A stochastic model for initial movement of sand grains by wind

A stochastic model for entrainment of sand grains by wind is presented through analysis of the forces exerted on a single spherical grain, coupled with fluctuations of wind velocity and the change in grain position on the surface. The structure of the stochastic model is consistent with experimental data in the literature. The probability of initial motion increases first, and then decreases, with grain size. It reaches a maximum at diameters of about 0·9 mm. Some sand grains are still in motion at less than the conventional threshold velocity, even at very low velocities. The probability of sand grain movement reaches unity at twice the conventional threshold velocity. Considerable discrepancies amongst conventional threshold formulae may result from the different probabilities of initial movement implied in these formulae. Copyright © 2008 John Wiley & Sons, Ltd.     

Perturbations of the upper atmosphere in the cleft region

In the dayside polar region—loosely referred to as the cleft region—particle precipitation and Joule heating cause significant perturbations of the upper atmosphere. Here Dynamics Explorer-2 satellite data are used to present a synopsis of these disturbance effects. Documented are an increase in electron temperature and a decrease in electron density; increases in ion drift speed and ion temperature; an increase in the upward-directed ion velocity; increases in zonal wind speed and neutral gas temperature; and changes in the neutral gas composition and mass density. It is suggested that the increase in electron temperature is partly controlled by the decrease in electron density; that the ion upflow velocity mainly depends on the electron temperature, less frequently on the ion temperature; and that the observed decrease in thermospheric mass density is due to a decrease in the atomic oxygen density, which in turn is caused by diverging wind flows.     

预静载对全级配混凝土梁动弯拉强度的影响

为了探讨预静载对全级配混凝土梁动弯拉强度的影响,本文从全级配混凝土的细观层次出发,将梁的纯弯段视为由水泥砂浆、粗骨料及两者间的黏结带所组成的复合材料。采用既考虑应变率强化效应又计及损伤弱化效应的动态本构模型反映细观单元的损伤退化。通过自编的以位移控制的有限元程序对全级配混凝土梁在不同预静载作用下的破坏机制进行了数值模拟,给出了相应的应力一应变曲线和动弯拉强度。计算结果显示：全级配混凝土梁的动弯拉强度随着预静载的增加而上升。     

Precise integration methods based on the Chebyshev polynomial of the first kind

This paper introduces two new types of precise integration methods based on Chebyshev polynomial of the first kind for dynamic response analysis of structures, namely the integral formula method （IFM） and the homogenized initial system method （HISM）. In both methods, nonlinear variable loadings within time intervals are simulated using Chebyshev polynomials of the first kind before a direct integration is performed. Developed on the basis of the integral formula, the recurrence relationship of the integral computation suggested in this paper is combined with the Crout decomposed method to solve linear algebraic equations. In this way, the IFM based on Chebyshev polynomial of the first kind is constructed. Transforming the non-homogenous initial system to the homogeneous dynamic system, and developing a special scheme without dimensional expansion, the HISM based on Chebyshev polynomial of the first kind is able to avoid the matrix inversion operation. The accuracy of the time integration schemes is examined and compared with other commonly used schemes, and it is shown that a greater accuracy as well as less time consuming can be achieved. Two numerical examples are presented to demonstrate the applicability of these new methods.     

桥梁结构损伤对其固有振动特性的影响

分析实际桥梁结构的各种损伤状况,采用矩阵摄动理论分析桥梁结构发生损伤时固有频率及振型向量的一阶、二阶摄动结果,并讨论了固有振动特性对结构单元损伤的灵敏性。在理论分析的基础上,编制桥梁结构固有振动特性及其在损伤下的摄动分析程序,并用以分析简支梁、连续梁及拱结构损伤引起的固有振动的摄动结果。分析结果表明:单一单元损伤引起固有频率的摄动结果很小;不同位置处单元损伤与固有频率摄动结果的关系曲线与对应振型曲线具有相似性,但在中间支承处或结构端部有扭转约束处单元损伤影响显著;而单元损伤对振型向量影响极小。     

饱水介质波动分析的谐波输入初始间断的处理方法

本文以饱水两相介质的土力学模型为研究对象,在假定两相介质为弹性介质条件下,采用了显式有限元法和透射边界进行了饱和弹性半空间动力响应问题的研究。为避免谐波输入初始间断的影响,文中提出了一个处理函数,并以弹性半空间为算例,对饱水介质和单相介质分别进行了在底边界P波垂直入射时的动力响应分析,验证了该处理函数的有效性和实用性。     

Rb–Sr isochron ages of the Cretaceous granitoids in the Ryoke belt, Kinki district, Southwest Japan

Abstract Granitoids are widely distributed in the Ryoke belt and have been divided into four main igneous stages based on their field setting. In this paper, we present Rb–Sr isochron ages for the younger Ryoke granitoids (second stage to fourth stage) in the Kinki district. The Yagyu granite (second stage) gave a Rb–Sr whole‐rock isochron age of 74.6 ± 10.9 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70938 ± 0.00016, and a Rb–Sr mineral isochron age of 71.8 ± 0.1 Ma. The Narukawa granite (second stage) yielded a Rb–Sr mineral isochron age of 79.5 ± 0.4 Ma. A Rb–Sr whole‐rock isochron age of 78.3 ± 3.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70764 ± 0.00014 was obtained for the Takijiri adamellite (third stage). The Katsuragi quartzdiorite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 85.1 ± 18.3 Ma (initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70728 ± 0.00006), and mineral isochron ages of 76.9 ± 0.5 Ma and 74.8 ± 0.5 Ma. The Minamikawachi granite (fourth stage) gave a Rb–Sr whole‐rock isochron age of 72.8 ± 2.0 Ma with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.70891 ± 0.00021. These age data indicate that the igneous activity in younger Ryoke granitoids of Kinki district occurred between 80 and 70 Ma, except for the Katsuragi quartz diorite. The isotopic data on the various igneous stages in Kinki district correspond with the relative timing from field observations. Based on the initial ⁸⁷Sr/⁸⁶Sr ratios, the granitoids of the Ryoke belt in Kinki district are spatially divided into two groups. One is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.707–0.708, distributed in the southern part of the Ryoke belt. The other is granitoids with initial ⁸⁷Sr/⁸⁶Sr ratio of 0.708–0.710 distributed in the northern part of the Ryoke belt. The initial ⁸⁷Sr/⁸⁶Sr ratios of granitoids increase with decreasing (becoming younger) Rb–Sr whole‐rock isochron ages.     

随机阻尼对结构抗风抗震动力响应的影响

本文讨论了结构系统阻尼的随机性,运用运动微分方程和二阶摄动地具有随机阻尼的结构系统在地震荷载作用下的动力响应进行了计算,给出随机阻尼对结构响应的影响。结果表明,随机阻尼对结构的位移、速度和加速度的物确存在,特别是阻尼的变异系数较大对结构的动力响应影响更为明显,应引起结构设计者的足够重视。     

Research on mode localization of reticulated shell structures

Reticulated shell structures (RSSs) are characterized as cyclically periodic structures. Mistuning of RSSs will induce structural mode localization. Mode localization has the following two features: some modal vectors of the structure change remarkably when the values of its physical parameters (mass or stiffness) have a slight change; and the vibration of some modes is mainly restricted in some local areas of the structure. In this paper, two quantitative assessment indexes are introduced that correspond to these two features. The first feature is studied through a numerical example of a RSS, and its induced causes are analyzed by using the perturbation theory. The analysis showed that internally, mode localization is closely related to structural frequencies and externally, slight changes of the physical parameters of the structure cause instability to the RSS. A scaled model experiment to examine mode localization was carried out on a Kiewit single-layer spherical RSS, and both features of mode localization are studied. Eight tests that measured the changes of the physical parameters were carried out in the experiment. Since many modes make their contribution in structural dynamic response, six strong vibration modes were tested at random in the experimental analysis. The change and localization of the six modes are analyzed for each test. The results show that slight changes to the physical parameters are likely to induce remarkable changes and localization of some modal vectors in the RSSs.     

滑动长孔螺栓摩擦阻尼器力学性能试验研究

本文在已有摩擦阻尼器的基础上,设计可应用于装配式剪力墙竖向接缝的阻尼器。通过改变摩擦材料、高强螺栓预紧力等参数,对阻尼器工作性能参数（起滑力、起滑位移、等效初始刚度等）进行对比分析。研究结果表明,当摩擦材料为黄铜板、铝板及碳纤维板时,2个受力方向的起滑力相差不大,而当摩擦材料为橡胶板时,由于橡胶自身强度较低,2个受力方向的起滑力差异较大,工作性能相对较差;4种材料起滑位移及等效初始刚度在不同预紧力下按照一定规律变化,表现出较好的工作性能。在实际应用中,应设置碟形垫片以保证预紧力的稳定,使阻尼器工作性能良好。