错位转换层位置对高层建筑抗震性能影响的研究

本文采用有限元程序对设有错位转换的高层结构进行了空间时程地震反应分析,论述了错位转换层整体位置、错位转换层相对位置对结构动力特性、动力反应及竖向构件内力的影响。分析结果表明,错位转换层整体位置的变动及相对变动对结构周期、主要振型和相对位移影响不大;随着错位转换层整体向上移动,在转换层处的层间位移角的突变会加剧;当两错位转换层相距3层左右时,高层结构地震反应最不利;错位转换层的存在会使两错位转换层附近的竖向构件内力发生突变,设计中应予以加强。     

Empirical formula for fundamental vibration periods of reinforced concrete buildings in Taiwan

More than 30 buildings around Taiwan have been selected to monitor the floor responses under seismic excitation. The structural array monitoring system in each building controls at most 27 channels of accelerometers distributed in several floors. Those buildings were triggered by many events during the past five years of operation. In each building, the records at the basement can be considered as the ground excitation, and the others at the upper floors are the structural responses. The frequency transfer functions of those buildings can be identified by ARX models, and then the fundamental vibration periods are estimated. The identified fundamental vibration periods using different events are compared in order to ensure the reliability of system identification. An empirical formula in predicting the fundamental vibration period is presented through the regression analysis to the identified fundamental vibration periods of 21 reinforced concrete (RC) moment‐resisting frame (MRF) buildings. It is found that the height of a building plays an important role in predicting the fundamental vibration period, compared with the length, width, and time after completion of the building. It is also found that the RC MRF buildings in Taiwan tend to be stiffer than those in the U.S. Copyright © 2000 John Wiley & Sons, Ltd.     

六种近地层湍流动量输送系数计算方案对比分析

选取30多年来近地层湍流通量研究中具有代表性的六种参数化方案, 应用GAME/Tibet试验中那曲通量观测站的实测资料, 对比分析了各方案计算所得的湍流动量输送系数 (C_M) 之间的差异。结果表明:六种参数化方案计算得到的湍流动量输送系数之间存在较大差异。对于那曲观测站稀疏短草下垫面而言, 稳定条件下当理查孙数小于0.1时, 除Businger71方案存在显著低估以外, 其他各方案均能较好估算湍流动量输送系数; 不稳定条件下, Dyer74方案对湍流动量输送系数的估算效果最好, 其次为Wang02, Launiainen95和Louis82方案, Businger71方案误差较大。     

Comparing methods for mapping canopy chlorophyll content in a mixed mountain forest using Sentinel-2 data

The Sentinel-2 Multi-Spectral Imager (MSI) has three spectral bands centered at 705, 740, and 783 nm wavelengths that exploit the red-edge information useful for quantifying plant biochemical traits. This sensor configuration is expected to improve the prediction accuracy of vegetation chlorophyll content. In this work, we assessed the performance of several statistical and physical-based methods in retrieving canopy chlorophyll content (CCC) from Sentinel-2 in a heterogeneous mixed mountain forest. Amongst the algorithms presented in the literature, 13 different vegetation indices (VIs), a non-parametric statistical approach, and two radiative transfer models (RTM) were used to assess the CCC prediction accuracy. A field campaign was conducted in July 2017 to collect in situ measurements of CCC in Bavarian forest national park, and the cloud-free Sentinel-2 image was acquired on 13 July 2017. The leave-one-out cross-validation technique was used to compare the VIs and the non-parametric approach. Whereas physical-based methods were calibrated using simulated data and validated using the in situ reference dataset. The statistical-based approaches, such as the modified simple ratio (mSR) vegetation index and the partial least square regression (PLSR) outperformed all other techniques. As such the modified simple ratio (mSR3) (665, 865) gave the lowest cross-validated RMSE of 0.21 g/m² (R² = 0.75). The PLSR resulted in the highest R² of 0.78, and slightly higher RMSE =0.22 g/m² than mSR3. The physical-based approach-INFORM inversion using look-up table resulted in an RMSE =0.31 g/m², and R² = 0.67. Although mapping CCC using these methods revealed similar spatial distribution patterns, over and underestimation of low and high CCC values were observed mainly in the statistical approaches. Further validation using in situ data from different terrestrial ecosystems is imperative for both the statistical and physical-based approaches' effectiveness to quantify CCC before selecting the best operational algorithm to map CCC from Sentinel-2 for long-term terrestrial ecosystems monitoring across the globe.     

IMU辅助下的单目视觉坐标传递

针对管线测量中由于GNSS RTK卫星信号遮挡或测站不易架设等原因无法提供坐标的场景,本文提出了一种IMU辅助下的单目视觉坐标传递方法,用于延伸RTK测得的精密坐标。首先在两个已知的RTK点上架设相机拍摄图像,同时采集IMU数据;然后对图像进行特征提取和匹配,恢复帧间旋转、平移及尺度;最后三角化待测点,以获得待测点在真实尺度的相机坐标系下的坐标。受IMU静基座初始对准方法的启发,本文利用重力矢量和相机光心间矢量分别在地理坐标系和相机坐标系下的投影求解这两个坐标系间的旋转矩阵,进而求得待测点在地理坐标系下的三维坐标。相对于其他测量手段,本文方法灵活便捷、设备轻便、操作简单,且一次采集即可获取相机视场中的任意共视点坐标。试验结果表明,本文提出的坐标传递方法所得的平面坐标的误差平均值为0.12 m,高程误差的平均值为0.2 m。     

土地利用时空变化与城市化发展分析

城市化发展与科学合理利用土地已经成为社会发展最为关注的问题。土地利用结构发生变化后,不合理的土地利用规划等问题将会导致一系列的生态环境问题,因此,监测土地利用的时空变化,为土地利用规划及城市发展规划提供有效的数据支持很有必要。本文通过年度区间土地利用变化数据,利用转移矩阵、动态度、土地利用重心及土地利用结构信息熵分析土地利用变化情况,经过试验表明能够真实、全面反映土地利用变化状态,充分反映城市发展与土地利用关系,有利于提高分析的准确性和可靠性。     

南海西部风驱离岸急流次中尺度锋面的动力学分析

本文利用卫星观测资料和500 m分辨率数值模拟结果,结合理论分析,对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明,南海西部（WSCS）存在侧向尺度为O（1-10）km的次中尺度锋面,在地转和非地转运动的共同作用下,次中尺度密度锋面具有一阶Rossby（Ro）和Richardson（Ri）数。锋面诊断结果显示,沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送,有效地在跨锋面方向将表层冷水平流输送至暖水侧,导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡（PV）,表明该密度锋面易受次中尺度对称不稳定（SI）的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度,其最大值可达100 m·d^-1。能量评估结果表明,次中尺度湍流的两个主要能量源,即地转剪切项（GSP）和垂向浮力通量（BFLUX）在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下,大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此,次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量,可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。     

青藏高原地区城乡虚拟水贸易格局与影响因素

青藏高原是亚洲水塔,其水资源与水生态环境保护意义重大。从虚拟水视角,研究青藏高原与外部的水资源贸易关系和影响因素,有助于理解该地区的水资源问题、制定虚拟水贸易策略、优化区域城乡水资源配置、保障亚洲水塔功能。依托2012年中国区域间投入产出表成果,本文测算了青藏高原与中国其他区域之间的虚拟水贸易关系,建立了中国区域城镇与农村地区的虚拟水贸易网络,采用对数平均迪氏指数模型分析了青藏高原对其他区域虚拟水贸易不平衡的影响因素。结果表明,青藏高原向中国其他区域净输出虚拟水2.25亿m³,其中向西南、华北、华中、华东、华南等5个区域净输出虚拟水,从西北和东北2个区域净输入虚拟水。城乡之间虚拟水贸易联系非常紧密,农村地区生产水足迹较高,而城镇地区由于人口密度较高、消费水平较高,是虚拟水最终消费的热点区域,青藏高原农村地区的虚拟水贸易量大于城镇地区的虚拟水贸易量。青藏高原贸易输出结构以农产品为主导,虚拟水净输出12.7亿m³;青藏高原与其他区域贸易存在逆差,贸易量因素导致虚拟水净输入8.6亿m³;用水效率在青藏高原与不同区域虚拟水贸易中的正负效应不一,总体带来青藏高原虚拟水净输入1.8亿m³。未来,应重点通过灌溉节水减少农业水足迹,引导城镇居民向低水足迹生活方式转变,鼓励内地为青藏高原提供物质与技术支援,实行水资源生态补偿政策,以保护青藏高原水资源,促进区域水资源可持续利用。     

一种公交换乘优化算法设计

目前,在智能交通系统应用中,公交查询亟待解决其算法效率和换乘两大问题。对此,本文首先在分析乘客出行因素和公交网络特性的基础上,提出了关键站点的公交网络通达矩阵,分析了直达矩阵的计算原理;其次,阐述了城市道路网络最短路径算法,引入直达矩阵和公交网络通达矩阵,设计了一种公交换乘优化算法。该算法的基本原理:利用公交网络通达矩阵来确定哪些临时标记节点是潜在的永久标记结点,并始终优先考虑直达节点作为最短路径上的节点,从而实现对经典最短路径算法--Dijkstra算法的改进;最后,通过一个案例对该算法进行验证。实验表明,该算法不仅能够获得优于Dijkstra算法的性能,而且换乘次数更加合理。该算法能够适用于一般公交网络,对于换乘代价较高的公交网络更有优势。     

Steady streaming around a circular cylinder in an oscillatory flow

Steady streaming induced by an oscillatory flow around a circular cylinder is investigated using a numerical method. Two-dimensional Reynolds-averaged Navier-Stokes equations are solved using a finite element method with a k-ω turbulent model closure. The range of the Keulegan-Carpenter (KC) number investigated is between 2 and 40, which is substantially higher than those reported in literature related to steady streaming to date. A constant value of Stokes number (β) of 196 is chosen in this study. The steady streaming structures and velocity distribution are analysed in detail. It is found that the characteristics of steady streaming are strongly related to the vortex shedding flow regimes.