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1.
切变湍流的相干结构是湍流研究中的重大发现,它表明湍流在表面上看来不规则运动中具有可检测的有序运动,这种相干结构在切变湍流的脉动生成和发展中起着主宰作用.因此识别和提取相干结构对于认识和研究湍流是非常重要的.用数字滤波法将包含相干结构的大尺度信号提取出来以后,再用子波分析,根据子波能量极大值的判别方法,分别确定出大气湍流三个方向上的速度脉动信号相干结构的频率或时间尺度,然后由确定尺度上的连续子波反演公式,提取出大气湍流三个方向上的速度脉动信号相干结构所对应的波形. 相似文献
2.
高风速相干结构对通量输送影响的实验研究 总被引：2，自引：0，他引：2
切变湍流的相干结构是湍流研究中的重大发现,它表明湍流运动并非完全随机,其中具有可检测的有序结构.本文通过处理南京浦口地区大气边界层观测数据,来分析不稳定层结中高风速相干结构特征.本次观测项目包括对场地中央的气象铁塔上2 m和40 m高度上超声风速仪的脉动速度、温度测量以及风廓线雷达对边界层风速廓线的测量.对超声水平风速时间序列数据进行小波变换 (时间尺度400 s),通过阈值来识别这种高风速相干结构.与多普勒风廓线雷达测量结果对比后发现,这种方法确定的相干结构符合常规的认识,具有较长的时间尺度和较大的垂直尺度 (接近边界层厚度).分析三天相干结构特性得到无量纲空间间隔约为6,即每隔6个边界层厚度的水平位置出现一个高速相干结构.通过与垂直风速小波系数的比较,发现高风速相干结构与向下垂直风速之间有较好相关,这与湍流中 “阵风” 现象的研究结论相似.使用四象限分析方法分类得到两种动量通量输送为负的运动:较小水平风速的上扬 (ejection) 运动 (简称为上扬运动) 和较大水平风速的下扫 (sweep) 运动 (简称为下扫运动),这两种运动在整个湍流活动中处于主导地位.高风速相干结构通过促进下扫运动和抑制上扬运动来影响动量通量的输送. 相似文献
3.
Christian Barthlott Philippe Drobinski Clément Fesquet Thomas Dubos Christophe Pietras 《Boundary-Layer Meteorology》2007,125(1):1-24
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. 相似文献
4.
In the present study, three wavelet basis functions (Mexican-hat, Morlet, and Wave) were used to analyze atmospheric turbulence data obtained from an eddy covariance system in order to determine effect of six meteorological elements (three-dimensional wind speed, temperature, and CO2 and H2O concentrations) on the time scale of coherent structures. First, we used the degree of correlation between original and reconstructed waveforms to test the three wavelets’ performance when determining the time scale of coherent structures. The Wave wavelet’s reconstructed coherent structure signal best matched the original signal; thus, it was used for further analysis of the time scale, number, and time cover of the meteorological elements. We found similar results for all elements, though there was some internal variation, suggesting that coherent structures are not inherently dependent on these elements. Our results provide a basis for proper coherent structure detection in atmospheric turbulence and improve the understanding of similarities and differences between coherent structure characteristics of different meteorological elements, which is helpful for further research into atmospheric turbulence and boundary layers. 相似文献
5.
K. G. McNaughton 《Boundary-Layer Meteorology》2004,111(1):1-18
We investigate the production components of turbulent spectra within logarithmic layers over flat ground. This assumes that the turbulence giving rise to these spectra consists of active coherent structures (eddies) that are attached to the ground, and whose properties display perfect statistical self-similarity under inner scaling. That is, we take the extreme view that active coherent structures not only contribute to turbulence production spectra but explain the whole of them, so that neither detached eddies nor unstructured motions make any significant contribution. Perfect self-similarityis held to apply only to eddies that are themselves formed totally within the log layer, so the theory applies in the limit of spectra obtained at the hearts of very deep log layers. The model predicts that spectral variance and covariance should become independent of wavenumber at small wavenumbers. This asymptotic behaviour is observed in all neutral spectra from the Kansas experiment. The model also interprets the various positions of the spectral peaks observed at Kansas and in aircraft flights over the sea as consequences of the eddies being aggregated into files aligned with the wind. The observed spectra are therefore consistent with large-scale wedge-like structures being the principal component of active turbulence in the neutral atmospheric surface layer. 相似文献
6.
7.
Barbara Hennemuth 《Boundary-Layer Meteorology》1978,15(4):489-506
Statistical functions such as correlation functions and spectra yield length scales, each of which describes a certain aspect of the large-scale structure of turbulence. In this study not only statistics of wind fluctuations measured at the same place have been analysed but also statistics of fluctuations at different heights. Thus a spatial picture of the energy-containing and the momentum-transporting eddies has been obtained and some features of the momentum transport are described. 相似文献
8.
Mitsuaki Horiguchi Taiichi Hayashi Ahoro Adachi Shigeru Onogi 《Boundary-Layer Meteorology》2012,144(2):179-198
Large-scale turbulence structures in the near-neutral atmospheric boundary layer (ABL) are investigated on the basis of observations made from the 213-m tall meteorological tower at Tsukuba, Japan. Vertical profiles of wind speed and turbulent fluxes in the ABL were obtained with sonic anemometer-thermometers at six levels of the tower. From the archived data, 31 near-neutral cases are selected for the analysis of turbulence structures. For the typical case, event detection by the integral wavelet transform with a large time scale (180 s) from the streamwise velocity component (u) at the highest level (200 m) reveals a descending high-speed structure with a time scale of approximately 100 s (a spatial scale of 1 km at the 200-m height). By applying the wavelet transform to the u velocity component at each level, the intermittent appearance of large-scale high-speed structures extending also in the vertical is detected. These structures usually make a large contribution to the downward momentum transfer and induce the enhancement of turbulent kinetic energy. This behaviour is like that of “active” turbulent motions. From the analysis of the two-point space–time correlation of wavelet coefficients for the u velocity component, the vertical extent and the downward influence of large-scale structures are examined. Large fluctuations in the large-scale range (wavelet variance at the selected time scale) at the 200-m level tend to induce the large correlation between the higher and lower levels. 相似文献
9.
10.
Summary This study presents a discussion of a method for automated and quasi-online analysis of coherent structures using wavelet transform. The method is optimised for rapid processing of vector and scalar variables obtained over tall vegetation. It has been designed to assess long-term statistics of coherent structures, as it is applicable over a wide range of atmospheric conditions. Data of artificial and real turbulent signals are used to perform the analysis and to evaluate the presented method.Different wavelet functions are used for filtering the original signals, determining characteristic time scales, and detecting individual coherent structures. On this basis, statistics of temporal separation of coherent structures and phase shift between different variables can be calculated.Background turbulence and spikes are found to be efficiently removed without changing the shape, particularly the sharp localised gradients, of coherent structures. The determined peak in the calculated wavelet variance spectrum is observed to correspond very well to characteristic event durations and to satisfy the definition of coherent structures present in vector and scalar variables. The detection algorithm was successful in analysing data covering a wide range of atmospheric conditions. Detected individual coherent structures provide a parallel temporal pattern for scalar variables, but a phase shift between scalar and vector components. 相似文献
11.
Christoph Thomas Jens-Christopher Mayer Franz X. Meixner Thomas Foken 《Boundary-Layer Meteorology》2006,119(3):563-587
This study applies acoustic sounding to observe coherent structures in the roughness sublayer (RSL) above tall vegetated surfaces. Data were collected on 22 days during two separate field experiments in summer 2003. A quality control scheme was developed to ensure high data quality of the collected time series. The data analysis was done using both discrete and continuous wavelet transform. The flow in the RSL was found to be a superposition of dynamic Kelvin–Helmholtz instabilities and convective mixing. The characteristic time scales for coherent structures resulting from the dynamic instabilities were observed to be approximately 20–30 s while thermal eddies have much larger time scales of 190–210 s. The degree of vertical coherency in the RSL increases with the flow evolving from neutral to near-convective conditions. This increase in the degree of organisation is attributed to the evolution of attached thermal eddies. The coherent structures resulting from instabilities were found to be present throughout the RSL but do not contribute to the increased vertical coherency. An alternative conceptual approach for the definition of the RSL is proposed, which yields its maximum vertical extent to five times the canopy height. 相似文献
12.
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。 相似文献
13.
The fluctuations of the instantaneous values of line integrated concentrations across plumes from point sources diffusing in turbulent shear flows, and in grid generated turbulence, have been studied experimentally using a fast response system which measured the attenuation of the intensity of an infrared beam crossing the plume. Analysis of the measurements show that the dimensionless statistical properties of the fluctuations at different distances from the source at each flow are approximately similar, in the sense that they depend primarily on the relative off-center location of the line of integration and almost independent of the distance from the source and the nature of the turbulence in the flows, as long as the characteristic length of the mean plume is not large compared to the size of the large eddies. The characteristic time of the fluctuations, on the other hand, was found to grow with the distance from the source and the autocorrelations of the fluctuations, particularly in the case of a plume diffusing in grid generated turbulence, were it found to be proportional to the lateral size of the mean plume. A—5/3 decay law of the power spectrum of the fluctuations was observed in the low frequency range which corresponds to the scale of the large eddies. The decay of the fluctuations caused by smaller eddies was much faster, as expected. 相似文献
14.
The Effects of Vegetation Density on Coherent Turbulent Structures within the Canopy Sublayer: A Large-Eddy Simulation Study 总被引：1，自引：0，他引：1
Large-eddy simulation has become an important tool for the study of the atmospheric boundary layer. However, since large-eddy
simulation does not simulate small scales, which do interact to some degree with large scales, and does not explicitly resolve
the viscous sublayer, it is reasonable to ask if these limitations affect significantly the ability of large-eddy simulation
to simulate large-scale coherent structures. This issue is investigated here through the analysis of simulated coherent structures
with the proper orthogonal decomposition technique. We compare large-eddy simulation of the atmospheric boundary layer with
direct numerical simulation of channel flow. Despite the differences of the two flow types it is expected that the atmospheric
boundary layer should exhibit similar structures as those in the channel flow, since these large-scale coherent structures
arise from the same primary instability generated by the interaction of the mean flow with the wall surface in both flows.
It is shown here that several important similarities are present in the two simulations: (i) coherent structures in the spanwise-vertical
plane consist of a strong ejection between a pair of counter-rotating vortices; (ii) each vortex in the pair is inclined from
the wall in the spanwise direction with a tilt angle of approximately 45°; (iii) the vortex pair curves up in the streamwise
direction. Overall, this comparison adds further confidence in the ability of large-eddy simulation to produce large-scale
structures even when wall models are used. Truncated reconstruction of instantaneous turbulent fields is carried out, testing
the ability of the proper orthogonal decomposition technique to approximate the original turbulent field with only a few of
the most important eigenmodes. It is observed that the proper orthogonal decomposition reconstructs the turbulent kinetic
energy more efficiently than the vorticity. 相似文献
15.
Large-eddy simulation has become an important tool for the study of the atmospheric boundary layer. However, since large-eddy
simulation does not simulate small scales, which do interact to some degree with large scales, and does not explicitly resolve
the viscous sublayer, it is reasonable to ask if these limitations affect significantly the ability of large-eddy simulation
to simulate large-scale coherent structures. This issue is investigated here through the analysis of simulated coherent structures
with the proper orthogonal decomposition technique. We compare large-eddy simulation of the atmospheric boundary layer with
direct numerical simulation of channel flow. Despite the differences of the two flow types it is expected that the atmospheric
boundary layer should exhibit similar structures as those in the channel flow, since these large-scale coherent structures
arise from the same primary instability generated by the interaction of the mean flow with the wall surface in both flows.
It is shown here that several important similarities are present in the two simulations: (i) coherent structures in the spanwise-vertical
plane consist of a strong ejection between a pair of counter-rotating vortices; (ii) each vortex in the pair is inclined from
the wall in the spanwise direction with a tilt angle of approximately 45°; (iii) the vortex pair curves up in the streamwise
direction. Overall, this comparison adds further confidence in the ability of large-eddy simulation to produce large-scale
structures even when wall models are used. Truncated reconstruction of instantaneous turbulent fields is carried out, testing
the ability of the proper orthogonal decomposition technique to approximate the original turbulent field with only a few of
the most important eigenmodes. It is observed that the proper orthogonal decomposition reconstructs the turbulent kinetic
energy more efficiently than the vorticity. 相似文献
16.
Wind-Tunnel Experiment on Logarithmic-Layer Turbulence under the Influence of Overlying Detached Eddies 总被引：1，自引：1，他引：0
Yasuo Hattori Chin-Hoh Moeng Hitoshi Suto Nobukazu Tanaka Hiromaru Hirakuchi 《Boundary-Layer Meteorology》2010,134(2):269-283
A wind-tunnel experiment was carried out to test a hypothesis that the turbulence characteristics in the near-neutral surface
layer are largely determined by detached eddies from above. The surrogate detached eddies were generated by using an active
turbulence grid installed at the front of the test section and the parameters of the grid were chosen such that the fully
developed logarithmic layer downstream consists of a turbulent flow that has similar normalized intensity to that typically
observed in the near-neutral atmospheric surface layer. The effects of the detached eddies on turbulence characteristics were
investigated by comparison with a second experiment without detached eddies. The influence of the detached eddies on the logarithmic
layer was mostly on the coherent structures; the logarithmic layer with the detached eddies revealed a multi-layer structure
similar to that found in the atmosphere where the lower part of the surface layer is dominated by sweep-like events and the
upper part by ejection-like events. Our experiments show that the mean velocity gradient and the Reynolds shear stress were,
however, not affected significantly by the detached eddies and hence the eddy viscosity. 相似文献
17.
Wind component fluctuations have been decomposed into contributions of large and small-scale eddies to study turbulence structures in the atmospheric surface layer over snow-covered pack ice for a case with snow drift and another without. It was found that on average, the large eddies contribute little to the covariance, but significantly to the variance of the horizontal wind components. The turbulent kinetic energy and the friction velocity show clearly expressed energy and flux variations. The time series of turbulent statistics, calculated on the basis of large eddies only, show the same behavior. However, the time variations of the turbulent kinetic energy and the friction velocity, determined by the small eddies, are suppressed when there is drifting snow.The occurrence of different types of motions (ejections, sweeps and interactions) was detected by the quadrant-technique. It was found that events with strong drag reduction during snow drift correspond to a strong increase of contributions from the first and third quadrant. However, strong drag reduction events in the case without snow drift are caused by a general decrease of contributions from all four quadrants. It was also found that interactions are significantly less correlated with ejections and sweeps in the case with snow drift than in the case without. 相似文献
18.
大气边界层阵风相干结构的产生条件 总被引：1，自引：0，他引：1
壁湍流相干结构的发现是近代湍流研究的重大进展之一,从20世纪50年代开始,在大气边界层湍流中也发现了相干结构——对流云街,并进行了系统的研究。近些年来,人们发现在近地层湍流中也存在相干结构。利用北京325 m气象塔对城市下垫面中大风和小风天气的风速分析,发现较有规律的周期3~6 min的阵风,且有明显的相干结构,而对不同下垫面的阵风研究,均发现存在这种相干结构,这种阵风相干结构对通量输送有不可忽视的作用。本文利用2012年4月甘肃省民勤县巴丹吉林沙漠观测塔的超声风速和平均场风速、温度观测资料,对阵风相干结构的产生条件进行了分析。采用傅立叶变换,将三维超声风速按频率分成基流(周期10分钟以上)、阵风扰动(周期1到10分钟)、湍流脉动(周期小于1分钟)三部分,结合平均场的资料分析发现:阵风相干结构出现在静力中性、不稳定甚至略微稳定的条件下,或者说机械作用主导的大气边界层,阵风区就会出现相干结构,热力作用对其有抑制和干扰的作用。从而,阵风的相干结构和壁面相干结构都出现在中性条件下,是机械湍流的现象,都主导着动量能量的输运。阵风区的相干结构并不等同于对流云街,他们出现在不同的大气稳定度条件下且尺度不同。 相似文献
19.
This paper describes how to measure turbulence in the atmospheric boundary layer (ABL) in order to address certain problems in modern atmospheric physics. These problems mainly relate to the Earth's energy budget (including the hydrological cycle) and biogeochemical cycles. Starting from the main characteristic numbers and the basic equations of atmospheric turbulent flow, we show what turbulence parameters are important to measure. Special attention is given to the various methods used to compute the turbulent fluxes. We analyse the range of scales which has to be measured to properly capture the eddies contributing to the turbulent transfers. This range of scales determines what sensors can be used in the atmospheric surface layer and in the ABL. We describe the most widely used instruments and their performances. The principal platforms used to deploy these instruments are examined. Aircraft are described in more details, because they allow a thorough exploration of the ABL. In the last section, some examples of ABL turbulence signals measured in various conditions are presented. These examples illustrate horizontally homogeneous turbulence as well as inhomogeneous signals for which standard analysis techniques cannot be used. We show how some recent techniques, like wavelet transforms, can help to investigate this kind of signal. At the end, we present what would be interesting to do in the near future for the study of ABL turbulence. 相似文献
20.
Clment Fesquet Philippe Drobinski Christian Barthlott Thomas Dubos 《Atmospheric Research》2009,94(2):254-269
This study investigates the impact of terrain heterogeneity on local turbulence measurements using 18 months of turbulence data taken on a 30 m tower at the SIRTA mixed land-use observatory under varying stability conditions and fetch configurations. These measurements show that turbulence variables such as the turbulent kinetic energy or momentum fluxes are strongly dependent on the upstream complexity of the terrain (presence of trees or buildings, open field). However, using a detection technique based on wavelet transforms which permits the isolation of the large-scale coherent structures from small-scale background fluctuations, the study shows that, for all stability conditions, whatever the upstream complexity of the terrain, the coherent structures display universal properties which are independent of the terrain nature: the frequency of occurrence, time duration of the coherent structures, the time separation between coherent structures and the relative contribution of the coherent structures to the total fluxes (momentum and heat) appear to be independent of the upstream roughness. This is an important result since coherent structures are known to transport a large portion of the total energy. This study extends to all stability conditions a numerical study by Fesquet et al. [Fesquet, C., Dupont, S., Drobinski, P., Barthlott, C., Dubos, T., 2008. Impact of terrain heterogeneities on coherent structures properties: experimental and numerical approaches. In: 18th Symposium on Boundary Layers and Turbulence. No. 11B.1. Stockholm, Sweden., Fesquet, C., Dupont, S., Drobinski, P., Dubos, T., Barthlott, C., in press. Impact of terrain heterogeneity on coherent structure properties: numerical approach. Bound.-Layer Meteorol.] conducted in neutral conditions which shows that a reason for such behavior is that the production of local active turbulence in an internal boundary layer associated with coherent structure originating from the outer layer and impinging onto the ground is not sensitive to the nature of the terrain. 相似文献