切变线降水系统微物理特征及隆水机制个例分析

利用机载云粒子探测系统(PMS).对2004年7月1日影响吉林省的一次切变线降水过程进行了探测飞行,利用所获取的宏微观资料对此次降水过程的微物理结构、降水机制进行综合分析.结果表明:此次切变线降水云系主要由高层云、雨层云、碎云构成,高层云和雨层云中间夹有1100m左右的无云区;3类云中平均云滴浓度、平均云滴直径各不相同;云水含量随高度分布不均匀,云的不同部位云水含量起伏较大;冰晶浓度平均为17.3个/L;此次探测的降水云系符合Bergeron提出的催化云一供水云相互作用导致降水的概念.根据云图及其他探测资料综合分析,冰晶主要产生于高层云上部或卷层云的冰晶播撒,供水云为高层云中下部和雨层云.     

TEMPORAL AND SPATIAL DYNAMICS OF DIFFERENT PARTICLE SIZE OF SUSPENDED SEDIMENT IN THE LIU RIVER CATCHMENTS,CHINA

1 INTRODUCTION The particle size of sediment eroded from basins can provide basic information about erosion processes (Meyer et al., 1980), which can be divided into sheet wash sediment processes on hill slopes and fluvial sediment processes in rivers. In…     

车辆碾压作用下戈壁地表起尘浓度

通过野外观测车辆碾压作用下戈壁地表PM₁₀释放浓度,探讨其主要影响因素。针对戈壁区砾石、砂砾、砂质和砂土4种路面类型,应用TSI粉尘仪多次重复观测车辆碾压产生的PM₁₀浓度,进而阐明碾压次数、车速和地表水分等因素对起尘浓度的影响。结果表明：4种路面的平均起尘浓度依次为5.339、9.089、16.944、50.251 mg·m^-3,是自然状态下的5~50倍;起尘浓度随碾压次数增多而增大,其中砂土路情况下的增长最显著;随土层厚度的增大呈现出幂函数的增长趋势;降雨后路面起尘浓度显著减小,且降雨量会影响降雨后起尘浓度的变化趋势。最后提出了针对性的建议和措施以减弱车辆碾压导致的戈壁地表起尘。     

胶州湾潮余流和物质输送之间的关系

基于MIKE21的HD模型,通过模拟得到胶州湾的潮流场,胶州湾在涨急时最大流速1.04m/s,落急时最大流速约为0.96m/s。胶州湾余流总体较小,平均为0.03m/s左右。并在湾内不同位置释放自由粒子,以MIKE21的Particle tracking模型计算出其在潮流作用下的运移轨迹,结果表明粒子大多数运移到湾内近岸区域,少部分在湾口区域附近;在潮流场基础上计算了欧拉余流场,并和粒子运移结果进行对比,表明欧拉余流场在区域流向比较一致时可以表示粒子运动的趋势,为物质迁移、控制污染等方面提供了一定的参考借鉴意义。     

Particle trajectories of nonlinear gravity waves in deep water

The paper presents a numerical method for calculating the particle trajectories of nonlinear gravity waves in deep water. Particle trajectories, mass-transport velocity and Lagrangian wave period can be accurately determined by the proposed method. The high success rate of the proposed method is examined by comparing the present results with those of (a) Longuet-Higgins, M.S., 1986, 1987. Eulerian and Lagrangian aspects of surface waves. Journal of Fluid Mechanics 173, 683-707 and (b) Lagrangian moments and mass transport in Stokes waves. Journal of Fluid Mechanics 179, 547-555. It is shown that the dimensionless mass-transport velocity can exceed 10% for large waves, and the Lagrangian wave period is much larger than the Eulerian wave period for large waves.     

Comparison of flow patterns in the near wake of a circular cylinder and a square cylinder placed near a plane wall

The flow patterns in the near wake of a cylinder (either circular or square in shape, D=25 mm) placed in the proximity of a fully developed turbulent boundary layer (thickness δ=0.4D) are investigated experimentally using particle image velocimetry (PIV). The effects of changing the gap height (S) between the cylinder bottom and the wall surface, over the gap ratio range S/D=0.1–1.0, have been investigated. The results show that both the ensemble-averaged and instantaneous flow fields are strongly dependent on S/D. The flow patterns for the two types of cylinders share many similarities with respect to the change in S/D, such as the reduced recirculation length and increased velocity fluctuation in the near wake with increasing S/D, as well as the trend of suppression of vortex shedding at small S/D and onset of vortex shedding at large S/D. However, developments of the shear layers, in terms of wake width, flow curvature, etc., are considerably different for these two types of cylinders. In general, the wake development and momentum exchange for the square cylinder are slower those for the circular cylinder at the same gap ratio. Correspondingly, it is shown that the periodic vortex shedding is delayed and weakened in the case of square cylinder, as compared to that of the circular cylinder at the same S/D.     

Modified Moving Particle Semi-implicit methods for the prediction of 2D wave impact pressure

As a gridless particle method, the MPS (Moving Particle Semi-implicit) method has proven useful in a wide variety of engineering applications including free-surface hydrodynamic flows. Despite its wide range of applicability, the MPS method suffers from some shortcomings such as non-conservation of momentum and spurious pressure fluctuation. By introducing new formulations for the pressure gradient and a new formulation of the source term of the Poisson Pressure Equation (PPE), and by allowing a slight compressibility, we have proposed modified MPS methods for the prediction of wave impact pressure on a coastal structure. The improved performance of the modified methods is shown through the simulation of numerous wave impact problems (including the impacts by a dam break flow, a flip-through and two cases of slightly-breaking waves) in comparison with the experimental data.     

A constitutive model for coal-fouled ballast capturing the effects of particle degradation

Rail tracks undergo degradation owing to particle breakage and fouling of ballast by various fines including coal and subgrade soil. As the ballast becomes fouled, its strength and drainage capacity are compromised, sometimes resulting in differential settlement and reduced track stability. This paper demonstrates a continuum mechanics based framework to evaluate the detrimental effect of fines on the strength, deformation and degradation of coal-fouled ballast under monotonic loading. An elastoplastic constitutive model that considers the effect of fines content and energy consumption associated with particle breakage during shearing is presented. This multiphase constitutive model is developed within a critical state framework based on a kinematic-type yield locus and a modified stress-dilatancy approach. A general formulation for the rate of ballast breakage and coal particle breakage during triaxial shearing is presented and incorporated into the plastic flow rule to accurately predict the stress–strain response of coal-fouled ballast at various confining pressures. The behaviour of ballast at various levels of fouling is analysed and validated by experimental data.     

粒子滤波同化在厄尔尼诺-南方涛动目标观测中的应用

针对粒子滤波—目标观测方法的局限性,提出了能够克服该局限性的新目标观测方法,并将其应用于厄尔尼诺—南方涛动可预报性研究,揭示了东太平洋型厄尔尼诺事件和中太平洋型厄尔尼诺事件的目标观测敏感区。通过同化该敏感区的目标观测,显著减小了两类厄尔尼诺事件的预报不确定性,验证了新方法揭示的目标观测敏感区在改进厄尔尼诺预报技巧中具有更加重要的作用。     

Using global node-based velocity in random walk particle tracking in variably saturated porous media: application to contaminant leaching from road constructions

Precise and efficient numerical simulation of transport processes in subsurface systems is a prerequisite for many site investigation or remediation studies. Random walk particle tracking (RWPT) methods have been introduced in the past to overcome numerical difficulties when simulating propagation processes in porous media such as advection-dominated mass transport. Crucial for the precision of RWPT methods is the accuracy of the numerically calculated ground water velocity field. In this paper, a global node-based method for velocity calculation is used, which was originally proposed by Yeh (Water Resour Res 7:1216–1225, 1981). This method is improved in three ways: (1) extension to unstructured grids, (2) significant enhancement of computational efficiency, and (3) extension to saturated (groundwater) as well as unsaturated systems (soil water). The novel RWPT method is tested with numerical benchmark examples from the literature and used in two field scale applications of contaminant transport in saturated and unsaturated ground water. To evaluate advective transport of the model, the accuracy of the velocity field is demonstrated by comparing several published results of particle pathlines or streamlines. Given the chosen test problem, the global node-based velocity estimation is found to be as accurate as the CK method (Cordes and Kinzelbach in Water Resour Res 28(11):2903–2911, 1992) but less accurate than the mixed or mixed-hybrid finite element methods for flow in highly heterogeneous media. To evaluate advective–diffusive transport, a transport problem studied by Hassan and Mohamed (J Hydrol 275(3–4):242–260, 2003) is investigated here and evaluated using different numbers of particles. The results indicate that the number of particles required for the given problem is decreased using the proposed method by about two orders of magnitude without losing accuracy of the concentration contours as compared to the published numbers.