A parallel mesh-free contaminant transport model based on the Analytic Element and Streamline Methods

This paper introduces a new method for simulating large-scale subsurface contaminant transport that combines an Analytic Element Method (AEM) groundwater flow solution with a split-operator Streamline Method for modeling reactive transport. The key feature of the method is the manner in which the vertically integrated AEM flow solution is used to construct three-dimensional particle tracks that define the geometry of the Streamline Method. The inherently parallel nature of the algorithm supports the development of reactive transport models for spatial domains much larger than current grid-based methods. The applicability of the new approach is verified for cases with negligible transverse dispersion through comparisons to analytic solutions and existing numerical solutions, and parallel performance is demonstrated through a realistic test problem based on the regional-scale transport of agricultural contaminants from spatially distributed sources.     

Uncertainty propagation of hydrodispersive transfer in an aquifer: an illustration of one-dimensional contaminant transport with slug injection

It is evident that the hydrodynamic dispersion coefficient and linear flow velocity dominate solute transport in aquifers. Both of them play important roles characterizing contaminant transport. However, by definition, the parameter of contaminant transport cannot be measured directly. For most problems of contaminant transport, a conceptual model for solute transport generally is established to fit the breakthrough curve obtained from field testing, and then suitable curve matching or the inverse solution of a theoretical model is used to determine the parameter. This study presents a one-dimensional solute transport problem for slug injection. Differential analysis is used to analyze uncertainty propagation, which is described by the variance and mean. The uncertainties of linear velocity and hydrodynamic dispersion coefficient are, respectively, characterized by the second-power and fourth-power of the length scale multiplied by a lumped relationship of variance and covariance of system parameters, i.e. the Peclet number and arrival time of maximum concentration. To validate the applicability for evaluating variance propagation in one-dimensional solute transport, two cases using field data are presented to demonstrate how parametric uncertainty can be caught depending on the manner of sampling.     

大型海底、海岸和沙漠沙丘的形态和迁移特征

海底、海岸和沙漠沙丘的迁移与其波高、波长等形态参数有关。对一些代表性大型沙丘的分析表明,海底、海岸和沙漠沙丘的形态特征存在着较大的差异,在相同波长条件下,海底沙丘波高最小,海岸沙丘波高次之,沙漠沙丘波高最大。其原因主要是流体厚度的差异,其次是物质供给条件的不同:浅海地区的水深限制了沙丘向上生长,而陆地气流厚度较大,给沙丘的向上生长提供了较大的空间;沙漠环境的沉积物供给远大于海岸环境,因而导致沙丘高度的差异。沙丘形态参数经常偏离统计的波长 波高曲线,数值实验结果显示,这与推移质输运率有关。推移质输运率的沿程变化可以使形态参数系统性地偏离波长 波高曲线;推移质输运率的沿程突变可使沙丘迁移受阻,造成沉积物的垂向堆积,形成超高的沙丘。由于大型沙丘的迁移特征受控于推移质输运率和沙丘高度,因此可以通过波高设计来计算迁移距离 历时曲线,进而控制沙丘迁移动态。文中给出了一个获取迁移距离 历时曲线的算例。设计波高可通过人工篱笆、隔挡墙、沟渠、表面护层等措施而实现。     

Sand transport under increased lateral jetting of raindrops induced by wind

Wind tunnel experiments for ‘Raindrop Detachment and Wind-Driven Transport’ (RD–WDT) process were conducted under improved lateral jetting induced by wind velocities of 6.4, 10, and 12 m s^− 1 at nozzle operating pressures of 75, 100, and 150 kPa. Wind-driven rainfalls were also incident on the windward and leeward slopes of 4° and 9° to have a broad variation in the angle of incidence. The objective of this experimental set-up was to distinguish the roles of both impact components of obliquely striking wind-driven raindrops on RD and wind on WDT. Raindrop impact components and reference horizontal wind were quantified by normal (E_tz) and horizontal (E_tx) kinetic energy fluxes and wind shear velocity (u), respectively, to physically model the process of RD–WDT. The results showed, at each level of u, differential sand transport rates by RD–WDT (q_m(RD–WDT)) occurred depending on the magnitude of raindrop impact components, and q_m(RD–WDT) increased as the relative contribution of E_tz increased. Although E_tx was more correlated with q_m(RD–WDT) than E_tz, the extreme increases in E_tx at the expense of E_tz brought about no increases but decreases in q_m(RD–WDT). An RD–WDT model was built under the process of examining the discrete effects of E_tz and E_tx on RD together with u and resulted in a better coefficient of determination (R² = 0.89) than only total kinetic energy (E_t) did alone with u (R² = 0.84). In this study, E_tx was strongly related to u and not to E_tz, which was the principal difference from the previous rainsplash studies, which relied on the compensatory lateral jet development by the compressive pressure build-up at the raindrop–soil interface. Including E_tx in the RD–WDT model both separated the distinct role of each raindrop impact component in RD and improved the performance of u in WDT by better distinguishing its interaction with E_tx, which was not explicitly separated in previous models of RD–WDT.     

Bedforms and bed material transport pathways in the Changjiang (Yangtze) Estuary

Bed features were mapped using a thermal depth recorder and a side-scan sonar in the Changjiang (Yangtze) Estuary, and flows were observed and bed surface materials were sampled to investigate bed material transport pathways. Distinct scales and dimensions of dunes were observed in terms of the classification scheme of Ashley [Ashley, G.M., 1990. Classification of large-scale sub-aqueous bedforms: A new look at an old problem. Journal of Sedimentary Research, 61, 160–172], forming a dune set in the flood season. Large dunes occurred predominately in the surveyed area of the estuarine zone, and two-dimensional dunes predominated in the dry season while three-dimensional dunes prevailed in the flood season. Sharp lee sides of dunes have low angles, indicating a significant suspended-load transport for the bedform migration, and the majority of them face down-estuary reflecting the controlling process by the ebb current. The bed material migrations in terms of relative maximum gross transport show a satisfactory agreement with the sonar mapping in the dry season. The relative maximum gross transport as an independent estimator of migration direction, can be used to effectively determine the bed material transport directions in the Changjiang Estuary.     

中国铁路客运网络可达性空间格局

基于铁路客运的基本特点,文章采用空间距离、时间距离、连接性、可选择性等多项指标对我国铁路客运网络可达性空间格局进行了分析。各项指标侧重点不同导致其反映的空间格局的明显差异。空间、时间距离指标均呈现以郑州为核心的中心－外围格局,但前者由中心向外围呈环状各向同性衰减,后者表现为各向异性,沿若干铁路干线衰减速度较慢; 连接性指标中东部地区优于西部,中心地区优于边缘,而可选择性指标东西分异显著,京广线以东(含京广沿线)地区明显优于西部地区。从空间均衡角度,主要体现铁路客运基本服务的空间距离、时间距离、连接性三指标相对较好,但偏重于高级服务的可选择性空间分布相对不均衡。主要铁路枢纽中,北京、上海、郑州、武汉等全国性铁路枢纽可达性明显优于广州、成都、兰州和沈阳等区域性枢纽。     

中国区域经济差异形成的三次地理要素

关于地理要素对经济差异的影响,国际上已经做了大量研究。为了探讨地理要素对中国经济差异的影响程度,在Krugman的关于城市聚集的两大自然理论基础之上,提出了影响经济差异的三大地理要素,分别为自然地理环境要素、交通和区位要素以及人力资本和研发水平要素,并通过建模,从全国和三大地带的尺度上细致地分析了各地理要素对经济分异的影响。研究发现,在中国,自然地理环境对经济发展的作用得到了验证和充实,这为揭示地理环境作用提供了主要依据,扩展了国际上Sachs等人关于地理要素对经济发展的认识。目前我国经济正处于转型期,在各地理要素中,影响经济差异最大的为交通便利性指数,影响程度为0.374,因此交通基础建设投资应继续加大。     

黑河下游植被对调水的响应

由于黑河下游来水量的减少,黑河下游绿洲面积急剧退化.为了改善黑河下游的生态环境问题,遏制绿洲进一步退化的趋势,国务院批复于2000年开始对黑河下游进行调水,通过9年调水,黑河下游水资源时空分布发生了很大变化.为了客观评价黑河下游植被对调水的响应,本文通过实地调查、遥感等手段,分析了黑河下游1995-2004年地下水埋深、典型植被、景观类型及东居延海在调水以来的变化情况.研究结果表明:黑河下游生态环境对调水响应明显,下游生态环境向良好的方向转变.     

多孔介质中非水相流体运移的数值模拟

针对多孔介质中水、气和非水相流体(NAPLs)的多相流动特点,建立了非水相流体(NAPLs)污染物迁移模型,分析了非水相流体在土壤非饱和区和地下水系统中的运移规律。通过有限元数值解对轻非水相流体和重非水相流体在土壤系统中的迁移过程进行模拟,得到了污染物的时空分布特征和污染范围。计算结果表明,数值模拟方法能够合理地描述非水相流体的运移过程和污染特征。土体渗透性和污染物残余饱和度是其重要影响因素。     

海盐粒子沉积下碳钢的大气腐蚀初期行为

利用体视显微镜和电化学极化技术原位考察了碳钢上NaCl盐粒的吸潮及其液滴的扩展行为。实验结果表明,NaCl液滴的吸潮水解过程主要受环境相对湿度的影响。相对湿度越高,水解越容易。在极化状态下,NaCl液滴的扩展随极化电流的增加而线性加速。NaCl液滴的扩展与碳钢大气腐蚀的电化学过程相互影响。一方面,NaCl液滴扩展的动力来自于大气腐蚀过程所形成的腐蚀电流,另一方面,NaCl液滴的扩展加大了大气腐蚀过程的阴极面积。进而加速了碳钢的腐蚀过程。