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排序方式: 共有128条查询结果,搜索用时 828 毫秒
1.
Distribution of dissolved species and suspended particulate copper in an intertidal ecosystem affected by copper mine tailings in Northern Chile 总被引:1,自引:0,他引:1
The coastline near Chañaral in Northern Chile is one of the most highly Cu-contaminated zones in the world due to discharges from mining activities for more than 60 years. The speciation of Cu has been studied to determine the importance of organic complexation in highly contaminated areas, and to assess the likely physiological impacts of Cu on marine organisms. Dissolved Cu concentrations of up to 500 nM were measured, completely saturating organic ligands and leading to free Cu2+ concentrations in excess of 10− 8 M. These values are higher than those reported in any other marine environment, and because they occur over an extensive area, provide a unique opportunity to study the effects of Cu on marine ecosystems and to see how Cu behaves when its speciation is predominantly inorganic. We found strong gradients in free Cu2+ between Chañaral and adjacent areas with lower Cu, where speciation is dominated by organic complexation. There is also a significant increase in the partitioning of Cu onto suspended particles in the contaminated areas, consistent with previous studies that showed that organic ligands stabilize Cu in the dissolved phase, whilst “excess” Cu is rapidly scavenged. Those high dissolved Cu concentrations persist in spite of solid phase partitioning and advective processes along this open-ocean coastline, suggesting that Cu inputs into the system are still very large. Measurements were made using anodic stripping voltammetry with a thin mercury film coated with Nafion, which previous workers have shown can mitigate ambiguity in the data arising from inadvertent reduction of organic complexes. Our findings suggest that this is a useful methodology for contaminated systems. 相似文献
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Wind velocity and sand transport on a barchan dune 总被引:2,自引:0,他引:2
G. Sauermann J. S. Andrade Jr. L. P. Maia U. M. S. Costa A. D. Araújo H. J. Herrmann 《Geomorphology》2003,54(3-4):245-255
We present measurements of wind velocity and sand flux performed on the windward side of a large barchan dune in Jericoacoara, northeastern Brazil. From the measured profile, we calculate the air shear stress using an analytical approximation and treat the problem of flow separation by an heuristic model. We find that the results from this approach agree well with our field data. Moreover, using the calculated shear velocity, we predict the sand flux according to well-known equilibrium relations and with a phenomenological continuum saltation model that includes saturation transients and thus allows for nonequilibrium conditions. Based on the field data and theoretical predicted results, we indicate the principal differences between saturated and nonsaturated sand flux models. Finally, we show that the measured dune moves with invariant shape and predict its velocity from our data and calculations. 相似文献
4.
The resistivity method is often used in cave prospecting. In this paper the pole–dipole array ability to detect cavities at different depths and with different water contents is investigated. The research was performed using analogical and numerical modelling. According to the results, empty caves at a depth less than four times its diameter can be easily detected. The ability of the pole–dipole array to detect water-filled caves reaches a minimum at filling percentages of 30–50%. Overburden effects research shows that low resistivity overburden reduces the resolution capability of the array. This study shows that equivalent results can be obtained by modelling the empty caves as infinite resistivity bodies or, alternatively, as very high resistivity ones. The analysis of field data acquired in the Maciço Calcário Estremenho (Portugal) shows the practical importance of the pole–dipole array in cavities prospecting. 相似文献
5.
Throughfall patterns in sugarcane and riparian forest: understanding the effect of sugarcane age and land use conversion 下载免费PDF全文
Robson Willians da Costa Silva Luiz Felippe Salemi Rafael Pires Fernandes Tatiana Morgan Berteli de Andrade Jorge Marcos de Moraes Plínio Barbosa de Camargo Luiz Antonio Martinelli 《水文研究》2016,30(15):2579-2589
Sugarcane is an annual crop with a dynamic canopy that changes over time mainly because of genetic adaptation. There is uncertainty about the temporal trends of throughfall (TF) in this important commercial crop. In the present paper, we used troughs to measure TF in a third and fourth ratoon and subsequently in a fourth and fifth ratoon. Additional measurements were carried out in an adjacent riparian forest. There were no significant differences between cycles of sugarcane, growth phases and riparian forest. The TF results for ratoon crop and riparian forest in 2011/2012 were 76% and 79.5% of gross rainfall, respectively, while in 2012/2013, they were 79% and 78%, respectively. However, TF was remarkably lower in the riparian forest relative to ratoon from the second half of the culm formation and elongation phase (280 days after harvest) until harvest. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
6.
This paper endows the recently‐proposed granular element method (GEM) with the ability to perform 3D discrete element calculations. By using non‐uniform rational B‐Splines to accurately represent complex grain geometries, we proposed an alternative approach to clustering‐based and polyhedra‐based discrete element methods whereby the need for complicated and ad hoc approaches to construct 3D grain geometries is entirely bypassed. We demonstrate the ability of GEM in capturing arbitrary‐shaped 3D grains with great ease, flexibility, and without excessive geometric information. Furthermore, the applicability of GEM is enhanced by its tight integration with existing non‐uniform rational B‐Splines modeling tools and ability to provide a seamless transition from binary images of real grain shapes (e.g., from 3D X‐ray CT) to modeling and discrete mechanics computations.Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
7.
We introduce a new DEM scheme (LS-DEM) that takes advantage of level sets to enable the inclusion of real grain shapes into a classical discrete element method. Then, LS-DEM is validated and calibrated with respect to real experimental results. Finally, we exploit part of LS-DEM potentiality by using it to study the dependency of critical state (CS) parameters such as critical state line (CSL) slope \(\lambda \), CSL intercept \(\varGamma \), and CS friction angle \(\varPhi _{\mathrm{CS}}\) on the grain’s morphology, i.e., sphericity, roundness, and regularity. This study is carried out in three steps. First, LS-DEM is used to capture and simulate the shape of five different two-dimensional cross sections of real grains, which have been previously classified according to the aforementioned morphological features. Second, the same LS-DEM simulations are carried out for idealized/simplified grains, which are morphologically equivalent to their real counterparts. Third, the results of real and idealized grains are compared, so the effect of “imperfections” on real particles is isolated. Finally, trends for the CS parameters (CSP) dependency on sphericity, roundness, and regularity are obtained as well as analyzed. The main observations and remarks connecting particle’s morphology, particle’s idealization, and CSP are summarized in a table that is attempted to help in keeping a general picture of the analysis, results, and corresponding implications. 相似文献
8.
By incorporating the fabric effect and Lode’s angle dependence into the Mohr–Coulomb failure criterion, a strength criterion for cross-anisotropic sand under general stress conditions was proposed. The obtained criterion has only three material parameters which can be specified by conventional triaxial tests. The formula to calculate the friction angle under any loading direction and intermediate principal stress ratio condition was deduced, and the influence of the degree of the cross-anisotropy was quantified. The friction angles of sand in triaxial, true triaxial, and hollow cylinder torsional shear tests were obtained, and a parametric analysis was used to detect the varying characteristics. The friction angle becomes smaller when the major principal stress changes from perpendicular to parallel to the bedding plane. The loading direction and intermediate principal stress ratio are unrelated in true triaxial tests, and their influences on the friction angle can be well captured by the proposed criterion. In hollow cylinder torsional shear tests with the same internal and external pressures, the loading direction and intermediate principal stress ratio are related. This property results in a lower friction angle in the hollow cylinder torsional shear test than that in the true triaxial test under the same intermediate principal stress ratio condition. By comparing the calculated friction angle with the experimental results under various loading conditions (e.g., triaxial, true triaxial, and hollow cylinder torsional shear test), the proposed criterion was verified to be able to characterize the shear strength of cross-anisotropic sand under general stress conditions. 相似文献
9.
L. C. Garcia de Andrade 《Astrophysics and Space Science》2007,310(1-2):25-29
Riemannian geometrical effects on the expansion of the electron magnetohydrodynamical (EMH) superconductivity modeled twisted
nonplanar thin magnetic flux tubes are considered. A solution is found which represents almost incompressible plasma flows,
where the twist of flux tube is computed in terms of the continuous variation of its cross-section. It is shown that the twist
increases in regions where twisted flux tube expands as in Parker’s conjecture. From computation of compression along the
tube we show that when the torsion is weak a centrifugal or vorticity effect on the longitudinal direction of the tube enhances
the screening effect on the “superconductor”. Throughout the paper we consider helical flux tubes where torsion and curvature
of the tube are constants. Thus we show that the Parker’s conjecture is valid in a continuos manner for these type II superconducting
twisted flux tubes. Throughout the paper we adopt the approximation that the radial component of the magnetic field varies
so slowly along the tube axis that it can be approximated to zero along the tube. It is suggested that the models discussed
here may also be applied to DNA and nanotubes. 相似文献
10.