Balancing the source terms in a SPH model for solving the shallow water equations

A shallow flow generally features complex hydrodynamics induced by complicated domain topography and geometry. A numerical scheme with well-balanced flux and source term gradients is therefore essential before a shallow flow model can be applied to simulate real-world problems. The issue of source term balancing has been exhaustively investigated in grid-based numerical approaches, e.g. discontinuous Galerkin finite element methods and finite volume Godunov-type methods. In recent years, a relatively new computational method, smooth particle hydrodynamics (SPH), has started to gain popularity in solving the shallow water equations (SWEs). However, the well-balanced problem has not been fully investigated and resolved in the context of SPH. This work aims to discuss the well-balanced problem caused by a standard SPH discretization to the SWEs with slope source terms and derive a corrected SPH algorithm that is able to preserve the solution of lake at rest. In order to enhance the shock capturing capability of the resulting SPH model, the Monotone Upwind-centered Scheme for Conservation Laws (MUSCL) is also explored and applied to enable Riemann solver based artificial viscosity. The new SPH model is validated against several idealized benchmark tests and a real-world dam-break case and promising results are obtained.     

温度对藨草(Scirpus triqueter)分解的影响

在5℃、15℃和25℃温度下,研究蔗草(Scirpus triqueter)烘干样品和鲜样品的分解状况,探讨夏季受淹死亡的藨草分解动态及其与正常凋落物分解的差异.结果表明,5个月后,25℃处理下,藨草剩余干物质质量分别为初始时的31.20％(烘干样品)和19.30％(鲜样品);5℃处理下,蔗草剩余干物质质量分别为初始时的65.15％(烘干样品)和62.12％(鲜样品);25℃处理下的蔗草剩余干物质质量明显低于5℃处理下的蔗草剩余干物质质量;藨草的前处理亦会影响其分解过程,在5℃和15℃处理下,藨草鲜样品在前期的分解速度低于藨草烘干样品,但它们后期的分解速度无明显差别,而在25℃处理下,蔗草鲜样品的分解速度明显大于烘干样品.因此,在夏季丰水期,具有明显水文动态的河流中的矮小水生植物,在受淹死亡后,其分解速度较快,能对生态系统的物质循环和水质环境产生影响.     

Study of the soil–atmosphere moisture exchanges through convective drying tests in non‐isothermal conditions

This paper deals with the moisture exchanges occurring between soils and the surrounding atmosphere. Convective drying tests are performed on Awans silts at different drying temperatures and air relative humidities in order to reproduce the natural drying conditions. The experiments improve the understanding of the vapour transfers kinetics between the soil samples and the atmosphere. The experimental results are analysed assuming that the transfers take place in a boundary layer existing at the surface of the porous medium. The influence of the thermal conditions on the evaporation is also taken into account. In our model, coupled vapour and energy exchanges are controlled by mass and heat transfer coefficients characterizing the boundary layer. These coefficients are determined from the drying experiments. The modelling of the drying tests in non‐isothermal conditions is performed in order to validate the formulation of the vapour and heat exchanges. The numerical results present a good agreement with the kinetic of the materials desaturation determined during the tests. The analysis of the moisture transport mechanisms into the sample and at the boundary shows that at the beginning of the test, the drying is first achieved by the transport of moisture in its liquid form and its evaporation at the sample outer boundary in contact with the atmosphere. In a second step, vapour diffusion becomes predominant into the sample and it corresponds to the most important decrease of relative humidity. Copyright © 2009 John Wiley & Sons, Ltd.     

Cracking risk of partially saturated porous media—Part II: Application to drying shrinkage

Drying of deformable porous media results in their shrinkage, and it may cause cracking provided that shrinkage deformations are hindered by kinematic constraints. Herein, we focus on slow drying of an initially water‐saturated sample of a microheterogeneous poroelastic material damaged by parallel mesocracks. The cracking risk is analyzed by means of the thermodynamics‐based microporoelasticity model described in the companion paper (Part I), which is extended toward consideration of the hierarchical organization of cracked argillite. Drying of a material sample is studied in a framework where macrodisplacements in direction of the crack normal are blocked, while elsewise macrostress‐free boundary conditions prevail. The model implies that the opening/closure behavior of the cracks is governed by an effective pressure, in which the average crack (under)pressure, making the crack opening smaller, competes with the average micropore (under)pressure that makes the crack opening larger. The driving force for crack propagation is a quadratic function of this effective pressure. The model proposes that if drying shrinkage deformations are hindered by kinematic constraints, onset of cracking becomes possible once air entry into the cracks is observed. Copyright © 2009 John Wiley & Sons, Ltd.     

Estimation of canopy drying time after rainfall using sap flow measurements in an emergent tree in a lowland mixed‐dipterocarp forest in Sarawak,Malaysia

This study emphasizes the importance of canopy drying time (CDT) after rainfall in a lowland tropical rain forest. In this study, we estimate CDT using sap flow velocities measured by a heat‐pulse method in an emergent tree in a lowland mixed‐dipterocarp forest. Estimated CDT (ECDT) for each rain event has been defined as the time from rainfall cessation to the specific time derived from the difference between diurnal courses of sap flow velocities on a rainy day versus bright days. ECDT could be derived for 22 rain events that were grouped into two types, depending on whether rainfall ceased before or after noon. The ECDTs were distributed more widely and with greater values when rainfall ceased before noon (Type 1) than after noon (Type 2). The ECDTs of both Type 1 and Type 2 decreased with increases in net radiation (Rn) and vapour pressure deficit (VPD) after rainfall. This result shows that ECDT is determined mainly by post‐rainfall evaporation rates. The sap flow velocity as a detector of canopy wetness worked out well because of the specific rainfall characteristics at this site. The practical limitations of the method using sap flow velocities are discussed in relation to rainfall characteristics and time lags between transpirations and sap flow velocities. Copyright © 2005 John Wiley & Sons, Ltd.     

利用Van Genuchten模型拟合土壤水分特征曲线

利用Van Genuchten模型对用砂性漏斗测量中砂的吸湿过程和脱湿过程土壤水分特征曲线进行拟合,结果表明实测曲线和拟合曲线吻合很好,并得出Van Genuchten模型中的残余含水量、特性参数α和n,模拟计算表明,中砂有αw=1.69αd的关系,为研究水分和污染物在非饱和土壤中运移问题提供土壤水分运动键参数.     

华北地区气候暖干化的农业适应性对策研究

通过对华北地区1951-2000年的气温和降水的统计资料进行分析,并作出年平均气温距平变化图和平均降水变化图,得出华北地区50多年来气候呈现暖干化趋势,气温明显升高,冬季表现最为明显,降水明显减少,夏季表现最为明显。接着采用降水量减去同时期蒸发力的方法,从水分平衡的角度分析了气候暖干化对地表水资源的影响,得出近50年来,华北地区年及季的水分亏缺量总体呈增加趋势,春季亏缺尤为严重,加剧了对农业生产的不利影响。然后通过大量农业实验数据,分析了气候暖干化对农作物产量、品质、病虫害及化肥农药的使用的影响,得出气候暖干化对农业生产的影响害处远大于利处。在此基础上分析得出华北地区农业应对气候暖干化的主要问题是水问题,然后通过具体实例和对实验数据的分析,提出以下具体建议与适应性对策:积极推广和普及农业节水技术;推广集水保水技术;因水制宜调整农作物类型,发展优势作物;合理调整种植结构,优化作物布局;加强宣传教育,增强农民气候适应意识。     

Influence of cyclic wetting and drying on swelling behavior of mudstone in south west of Iran

Mudstone experience periodic swell and shrink behavior due to alternate wetting and drying cycles at arid and semi-arid regions with distinctive seasonal changes. This paper examines the influence of cyclic wetting and drying on swelling strain and swelling pressure to simulate the behavior of such rocks under heavy structures (such as building foundations) and behind the stiff support systems (such as concrete linings). Mudstone samples from Bakhtiari formation of Masjed-Soleiman region in south west of Iran were chosen to perform the laboratory tests. The swelling strain under different dead pressures and also swelling pressure under oedometeric condition were measured over time. The maximum swelling strain and pressure increased with each cycle but it had a bound. Increasing the number of wetting and drying cycles reduces the time required to reach ultimate swelling of mudstone. This is a very important conclusion which helps to determine ultimate swelling behavior in a faster way as opposed to performing standard swelling tests for very long periods. The laboratory’s results are also in good correspondence with field measurements.     

油页岩灰渣提取白炭黑的方法

以油页岩灰渣制备的水玻璃为原料,结合传统沉淀法和热风干燥工艺成功地制备了白炭黑。实验过程中研究了各种因素对白炭黑性质的影响,并确定了白炭黑的最佳制备工艺：水玻璃中w（SiO₂）为8.1%、c（H₂SO₄）为1.5 mol/L,陈化时间为8 h,反应温度为70～80 ℃,pH值为8.0。透射电镜分析结果表明：采用热风干燥技术制备的白炭黑平均粒径约为50 nm,而且团聚体较少,性能优于烘箱干燥制备的白炭黑。     

失水过程孔隙结构、孔隙比、含水率变化规律

当土体总应力状态保持不变时,基质吸力的提高是导致孔隙水排出、土样收缩、孔隙结构改变的主要原因。对于特定吸力下的土样,其微观孔隙结构决定了土壤孔隙比;而土壤孔隙结构,土壤基质吸力共同决定了土壤的含水率。假定在失水过程中,当孔隙水尚未排出时,其土壤孔隙孔径不会收缩,孔隙体积保持不变。可通过吸力建立起变化的土壤孔隙体积曲线和土-水特征曲线、土壤收缩曲线之间的对应关系：（1）累计孔隙体积曲线的包络线即为实际发生的土水特征曲线;（2）基于累计孔隙体积曲线,能够有效确定土样收缩曲线的上下限范围