Coherent structures of the flow around a surface-mounted cubic obstacle in turbulent channel flow

The coherent structures of the flow developing around a surface-mounted cubic obstacle placed in turbulent channel flow (Re=40.000 based on bulk velocity and channel half-width) are investigated. The technique of the proper orthogonal decomposition is applied to extract the coherent structures of the flow from a database obtained with the use of a finite volume computational code for the numerical integration of the three-dimensional time-dependent incompressible Navier-Stokes equations; for turbulence modeling the LES approach is followed and the RNG subgrid-scale closure is used. The three-dimensional time-dependent velocity field is computed and 100 non-dimensional time steps of the turbulent statistically steady state are considered. The decomposition is performed on two appropriate subdomains around the cubic obstacle and a “reduced” velocity field, reflecting the contribution of the most energetic eigenfunctions of the decomposition, containing up to 95% of the turbulent kinetic energy of the original flow field, is reconstructed; results are presented in terms of vorticity showing the temporal dynamics of the coherent structures in the selected subdomains.     

Spanwise pressure coherence on prisms using wavelet transform and spectral proper orthogonal decomposition based tools

This paper presents new approaches to clarifying spanwise pressure coherence on typical prisms using some advanced tools based on continuous wavelet transform and spectral-branched proper orthogonal decomposition. Wavelet coherence and coherence modes have been developed for mapping characteristics of spanwise coherence of pressure and turbulence. Temporal-spectral spanwise coherence maps have been represented in the time-frequency plane and spatial-spectral spanwise coherence maps have been expressed in the space-frequency plane. Some new findings are that spanwise pressure coherence not only depends on spanwise separation, frequency and turbulent conditions, but is also influenced by bluff body flow and time. Intermittent and time-dependent pressure coherence in the time domain has been investigated as the nature of pressure coherence. Furthermore, distribution and intermittency of pressure coherence are significantly influenced by analyzed time-frequency resolutions and parameters of the analyzed wavelet function. The coherence mode has been proposed for better understanding of the effect of bluff body flow on pressure coherence. Physical measurements of surface fluctuating pressure and turbulence have been carried out on typical prisms with slenderness ratios of B/D=1 and 5 in turbulent flow.     

Estimation of the vortex shedding frequency of a 2-D building using correlation and the POD methods

A method was developed to estimate the frequency of vortex shedding in turbulent flow. In this method, a Particle Image Velocimetry (PIV) system was used to acquire the original complex turbulent flow field. Proper orthogonal decomposition (POD) analysis was applied to the PIV data in order to extract the main structure of the overall physical characteristics from the measured wind velocity field. A bivariate correlation was introduced to the POD results to determine the correlation of the vortex structures. Fast Fourier Transform (FFT) was employed to perform a power spectra analysis of these correlation coefficients to obtain the frequency of vortex shedding from the power peak. A PIV experiment was carried out within a wind tunnel simulated atmospheric surface layer, which involved a two-dimensional building problem with wind from two directions. The developed method was applied to the PIV experimental data and the frequency of vortex shedding was successfully extracted. The results show that the frequency of vortex shedding increases with increase in incoming flow velocity for both wind directions.     

A hybrid RANS and kinematic simulation of wind load effects on full-scale tall buildings

Up till recent years, predicting wind loads on full-scale tall buildings using Large Eddy Simulation (LES) is still impractical due to a prohibitively large amount of meshes required, especially in the vicinity of the near-wall layers of the turbulent flow. A hybrid approach is proposed for solving pressure fluctuations of wind flows around tall buildings based on the Reynolds Averaged Navier-Stokes (RANS) simulation, which requires coarse meshes, and the mesh-free Kinematic Simulation (KS). While RANS is commonly used to provide mean flow characteristics of turbulent airflows, KS is able to generate an artificial fluctuating velocity field that satisfies both the flow continuity condition and the specific energy spectra of atmospheric turbulence. The kinetic energy is split along three orthogonal directions to account for anisotropic effects in atmospheric boundary layer. The periodic vortex shedding effects can partially be incorporated by the use of an energy density function peaked at a Strouhal wave number. The pressure fluctuations can then be obtained by solving the Poisson equation corresponding to the generated velocity fluctuation field by the KS. An example of the CAARC building demonstrates the efficiency of the synthesized approach and shows good agreements with the results of LES and wind tunnel measurements.     

Orthogonal expansion of Gaussian wind velocity field and PDEM-based vibration analysis of wind-excited structures

An effective procedure for simulation of random wind velocity field by the orthogonal expansion method is proposed in this paper. The procedure starts with decomposing the fluctuating wind velocity field into a product of a stochastic process and a random field, which represent the time property and the spatial correlation property of wind velocity fluctuations, respectively. By an innovative orthogonal expansion technology, the stochastic process for wind velocity fluctuations may be represented as a finite sum of deterministic time functions with corresponding uncorrelated random coefficients. Similarly, the random field can be expressed as a combination form with only a few random variables by the Karhunen-Loeve decomposition. This approach actually simulates the wind velocity field with stochastic functions other than methods such as spectral representation and proper orthogonal decomposition. In the second part of the paper, the probability density evolution method (PDEM) is employed to predict the stochastic dynamic response of structures subjected to wind excitations. In the PDEM, a completely uncoupled one-dimensional partial differential equation, the generalized density evolution equation, plays a central role in governing the stochastic responses of structures. The solution of this equation will give rise to instantaneous probability density function of the responses. Finally, the accuracy and effectiveness of the approach in representing the random wind velocity field and PDEM-based dynamic response of wind-excited building are investigated.     

全桥气弹模型颤振导数识别

将节段模型颤振导数识别方法用于全桥气弹模型,对其可行性和便利性进行详细论述;给出气弹模型模态质量的确定方法;采用随机子空间方法和随机搜索方法识别均匀流场和紊流场中苏通大桥气弹模型的18颤振导数,并和特征系统实现算法识别结果进行对比分析。研究结果表明:节段模型颤振导数识别方法是识别气弹模型颤振导数的有效和实用方法;模型模态质量可以方便地根据几何缩尺比由实桥对应值直接推算得到,且比通过模型试验实测精度更高;不同方法识别得到的均匀流场中苏通大桥气弹模型大部分颤振导数基本吻合;相对于紊流场中的气弹模型而言,均匀流场中的节段模型颤振导数识别精度更高。     

泥石流冲击力的野外测量

冲击作用是泥石流最为剧烈的一种破坏方式。泥石流的冲击力因此也成为泥石流工程防治和危险性分区中的重要参数。然而,由于其破坏力巨大,可靠的野外泥石流的冲击力数据比较少。2004年通过在云南蒋家沟建立的泥石流冲击力野外测试装置和新研制的力传感器以及数据采集系统,首次测得不同流深位置、长历时、波形完整的泥石流冲击力信号。原始信号经过低通滤波处理后,得到真实的泥石流冲击力数据。对数据的初步分析发现,在同等流速的条件下,连续流的冲击力要比阵性流的大得多。单就阵性流而言,泥石流的冲击力最大值也不是随流速而单调增加的,而应该跟它所携带的固体物质的大小有很密切的关系。最后,对不同位置的冲击力过程线的分析说明泥石流中中等粒径的石块多集中在龙头和流体表面,而大粒径的石块应该是在泥石流体中半悬浮运动。     

A wavelet-based method to generate artificial wind fluctuation data

A method using the inverse wavelet transform is proposed to generate artificial wind velocity fluctuations. At first, in order to investigate the time-scale structure of natural wind, the wavelet transform is applied to the time history of a measured wind velocity data. Taking the results into account, the wavelet-based method is constructed such that the created time history possesses the characteristics similar to those of the natural wind data. The time histories are in particular synthesized to have a target power spectrum and intermittency similar to measured time histories. The characteristics of the time histories produced with the proposed method are discussed.     

Using projection pursuit and proper orthogonal decomposition to identify independent flow mechanisms

In this paper, we develop a new dimension reduction technique, using the projection pursuit approach, to identify underlying physical mechanisms of the flow. By first applying the standard dimension reduction technique-proper orthogonal decomposition (POD), a low-dimensional subspace is defined. POD models of pressure fields have, in the past, been challenged with questions of interpretation in terms of flow mechanisms. To address this issue, the projection criterion is applied to decompose independent physical mechanisms in the non-Gaussian pressure field. This procedure leads to a non-orthogonal subspace decomposition that provides a suitable subspace for identification of physical mechanisms in intermittent flows. This approach provides a new tool to further our understanding of the fundamental nature of intermittent and independent phenomena in fluid flows. Finally, this technique is tested with experimental data collected at Texas Tech University's Wind Engineering Research Field Laboratory.     

Wavelet Packet-Autocorrelation Function Method for Traffic Flow Pattern Analysis

Abstract:   Accurate and timely forecasting of traffic flow is of paramount importance for effective management of traffic congestion in intelligent transportation systems. A detailed understanding of the properties of traffic flow is essential for building a reliable forecasting model. The discrete wavelet packet transform (DWPT) provides more coefficients than the conventional discrete wavelet transform (DWT), representing additional subtle details of a signal. In wavelet multiresolution analysis, an important decision is the selection of the decomposition level. In this research, the statistical autocorrelation function (ACF) is proposed for the selection of the decomposition level in wavelet multiresolution analysis of traffic flow time series. A hybrid wavelet packet-ACF method is proposed for analysis of traffic flow time series and determining its self-similar, singular, and fractal properties. A DWPT-based approach combined with a wavelet coefficients penalization scheme and soft thresholding is presented for denoising the traffic flow. The proposed methodology provides a powerful tool in removing the noise and identifying singularities in the traffic flow. The methods created in this research are of value in developing accurate traffic-forecasting models .     

Proper orthogonal decomposition of two vertical profiles of full-scale nonstationary downburst wind speeds[lzcl]

The paper proposes a novel analysis framework for nonstationary wind speeds, from which accumulated results can potentially lead to or enhance empirical nonstationary wind speed models such as the hybrid one for downbursts in (Eng. Struct. 26 (2004) 619). This framework is motivated by the proper orthogonal decomposition (POD) technique and consists of four major steps: separation, POD, approximation and property inference. In the first step, the original wind speed time histories are separated into their time-varying mean speeds and fluctuating speeds through wavelet shrinkage, which is a promising new tool for smoothing; fluctuating speeds are further expressed as the product of their time-varying standard deviations and normalized fluctuating speeds with unit variance. In the second step, the POD is applied to the time-varying means, standard deviations and normalized fluctuation. In the third step, by properly selecting the number of retained modes, the original time series can be approximated to a numerical model. Finally, wind properties such as velocity vertical profiles and turbulence vertical profiles can be calculated from the model. The POD employed herein is a specific POD for arbitrary multivariate data representation. The normalized fluctuations are characterized by both their power spectral densities (PSD) and evolutionary power spectral densities (EPSD). The concept of EPSD for nonstationary processes is briefly presented and one EPSD estimator is given. Specifically, the PSD/EPSD of an original process is related to the PSD/EPSD of the principal coordinate process obtained by the POD. This framework is applied to two sets of nonstationary full-scale thunderstorm downburst wind speed time series. Many appealing downburst properties are obtained and the benefit of the POD is demonstrated.     

Direct identification of flutter derivatives and aerodynamic admittances of bridge decks

Flutter derivatives and aerodynamic admittances provide basis of predicting the critical wind speed in flutter and buffeting analysis of long-span cable-supported bridges. In this paper, one popular stochastic system identification technique, covariance-driven stochastic subspace identification (SSI in short), is first presented for estimation of the flutter derivatives and aerodynamic admittances of bridge decks from their random responses in turbulent flow. Numerical simulations of an ideal thin plate are adopted to extract these aerodynamic parameters to evaluate the applicability of the present method. Then wind tunnel tests of a streamlined thin plate model and a Π type blunt bridge section model were conducted in turbulent flow and the flutter derivatives and aerodynamic admittances are determined by the SSI technique. The identified aerodynamic parameters are compared with the theoretical ones and the results indicate the applicability of the current method.     

An analysis of extreme non-synoptic winds

This paper presents an analysis of wind velocity data relating to 11 extreme non-synoptic events obtained from velocity time series pertaining to flow over rural and coastal terrain. For the purposes of the current analysis, an extreme event is deemed to have occurred when the local velocity increases rapidly by 50% or more and decreases within a relatively short time. The data were measured at a number of locations within Northern Europe and in all cases a sufficient array of anemometry was present to obtain an indication of the velocity profile. The velocity profile is shown to alter significantly during such events with three different trends identified. Despite the large variations in wind speed and velocity profile, wavelet analysis is used to illustrate that the underlying turbulent structures exhibit similar behaviour to those expected to occur during extreme wind speed events in boundary layer flow.     

3D flow around a rectangular cylinder: A computational study

The aim of this paper is to provide a contribution to the analysis of the 3D, high Reynolds number, turbulent, separated and reattached flow around a fixed sharp-edged rectangular cylinder with a chord-to-depth ratio equal to 5. The work is developed in the perspective of the benchmark on the aerodynamics of a rectangular cylinder (BARC), in terms of an exploratory computational study. First, the adopted flow modelling and computational approach are shortly described. Second, the obtained main aerodynamic integral parameters are compared with other results proposed in the literature. Hence, the 3D flow features around the nominally 2D cylinder are investigated by means of both proper orthogonal decomposition and coherence function of the side-surface fluctuating pressure field. Once the main 2D nature of the flow has been pointed out, some of the 2D mechanisms that are responsible for the variation of the fluctuating aerodynamic forces are scrutinised: the computational approach post-processing facilities are employed to look for significant relationships between the flow structures, the pressure field and the aerodynamic force components.     

Effective pavement skid resistance measurement using multi-scale textures and deep fusion network

Pavement skid resistance measurement is a fundamental component of roadway management and maintenance. Most traditional approaches rely on manual operations or heavy devices, which lead to a labor-intensive, inefficient, and vulnerable testing environment. Precise laser scanning technology lays a solid foundation for effective and continuous pavement friction measurement. This paper proposed an automated pavement friction estimation model using 3D point cloud data and a deep neural network. The fine-grained texture data of over 800 pavement sections with various anti-skidding abilities were collected. The impact of the multi-scale textures on pavement friction was separated and analyzed via two-dimensional wavelet decomposition. A multi-input fusion network with deep aggregation modules was designed to fuse the features of sub-images generated by wavelet decomposition. The results show that the average prediction error is 0.0935, outperforming most state-of-the-art models. The impact of different texture scales on friction estimation is then revealed. The proposed method provides a new tool for effective and large-scale pavement friction evaluation.     

Low-level atmospheric jets as main mechanism of long-range transport of power plant plumes in the Lake Baikal Region

This paper considers some peculiarities of anthropogenic admixtures propagation from large regional Coal Power Plants towards the South Baikal using experimental data and model estimates. Results of observations show that the main mechanism for transfer of atmospheric admixtures towards the South Baikal is low-level atmospheric jets having high velocity and a weak turbulent mixing. A weak mixing of emitted plume with the surrounding air (within a jet flow) results in deficiency of oxidants in it and in slowing of chemical transformation of some admixtures during the transfer.     

基于小波分解的建筑物变形监测数据处理

基于小波分析理论，利用小波去噪技术，对一组建筑物变形监测数据进行了去噪处理。实际计算结果表明，小波去噪合理有效，能够敏感识别观测噪声和有用信息，不需要待检测信号的先验知识，特别适合于建筑物变形监测的数据处理。     

Assessment of Numerical Simulation Capabilities of the Fire Dynamics Simulator (FDS 6) for Planar Air Curtain Flows

Computational Fluid Dynamics (CFD) results are discussed for momentum driven planar jet flows, resembling configurations in use for air curtain in the context of smoke control in building fire. The CFD package Fire Dynamics Simulator (FDS) is used. Special focus is given to the impact of grid resolution, synthetic turbulent inflow boundary condition and sub-grid scale eddy viscosity models. The computational results are compared with summarized literature data. Investigation of different set-ups of inlet boundary conditions, including the inlet duct length, velocity profile and method of generation of turbulence at the level of the inflow, reveals that the inlet boundary condition is the most influential factor governing the flow downstream. The FDS results successfully reproduce the planar jet flows, both in terms of mean variables and second-order statistics. ‘Reference’ results have been obtained with a fully developed turbulent flow emerging from a long inlet duct. By reducing the inlet duct length and applying the Synthetic Eddy Method (SEM) at the inflow boundary condition, the ‘reference’ results have been reproduced with a reduction in the computing times of approximately 20%. However, care must be taken when choosing the parameters of SEM, in particular the number of eddies and their length scale. The impact of turbulent viscosity model is noticeable, but not of primary importance for the flow at hand, provided that a sufficiently fine computational mesh is used.