied routine contribute less to total turbulent fluxes at the edge than within the forest. Thus, coherent structures with time scales between 10 and 40 s are not the dominant exchange mechanism at the forest edge. With respect to the horizontal direction, a consistent picture of coherent transport could be derived: along the forest edge there is mainly good coupling by coherent structures, whereas perpendicular to the forest edge there is mainly decoupling. Finally, it was found that there is a systematic modulation of coherent structures directly at the forest edge: strong ejection motions appear in all time series during the daytime, whereas strong sweeps dominate at night. An effect of wind direction relative to the forest edge is excluded. Consequently, it is hypothesized that this might be an indication of a quasi-stationary secondary circulation above the clear cutting that develops due to differences in surface temperature and roughness. Such circulations might be a relevant turbulent transport mechanism for ecosystem-atmosphere exchange in heterogeneous landscapes.     

近壁湍流边界层相干结构形成的动力学分析

以壁面脉冲扰动来构建湍流边界层近壁区对称与非对称相干结构理论模式,采用直接数值模拟的方法,研究了湍流边界层近壁区对称与非对称相干结构形成的理论机制和演化规律.计算结果显示非对称相干结构比对称相干结构更容易增长,这是由于非对称型结构能在湍流边界层近壁区域诱导产生较大的展向扰动速度,且不同初始结构类型分布对湍流边界层近壁区相干结构形成的动力学机制不同所致,从而加深了对近壁湍流边界层单个相干结构形成的某种动力学机制的认识理解.     

Wave-induced velocities and turbulent reynolds stresses above an air-water interface

The progressive, sinusoidal (1.0 Hz) water-wave-induced perturbations in a turbulent air flow above an air-water interface were studied in a large laboratory facility. The velocity field was measured in both fixed and wave-following reference frames with a cross hot-film probe. Both the magnitude and phase of the wave-induced velocities and turbulent Reynolds stresses were obtained across the turbulent boundary layer for different free stream velocities.     

Scaling of Wall-Normal Turbulence Intensity and Vertical Eddy Structures in the Atmospheric Surface Layer

Adequate high-quality data on three-dimensional velocities in the atmospheric surface layer (height \(\delta \)) were acquired in the field at the Qingtu Lake Observation Array. The measurement range occupies nearly the entire logarithmic layer from approximately \(0.006\delta \)–\(0.2\delta \). The turbulence intensity and eddy structures of the velocity fluctuations in the logarithmic region were primarily analyzed, and their variations in the z (wall-normal) direction were revealed. The primary finding was that the turbulent intensity of wall-normal velocity fluctuations exhibits a sharp upswing in the logarithmic region, which differs from classic scaling law and laboratory results. The upswing of the wall-normal turbulence intensity in the logarithmic region is deemed to be linear based on an ensemble of 20 sets of data. In addition, the wall-normal extent of the correlated structures and wall-normal spectra were compared to low Reynolds number results in the laboratory.     

Concurrency of Coherent Structures and Conditionally Sampled Daytime Sub-canopy Respiration

We investigated an alternative means for quantifying daytime ecosystem respiration from eddy-covariance data in three forests with different canopy architecture. Our hypothesis was that the turbulent transport by coherent structures is the main pathway for carrying detectable sub-canopy respiration signals through the canopy. The study extends previously published work by incorporating state-of-the-art wavelet decomposition techniques for the detection of coherent structures. Further, we investigated spatial and temporal variability of the respiration signal and coherent exchange at multiple heights, for three mature forest sites with varying canopy and terrain properties for one summer month. A connection between the coherent structures and identified sub-canopy respiration signal was clearly determined. Although not always visible in signals collected above the canopy, certain cases showed a clear link between conditionally sampled respiration events and coherent structures. The dominant time scales of the coherent structure ejection phase (20?C30 s), relative timing of maximum coincidence between respiration events and the coherent structure ejection phase (at approximately ?10 s from detection) and vertical transport upward through the canopy were shown to be consistent in time, across measurement heights and across the different forest sites. Best results were observed for an open canopy pine site. We conclude that the presented method is likely to be applicable at more open rather than dense (closed) canopies. The results provided a confirmation of the connection between below- and above-canopy scalar time series, and may help the development or refinement of direct methods for the determination of component fluxes from observations above the canopy.     

Long-term study of coherent structures in the atmospheric surface layer

A long-term study of coherent turbulence structures in the atmospheric surface layer has been carried out using 10 months of turbulence data taken on a 30-m tower under varying meteorological conditions. We use an objective detection technique based on wavelet transforms. The applied technique permits the isolation of the coherent structures from small-scale background fluctuations which is necessary for the development of dynamical models describing the evolution and properties of these phenomena. It was observed that coherent structures occupied 36% of the total time with mean turbulent flux contributions of 44% for momentum and 48% for heat. The calculation of a transport efficiency parameter indicates that coherent structures transport heat more efficiently than momentum. Furthermore, the transport efficiency increases with increasing contribution of the structures to the overall transport.     

Seasonal and diurnal variations of coherent structures over a deciduous forest

Coherent structures in turbulent flow above a midlatitude deciduous forest are identified using a wavelet analysis technique. Coupling between motions above the canopy (z/h=1.5, whereh is canopy height) and within the canopy (z/h=0.6) are studied using composite velocity and temperature fields constructed from 85 hours of data. Data are classified into winter and summer cases, for both convective and stable conditions. Vertical velocity fluctuations are in phase at both observation levels. Horizontal motions associated with the structures within the canopy lead those above the canopy, and linear analysis indicates that the horizontal motions deep in the canopy should lead the vertical motions by 90°. On average, coherent structures are responsible for only about 40% of overall turbulent heat and momentum fluxes, much less than previously reported. However, our large data set reveals that this flux fraction comes from a wide distribution that includes much higher fractions in its upper extremes. The separation distanceL _s between adjacent coherent structures, 6–10h, is comparable to that obtained in previous observations over short canopies and in the laboratory. Changes in separation between the summer and winter (leafless) conditions are consistent withL _s being determined by a local horizontal wind shear scale.     

Coherent Structures and the Dissimilarity of Turbulent Transport of Momentum and Scalars in the Unstable Atmospheric Surface Layer

Atmospheric stability effects on the dissimilarity between the turbulent transport of momentum and scalars (water vapour and temperature) are investigated in the neutral and unstable atmospheric surface layers over a lake and a vineyard. A decorrelation of the momentum and scalar fluxes is observed with increasing instability. Moreover, different measures of transport efficiency (correlation coefficients, efficiencies based on quadrant analysis and bulk transfer coefficients) indicate that, under close to neutral conditions, momentum and scalars are transported similarly whereas, as the instability of the atmosphere increases, scalars are transported increasingly more efficiently than momentum. This dissimilarity between the turbulent transport of momentum and scalars under unstable conditions concurs with, and is likely caused by, a change in the topology of turbulent coherent structures. Previous laboratory and field studies report that under neutral conditions hairpin vortices and hairpin packets are present and dominate the vertical fluxes, while under free-convection conditions thermal plumes are expected. Our results (cross-stream vorticity variation, quadrant analysis and time series analysis) are in very good agreement with this picture and confirm a change in the structure of the coherent turbulent motions under increasing instability, although the exact structure of these motions and how they are modified by stability requires further investigation based on three-dimensional flow data.     

用小波系数谱方法分析湍流湿度脉动的相干结构

小波系数谱分析方法是结合小波分析和高分辨率谱分析的一种统计方法,可以用来同时识别时间序列中相干结构的生命尺度和出现周期,可以很好地描述相干结构的演变过程。基于此方法,作者分析了2004年11月在河北省白洋淀地区的陆地和岛上两个观测点（分别代表陆地和水面两种不同下垫面）湍流湿度脉动的相干结构特征,结果表明陆地和水上湿度序列的相干结构尺度分布相似,并且尺度与周期之间的关系一致:小于5 s的相干结构不连续出现,而且通常伴有更大尺度的相干结构,而5~30s的相干结构有与其尺度差不多的周期。在寻找更大尺度的相干结构时发现存在一个尺度,当大于某个周期时,在各个周期上这个尺度的相干结构都显著;与正交小波变换识别相干结构主尺度的方法识别的相干结构主尺度一致。另外,小尺度结构不连续出现也可以解释小尺度湍流能量变化比较大。     

Boundary-Layer Stress Instabilities in Neutral,Rotating Turbulent Flows

Boundary-layer instabilities are studied by analyzing the results of laboratory simulations of wall turbulence in a shear-driven rotating flow. The experiments were carried out in the Turin University Laboratory rotating water tank, where a circular flow was generated by either increasing (spin-up) or decreasing (spin-down) the rotation speed of the platform. The flow was measured using a Particle Image Velocimetry technique and the developed turbulence analyzed. Two cases were accounted for, in the former the measurements were performed over a smooth surface (bottom of the tank), while in the latter a rough-to-smooth transition was considered. The turbulent boundary layer developed inside the tank is analyzed by means of vertical profiles of mean and turbulent quantities and on the basis of drag coefficients. Then turbulent structures developed in the different cases are shown and discussed in terms of the vorticity fields. Finally, an analysis based on the concept of swirling strength was carried out to select among the vortex extremes those associated with a coherent structure.     

Turbulence in Sparse, Organized Vegetative Canopies: A Large-Eddy Simulation Study

A large-eddy simulation study was performed to characterize turbulence in sparse, row-oriented canopies. This was accomplished by simulating a set of heterogeneous row-oriented canopies with varying row vegetation density and spacing. To determine the effects of heterogeneity, results were compared to horizontally homogeneous canopies with an equivalent ‘effective’ leaf area index. By using a proper effective leaf area index, plane-averaged mean velocities and bulk scaling parameters contained only small errors when heterogeneity was ignored. However, many cases had significantly larger second- and third-order velocity moments in the presence of heterogeneity. Some heterogeneous canopies also contained dispersive fluxes in the lower canopy that were over 20 % as large as the turbulent flux. Impacts of heterogeneity were most pronounced in the cases of large row leaf area density and widely spaced rows. Despite the substantial amount of open space in the sparse canopies, vertical velocity skewness and quadrant-hole analysis indicated that the flow behaved predominantly as a canopy layer even though integral length scales at the canopy top no longer followed mixing-layer scaling. This was supported by the fact that similar composite-averaged coherent structures could be readily identified in both the heterogeneous and homogeneous canopies. Heterogeneity had an effect on coherent structures, in that structure detection events were most likely to occur just upwind of the vegetation rows. In simulations with large row spacing, these structures also penetrated deeper into the canopy when compared to the equivalent homogeneous canopy.     

The Effect of Coherent Structures in the Atmospheric Surface Layer on Blowing-Snow Transport

While turbulent bursts are considered critical for blowing-snow transport and initiation, the interaction of the airflow with the snow surface is not fully understood. To better characterize the coupling of turbulent structures and blowing-snow transport, observations collected in natural environments at the necessary high-resolution time scales are needed. To address this, high-frequency measurements of turbulence, blowing-snow density and particle velocity were made in the Canadian Rockies. During blowing-snow storms, modified variable-interval time averaging enabled identification of periods of near-surface blowing-snow coupling with shear-stress-producing motions in the lowest 2 m of the atmospheric surface layer. The identification of those turbulent motions responsible for blowing snow yields a better understanding of the event-driven mechanics of initiation and sustained transport. The type of coherent structures generating the Reynolds stress are just as important as the magnitude of the Reynolds stress in initiating and sustaining near-surface blowing snow. Our results suggest that blowing-snow models driven by merely the time-averaged shear stress lack physical realism in the near-surface region. The next phase of the development of blowing-snow models should incorporate parametrizations of coherent turbulent structures.     

蒙古气旋爆发性发展导致的强沙尘暴个例研究

对2001年4月6～7日发生在中国北方的一次强沙尘暴过程,从沙尘源、干旱气候背景、天气系统及起沙和扬沙的动力机制进行了初步研究,揭示了强沙尘暴与蒙古气旋和高空急流活动的关系.得出:位于内蒙古西部的巴丹吉林沙漠和中部的浑善达克沙地,是沙尘暴的主要沙尘源区;持续两年的干旱是强沙尘暴形成的气候背景;蒙古气旋的爆发性发展和冷锋后大风是起沙的主要动力;湍流输送和高空急流出口区左侧气流辐散强迫形成的干对流上升气流是沙尘向高空输送的动力机制等结论.作者认为强沙尘暴是挟带大量沙尘的强干对流风暴.