Structural identification with incomplete output‐only data and independence of measured information for shear‐type systems

Structural identification is the inverse problem of estimating physical parameters of a structural system from its vibration response measurements. Incomplete instrumentation and ambient vibration testing generally result in incomplete and arbitrarily normalized measured modal information, often leading to an ill‐conditioned inverse problem and non‐unique identification results. The identifiability of any parameter set of interest depends on the amount of independent available information. In this paper, we consider the identifiability of the mass and stiffness parameters of shear‐type systems in output‐only situations with incomplete instrumentation. A mode shape expansion‐cum‐mass normalization approach is presented to obtain the complete mass normalized mode shape matrix, starting from the incomplete non‐normalized modes identified using any operational modal analysis technique. An analysis is presented to determine the minimum independent information carried by any given sensor set‐up. This is used to determine the minimum necessary number and location of sensors from the point of view of minimum necessary information for identification. The different theoretical discussions are illustrated using numerical simulations and shake table experiments. It is shown that the proposed identification algorithm is able to obtain reliably accurate physical parameter estimates under the constraints of minimal instrumentation, minimal a priori information, and unmeasured input. The sensor placement rules can be used in experiment design to determine the necessary number and location of sensors on the monitored system. John Wiley & Sons, Ltd.     

Modelling exhaust plume mixing in the near field of an aircraft

A simplified approach has been applied to analyse the mixing and entrainment processes of the engine exhaust through their interaction with the vortex wake of an aircraft. Our investigation is focused on the near field, extending from the exit nozzle until about 30 s after the wake is generated, in the vortex phase. This study was performed by using an integral model and a numerical simulation for two large civil aircraft: a two-engine Airbus 330 and a four-engine Boeing 747. The influence of the wing-tip vortices on the dilution ratio (defined as a tracer concentration) shown. The mixing process is also affected by the buoyancy effect, but only after the jet regime, when the trapping in the vortex core has occurred. In the early wake, the engine jet location (i.e. inboard or outboard engine jet) has an important influence on the mixing rate. The plume streamlines inside the vortices are subject to distortion and stretching, and the role of the descent of the vortices on the maximum tracer concentration is discussed. Qualitative comparison with contrail photograph shows similar features. Finally, tracer concentration of inboard engine centreline of B-747 are compared with other theoretical analyses and measured data.     

Characteristics of inertia-gravity waves around jet stream from radiosonde observations in Wuhan (30.5°N, 114.4°E)

The 5 years’ radiosonde data obtained from January 2000 to December 2004 in Wuhan (30.5°N, 114.4°E) have been used for studying the behaviors of inertia-gravity waves in the vicinity of the jet stream. It is observed that the wave intensity has a similar seasonal variation with the jet stream intensity with a strong winter maximum and a summer minimum. Moreover, a similar inter-annual trend for both the wave intensity and jet stream intensity is also found. These results suggest that the jet stream may be the predominant source of the inertia-gravity waves in the troposphere and lower stratosphere over Wuhan in the period of the 5 years. It is noticed from 28 radiosonde profiles during wintertime that the energy of inertia-gravity waves exhibits upward and downward propagation respectively above and below the jet stream. This indicates that the source of the inertia-gravity waves is within the jet stream. In these cases, the twin waves below and above the jet stream usually hold similar amplitudes. The horizontal propagation of the twin waves also shows some interesting relationship.     

区域一维速度模型对地震定位的定量影响

利用2017年8月1日至2017年12月31日四川地震台网和甘肃地震台网记录到的发生在青藏高原东缘的731个地震事件的9 284条Pg震相到时数据,首先反演了该地区的“最小一维速度模型”,并将该模型和选取的速度模型建立对比模型,以九寨沟地震序列为研究目标,定量讨论了两种速度模型分别在绝对定位和相对定位方法中对定位结果的影响。所得定位结果表明:反演获得的“最小一维速度模型”在重定位中可以有效地减小地震走时均方根残差;绝对定位比相对定位更加依赖于一维速度模型,一维速度模型会直接影响绝对定位结果中的震源分布形态,但在相对定位结果中仅起到调整地震事件相对位置的作用;在地震绝对定位中,震级越大的地震对于速度模型越敏感,而这一特点在相对定位中表现得并不明显。通过本项研究可知,在地震定位研究中,联合采用绝对定位和相对定位方法是最佳策略。      

北半球电离层中纬槽发生率及其槽极小位置变化的统计研究

本文利用2014年9月到2017年8月全球高时空分辨率TEC数据对北半球四个经度带电离层中纬槽的发生率和槽极小位置的变化进行了统计研究.基于Kp指数,我们引入了一个包含地磁活动变化历史效应的地磁指数(Kp 9)来分析中纬槽位置变化与地磁活动水平的关系.通过与其他地磁活动指数的对比,发现槽极小纬度与Kp 9指数的相关性最好.此外,本文重点分析了中纬槽发生率及槽极小纬度的经度差异、季节变化、地方时变化以及与地磁活动强度等的关系.结果表明,中纬槽的发生率与经度关系不大,主要受到季节、地方时与地磁活动的影响.午夜中纬槽发生率在夏季较低,其随地方时的变化则呈现出负偏态分布的特点,在后半夜发生率更高,而地磁活动增强对中纬槽的发生具有明显的促进作用.对于槽极小纬度,其在四个经度带的分布差异不大,但月变化各不相同,其中-120°经度带呈单峰分布,在夏季槽极小纬度更高,而0°经度带夏季槽极小纬度更低.槽极小的位置显著依赖于地磁活动、地方时以及季节变化.一般说来,地磁活动越强,中纬槽纬度越低.中纬槽位置随地方时的变化有明显的季节差异,冬季昏侧槽极小纬度随地方时变化较快,弱地磁活动条件下22∶00 LT前即达到最低纬度,其后位置几乎保持不变,而两分季槽极小纬度从昏侧至午夜都在降低,夏季槽极小纬度从昏侧连续下降至03∶00 LT左右.     

Variations in turbulent kinetic energy at a point source submarine groundwater discharge in a reef lagoon

The influence of sea level variations due to tides and wave setup on turbulent kinetic energy (TKE) was observed at a point source submarine groundwater discharge in a fringing coral reef lagoon. Tidal and wave setup variations modulated speed, TKE, TKE dissipation, and water temperature and salinity at the buoyant jet. The primary driver of jet TKE and speed variations was tides, while wave setup was a minor contributor. An inverse relationship between surface elevation and TKE was explained with an exponential equation based on sea level variations. During low tides, peak jet speeds (up to 0.3 m s^?1) and TKE per unit mass (up to 0.4 m² s^?2) were observed. As high tide approached, the jet produced minimum TKE of ~0.003 m² s^?2 and TKE dissipation ranged from 2 to 8×10^?4 m² s^?3. This demonstrated the sensitivity of the jet discharge to tides despite the small tidal range (<20 cm). Jet temperatures and salinities displayed semidiurnal oscillations with minimum salinity and temperature values during maximum discharge. Jet salinities increased throughout low tides while temperatures decreased. This pattern suggested the jet conduit was connected to a stratified cavity within the aquifer containing cool fresh water over cool salty water. As low tides progressed, jet outflow increased in salinity because of the mixing within the conduit, while lower jet temperatures suggested water coming from further or deeper in the aquifer. The presence of such a cavity has been recently confirmed by divers.     

实验室声发射三维定位软件

简要介绍了地震动力学实验室新一代声发射定位软件的主要功能、定位算法的实现方案以及定位效果的评测结果。新软件配有多种通用数据库接口,具有与设备无关的处理能力。它除了可以进行三维定位之外,还可以进行面定位与线定位。定位时,可以给定样品的波速,也可以把波速当未知数求解。定位算法的主要改进有两个方面:1)以稳健的走时绝对偏差最小作为定位目标函数;2)方程组求解优化过程采用了模拟退火法直接搜索目标函数的全局最小值。新软件显著提高了定位的精度,同时提供了一个方便快捷的数据处理平台     

基于COSMIC卫星观测数据的平流层重力波的全球分布特征研究

本文利用2007年1月至2012年12月的COSMIC卫星温度剖线,从中提取了垂直波长在3~10 km的重力波扰动信息,进而分析了全球平流层大气重力波的分布特征.赤道地区低平流层重力波表现出明显的准两年变化,这种变化与风场的准两年变化具有明显的相关性,向下发展速度约为1 km/月;赤道地区高平流层(35 km以上区域)的重力波活动则存在明显的半年变化.中高纬度重力波活动主要表现为冬季强夏季弱.在南极地区存在着与急流的时间、空间以及强度变化密切相关的重力波分布特征,这说明在南极极夜急流是非常重要的一个重力波源;而在北极极夜急流的作用则没有那么强.此外,通过考察不同高度的重力波活动特征,我们发现:30 km以下重力波活动较强区域主要在赤道地区且与强对流区分布基本吻合,地形诱发的以及与天气系统相关的强重力波活动在该高度范围内同样出现;而在30 km以上的区域重力波活动强度分布则会出现与平流层爆发性增温以及极夜急流有关的变化.     

煤矿矿震定位中异向波速模型的构建与求解

针对煤矿上覆岩层层状赋存和离层带的特点,构建矿井尺度的微震监测系统异向波速模型,模型中波速向量由地面探头速度与井下探头速度组成.研究了在只有强矿震信号和混有爆破信号两种条件下,以到时残差最小为目标和震源定位误差最小为目标的两种求解模型,模型求解选用具有全局寻优特性的遗传算法与CMEAS算法结合的混合算法.现场实际应用得出,只使用爆破信号的到时残差法最优,混有强矿震信号的到时残差法其次;与爆破信号定位所用的统一简化波速模型相比,震源定位误差大幅度降低.在此基础上进一步减低定位误差,还需从微震台网的优化布设方面解决.     

三维复杂速度模型中地震事件震源轨迹的计算

地球内部三维速度图像的广泛建立为进行高精度的地震定位提供了良好条件.使用震源轨迹确定震源位置不仅稳健而且直观,但三维复杂速度模型中的震源轨迹难以给出解析解.为此,本文提出了一种较准确地计算三维复杂速度模型中震源轨迹的数值方法.根据震源轨迹在残差场中的特点：（1）震源轨迹位于残差正负极性彼此不同的邻点之间;（2）绝对梯度在震源轨迹的法线方向最大;（3）在法线方向上越靠近震源轨迹残差绝对值越小,对于每个模型节点分别和残差正负极性与其不同的邻点组成的点对,将其中绝对梯度最大的点对作为震源轨迹法线点对,选取法线点对中残差绝对值较小的点（即震源轨迹所在模型单元的节点）作为震源轨迹代表点;在绝对残差场中数值较小的连通区域（可能有多个）内,利用最小走时树算法依次计算出每个连通区域中地震波从绝对残差最小点至同一连通区域内震源轨迹代表点的射线路径作为震源轨迹.算例表明：本文方法适用于三维复杂速度模型,对震源轨迹的稳定性及构成段数没有限制,计算的震源轨迹精细且较完整、可用于高精度的地震定位.     

Optimal location of sensors for reconstruction of seismic responses through spline function interpolation

In this paper, a criterion is proposed for the choice of the optimal location of a limited number of recording sensors for reconstruction of seismic responses of multistorey frames. Reconstruction of unknown responses is performed by modelling through a spline shape function the evolution of the relative acceleration along the height of the building. The coefficients of the spline function are determined by interpolating available responses in terms of absolute accelerations taking into account boundary conditions. The optimal location for a given number of recording sensors is defined as the one corresponding to the minimum value of the global error evaluated on the whole set of reconstructed responses. The criterion has been verified by using simulated responses of numerical models of multistorey frames and responses recorded on real buildings during the Northridge earthquake. A physical characterization of the location of available sensors leading to the minimum global error between calculated and real responses is proposed in terms of effective modal participation factors relevant to the principal modes of the structure. This characterization allows for frames of known modal characteristics the definition of a criterion for the choice of the location of a limited number of sensors for monitoring or vibration control purposes. Copyright © 2003 John Wiley & Sons, Ltd.     

Numerical Green’s function method:Application to quantifying ground motion variations of M7 earthquakes

The concept of "numerical Green’s functions" (NGF or Green’s function database) is developed. The basic idea is: a large seismic fault is divided into subfaults of appropriate size, for which synthetic Green’s functions at the surface (NGF) are calculated and stored. Consequently, ground motions from arbitrary kinematic sources can be simulated, rapidly, for the whole fault or parts of it by superposition. The target fault is a simplified, vertical model of the Newport-Inglewood fault in the Los Angeles basin. This approach and its functionality are illustrated by investigating the variations of ground motions (e.g. peak ground velocity and synthetic seismograms) due to the source complexity. The source complexities are considered with two respects: hypocenter location and slip history. The results show a complex behavior, with dependence of absolute peak ground velocity and their variation on source process directionality, hypocenter location, local structure, and static slip asperity location. We concluded that combining effect due to 3-D structure and finite-source is necessary to quan- tify ground motion characteristics and their variations. Our results will facilitate the earthquake hazard assessment projects.     

吉林前郭地区一维P波速度模型研究

基于2013年10月31日至2014年8月25日吉林前郭地区发生的290次地震事件的P波到时数据,利用Kissling提出的VELEST程序,获取了前郭地区最小一维P波速度模型。并将新模型应用到前郭地震重定位和PTD定震源深度中,二者综合显示获取的最小一维P波速度模型对于提高地震定位的精度有一定的现实意义。     

Electrical resistivity and seismic refraction tomography to detect buried cavities

Near‐surface cavities can pose serious hazards to human safety, especially in highly urbanized town centres. The location of subsurface voids, the estimation of their size and the evaluation of the overburden thickness are necessary to assess the risk of collapse. In this study, electrical resistivity tomography (ERT) and seismic refraction tomography data are integrated in a joint interpretation process for cavity location in the city of Rome. ERT is a well established and widely employed method for cavity detection. However, additional information provided by seismic refraction tomography is capable of eliminating some potential pitfalls in resistivity data interpretation. We propose that the structure of the cavities defined by ERT can be used as a base to optimize seismic refraction tomography investigations within the framework of a joint interpretation process. Data integration and the insertion of a priori information are key issues for reducing the uncertainties associated with the inversion process and for optimizing both acquisition procedures and computation time. Herein, the two geophysical methods are tested on both synthetic and real data and the integration of the results is found to be successful in detecting isolated cavities and in assessing their geometrical characteristics. The cavity location inferred by geophysical non‐invasive methods has been subsequently confirmed by direct inspection.     

Jet flow over foredunes

Jet flows, which are localized flows exhibiting a high speed maxima, are relatively common in nature, and in many devices. They have only been occasionally observed on dunes, and their dynamics are poorly known. This paper examines computational fluid dynamic (CFD) two‐dimensional (2D) modelling of jet flow over a foredune topography. Flow was simulated in 10° increments from onshore (0°) to highly oblique alongshore (70°) incident wind approach angles. CFD modelling reveals that the formation of a jet is not dependent on a critical wind speed, and an increase in incident wind velocity does not affect the magnitude of jet flow. A jet is first formed at ~1.0 m seawards of the foredune crest on the Prince Edward Island foredune morphology example examined here. A jet is not developed when the incident wind is from an oblique approach angle greater than ~50° because there is significantly less flow acceleration across a much lower slope at this incident angle. The presence of a scarp does influence the structure of the crest jet, in that the jet is more pronounced where a scarp is present. Surface roughness affects the magnitude of jet expansion and jets are better developed on bare surfaces compared to vegetated ones. Copyright © 2016 John Wiley & Sons, Ltd.     

模拟退火算法在地震定位中的应用

阐述了模拟退火算法及其在地震定位中的原理,以震相观测到时与理论到时之差(到时残差)为目标函数,利用模拟退火算法求解目标函数的最小值以获得地震的震源参数。在模拟退火算法的地震定位中根据震相的可靠程度对不同震相设置了相应的定位权重;对震相的走时误差设置了3组不同的值,以分析走时误差对定位结果的影响,对随机生成的1000个模拟地震进行模拟退火算法再定位。结果表明,走时误差越小,定位结果越准确。最后,通过地震震例进一步验证了模拟退火算法在地震定位中的可用性。     

Evaluation of infrared emission spectra of aircraft exhaust with the FitFas software

A Fourier transform spectrometer was used to measure infrared spectra of the exhaust gas of an aircraft’s jet engine. The measured spectra were modelled by use of the program FASCODE. For this simulation, the inhomogeneous gas mixture is divided into several homogenous layers which are characterized by their geometrical extents, temperatures, pressures and chemical compositions. To obtain values for the temperatures and the CO, NO, H₂O and CO₂ concentrations of the layers a nonlinear least-squares algorithm was implemented. The program (FITFAS) not only changes the parameters to find the minimum of the squared differences between measurement and calculation; it also provides the variances and covariances of the parameters. Thus information is obtained to which parameters (besides the interesting ones) must be fitted (or be accurately known). It also tells us whether or not another spectral interval is more suitable for the determination of a specific parameter.     

与东北冷涡相伴的高空急流诱发平流层重力波的数值模拟研究

使用中尺度数值模式WRF-ARW,针对2010年6月发生在中国东北地区一例伴随对流层高空西风急流（位于~9 km高度）演变过程出现的平流层重力波活动特征开展了数值模拟. 事件发生期间,对流层区域环流处在一个东北冷涡系统的控制之下. 模拟结果再现了该东北冷涡的发展和维持过程,以及与之相伴的高空急流的特征. 模拟结果揭示出在急流区域上空的平流层中存在显著重力波活动现象. 分析结果显示,重力波活动与急流存在紧密联系,在水平方向上,重力波呈显著的二维结构,出现在急流出口区上部并逆背景流向西传播. 功率谱分析结果表明盛行波动具有~700 km水平尺度、9~12 h时间尺度以及4~5 km垂直波长. 由于急流的存在,造成其与平流层中下部之间存在显著的水平风速垂直切变,与切变相伴的耗散使得上传的重力波动量通量数值随着高度升高而递减. 同时,在18~20 km高度间出现的西风-东风转换带极大地抑制了波动在垂直方向的传播,形成显著动量通量沉积效应. 估算结果表明,在11~20 km高度之间,这种效应的整体作用相当于对该层背景流施加强度为0.86 m·s^-1·day^-1的动力阻曳.     

Erodibility of cohesive streambeds in the loess area of the midwestern USA

Excess stress parameters, critical shear stress (τ_c) and erodibility coefficient (k_d), for degrading channels in the loess areas of the midwestern USA are presented based on in situ jet‐testing measurements. Critical shear stress and k_d are used to define the erosion resistance of the streambed. The jet‐testing apparatus applies hydraulic stresses to the bed and the resulting scour due to the impinging jet is related to the excess stress parameters. Streams tested were primarily silt‐bedded in texture with low densities, which is typical of loess soils. Results indicate that there is a wide variation in the erosion resistance of streambeds, spanning six orders of magnitude for τ_c and four orders of magnitude for k_d. Erosion resistance was observed to vary within a streambed, from streambed to streambed, and from region to region. An example of the diversity of materials within a river system is the Yalobusha River Basin in Mississippi. The median value of τ_c for the two primary bed materials, Naheola and Porters Creek Clay Formations, was 1·31 and 256 Pa, respectively. Streambeds composed of the Naheola Formation are readily eroded over the entire range of shear stresses, whereas only the deepest flows generate boundary stresses great enough to erode streambeds composed of the Porters Creek Clay Formation. Therefore, assessing material resistance and location is essential in classifying and modelling streambed erosion processes of these streams.     

东亚地区夏季7～8月大气环流季节演变异常的内部动力学过程

利用NCEP/NCAR 1958～1999年月平均再分析数据集,本文分析了7～8月东亚大气环流季节演变异常,包括提前和延迟这两种情形,及其与沿副热带西风急流的内部动力学过程的联系,比较了与气候季节演变进程的差异.结果表明,存在着两个地理位置固定的遥相关型,探讨了二者位相与副热带西风急流的关系,讨论了此类波列的出现与内部动力学过程的可能联系.