部分饱和岩石的黏弹性Chapman-Kelvin模型的正演模拟方法研究

在讨论地震波传播理论时,大部分情况下是把地震波看作弹性波.事实上,地下介质是非完全弹性介质,这使得传统的均匀、完全弹性介质理论受到了严重挑战.地震波在裂隙岩石中传播受裂隙系统和流体含量的影响较大,以往许多关于频变各向异性的理论大多局限于单相流体假设,但是几乎所有储层中通常被一种以上的流体部分饱和或完全饱和.本文在Chapman理论基础上,提出了基于部分饱和的黏弹性Chapman-Kelvin模型的正演模拟新方法,以提高对部分饱和岩石的黏弹性地震波频变各向异性的认识.该方法基于Chapman和Kelvin理论模型,计算了双相不混溶流体饱和裂隙岩石中的黏弹性波频变各向异性弹性系数的表达式,提出包含喷射流和斑块效应的统一地震波传播的黏弹性Chapman-Kelvin新模型.通过对新模型进行数值试验,讨论了无裂缝和存在裂缝两种情况下,含裂隙储层部分饱和岩石中耦合的喷射流和斑块效应对黏弹性介质频变各向异性的影响.试验结果反映出黏弹性介质下地震波频变各向异性的变化规律,验证了本文提出的新方法和新模型正确.本文将黏弹性介质各向异性与裂缝中的流体流动参数相联系,有利于提高对含裂缝储层部分饱和岩石的黏弹性波频变地震各向异性的认识,以及地震学与油藏工程的结合程度.

基于Kelvin模型的圆形巷道位移反分析的唯一性

讨论了黏弹性岩体中圆形巷道位移反分析的唯一性问题。以Kelvin模型为研究对象,在圆形巷道位移解析解的基础上,利用参数可辨识条件,对岩体参数进行了位移反分析唯一性的研究。结果表明:无论测量出多少个测点的位移,都不能唯一地反分析出所有岩体参数,至少要已知2个参数,才有可能唯一地反分析出其余参数;随着已知参数的增多,各参数的可辨识性逐渐增强;竖向地应力与水平地应力是否相等,且二者作为已知条件时其大小是否为0对反分析结果有较为明显的影响;适当减少待反演参数的数量对提升反分析结果的唯一性有显著影响;所有参数中地应力具有最好的可辨识性,其次是弹性模量,泊松比及黏滞系数最差。

A Realistic 3D Non-Stationary Channel Model for UAV-to-Vehicle Communications Incorporating Fuselage Posture

Considering the unmanned aerial vehicle(UAV) three-dimensional(3D) posture, a novel 3D non-stationary geometry-based stochastic model(GBSM) is proposed for multiple-input multipleoutput(MIMO) UAV-to-vehicle(U2V) channels. It consists of a line-of-sight(Lo S) and non-line-of-sight(NLo S) components. The factor of fuselage posture is considered by introducing a time-variant 3D posture matrix. Some important statistical properties, i.e.the temporal autocorrelation function(ACF) and spatial cross correlation function(CCF), are derived and investigated. Simulation results show that the fuselage posture has significant impact on the U2V channel characteristic and aggravate the non-stationarity. The agreements between analytical, simulated, and measured results verify the correctness of proposed model and derivations. Moreover, it is demonstrated that the proposed model is also compatible to the existing GBSM without considering fuselage posture.

基于黏弹性Chapman-Kelvin模型的裂缝储层频变AVAZ反演研究

为了在地震资料和裂缝储层特征之间建立联系,对裂缝储层采用了等效介质模型.而传统的等效介质模型未充分考虑非完全弹性介质理论和基于频变各向异性理论的双相或多相流体假设,也不能对实际裂缝储层中的地震波频散和衰减现象提供准确合理的解释,并且储层参数的反演研究对裂缝储层的定性预测和定量描述举足轻重.为此,本文首先根据所提出的黏弹性Chapman-Kelvin动态等效介质模型,该模型考虑了耦合的双相流体假设、黏弹性理论、喷射流以及斑块效应,并在此基础上分析了裂缝储层参数(主要为裂缝密度、裂缝长度、孔隙度和含水饱和度)对地震波频变特征的影响.然后基于黏弹性Chapman-Kelvin模型与Schoenberg和Protazio概括的Zoeppritz方程所计算出的频变反射系数,分析了反射PP波和PS波的频变AVAZ(Amplitude Versus Angle and Azimuth)特性和PP波频变反射系数与裂缝储层参数的关系.同时考虑到发生地震频散时,反射系数和频率产生关系,构建了在角度、方位和时间域内的新型正演方程.最后,基于PP波频变反射系数对裂缝密度、裂缝长度、孔隙度和含水饱和度的变化有较好的敏感性特点,进行了两种反演方法研究:其一是基于贝叶斯理论直接反演方法,其中以对数绝对范数作为似然函数和高斯分布,即L2范数度量作为先验约束;其二是基于频变反射系数的MCMC(Markov Chain Monte Carlo)随机反演方法.通过频变AVAZ进行裂缝储层参数的反演研究,结果表明,MCMC随机反演方法在缺失先验的储层参数信息时,反演结果的不确定性较强.当存在有效且足够的先验信息时,反演结果的可靠性进一步提升.而基于贝叶斯理论直接反演方法,则显示出基于频变AVAZ来区别大尺度裂缝和微尺度裂隙的潜力.

基于Kelvin-Voigt模型的黏弹性岩体位移反分析唯一性研究

在岩土工程中,准确的岩体力学参数对于工程项目的建设和维护极为重要,相较于现场原位试验、实验室试验、数值模拟等获取参数的方法,岩体位移反分析方法具有方便操作、条件获取难度小、数据准确等特点,而反分析结果的唯一性是位移反分析能否成功的重要影响因素。为了研究位移反分析结果的唯一性问题,使用Laplace变换和蠕变柔量的方法,推导了在圆形巷道中符合Kelvin-Voigt模型的黏弹性岩体的位移解析解,并运用参数可辨识准则研究了已知参数对黏弹性岩体反分析唯一性的影响。研究结果表明,无论在任何条件下,都无法唯一地反演出所有6个参数,在至少有2个参数是已知的情况下,才可能唯一地反演出其他参数;地应力分量被唯一反演的概率最大,弹性模量、泊松比和黏滞系数根据控制条件的不同,其可辨识性存在差异,并随着已知参数数量的增加,参数可辨识性也随之增强;地应力是否为0对反演结果的唯一性有着显著的影响。工程算例表明,研究结果对于岩体参数的反分析计算具有重要的指导意义。

Modeling Rainfall Intensity-Duration-Frequency (IDF) and Establishing Climate Change Existence in Uyo-Nigeria Using Non-Stationary Approach

This study aims at establishing if climate change exists in the Niger Delta environment using non-stationary rainfall Intensity-Duration-Frequency (IDF) modelling incorporating time-variant parameters. To compute the intensity levels, the open-access R-studio software was used based on the General Extreme Value (GEV) distribution function. Among the four linear parameter models adopted for integrating time as a covariate, the fourth linear model incorporating scale and location with the shape function constant produced the least corrected Akaike Information Criteria (AICc), varying between 306.191 to 101.497 for 15 and 1440 minutes, respectively, selected for calibration of the GEV distribution equation. The non-stationary intensities yielded higher values above those of stationary models, proving that the assumption of stationary IDF models underestimated extreme events. The difference of 13.71 mm/hr (22.71%) to 14.26 mm/hr (17.0%) intensities implies an underestimation of the peak flood from a stationary IDF curve. The statistical difference at a 95% confidence level between stationary and non-stationary models was significant, confirming evidence of climatic change influenced by time-variant parameters. Consequently, emphasis should be on applying shorter-duration storms for design purposes occurring with higher intensities to help reduce the flood risk and resultant infrastructural failures.

A modified Kelvin impact model for pounding simulation of base-isolated building with adjacent structures

Base isolation can effectively reduce the seismic forces on a superstructure, particularly in lowto medium-rise buildings. However, under strong near-fault ground motions, pounding may occur at the isolation level between the baseisolated building （BIB） and its surrounding retaining walls. To effectively investigate the behavior of the BIB pounding with adjacent structures, after assessing some commonly used impact models, a modified Kelvin impact model is proposed in this paper. Relevant parameters in the modified Kelvin model are theoretically derived and numerically verified through a simple pounding case. At the same time, inelasticity of the isolated superstructure is introduced in order to accurately evaluate the potential damage to the superstructure caused by the pounding of the BIB with adjacent structures. The reliability of the modified Kelvin impact model is validated through numerical comparisons with other impact models. However, the difference between the numerical results from the various impact analytical models is not significant. Many numerical simulations of BIBs are conducted to investigate the influence of various design parameters and conditions on the peak inter-story drifts and floor accelerations during pounding. It is shown that pounding can substantially increase floor accelerations, especially at the ground floor where impacts occur. Higher modes of vibration are excited during poundings, increasing the inter-story drifts instead of keeping a nearly rigid-body motion of the superstructure. Furthermore, higher ductility demands can be imposed on lower floors of the superstructure. Moreover, impact stiffness seems to play a significant role in the acceleration response at the isolation level and the inter-story drifts of lower floors of the superstructure. Finally, the numerical results show that excessive flexibility of the isolation system used to minimize the floor accelerations may cause the BIB to be more susceptible to pounding under a limited seismic gap.

A Novel 3D Non-Stationary UAV-MIMO Channel Model and Its Statistical Properties

The wireless communication systems based on Unmanned Aerial Vehicles(UAVs) have found a wide range of applications recently. In this paper, we propose a new three-dimensional(3 D) non-stationary multiple-input multiple-output(MIMO) channel model for the communication links between the UAV and mobile terminal(MT). The new model originates the traditional geometry-based stochastic models(GBSMs) but considers the non-stationary propagation environment due to the rapid movements of the UAV, MT, and clusters. Meanwhile, the upgrade time evolving algorithms of time-variant channel parameters, i.e., the path number based on birth-death processes of clusters, path delays, path powers, and angles of arrival and departure, are developed and optimized. In addition, the statistical properties of proposed GBSM including autocorrelation function(ACF), cross-correlation function(CCF), and Doppler power spectrum density(DPSD) are investigated and analyzed. Simulation results demonstrate that our proposed model provides a good agreement on the statistical properties with the corresponding derived theoretical ones, which indicates its usefulness for the performance evaluation and validation of the UAV based communication systems.

A Geometry-Based Non-Stationary MIMO Channel Model for Vehicular Communications

This paper derives a non-stationary multiple-input multiple-output(MIMO) from the one-ring scattering model. The proposed channel model characterizes vehicular radio propagation channels with considerations of moving base and mobile stations, which makes the angle of arrivals(AOAs) along with the angle of departures(AODs) time-variant. We introduce the methodology of including the time-variant impacts when characterizing non-stationary radio propagation channels through the geometrical channel modelling approach. We analyze the statistical properties of the proposed channel model including the local time-variant autocorrelation function(ACF) and the space cross-correlation functions(CCFs). We show that the model developed in this paper for non-stationary scenarios includes the existing one-ring wide-sense stationary channel model as its special case.

On-Line Structural Breaks Estimation for Non-stationary Time Series Models

Non-stationary time series could be divided into piecewise stationary stochastic signal. However, the number and locations of breakpoints, as well as the approximation function of the respective segment signal are unknown. To solve this problem, a novel on-line structural breaks estimation algorithm based on piecewise autoregressive processes is proposed. In order to find the "best" combination of the number, lengths, and orders of the piecewise autoregressive (AR) processes, the Akaikes Information Criterion (AIC) and Yule-Walker equations are applied to estimate an AR model fit to the data. Numerical results demonstrate that the proposed estimation algorithm is suitable for different data series. Furthermore, the algorithm is used in a clinical study of electroencephalogram (EEG) with satisfactory results, and the ability to deal with real-time data is the most outstanding characteristic of on-line structural breaks estimation algorithm proposed.

黏弹性地基中纳米输流管道振动与失稳分析

该文基于哈密顿原理与非局部弹性理论,针对黏弹性地基中轴向磁场作用固支输流碳纳米管的振动失稳特性,建立了该系统的振动微分方程,其中黏弹性地基分别采用了Kelvin-Voigt模型与Maxwell模型.应用微分求积法(DQM)离散了该微分方程,得到了求解系统振动特性的广义特征值问题.数值计算结果表明:外加轴向磁场在任何参数情况下均能提高系统振动频率,从而提升系统稳定性;小尺度效应弱化系统刚度,从而使系统更易产生发散失稳.尤为关键的是,黏弹性地基若采用不同的地基模型,其与纳米管间的作用效果存在差异:Kelvin-Voigt模型阻尼参数的增加降低系统稳定性,弹性系数的增加提升系统稳定性;Maxwell模型随两参数的增加,系统稳定性均表现出“先升后抑”的效果.

Kelvin尾迹SAR图像仿真研究

一方面利用Kelvin尾迹水动力模型,通过对同一吃水深度(4 m)、不同船速(1-20 m/s)下的Kelvin尾迹波的仿真,提出了此模型适用的船只航行速度范围以及随着船速的变化,Kelvin尾迹波的变化情况。另一方面结合海表面波模型和雷达后向散射模型,比较同一船速、不同吃水深度(4 m、6 m和8 m)下的SAR仿真图像。结果表明,仅考虑Kelvin尾迹水动力模型时,当吃水深度不变时,在一定速度范围内,随船速的增大,Kelvin尾迹的横波波长逐渐增大,尖波变的逐渐清晰,超出此范围,Kelvin尾迹水动力模型就不再适用了;当同时考虑3种模型时,不同吃水深度下,在一定速度范围内,同一船速所仿真出的Kelvin尾迹SAR图像是不一样的,随着吃水深度的加深,Kelvin尾迹横波也随之清晰,超出此速度范围则无法看到横波。

修正广义Kelvin蠕变固结模型研究

软土变形的时效特性产生于蠕变和固结,但蠕变和固结基于不同的变形机制,因此研究蠕变固结耦合效应的计算模型对于软土工程来说是十分重要的。采用将非线性本构模型和Biot固结模型耦合的方法,建立修正的广义Kelvin蠕变固结模型,即非线性弹-黏弹固结模型。首先,根据软土室内试验成果分析变形机制和应变随应力和时间的变化规律,找到函数拟合关系;其次,将Duncan模型和修正的广义Kelvin模型相结合,建立描述上述函数拟合关系的本构模型。然后,将该本构模型代入Biot固结理论的平衡方程,并提出基于非线性变形特征的蠕变固结模型。该模型共有14个模型参数,包括8个Duncan参数,均可通过常规三轴试验确定。通过一个实际工程的计算,完成该模型的计算流程,并对计算结果进行分析和比较,验证所建模型的有效性,探讨软土蠕变对地基中孔隙水压力及应力水平的影响,在此基础上获得一些有益的结论。研究结果表明,采用修正的广义Kelvin蠕变固结模型进行软土变形特性分析是一种合理的方法,具有模型机制明确、参数确定方法简单的优点。另外,虽然蠕变和固结都会给工程带来工后沉降问题,但两者对工程的影响是不一样的,改善地基的排水条件可弱化蠕变效应并提高工程的安全度。

地震时桥梁碰撞分析的等效Kelvin撞击模型

建立地震时桥梁碰撞分析的Kelvin撞击模型的参数确定方法。基于Hertz接触理论,考虑波动效应,按照最大撞击力与最大撞击变形的比值,确定Kelvin撞击模型的碰撞刚度;数值分析了影响Kelvin撞击模型阻尼系数取值的邻梁碰撞恢复系数。结果表明:Kelvin撞击模型的碰撞刚度随Hertz接触刚度、撞击速度以及短梁与长梁的长度比的增大而增大;波动效应对碰撞刚度的影响明显,不能忽略;邻梁碰撞恢复系数随撞击速度增大而减小,随短梁与长梁的长度比减小而减小;确定Kelvin撞击模型的碰撞刚度和邻梁碰撞恢复系数对城市桥梁的合理取值范围应分别为3×105kN/m―6×105kN/m和0.7―0.95。

不同条件下海面Kelvin尾迹的红外仿真

针对舰船在海上航行时形成的尾迹包含多类信息,利用Kelvin尾迹模型得到舰船速度为5m/s、10m/s和15m/s的尾迹最大波高分别为0.5m、1.5m和2.5m。通过海浪PiersonMoscowitz谱模型描述海面风速分别为5m/s、8m/s和10m/s下的复杂海面背景模型。基于海面背景和尾迹区域的几何差异,建立了Kelvin尾迹的红外发射模型,得到不同海面风速、不同舰船航速和不同探测天顶角下的红外特征仿真图像。仿真结果表明,红外图像的灰度极大值位于尾迹波峰处,海面风速由5m/s增加到10m/s时,尾迹区域与海面背景平均灰度差值由100逐渐减小,直至无差异。相同海面风速下,舰船速度由5m/s增加到15m/s,尾迹波长由10m增加到40m,红外特征明显。改变探测器天顶角分别为0°和30°时,红外图像差异较小,当天顶角增大到60°时,尾迹区域灰度值接近海面背景,差值小于30,尾迹的辨识难度增加。

Kelvin模型阻尼层合板的振动分析

基于Kelvin模型粘弹性材料本构关系导出了阻尼层合板的动力学微分方程组,给出了四边简支阻尼层合板的固有频率和损耗因子的解析解。与文献结果比较表明,将Kelvin模型应用于粘弹结构的动力特性问题求解,计算模型简便,且计算结果比常复数模型更为精确。分析了阻尼层参数变化对结构振动特性的影响。分析结果表明:增加阻尼层厚度,可以有效提高结构损耗因子;增加阻尼层材料的剪切模量,结构损耗因子增大到一定值后又逐渐减小,在减振设计中阻尼层的模量存在最佳值。

The non-stationary and spatially varying associations between hand,foot and mouth disease and multiple environmental factors:A Bayesian spatiotemporal mapping model study

The transmission and prevalence of Hand,Foot and Mouth Disease(HFMD)are affected by a variety of natural and socio-economic environmental factors.This study aims to quantitatively investigate the non-stationary and spatially varying associations between various environmental factors and HFMD risk.We collected HFMD surveillance cases and a series of relevant environmental data from 2013 to 2021 in Xi'an,Northwest China.By controlling the spatial and temporal mixture effects of HFMD,we constructed a Bayesian spatiotemporal mapping model and characterized the impacts of different driving factors into global linear,non-stationary and spatially varying effects.The results showed that the impact of meteorological conditions on HFMD risk varies in both type and magnitude above certain thresholds(temperature:30°C,precipitation:70 mm,solar radiation:13000 kJ/m^(2),pressure:945 hPa,humidity:69%).Air pollutants(PM_(2.5),PM_(10),NO_(2))showed an inverted U-shaped relationship with the risk of HFMD,while other air pollutants(O_(3),SO_(2))showed nonlinear fluctuations.Moreover,the driving effect of increasing temperature on HFMD was significant in the 3-year period,while the inhibitory effect of increasing precipitation appeared evident in the 5-year period.In addition,the proportion of urban/suburban/rural area had a strong influence on HFMD,indicating that the incidence of HFMD firstly increased and then decreased during the rapid urbanization process.The influence of population density on HFMD was not only limited by spatial location,but also varied between high and low intervals.Higher road density inhibited the risk of HFMD,but higher night light index promoted the occurrence of HFMD.Our findings further demonstrated that both ecological and socioeconomic environmental factors can pose multiple driving effects on increasing the spatiotemporal risk of HFMD,which is of great significance for effectively responding to the changes in HFMD epidemic outbreaks.

亚临界区圆柱绕流相干结构壁面模化混合RANS/LES模型

本文发展了一种具有壁面模化大涡模拟能力的雷诺平均纳维-斯托克斯(RANS)和大涡模拟(LES)方法的混合模型(简称WM-HRL模型),致力于对亚临界区雷诺数钝体绕流相干结构这类复杂流动现象进行高置信度的CFD解析模拟研究.该方法通过一个仅与当地网格空间分布尺寸有关的湍动能解析度指标参数rk即可实现从RANS到LES的无缝快速转换,并且RANS/LES混合转换区的边界位置及其各个分区(包括RANS区、LES区及RANS/LES混合转换区)对湍动能的解析能力均可通过两个指标参数nrk1-Q和nrk2-Q准则进行预先设定.通过对雷诺数Re=3900下圆柱绕流场的系列数值模拟研究,获得了能够高置信度解析并捕捉其绕流场中三维时空瞬态发展相干结构特性的湍动能解析度指标参数nrk1-Q和nrk2-Q准则的组合条件.研究表明,该WM-HRL模型不仅能够准确获取圆柱绕流场中剪切层小尺度K-H不稳定性结构的精细谱结构,而且在同一套网格系统下通过变化湍动能解析度指标参数nrk2-Q和nrk1-Q准则的组合条件,还可以精细解析圆柱绕流场中两类不同回流区的长度结构特征,及其对应的圆柱尾部近壁面处V和U形两个平均流向速度剖面的分支结构特性.

机织建筑膜材料的广义Kelvin-Voigt蠕变模型

对平纹组织为基布的PVC涂层膜材料进行了经纬两方向各自5种不同应力条件下的单轴向拉伸蠕变试验,给出了广义Kelvin-Voigt模型的具体蠕变方程,并利用广义三参数、五参数与七参数Kelvin-Voigt的蠕变方程对实验数据进行了分析.结果显示:随着模型参数个数的增加,预测效果也呈现出递增趋势;当模型参数为7个时,各应力拟合的确定系数均在99.9%以上,表明七参数广义Kelvin-Voigt蠕变模型已可以很好地预测膜材料的蠕变特性.

确定Kelvin模型粘弹性材料参数的一种实验方法

从Kelvin模型和Euler-bernoulli梁理论出发,利用弹性-粘弹性相应原理和弹性材料动力学理论得出粘弹性梁的动力学方程。利用中心差分法分析了粘弹性材料悬臂梁的动态函数。根据振动特性,将材料参数与梁应变响应周期及振幅衰减相联系;利用这种联系,提出了测定Kelvin模型粘弹性材料参数的一种简单实验方法;分别取不同长度的橡胶悬臂梁进行了相应的实验,测出了材料的剪切模量和粘性系数;分析实验结果,得出一些有益的结论。