Overview of the seismic input at dam sites in China

The current Chinese national standard, the Standard for Seismic Design of Hydraulic Structures (GB51247), released in 2018, is strictly based on China’s national conditions and dam engineering features. A comprehensive and systematic overview of the basis of the seismic fortification requirements, the framework of the fortification criteria, and the mechanisms of seismic input related to the seismic design of dams are presented herein. We first analyzed and clarified several conceptual aspects in traditional seismic design of dams. Then, for the seismic input at the dam site described in the first national standard for hydraulic structures, we expounded innovative concepts, ideas, and methods to make relevant provisions more realistic and practical and discussed whether reservoir earthquakes must be included in the seismic fortification framework of dams. This study seeks to incorporate seismic input at the dam site into traditional seismic design practice to promote its improvement from the quasi-static method to the dynamic method and from the closed vibration system to an open wave propagation system, to ensure that the seismic design of dams becomes more reasonable, reliable, scientific, and economic.     

伪随机源时间域航空电磁响应系统辨识

在时间域航空电磁系统中,采用高频伪随机序列作为激励源是实现浅层高分辨地质探测的有效途径。为了提高大地脉冲响应的辨识精度,在基本相关辨识方法的基础上,本文提出了一种基于时间域循环相关计算的高精度辨识方法,通过消除旁瓣效应影响和在时间域下求解维纳-霍夫方程获取大地脉冲响应。数值模拟仿真结果表明,辨识大地响应相对误差均方根为2.2×10^-7,早期大地响应辨识误差小于10^-10。该高精度辨识方法具有较好的噪声压制能力,在观测时间较短时,单周期辨识提升的信噪比为8.7 dB,优于同等条件下线性叠加降噪方法提升的信噪比6.1 dB。     

Increases in fire risk due to warmer summer temperatures and wildland urban interface changes do not necessarily lead to more fires

Forest fire frequency in Mediterranean countries is expected to increase with land cover and climate changes as temperatures rise and rainfall patterns are altered. Although the cause of many Mediterranean fires remains poorly defined, most fires are of anthropogenic origin and are located in the wildland urban interface (WUI), so fire ignition risk depends on both weather and land cover characteristics. The objectives of this study were to quantify the overall trends in forest fire risk in the WUI of the Alpes-Maritimes department in SE France over a period of almost 50 years (about 1960–2009) and relate these to changes in land cover and temperature changes. Land cover for two contrasting reference catchments (236 km² and 289 km², respectively) was mapped from available aerial photographs. Changes in fire risk over time were estimated using statistical relationships defined for each type of WUI, where isolated and scattered housing present a greater risk than dense and very dense housing. Summer monthly temperatures and spring and summer precipitation were quantified over the same temporal period as land cover. Finally, trends in fire frequency and burned area were analyzed over a shorter 37 year period (1973–2009) due to the lack of available fire data before 1973. Fire risk associated with WUI expansion increased by about 18%–80% over the 1960–2009 period (depending on the catchment). Similarly, mean summer minimum and maximum monthly temperatures increased by 1.8 °C and 1.4 °C, respectively, over the same period. Summer rainfall appears to decrease over time since about the 1970's but remains highly variable. Land cover and weather changes both suggest an overall increase in fire risk. However, the number of fires and burned area have decreased significantly since about 1990. This paradoxical result is due to a change in fire-fighting strategy which reinforced the systematic extinction of fires in their early stages. Technical support in the form of improved radio communication and helicopters contributed greatly to reducing fire frequency and burned area. Surveillance and legal reforms included the introduction of field patrols and restricted access to forests during high risk periods. Although this has proven highly successful in the short term, the risk of fuel load accumulation over time remains a risk which might contribute to the development of mega-fires in extreme climatic conditions in the future.     

3D coupled scaled boundary finite-element/finite-element analysis of ground vibrations induced by underground train movement

When trains move through the tunnels, vibrations are generated and transmitted to the surrounding ground causing vibrations and noises in adjacent structures. The vibrations and noises can sometimes reach such a high level that can hardly be tolerated by the neighboring residents especially in densely populated urban environments. Consequently, the problem of train-induced vibrations has received increasing attention from both engineers and researchers. This paper presents the application of the scaled boundary finite-element method (SBFEM) to the 3D analysis of ground vibrations induced by underground trains. The media around the tunnel is simulated by the SBFEM. The train track is modeled using the finite-element method. 3D examples in both frequency and time domains are addressed to validate the accuracy and applicability of the method.     

细长输流管内外流耦合振动特性研究

管内流动会影响输流管的振动响应,目前关于输流弹性管涡激振动方面的研究较少。基于计算流体力学(CFD)方法,开展内外流对细长输流弹性管振动特性影响的研究。首先在不考虑内流的情况下将弹性管涡激振动数值预报结果与模型试验数据进行对比,验证了数值方法的可靠性。再者考虑内外流耦合作用情况下,对不同内流流速下细长输流弹性管振动位移时—空分布、顺流向最大平均偏移、振动轨迹、内部横向涡的形成与分布等进行了对比分析。结果发现,与外流流速相比,内流流速的增加虽然难以改变弹性管的主振模态,但对沿管体的振动强度影响显著。顺流向最大偏移处管体运动轨迹发生明显的变形和跳跃。在剪切外流和均匀内流对弹性管的联合作用下,沿管跨方向模态间能量转换频繁,伴随着间歇性出现或消失的沿弹性管传播的行波组分,这主要归因于复杂的双重流固耦合系统(外流—管体,内流—管体)。在内流以附加质量力、离心力和科氏力形式的激励下,弹性管内二次流现象明显。在振动过程中,内部横向涡沿管壁生成、脱落并逐渐散布于整个横截面。     

山东冰雹形成机制及雹云催化技术模拟——个例研究

利用中国科学院大气物理研究所开发的三维全弹性冰雹云模式,对2006年7月5日山东境内一次以冰雹、雷雨大风为主的强对流过程进行模拟,分析了冰雹的形成机制。结果表明:79%左右的冰雹胚胎是霰,雹胚以霰为主,霰主要来自于冰雪晶与过冷雨水碰撞冻结以及雪的自动转换过程,霰形成后主要靠碰并过冷云水、雨水增长,而冰雹主要质量来源是霰的自动转化以及碰并过冷云水增长。人工催化试验表明:在强对流云中冰雹含量达到0.1 g·m^-3前1～4 min进行催化,能有效抑制冰雹粒子的增长,在过冷水含量中心(5.5 km)上方1.0～1.5 km催化防雹效果较好,在其下方催化防雹效果较差;剂量越大防雹效果越好。AgI主要以凝华核的作用产生人工冰晶,冰晶凝华增长导致过冷云水含量降低。催化后雹胚特别是霰胚的数量增多,对过冷云水的竞争增强,其平均尺度、质量均减小,降低了雹胚向冰雹的转化率。冰雹碰并过冷云水增长也被减弱,导致冰雹总质量进一步减少。     

深海顶张紧式立管涡激振动疲劳可靠性研究

以深海顶端张紧式立管为研究对象,基于圆柱体受迫振荡实验数据和能量平衡原理预报立管涡激振动响应及其诱发的疲劳损伤度,各主要参数随机化,应用响应面法建立了涡激振动疲劳安全系数与极限状态方程关系,研究了疲劳安全系数对结构可靠性的影响,并分析了各随机变量的灵敏度。研究结果表明:1)随着疲劳安全系数的增大,结构失效概率逐渐减小,且失效概率的减小幅度随疲劳安全系数的增加而趋缓。疲劳安全系数SF无需超过20。2)增加疲劳安全系数SF可降低疲劳载荷不确定量B、S-N曲线参数A、顶张力、流速、外径和壁厚等立管及环境随机变量对结构涡激振动疲劳损伤可靠性的影响。3)立管各参数中,流速、外径和壁厚均值越大,立管结构安全度越低,顶张力对结构影响相反;各参数的标准差越大,结构可靠度越低。均值灵敏度由大到小依次为顶张力、流速、外径和壁厚,标准差灵敏度由大到小依次为流速、张力、外径和壁厚。4)壁厚对深海顶张紧式立管涡激振动所致疲劳损伤影响很小,可忽略其影响。研究成果可为深海顶张紧式立管涡激振动疲劳安全系数的选取提供参考。     

Non-stationary response of structures excited by random seismic processes with time variable frequency content

Seismic random processes are characterized by high non-stationarity and, in most cases, by a marked variability of frequency content. The hypothesis modeling seismic signal as a simple product of a stationary signal and a deterministic modulation function, consequently, is hardly ever applicable. Several mathematical models aimed at expressing the recorded process by means of a system of stationary random processes and deterministic amplitude and frequency modulations are proposed. Models oriented into the frequency domain with subsequent response analysis based on integral spectral resolution and models oriented into the time domain based on the multicomponent resolution are investigated. The resolution into individual components (non-stationary signals) is carried out by three methods. The resolution into intrinsic mode functions seems to possess the best characteristics and yields results almost not differing from the results obtained by stochastic simulation. An example of the seismic response of an existing bridge obtained by two older models and three variants of multicomponent resolution is given.     

Dynamic characteristics of a base isolated building from ambient vibration measurements and low level earthquake shaking

Ambient vibration tests were conducted on a base-isolated apartment building in Takamatsu, Japan, to determine the mode shapes and the associated natural frequencies and damping ratios at very low levels of excitation. The latest developments in signal analysis for modal decomposition are used to analyze the ambient response data. A finite element model of the building and isolators was calibrated and refined using the experimental results from the ambient vibration tests. This model was then used to simulate the recorded response of the building under excitation from a small earthquake. The finite element model, calibrated by ambient vibration data and the low level of earthquake shaking, provides the starting point for modelling the non-linear response of the building when subjected to strong shaking.     

新疆博乐垦区人工防雹效果的统计评估

为了客观地评估博乐垦区多年开展人工防雹的作业效果，利用兵团博乐农五师垦区1976--2003年中，后15年开展防雹工作和前13年未开展防雹工作期间的年雹灾面积资料，运用简单序列检验法、不成对秩和检验法以及t-检验法等统计学的方法进行了人工防雹作业效果统计评估分析。结果为：人工防雹作业使平均年雹灾面积减少841．53hm^2，雹灾面积相对减少率为26．56％，统计显著性水平达到α=0．05。因此，博乐垦区人工防雹作业效果显著。