On the application of the boundary layer approximation for the simulation of density stratified flows in aquifers

Stratification of the density in groundwater flow stems from the contact between water which contains minerals in low concentration with water containing a high concentration of minerals. The flow in such a flow field should be simulated by solving simultaneously the equations of continuity, motion and solute transport, because solute concentration affects the dynamics of the flow. Such an approach is generally associated with complicated calculations and numerical schemes subject to problems of convergence and stability, as the basic equations are highly nonlinear.This study applies the phenomenological boundary layer approximation, and suggests a reference to three different zones in the flow field: (a) fresh water zone, (b) transition zone, and (c) mineralized water zone. In zones (a) and (c) it is assumed that the potential flow theory can be applied. In zone (b) the flow is nonpotential but the basic similarity conditions typical to boundary layers exist.The approach suggested in this study simplifies the mathematical models that should be used for the flow field simulation. This approach is especially attractive in cases where the Dupuit approximation is applicable. In such cases very often analytical solutions can be obtained for unidirectional flows. In cases that are too complicated for representation by analytical solutions, the method can be used for the creation of simplified numerical schemes.Various examples in this study demonstrate the application of the method for various field problems associated with steady state as well as unsteady state conditions.The simplicity of the method makes it useful for variety of problems. It can be used even by small institutions and small consulting firms, who have usually access to minicomputers and microprocessors.     

磁尾等离子体片中周期性高速流相伴随磁场超低频波

本文利用Cluster卫星2004年11月8日的观测数据,分析了磁尾等离子体片中与地向周期性高速离子流相伴随的ULF波.结果显示周期性高速流的速度波动与磁场和温度中的ULF波同时出现、同时增强、同时消失,而且波动的频率都集中在60~70 mHz.这说明磁场和温度ULF波与周期性高速流密切相关,周期性高速流是ULF波产生的来源.高速流波动的相位与磁场波动的相位大致反相关,与热离子温度波动的相位正相关,同时磁场波动与热离子温度波动呈相位反相关的特性.最小方差法分析的结果显示虽然波传播方向有地向分量,但其主要传播方向是向等离子体片中心传播,并与周期性高速流速度方向垂直.以上观测说明是高速流的周期性变化产生了磁场在Pi1频率范围内的ULF波.     

河北邢束地区地壳上地幔的热作用

通过研究邢束地区的地温场、大地热流和其它地球物理资料,对该区的地温场分布、深部结构及其相互关系有了较深入的认识。研究证实:该区地温场具有明显的横向不均匀性,并与地壳上地幔结构有较清楚的对应关系。地温场的分布与构造的关系表明,凸起区与凹陷区相比,前者具有较高的热流值和较大的地温梯度。对该区地震分布与地温场的关系分析以及热应力的数学模拟结果说明,热应力在地震发生过程中具有重要作用。     

山体遮挡对滇池风生流的影响初探

用二维风生流数值模型模拟滇池湖流运动。滇池在主导风向西南风作用下,假定湖面风场是均匀的,数值模拟的湖流流态与实测湖流结果相差很大。而考虑山体遮挡影响,根据实测湖流期间现有的风情资料,在湖面上构造一非均匀风场,数值模拟结果与实测值基本一致。山体遮挡对滇池风生流的影响是不容忽视的。建议进一步进行湖流和湖面风向、风速监测,并建立过山气流数学模型,深入研究山体遮挡对湖泊风生流的影响。     

Nonlinear dynamics of Kuroshio intrusion in the Luzon Strait

This study used a 1.5-layer reduced-gravity numerical model to investigate the nonlinear dynamics of Kuroshio intrusion into the Luzon Strait. The model results suggested that both basin-scale wind curl and lateral friction are the primary factors that control the transformation of the flow, although inertia also plays an important role. Using an idealized model, both the mechanism via which the flow pattern changes depending on the two primary factors and the occurrence of hysteresis were investigated. It was established that the transformation of the Kuroshio flow field between the four previously reported flow patterns (i.e., leaping across, current looping, eddy shedding, and branch intruding) can be explained under a unified theoretical framework. A diagram is proposed to explain how the flow field transforms between the four patterns from a certain prior state when varying the values of the controlling factors.     

A heat pulse technique for the determination of small‐scale flow directions and flow velocities in the streambed of sand‐bed streams

The hyporheic zone (HZ) has the capability to eliminate and attenuate nutrients and contaminants in riverine systems. Biogeochemical reactions and the potential elimination of contaminants are strongly controlled by the flow paths and dynamics in the HZ. Nevertheless, an easily applicable method for the field determination of flow patterns in the HZ is still lacking. Therefore, a heat pulse technique, which traces the movement of a short heat pulse in the upper part of the HZ and other sand beds, was developed. Five rods are vertically driven into the sediment of the streambed; one rod with a heater as point source located in about 10‐cm sediment depth and four rods with four temperature sensors in 3 cm distance, arranged concentrically with 7 cm diameter around the heating rod. Subsequently, a heat pulse is applied and the resulting breakthrough curves are indicative of flow velocities and flow directions in the streambed. A rough data analysis procedure is also suggested. In addition, laboratory experiments were performed to test the heat pulse technique. These experiments were validated based on coupled numerical modelling of flow and heat transport. First field tests of the method prove that the method is easily applicable under field conditions. These first field tests showed highly complex flow patterns with flow velocities from 1·8 to 4·9 cm min^?1 and flow directions from parallel to surface flow to opposite to surface flow. This suggests the need for a robust method to quantify hyporheic flow patterns in situ. Copyright © 2011 John Wiley & Sons, Ltd.     

Thermal Process in the Crust and Upper Mantle in Xingtai-Shulu area of Hebei Province,China

Study of geothermal field data,terrestrial heat flow values,and other geophysical data from the Xingtai-Shulu area of Hebei Province made us more understanding of the distribution of geothermal fields and deep structures and their interrelation.The study illustrates that the geothermal field has an apparent lateral inhomogeneity and is evidently correlated by the structure of the crust and upper mantle in the area.The relation of the geothermal field distribution to the structure indicates that in comparison with the depression zone,the uplift zone has a higher heat flow value and a larger geothermal gradient.The analysis of the relation between distribution of earthquake epicenters and geothermal field and mathematical simulation of thermal stress in the area suggests that thermal stress plays an important role in the process of earthquake generation.     

Large-scale vortices in helical turbulence of incompressible fluid

Abstract

The interaction of a mean flow with a random fluctuation field is considered. This interaction is described by the averaged Navier-Stokes equation in which terms nonlinear in the fluctuation field are expressed in terms of the mean flow and the statistical properties of the fluctuation field, which is assumed to be homogeneous, isotropic, and helical. Averaged equations are derived using a functional technique. These equations are solved for a mean background flow that depends linearly on the position vector. The solutions show that large-scale vortices may arise in this system.     

Outflow of cold dense plasma associated with variation of convection in the outer magnetosphere

Cold dense plasma with the ionospheric origin is often observed in the outer magnetosphere with L values as large as 10. We have examined the electric field data accompanied by the cold dense plasma. The electric field data are obtained by the direct measurement of the drift motion of electrons released from electron guns. We get westward components of convection. In addition, there is an AC component of electric field in the ULF range larger than the DC component. If such a large variation of electric field exists in the vicinity of the stagnation point, the plasmaspheric plasma is expected to flow away to the outer magnetosphere. Then we have a test particle simulation by adding a potential variable in time. There is a particle outflow accompanied by the AC variation of electric field in the ULF range. If there are more complicated variations of electric field, it is possible that particles flow out frequently.     

于桥水库风季流场的计算与分析

本文依据水流运动的基本理论,并考虑风对水体的剪切作用,应用改进的移步ADI法,建立了风作用下流场的数值模似系统,其结果与连续四年的实测资料验证相吻合。计算预报了于桥水库三个典型水文年定常风不同风速、风向的流场,并由计算机绘制出彩色流场图。文中还针对计算应用成果进行了分析。     

Stability of the line preserving flows

We examine the equations that are used to describe flows which preserve field lines. We study what happens if we introduce perturbations to the governing equations. The stability of the line preserving flows in the case of the magneto-fluids permeated by magnetic fields is strictly connected to the non-null magnetic reconnection processes. In most of our study we use the Euler potential representation of the external magnetic field. We provide general expressions for the perturbations of the Euler potentials that describe the magnetic field. Similarly, we provide expressions for the case of steady flow as well as we obtain certain conditions required for the stability of the flow. In addition, for steady flows we formulate conditions under which the perturbations of the external field are negligible and the field may be described by its initial unperturbed form. Then we consider the flow equation that transforms quantities from the laboratory coordinate system to the related external field coordinate system. We introduce perturbations to the equation and obtain its simplified versions for the case of a steady flow. For a given system, use of this method allows us to simplify the considerations provided that some part of the system may be described as a perturbation. Next, to study regions favourable for the magnetic reconnection to occur we introduce a deviation vector to the basic line preserving flows condition equation. We provide expressions of the vector for some simplifying cases. This method allows us to examine if given perturbations either stabilise the system or induce magnetic reconnection. To illustrate some of our results we study two examples, namely a simple laboratory plasma flow and a simple planetary magnetosphere model.     

Conjugate observations of the day-side reconnection electric field: A GEM boundary layer campaign

Data from HF-radars are used to make the first simultaneous conjugate measurements of the day-side reconnection electric field. A period of 4 h around local magnetic noon are studied during a geospace environment modeling (GEM) boundary layer campaign. The interplanetary magnetic field (IMF) was southward whilst the eastward component (By) was variable. The flow patterns derived from the radar data show the expected conjugate asymmetries associated with IMF |By| ≥ 0. High-time resolution data (50 and 100 s) enable the flow of plasma across the open/closed field line boundary (the separatrix) to be studied in greater detail than in previous work. The latitude of the separatrix follows the same general trend in both hemispheres but shows a hemispherical difference of 4°, with the summer cusp at higher latitude, as expected from dipole tilt considerations. However, the short-time scale motion of the separatrix cannot be satisfactorily resolved within the best resolution (300 m s⁻¹) of the experiment. The orientation of the separatrix with respect to magnetic latitude is found to follow the same trend in both hemispheres and qualitatively fits that predicted by a model auroral oval. It shows no correlation with IMF By. However, the degree of tilt in the Northern (summer) Hemisphere is found to be significantly greater than that given by the model oval. The convection pattern data show that the meridian at which throat flow occurs is different in the two hemispheres and is controlled by IMF By, in agreement with empirically derived convection patterns and theoretical models. The day-side reconnection electric field values are largest when the radar’s meridian is in the throat flow or early afternoon flow regions. In the morning or afternoon convection cells, the reconnection electric field tends to zero away from the throat flow region. The reconnection electric field observed in the throat flow region is bursty in nature.     

Terrestrial heat flow in the natural steam field at Larderello

Summary Measurements of thermal gradients and rock conductivity in the steam field of Larderello, along a cross section of about 12 km length, reveal a great anomaly of terrestrial heat flow. Heat flow varies between 6 and 14 cal-cm² sec, while thermal gradients are between 200 and 800 deg C-km. The measurements show that heat flow values characterize better the productive areas than thermal gradient anomalies. Knowing the heat flow it is possible to set up the thermal balance of the steam producing area, which is of considerable importance in planning the production of the steam field.     

Geomagnetic cutoff rigidities of cosmic rays in a model of the bounded magnetosphere with the ring current

The cosmic ray geomagnetic cutoff rigidities are obtained by analytical calculations within an axisymmetric model of bounded magnetosphere, the magnetic field of which is created by the dipole field of the Earth and by two spheres located beyond the Earth with the currents that flow along the parallels and have a value proportional to the cosine of latitude. The inner sphere models the ring current flowing in the westerly direction; the outer sphere simulates the currents over the magnetopause, which flow in the easterly direction. The analytical results of calculations of variations in the geomagnetic cutoff rigidity for different levels of geomagnetic disturbances are given. The results are compared with the results of analytical calculations within the model of unbounded magnetosphere (when the outer sphere is absent).     

Influence of a convective generator on the diurnal behavior of the electric field strength in the near-Earth atmosphere in Kamchatka

Diurnal variations in the electric field strength, electrical conductivity, and temperature in the near-Earth atmosphere under “fair-weather” conditions at the Paratunka observatory (Kamchatka) are considered. It is shown that the morning maximum in the electric field diurnal behavior is caused by air convection in the near-surface layer. The difference in the atmospheric temperatures near the Earth’s surface and at an altitude of 25 m is chosen as a measure of the convective air flow. A high correlation of the values of the temperature difference for these altitudes with the diurnal behavior of the electric field strength is obtained.     

陆地下垫面与环境气流对锋面气旋发展过程影响的理想试验

地面摩擦和大尺度流场是影响锋面气旋结构演变的重要因子,本文使用WRF模式并采用湿物理方案,通过理想化试验,综合考虑陆面摩擦、气旋式扰动相对于急流位置和大尺度流场对锋面气旋结构变化的影响.结果表明：当仅考虑单一因子时,气旋式扰动位于急流南侧和辐合流流场有利于气旋形成Shapiro-Keyser（S-K）模型结构.当同时考虑地面摩擦和大尺度辐合时,气旋式扰动位于急流北侧的气旋发展整体向经典的挪威气旋模型转变;扰动位于急流南侧的的气旋发展则整体呈现S-K模型结构,此时辐合流场有利于S-K模型结构出现.当同时考虑地面摩擦和大尺度辐散时,扰动处于急流北侧的气旋呈现挪威气旋模型结构;由于气旋式扰动穿越急流和辐散流场同时有利于暖锋后弯发展以及冷暖锋距离加大和锋消,扰动处于急流南侧的气旋呈现典型S-K气旋模型结构.这个结果解释了在东亚大陆地区辐散场形势下出现的S-K模型结构气旋个例.     

西山遮挡对滇池风生流影响的数值模拟

对于湖泊风生流的模拟,湖 面风场数据是决定精度高低的关键因素, 而湖泊周围的山地等复杂地势常常引起湖面风速风向的不均匀分布,致使湖流流态因地形因素而变化较大,本文建立了一个适合于湖泊区域的中小尺度三维过山气流数值模型,用以计算受山体遮挡的风场合沿水深平均的二维湖泊风生流数值模型模拟了实际地形条件下的滇池风生流,并与均匀风     

Modeling the effects of nonlinear equilibrium sorption on the transport of solute plumes in saturated heterogeneous porous media

Transport of sorbing solutes in 2D steady and heterogeneous flow fields is modeled using a particle tracking random walk technique. The solute is injected as an instantaneous pulse over a finite area. Cases of linear and Freundlich sorption isotherms are considered. Local pore velocity and mechanical dispersion are used to describe the solute transport mechanisms at the local scale. This paper addresses the impact of the degree of heterogeneity and correlation lengths of the log-hydraulic conductivity field as well as negative correlation between the log-hydraulic conductivity field and the log-sorption affinity field on the behavior of the plume of a sorbing chemical. Behavior of the plume is quantified in terms of longitudinal spatial moments: center-of-mass displacement, variance, 95% range, and skewness. The range appears to be a better measure of the spread in the plumes with Freundlich sorption because of plume asymmetry. It has been found that the range varied linearly with the travelled distance, regardless of the sorption isotherm. This linear relationship is important for extrapolation of results to predict behavior beyond simulated times and distances. It was observed that the flow domain heterogeneity slightly enhanced the spreading of nonlinearly sorbing solutes in comparison to that which occurred for the homogeneous flow domain, whereas the spreading enhancement in the case of linear sorption was much more pronounced. In the case of Freundlich sorption, this enhancement led to further deceleration of the solute plume movement as a result of increased retardation coefficients produced by smaller concentrations. It was also observed that, except for plumes with linear sorption, correlation between the hydraulic conductivity and the sorption affinity fields had minimal effect on the spatial moments of solute plumes with nonlinear sorption.