Finite difference modeling of contaminant transport using analytic element flow solutions     

James R. Craig  Alan J. Rabideau 《Advances in water resources》2006

The two-dimensional implementation of the analytic element method (AEM) is commonly used to simulate steady-state saturated groundwater flow phenomena at regional and local scales. However, unlike alternative groundwater flow simulation methods, AEM results are not ordinarily used as the basis for simulation of reactive solute transport. The use of AEM-simulated flow fields is impeded by the discrepancy between a continuous representation of flow and a typically discrete representation of transport, and requires translation of the flow solution to a discrete analog. This paper presents a variety of methods for analytically calculating conservative discrete water fluxes and integrated components of the dispersion tensor across cell interfaces. An Eulerian finite difference method based on these AEM-derived parameters is implemented for use in simulation of 2D (vertically averaged) solute transport. This implementation is first benchmarked against existing methods that use standard finite difference flow solutions, then used to investigate the effects of an inaccurate discrete water balance. It is shown that improper translation of AEM fluxes leads to significant water balance errors and inaccurate simulation of contaminant transport.  相似文献   

Water clarity modeling in Lake Tahoe: Linking suspended matter characteristics to Secchi depth     

Theodore J. Swift  Joaquim Perez-Losada  S. Geoffrey Schladow  John E. Reuter  Alan D. Jassby  Charles R. Goldman 《Aquatic Sciences - Research Across Boundaries》2006,68(1):1-15

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Simulation of a Stably Stratified Atmospheric Boundary Layer Using One-Dimensional Turbulence     

Alan R. Kerstein  Scott Wunsch 《Boundary-Layer Meteorology》2006,118(2):325-356

One-dimensional turbulence (ODT) is a single-column simulation in which vertical motions are represented by an unsteady advective process, rather than their customary representation by a diffusive process. No space or time averaging of mesh-resolved motions is invoked. Molecular-transport scales can be resolved in ODT simulations of laboratory-scale flows, but this resolution of these scales is prohibitively expensive in ODT simulations of the atmospheric boundary layer (ABL), except possibly in small subregions of a non-uniform mesh.Here, two methods for ODT simulation of the ABL on uniform meshes are described and applied to the GABLS (GEWEX Atmospheric Boundary Layer Study; GEWEX is the Global Energy and Water Cycle Experiment) stable boundary-layer intercomparison case. One method involves resolution of the roughness scale using a fixed eddy viscosity to represent subgrid motions. The other method, which is implemented at lower spatial resolution, involves a variable eddy viscosity determined by the local mesh-resolved flow, as in multi-dimensional large-eddy simulation (LES). When run at typical LES resolution, it reproduces some of the key high-resolution results, but its fidelity is lower in some important respects. It is concluded that a more elaborate empirically based representation of the subgrid physics, closely analogous to closures currently employed in LES of the ABL, might improve its performance substantially, yielding a cost-effective ABL simulation tool. Prospects for further application of ODT to the ABL, including possible use of ODT as a near-surface subgrid closure framework for general circulation modeling, are assessed.  相似文献   

Progressive aqueous alteration of CM carbonaceous chondrites     

Alan E. Rubin  Josep M. Trigo-Rodríguez  Heinz Huber 《Geochimica et cosmochimica acta》2007,71(9):2361-2382

CM chondrites are aqueously altered rocks that contain ∼9 wt% H₂O⁺ (i.e., indigenous water) bound in phyllosilicates; also present are clumps of serpentine-tochilinite intergrowths (previously called “poorly characterized phases” or PCP), pentlandite and Ni-bearing pyrrhotite. We studied 11 CM chondrites that span the known range from least altered to most altered. We used various petrologic properties (many previously identified) that provide information regarding the degree of aqueous alteration. There are no known unaltered or slightly altered CM chondrites (e.g., rocks containing numerous chondrules with primary igneous glass). Some CM properties result from processes associated with early and intermediate stages of the alteration sequence (i.e., hydration of matrix, alteration of chondrule glass, and production of large PCP clumps). Other petrologic properties reflect processes active throughout the alteration sequence; these include oxidation of metallic Fe-Ni, alteration of chondrule phenocrysts, changes in PCP composition (reflecting an increase in the phyllosilicate/sulfide ratio), and changes in carbonate mineralogy (reflecting the development of dolomite and complex carbonates at the expense of Ca carbonate).On the basis of these parameters, we propose a numerical alteration sequence for CM chondrites. Because there are no known CM samples that display only incipient alteration, the least altered sample was arbitrarily assigned to subtype 2.6. The most altered CM chondrites, currently classified CM1, are assigned to subtype 2.0. These highly altered rocks have essentially no mafic silicates; they contain chondrule pseudomorphs composed mainly of phyllosilicate. However, their bulk compositions are CM-like, and they are closer in texture to other C2 chondrites than to CI1 chondrites (which lack chondrule pseudomorphs). Using several diagnostic criteria, we assigned petrologic subtypes (±0.1) to every CM chondrite in our study: QUE 97990, CM2.6; Murchison, CM2.5; Kivesvaara, CM2.5; Murray, CM2.4/2.5; Y 791198, CM2.4; QUE 99355, CM2.3; Nogoya, CM2.2; Cold Bokkeveld, CM2.2; QUE 93005, CM2.1; LAP 02277, CM2.0; MET 01070, CM2.0.The proposed CM numerical alteration sequence improves upon the existing scheme of Browning et al. (1996) in that it does not require a complicated algorithm applied to electron-microprobe data to determine the average matrix phyllosilicate composition. The new sequence is more comprehensive and employs petrologic subtypes that are easier to use and remember than mineralogic alteration index values.New neutron-activation analyses of QUE 97990, QUE 93005, MET 01070, Murchison and Crescent, together with literature data, confirm the compositional uniformity of the CM group; different degrees of alteration among CM chondrites do not lead to resolvable bulk compositional differences. This suggests that the textural differences among individual CM chondrites reflect progressive alteration of similar hypothetical CM3.0 starting materials in different regions of the same parent body, with minimal aqueous transport of materials over appreciable (e.g., meters) distances.  相似文献   

Petrogenesis of acapulcoites and lodranites: A shock-melting model     

Alan E. Rubin 《Geochimica et cosmochimica acta》2007,71(9):2383-2401

Acapulcoites are modeled as having formed by shock melting CR-like carbonaceous chondrite precursors; the degree of melting of some acapulcoites was low enough to allow the preservation of 3-6 vol % relict chondrules. Shock effects in acapulcoites include veins of metallic Fe-Ni and troilite, polycrystalline kamacite, fine-grained metal-troilite assemblages, metallic Cu, and irregularly shaped troilite grains within metallic Fe-Ni. While at elevated temperatures, acapulcoites experienced appreciable reduction. Because graphite is present in some acapulcoites and lodranites, it seems likely that carbon was the principal reducing agent. Reduction is responsible for the low contents of olivine Fa (4-14 mol %) and low-Ca pyroxene Fs (3-13 mol %) in the acapulcoites, the observation that, in more than two-thirds of the acapulcoites, the Fa value is lower than the Fs value (in contrast to the case for equilibrated ordinary chondrites), the low FeO/MnO ratios in acapulcoite olivine (16-18, compared to 32-38 in equilibrated H chondrites), the relatively high modal orthopyroxene/olivine ratios (e.g., 1.7 in Monument Draw compared to 0.74 in H chondrites), and reverse zoning in some mafic silicate grains. Lodranites formed in a similar manner to acapulcoites but suffered more extensive heating, loss of plagioclase, and loss of an Fe-Ni-S melt.Acapulcoites and lodranites experienced moderate post-shock annealing, presumably resulting from burial beneath material of low thermal diffusivity. The annealing process repaired damaged olivine crystal lattices, lending acapulcoites and lodranites the appearance of unshocked (i.e., shock-stage S1) rocks. Any high-pressure phases that may have formed during initial shock reverted to their low-pressure polymorphs during annealing. Some samples were subsequently shocked again; several acapulcoites reached shock-stage S2 levels, ALH 84190 reached S3, and the lodranite MAC 88177 reached S5.  相似文献   

Determination of the distribution of size of irregularly shaped particles from laser diffractometer measurements     

Claudio L. Schneider  Reiner Neumann  Alan S. Souza 《International Journal of Mineral Processing》2007

The formal stereological transformation equation for particle sieve size distribution from measurements in lower dimensional spaces is applied to laser diffractometer measurements. The transformation function for iron ore particles is measured experimentally, and modeled. The solution is tested against the measured transformation function data as well as synthetic composite distributions of the original sample. The natural size distribution of a sample taken from a grinding circuit stream was measured by a combination of standard sieving and cyclosizer, and the result is compared to the transformed size distribution calculated from laser diffractometer measurements. The stereological transformation technique performed well in all cases.  相似文献   

A three-dimensional finite-element model of wind effects upon higher harmonics of the internal tide     

Philip Hall  Alan M. Davies 《Ocean Dynamics》2007,57(4-5):305-323

A non-linear three-dimensional unstructured grid model of the M₂ tide in the shelf edge area off the west coast of Scotland is used to examine the spatial distribution of the M₂ internal tide and its higher harmonics in the region. In addition, the spatial variability of the tidally induced turbulent kinetic energy and associated mixing in the area are considered. Initial calculations involve only tidal forcing, although subsequent calculations are performed with up-welling and down-welling favourable winds to examine how these influence the tidal distribution (particularly the higher harmonics) and mixing in the region. Both short- and long-duration winds are used in these calculations. Tidal calculations show that there is significant small-scale spatial variability particularly in the higher harmonics of the internal tide in the region. In addition, turbulence energy and mixing exhibit appreciable spatial variability in regions of rapidly changing topography, with increased mixing occurring above seamounts. Wind effects significantly change the distribution of the M₂ internal tide and its higher harmonics, with appreciable differences found between up- and down-welling winds and long- and short-duration winds because of differences in mixing and the presence of wind-induced flows. The implications for model validation, particularly in terms of energy transfer to higher harmonics, and mixing are briefly discussed.  相似文献   

On the influence of stratification and tidal forcing upon mixing in sill regions     

Alan M. Davies  Jiuxing Xing 《Ocean Dynamics》2007,57(4-5):431-451

A cross-sectional non-hydrostatic model with idealized topography was used to examine the processes influencing tidal mixing in the region of sills. Initial calculations with appropriate parameters for the sill at the entrance to Loch Etive showed that the model could reproduce the main features of the observed mixing in the region. In particular, the hydraulic jump in the sill region was reproduced, as was an intense mid-water jet that was observed to separate from the lee side of the sill. Shear instabilities associated with the jet appeared to be a source of mixing within the thermocline. In addition, internal lee waves were generated on the lee side of the sill, with the observed amplification because of trapping during the flood stage. Their magnitude and hence the mixing increased with increasing Froude number (F _r). In the case of vertically varying buoyancy frequency, its value near the sill top determined the F _r number, with its value below influencing internal waves magnitude at depth. At high F _r values particularly with strong currents, short waves and overturning occurred.  相似文献   

What explains agricultural performance: climate normals or climate variance?     

Robert Mendelsohn  Alan Basist  Ariel Dinar  Pradeep Kurukulasuriya  Claude Williams 《Climatic change》2007,81(1):85-99

This paper measures the influence of climate normals (average long-term surface wetness and temperature) and interannual climate variance on farms in the United States and Brazil using satellite data. The paper finds that just climate normals or just climate variance variables can explain both net revenues and how much land is used for cropland. However, because they are correlated with each other, it is important to include both normals and variance in the same statistical model to get accurate measures of their individual contribution to farm outcomes. In general, higher climate variance increases the probability that land is used for cropland in both countries and higher temperatures reduce both cropland and land values. Other annual effects were not consistent across the two countries.  相似文献   

Mechanisms of orthopyroxene dissolution in silica-undersaturated melts at 1 atmosphere and implications for the origin of silica-rich glass in mantle xenoliths   总被引：2，自引：0，他引：2  

Shaw  Cliff S. J.  Thibault  Yves  Edgar  Alan D.  Lloyd  Felicity E. 《Contributions to Mineralogy and Petrology》1998,132(4):354-370

Experiments dissolving orthopyroxene (En₉₃) in a variety of Si-undersaturated alkaline melts at 1 atmosphere and variable f _O2 demonstrate that orthopyroxene dissolves to form olivine, Si-rich melt and clinopyroxene. These phases form a texturally and chemically distinct boundary layer around the partly dissolved orthopyroxene crystals. The occurrence of clinopyroxene in the boundary layer is due to inward diffusion of Ca from the solvent melt to the boundary layer causing clinopyroxene saturation. Compositional profiles through the solvent and the boundary layer for a number of experiments demonstrate rapid diffusion of cations across the boundary layer – solvent interface. SiO₂ diffuses outward from the boundary layer whereas CaO and Al₂O₃ diffuse toward the Si-enriched boundary layer melt. The rate of Al diffusion is slower under reducing conditions compared to the rates in experiments performed in air. Concentrations of FeO and MgO in the boundary layer and solvent are approximately equal indicating rapid diffusion and attainment of equilibrium despite ongoing crystallisation of clinopyroxene within the boundary layer. The behaviour of Na₂O and K₂O is strongly affected by f _O2. Under reducing conditions Na₂O and K₂O concentrations are approximately equal in the boundary layer and solvent indicating normal diffusion down the concentration gradient and attainment of equilibrium. Under oxidising conditions, K₂O and to a lesser extent Na₂O, have compositional profiles indicative of uphill diffusion likely due to their preference for more polymerised Si- and Al-rich melts. Under reduced conditions Al-enrichment in the boundary layer melt is not as extreme and uphill diffusion did not occur. The composition of the solvent melt after the experiments indicates that it was contaminated by the boundary layer by convective mixing due to the onset of hydrodynamic instabilities brought on by density and viscosity contrasts between the two melts. Despite using a wide variety of solvent melt compositions we find that the boundary layer melts converge toward a common composition at high SiO₂ contents. The composition of glass generated by orthopyroxene dissolution at 1 atmosphere is similar in many respects to Si-rich glass found in many orthopyroxene-rich mantle xenoliths that have been attributed to high pressure in situ processes including mantle metasomatism. The results of this study suggest that at least some Si-rich melts are likely to have formed by dissolution of xenolith orthopyroxene at low pressure possibly by their Si-undersaturated host magmas. Received: 30 August 1996 / Accepted: 15 April 1998  相似文献