喀斯特区域的水化学不稳定性——以黔中地区为例

黔中地区是岩溶作用发育的喀斯特区域.1993年秋和1994年春末采自该地区不同类型水样的水化学分析表明:碳酸盐岩裂隙泉水呈中偏碱性,为[C]Ca-Ⅱ型;秋季离子总量、HCO₃-及Ca2+浓度大于春季.裂隙泉水流经地表一定距离后,HCO₃-降低,pH及SO₄2-、Cl-、K+、Na+明显增高.黄果树的天星桥、水帘洞及落水潭三个部位河水的SO₄2-、Na+以及Fe3+、NO₃-等浓度的季节性变化也更加明显.春季因瀑布暴气,CO₂的逸出,钙华生成更强烈一些.红枫湖作为喀斯特区域地表水的汇集地,HCO₃-与SO₄2-的当量比值仅2.1～2.4;Ca2+与Mg2+比值上升为2.4～4.2,Cl-浓度较碳酸盐岩裂隙泉水增高1倍;Na+浓度增高一个数量级.说明流域内地表土层溶蚀及人为污染影响的增强.土层孔隙水属强矿化水,而湖水呈现过渡特征.碳酸盐沉淀作用、硫酸盐矿化作用及固氮氨化作用,导致水质组成的显着差异.     

Role of barotropic mechanism in the development of a monsoon depression: A MONEX study

The role of barotropic processes in the development of a monsoon depression, formed on 5 July 1979 during MONEX observational period, is studied by considering it as a quasi-geostrophic divergent barotropic instability problem of zonal flow of 3 July 1979 at 700 mb level. Numerical solutions are obtained by initial value approach. The preferred wave has a wavelength of 2750 km, an e-folding time of 4.3 days, a period of 6.5 days and an eastward phase speed of 4.9 ms^–1. Structure of preferred wave is found to be in good agreement with the observed horizontal structure of the depression at 700 mb. Poleward momentum transports are found to predominate over equatorward transports.Parts of this paper were presented at the National Symposium on Early Results of MONEX-1979. 9–12 March 1981, in New Delhi, India.     

Role of barotropic,baroclinic and combined barotropic-baroclinic instability for the growth of monsoon depressions and mid-tropospheric cyclones

A detailed barotropic, baroclinic and combined barotropic-baroclinic stability analysis has been carried out with mean monsoon zonal currents over western India, eastern India and S.E. Asia. The lower and middle tropospheric zonal wind profiles over western India are barotropically unstable. The structure and growth rate of these modes agree well with the observed features of the midtropospheric cyclones. Similar profiles over eastern India and S.E. Asia, however, are barotropically stable. This is attributed to weak horizontal shear, inherent to these profiles. The upper tropospheric profiles, on the other hand, are barotropically unstable throughout the whole region. The features of these unstable modes agree with those of observed easterly waves. The baroclinic and combined barotropic-baroclinic stability analyses show that the baroclinic effects are not important in tropics. Though the barotropic instability of the mean zonal current seems to be res ponsible for the initial growth of the mid-tropospheric cyclones, neither barotropic nor baroclinic instability of the mean zonal current seem to explain the observed features of the monsoon depressions.     

Some theoretical problems of the planetary-scale monsoons

Summary Some important theoretical problems of the planetary-scale monsoons which have arisen from recent advances of observational studies are reviewed. These include: (1) the requirement of a strong damping mechanism in the planetary scale vorticity budget of summer monsoon and a similar but weaker requirement for the winter monsoon; (2) the localized barotropic instability of the summer monsoon which is a result of the strong zonal asymmetry of the planetary-scale flow and causes significant nonlinear energy conversions; and (3) the oscillations of the planetary-scale monsoons. It is pointed out that these problems are inter-related and their understanding is also important for the proper simulation of other scales of motion of the monsoon circulation.     

Lagrangian meshfree particles method (SPH) for large deformation and failure flows of geomaterial using elastic–plastic soil constitutive model

Simulation of large deformation and post‐failure of geomaterial in the framework of smoothed particle hydrodynamics (SPH) are presented in this study. The Drucker–Prager model with associated and non‐associated plastic flow rules is implemented into the SPH code to describe elastic–plastic soil behavior. In contrast to previous work on SPH for solids, where the hydrostatic pressure is often estimated from density by an equation of state, this study proposes to calculate the hydrostatic pressure of soil directly from constitutive models. Results obtained in this paper show that the original SPH method, which has been successfully applied to a vast range of problems, is unable to directly solve elastic–plastic flows of soil because of the so‐called SPH tensile instability. This numerical instability may result in unrealistic fracture and particles clustering in SPH simulation. For non‐cohesive soil, the instability is not serious and can be completely removed by using a tension cracking treatment from soil constitutive model and thereby give realistic soil behavior. However, the serious tensile instability that is found in SPH application for cohesive soil requires a special treatment to overcome this problem. In this paper, an artificial stress method is applied to remove the SPH numerical instability in cohesive soil. A number of numerical tests are carried out to check the capability of SPH in the current application. Numerical results are then compared with experimental and finite element method solutions. The good agreement obtained from these comparisons suggests that SPH can be extended to general geotechnical problems. Copyright © 2008 John Wiley & Sons, Ltd.     

MHD instability due to non-homogeneity and non-uniformity of magnetic field in gravitating medium and its consequences in the central region of galaxies

The MHD wave instabilities due to non-uniform magnetic field and non-homogeneity of density have been studied. The reference (coordinate) system considered here is cylindrical type. The General Dispersion Relation (GDR) for the wave propagation in a gravitating but non-relativistic region has been derived. Similar to common knowledge, the said non-uniformities have been found to be responsible for the instability of the system. But interestingly many instability factors are produced due to presence of two types of non-uniformities simultaneously. This theory may add more clues for the event of instabilities, formation of hot plasma-bed in Galactic Central Region, and mass out-flow from there. Many conditions for instabilities could be obtained from GDR deduced here. However, a few conditions for critical wavelength of the MHD wave have been obtained in terms of system parameters (like gradient of magnetic field and rotation). This theory, in turn, may be helpful for the better understanding of the Explosion Theory of formation of outer structure of Galaxies like ours.     

Large-eddy simulation of the turbulent near wake behind a circular cylinder: Reynolds number effect

The purpose of the present work is <!–<query>The highlights are in an incorrect format. Hence they have been deleted. Please refer the online instructions: http://www.elsevier.com/highlights and provide 3-5 bullet points.</query>–>to study the effect of the Reynolds number on the near-wake structure and separating shear layers behind a circular cylinder. Three-dimensional unsteady large-eddy simulation is carried out and two different subgrid scale models are applied in order to evaluate the turbulent wake reasonably. The Reynolds number based on the free-stream velocity and the cylinder diameter is ranging from Re = 5500–41,300, corresponding to the full development of the shear-layer instability in the intermediate subcritical flow regime. For a complete validation of this numerical study, hydrodynamic bulk coefficients are computed and compared to experimental measurements and numerical studies in the literature. Special focus is made on the variations of both the large-scale near-wake structure and the small-scale shear-layer instability with increasing Reynolds numbers. The present numerical study clearly shows the broadband nature of the shear-layer instability as well as the dependence of the shear-layer frequency especially on the high Reynolds numbers.     

Modelling of the spatial evolution of extreme laboratory wave crest and trough heights with the NLS-type equations

The statistical properties of long-crested nonlinear wave time series measured in an offshore basin have been analyzed in different aspects such as the distributions of surface elevation, wave crest, wave trough, and wave period. Comparison with linear, second-order and third-order theoretical models indicates that although bound wave effects also contribute to the deviation from a Gaussian process, it is the modulational instability that primarily determines the discrepancy in the evolution process in the presence of strong nonlinearity. Interestingly enough, wave crest is more sensitive to the quasi-resonant four-wave interaction effect than wave trough and the scaled maximal wave crest presents a linear regression model with the coefficient of kurtosis. Meanwhile, the estimation of the observed statistical properties is reconstructed on the basis of an ensemble of 100 wave series simulated by the NLS-type equations and compared favourably with the experimental results in most cases. Moreover, with the increased third-order nonlinear effect the difference between NLS and Dysthe simulations is enlarged and mainly reflected on the distribution of wave crest.     

贵阳某些表层岩溶基坑边坡局部失稳机理分析

随着建设活动的日益频繁以及建筑范围的扩大,局部表层岩溶被揭露,出现不同于基坑整体失稳的局部岩溶充填物的失稳现象,此失稳现象往往和地下岩溶水相联系,因为处于局部,不能引起设计人员的重视,在沟槽中岩溶水动力的作用下失稳,可能产生较大的工程问题。     

某深基坑工程边坡变形失稳致因及处理对策

针对某深基坑工程在施工过程中,出现支护结构和邻近建筑物变形沉降增大、边坡有失稳滑移的危险状况,采用场地水文地质特性调查、边坡支护方案模拟计算与施工监测数据分析相结合的研究方法,确定场地中对施工扰动敏感性强的原状粉土和粉细砂层的沉降是导致基坑边坡失稳的主因,采用土层锚拉加固方案取代对地层扰动较大的注浆排险方案,确保了基坑安全开挖和边坡的最终稳定。