LES prediction of space-time correlations in turbulent shear flows

We compare the space-time correlations calculated from direct numerical simulation （DNS） and large-eddy simulation （LES） of turbulent channel flows. It is found from the comparisons that the LES with an eddy-viscosity subgrid scale （SGS） model over-predicts the space-time corre- lations than the DNS. The overpredictions are further quantified by the integral scales of directional correlations and convection velocities. A physical argument for the overpre- diction is provided that the eddy-viscosity SGS model alone does not includes the backscatter effects although it correctly represents the energy dissipations of SGS motions. This argument is confirmed by the recently developed elliptic model for space-time correlations in turbulent shear flows. It suggests that enstrophy is crucial to the LES prediction of spacetime correlations. The random forcing models and stochastic SGS models are proposed to overcome the overpredictions on space-time correlations.

Space-time correlations in turbulent Rayleigh–Bnard convection

The recent development of the elliptic model （He, et al. Phy. Rev. E, 2006）, which predicts that the space-time correlation function Cu（r, r） in a turbulent flow has a scaling form Cu（rE, 0） with re being a combined space-time separa- tion involving spatial separation r and time delay T, has stimulated considerable experimental efforts aimed at testing the model in various turbulent flows. In this paper, we review some recent experimental investigations of the space-time correlation function in turbulent Rayleigh-Benard convection. The experiments conducted at different representative locations in the convection cell confirmed the predictions of the elliptic model for the velocity field and passive scalar field, such as local temperature and shadowgraph images. The understanding of the functional form of Cu（r, v） has a wide variety of applications in the analysis of experimental and numerical data and in the study of the statistical properties of small-scale turbulence. A few examples are discussed in the review.

Space–time correlations in turbulent flow: A review

This paper reviews some of the principal uses, over almost seven decades, of correlations, in both Eulerian and Lagrangian frames of reference, of properties of turbulent flows at variable spatial locations and variable time in- stants. Commonly called space-time correlations, they have been fundamental to theories and models of turbulence as well as for the analyses of experimental and direct numerical simulation turbulence data.

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 remains valid.

Spacetime coded eigenbeamforming for downlink frequency-selective correlated fading channels

To improve the performance of space-time coding over downlink frequency-selective correlated fading channels, a novel transmission scheme combining eigenbeamfoming and OFDM is proposed. Provided that the channel correlated statistics are available at the transmitter, the wideband correlated fading channels can be converted into an independent FIR channel with 2 transmitting antennas and N receiving antennas by eigenbeamforming and dimension reduction. OFDM is utilized to convert the FIR channel into a group of independent parallel subchanneis to carry space-time codes. With the new structure, the performance of space-time coding over downlink wideband correlated fading channels is greatly improved and the system complexity is reduced. Validity of the proposed system is verified by simulations under different conditions. Comparison between the new structure and an available structure is made both theoretically and computationslly.

Selection of Optimal Embedding Parameters Applied to Short and Noisy Time Series from Rossler System

Throughout scientific research, the state space reconstruction that embeds a non-linear time series is the first and necessary step for characterizing and predicting the behavior of a complex system. This requires to choose appropriate values of time delay T and embedding dimension dE. Three methods are applied and discussed on nonlinear time series provided by the Rössler attractor equations set: Cao’s method, the C-C method developed by Kim et al. and the C-C-1 method developed by Cai et al. A way to fix a parameter necessary to implement the last method is given. Focus has been put on small size and/or noisy time series. The reconstruction quality is measured by using a criterion based on the transformation smoothness.

Features of aerosol optical depth and its relation to extreme temperatures in China during 1980–2001

Based on Total Ozone Mapping Spectrometer （TOMS） monthly aerosol optical thickness （AOT） measurements in 1980–2001 a study is made of space/time patterns and difference between land and sea of AOT 0.50 μm thick over China,which are put into correlation analysis with synchronous extreme temperature indices （warm/cold day and night）.Results suggest that 1） the long-term mean AOT over China is characterized by typical geography,with pronounced land-sea contrast.And AOT has significant seasonality and its seasonal difference is diminished as a function of latitude.2） On the whole,the AOT displays an appreciably increasing trend,with the distinct increase in the eastern Qinghai-Tibetan plateau and SW China,North China,the mid-lower Changjiang （MiLY） valley as well as the South China Sea,but marginal decrease over western/northern Xinjiang and part of South China.3） The AOT over land and sea is marked by conspicuous intra-seasonal and -yearly oscillations,with remarkable periods at one-,two-yr and more （as interannual periods）.4） Land AOT change is well correlated with extremely temperature indexes.Generally,the correlations of AOT to the extreme temperature indices are more significant in Eastern China with 110 ° E as the division.Their high-correlation regions are along the Southern China coastline,the Loess Plateau and the Sichuan Basin,and even higher in North China Plain and the mid-lower Changjiang River reaches.5） Simulations of LMDZ-regional model indicate that aerosol effects may result in cooling all over China,particularly in Eastern China.The contribution of aerosol change may result in more decrease in the maximum temperature than the minimum,with decrease of 0.11/0.08 K for zonal average,respectively.

Nonlinear Deterministic Chaos in Benue River Flow Daily Time Sequence

The Various physical mechanisms governing river flow dynamics act on a wide range of temporal and spatial scales. This spatio-temporal variability has been believed to be influenced by a large number of variables. In the light of this, an attempt was made in this paper to examine whether the daily flow sequence of the Benue River exhibits low-dimensional chaos;that is, if or not its dynamics could be explained by a small number of effective degrees of freedom. To this end, nonlinear analysis of the flow sequence was done by evaluating the correlation dimension based on phase space reconstruction and maximal Lyapunov estimation as well as nonlinear prediction. Results obtained in all instances considered indicate that there is no discernible evidence to suggest that the daily flow sequence of the Benue River exhibit nonlinear deterministic chaotic signatures. Thus, it may be conjectured that the daily flow time series span a wide dynamical range between deterministic chaos and periodic signal contaminated with additive noise;that is, by either measurement or dynamical noise. However, contradictory results abound on the existence of low-dimensional chaos in daily streamflows. Hence, it is paramount to note that if the existence of low-dimension deterministic component is reliably verified, it is necessary to investigate its origin, dependence on the space-time behavior of precipitation and therefore on climate and role of the inflow-runoff mechanism.

Convection and correlation of coherent structure in turbulent boundary layer using tomographic particle image velocimetry

The present experimental work focuses on a new model for space-time correlation and the scale-dependencies of convection velocity and sweep velocity in turbulent boundary layer over a flat wail. A turbulent boundary layer flow at Reo = 2460 is measured by tomographic particle image velocimetry （tomographic PIV）. It is demonstrated that arch, cane, and hairpin vortices are dominant in the logarithmic layer. Hairpins and hairpin packets are responsible for the elongated low-momentum zones observed in the instantaneous flow field. The conditionally-averaged coherent structures systemically illustrate the key roles of hairpin vortice in the turbulence dynamic events, such as ejection and sweep events and energy transport. The space-time correlations of instantaneous streamwise fluctuation velocity are calculated and confirm the new elliptic model for the space-time correlation instead of Taylor hypothesis. The convection velocities derived from the space-time correlation and conditionally-averaged method both suggest the scaling with the local mean velocity in the logarithmic layer. Convection velocity result based on Fourier decomposition （FD） shows stronger scale- dependency in the spanwise direction than in streamwise direction. Compared with FD, the proper orthogonal decomposition （POD） has a distinct distribution of convection velocity for the large- and small-scales which are separated in light of their contributions of turbulent kinetic energy.

Restoration of Time-Spatial Scales in Global Temperature Data

The objective of this paper is to utilize images of spatial and temporal fluctuations of temperature over the Earth to study the global climate variation. We illustrated that monthly temperature observations from weather stations could be decomposed as components with different time scales based on their spectral distribution. Kolmogorov-Zurbenko (KZ) filters were applied to smooth and interpolate gridded temperature data to construct global maps for long-term (≥ 6 years) trends and El Nino-like (2 to 5 years) movements over the time period of 1893 to 2008. Annual temperature seasonality, latitude and altitude effects have been carefully accounted for to capture meaningful spatiotemporal patterns of climate variability. The result revealed striking facts about global temperature anomalies for specific regions. Correlation analysis and the movie of thermal maps for El Nino-like component clearly supported the existence of such climate fluctuations in time and space.

体育文化资本对我国体育旅游经济的影响研究——基于31个省(区、市)的面板数据分析

在推动文旅融合、提高文化附加值、促进旅游市场繁荣的背景下,探讨体育文化资本对体育旅游经济的影响具有重要意义。该研究基于我国31个省(区、市)的面板数据,采用空间可视化对省域体育文化资本的存量与体育旅游经济发展的时空关联特征进行分析,并借助空间杜宾模型及空间效应分解方法探讨了体育文化资本各维度对体育旅游经济的影响作用。研究发现,我国体育文化资本存量与体育旅游经济发展之间存在明显的空间分异现象与稳定的时空关联特征。其中,固体化体育文化资本对本地区体育旅游经济具有正向促进作用,对邻近地区体育旅游经济呈现负向溢出效应;而产品化、能力化和制度化体育文化资本对本地区和邻近地区体育旅游经济都呈现正向促进和正向空间溢出效应。基于上述分析,该研究提出了相应的促进建议:提高产品化体育文化资本质量,塑造体育文化IP;加强人才培养与交流活动,为体育旅游提供可持续动力;完善体育文化支持与管理政策,营造良好的体育旅游发展环境;加强省域间体育事业合作,发挥空间溢出效应。

体育产业与旅游业融合水平的时空演化及影响机理研究——以广西国家体育旅游示范区为例

基于2015-2022年面板数据,借助耦合协调度模型和空间关联模型研究广西体育产业与旅游业融合水平的时空演化特征,并运用OLS回归方程进一步探究体育产业与旅游业耦合发展的影响因素。结果发现:①2015-2022年广西体育产业与旅游业综合发展水平总体为增长态势,体育产业展现出了较旅游业更为强劲的发展动力,并在空间上表现出“中间高四周低”的演变格局;②广西体育产业与旅游业耦合协调水平总体达到初始阶段并具有显著的正相关性,在空间上沿“东北—西南”方向下延伸发展;③广西体育产业与旅游业融合发展同外部环境联系紧密,地方经济和社会发展的相对滞后是制约广西体育产业与旅游业融合发展的关键因素。

Internet访问时间的混沌性解析

以CAIDA组织授权的海量数据为样本空间,利用相空间重构技术以及G-P算法,对时间维的Internet访问时间进行相空间重构,得到其混沌吸引子饱和关联维数为2.8308,证实Internet访问时间的演化过程具有混沌特征。在此基础上,根据关联维数及奇怪吸引子在相空间混沌轨道运动的特性,提出了三维微分方程组的Internet访问时间长期预测模型。

二态量子系统准确布居动力学的一类新型相空间表示及其与三角窗函数的关系

二态系统是最简单的且无经典对应的量子系统,对二态系统的同构表示的认识和研究能启发研究人员对其动力学与统计行为的更深刻理解.本文使用在[J.Chem.Phys.145,204105(2016);J.Chem.Phys.151,024105(2019);J.Phys.Chem.Lett.12,2496(2021)]等文章中发展的约束相空间严格理论、非协变相空间函数、含时权重函数与含时归一化因子来构建一类新型量子相空间表示.这类同构表示可以导出二态量子系统布居动力学的准确结果.约束相空间上的轨迹运动方程同构于含时薛定谔方程.每条相空间轨迹对于布居动力学所对应积分表达式的贡献严格半正定。进一步证明了在[J.Chem.Phys.145,144108(2016)]这篇文章中根据经验提出的三角窗函数方法在本质上可以对应于本文的这类新型相空间表示的一个特殊情况,因此同样是二态量子系统准确布居动力学的同构表示.

空时处理引起的GNSS信号失真分析与补偿方法

在复杂的干扰环境中,空时自适应处理(STAP)技术会扭曲全球卫星导航系统(GNSS)信号的相关峰、降低捕获和跟踪性能,导致接收机的定位精度下降。首先,通过理论和仿真分析STAP在最小方差无失真响应准则下的相关峰失真,结果表明抽头数和空间相关系数会直接影响到相关峰的畸变;其次,为优化抽头数和降低空间相关系数,提出基于天线阵列信干噪比(SINR)的自适应抽头算法和阵列稀疏配置算法;最后,通过仿真验证优化后的抽头数和阵列稀疏配置可以减少因STAP引起的相关峰畸变。

高维时序空间大数据关联特征精准挖掘算法

高维时序空间大数据关联特征较多且复杂,导致大数据关联特征挖掘难度大幅度上升,所以设计了高维时序空间大数据关联特征精准挖掘算法。根据体数据求解结果,确定高维时序空间大数据子序列表达式,计算大数据样本的时序特征。利用关联方差,推导精准序列矩阵,结合回归向量的取值范围对大数据关联特征进行精准挖掘,得到相关的挖掘结果。实验结果表明,随着大数据样本量的增加,所提方法的拟合误差参数最大值不超过2.1,不会影响挖掘指令执行能力,满足大数据关联特征精准挖掘的实际应用需求。

Direction-of-arrival estimation based on direct data domain (D3) method

A direction-of-arrival （DOA） estimation algorithm based on direct data domain （D3） approach is presented. This method can accuracy estimate DOA using one snapshot modified data, called the temporal and spatial two-dimensional vector reconstruction （TSR） method. The key idea is to apply the D3 approach which can extract the signal of given frequency but null out other frequency signals in temporal domain. Then the spatial vector reconstruction processing is used to estimate the angle of the spatial coherent signal source based on extract signal data. Compared with the common temporal and spatial processing approach, the TSR method has a lower computational load, higher real-time performance, robustness and angular accuracy of DOA. The proposed algorithm can be directly applied to the phased array radar of coherent pulses. Simulation results demonstrate the performance of the proposed technique.

Two Methods to Solve the Ionospheric Electron Concentration Horizontal Gradient at Chongqing

The electron concentration horizontal gradient vector of the ionosphere and its south-north and east-west components over Chongqing station are analyzed and calculated, using the first approximation, time correlation and space correlation and another approach introduced. And then, the validity of the two methods is analyzed and compared.

采用分布式算法的多区域电网风-光-储容量优化配置

针对新能源发电在整个电网中渗透率不断提高而造成的电能质量下降、电力系统无法可靠运行等一系列问题,基于随机规划理论,综合考虑风电、光伏等电源以及联络线、储能设备运行约束,以典型新能源出力时序特征序列下平均新能源消纳水平最大为目标,提出了一种计及电网新能源消纳承载能力的多区域风-光-储容量优化配置方法。该方法主要考虑将风电、光伏发电满足负荷需求后的无法消纳量在不超过储能设备容量的前提下进行储电,而储能设备将在新能源发电低谷期提供电能,充分发挥风、光的时空互补性和储能的调峰优势。首先构造了考虑时-空相关性的典型新能源出力时序特征序列不确定集,然后按典型序列将模型进行分解,采用分布式惩罚原始-对偶次梯度算法实现目标函数的分布式优化求解,最后基于IEEE-24、31节点系统进行算例分析,得到风-光-储容量配置结果,并建议多区域电网中储能设备应考虑建设在含有风光装机的区域。仿真结果表明:相比传统集中式优化方法和拉格朗日松弛法,所提分布式优化方法随着电网区域增多,计算效率优势会越来越明显,10个典型序列时所提分布式方法相比拉格朗日松弛法收敛速度提升了57.1%。所提方法可以为考虑新能源随机性的风-光-储多区域电网容量优化配置问题提供系统性解决方案。