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1.
Weight percentages of different size incremental particles in a mineral slurry is integrated into the ultrasonic attenuation model to derive a relation between ultrasonic attenuation and particle sizes. However, in the inertial and scattering regimes of frequency–radius space, irregularity and aggregation of particles can results in values of ultrasonic attenuation that are significantly different from those predicted by the derived model, which is based on the assumption of separated smooth spherical particles in mineral slurries. Experimental attenuation data obtained previously from mineral slurries of iron ores particles are compared with predicted values. It is shown that there is a negligible difference between predictions and experimental data. A new modification of theoretical model for ultrasonic attenuation is derived. The theory uses ωτv or κR as a fractal scale. It requires an empirical determination of the difference between the fractal dimension of the measured mineral slurry and that of a hypothetical slurry of spheres with the same particle size distribution. The new fractal modification of ultrasonic attenuation model is found to enable better agreement with measured attenuation as a function concentration for irregular particles than the theoretical model. Moreover, the fractal approach is found to discriminate between the effects of particle irregularity and aggregation. 相似文献
2.
The possible existence of clusters of microscopic gas-dust particles forming gigantic clouds is demonstrated in the framework of the photogravitational three-body problem. These clouds are pulsating formations representing heterogeneous ensembles of particles of various sizes, densities, and susceptibilities to the radiation pressure, with each of the particles moving along its own trajectory. The conditions for the existence of co-linear and triangular libration points are derived by analyzing the dynamical equations describing the motion of a test particle in the gravitational-repulsive field of two gravitating and radiating bodies moving in an elliptical orbit about their common center of mass. The stability of these libration points is studied for various eccentricities e, mass parameters µ, and reduction coefficients characterizing the action of the stellar radiation on particles, and the results generalized in diagrams. We demonstrate the formation of clouds of particles located at multiple stable libration points. The extent of these clouds and their distances from the binary components are estimated. Scenarios for the subdivision of the clouds based on variations of e and µ are presented. The results of computations for a binary star with characteristics similar to those for α Cen are presented. 相似文献
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4.
Proof of multiplicity of solutions for groundwater seepage in recharged heterogeneous unconfined aquifers 
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下载免费PDF全文 R.P. Chapuis 《国际地质力学数值与分析法杂志》2016,40(14):1988-2002
A new analytical proof is presented for steady‐state seepage in recharged heterogeneous unconfined aquifers. The paper also presents a detailed procedure and important rules for performing correctly numerical studies of unsaturated seepage. Once a numerical solution is calibrated with field data, using a set of spatially distributed values for hydraulic conductivity K and effective infiltration EI, any new numerical analysis with a set of αK and αEI values, where α is a constant, yields an equally good calibration. However, if the effective porosities of each layer are unchanged, the groundwater velocities are multiplied by α, whereas the travel times are divided by α, which may help to select α in order to match known travel time data. This is a clear example of multiple solutions to an inverse problem. The paper underlines the role and the need to finely mesh unsaturated zones and also contacts between layers to reach the asymptotic convergence range, as it was carried out to verify the proof and as it should be completed to study any seepage problem. A few consequences of the new analytical proof and the rigorous procedure are shown with examples. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
5.
Chan-Hee Park Christof Beyer Sebastian Bauer Olaf Kolditz 《Environmental Geology》2008,55(8):1755-1766
Precise and efficient numerical simulation of transport processes in subsurface systems is a prerequisite for many site investigation
or remediation studies. Random walk particle tracking (RWPT) methods have been introduced in the past to overcome numerical
difficulties when simulating propagation processes in porous media such as advection-dominated mass transport. Crucial for
the precision of RWPT methods is the accuracy of the numerically calculated ground water velocity field. In this paper, a
global node-based method for velocity calculation is used, which was originally proposed by Yeh (Water Resour Res 7:1216–1225,
1981). This method is improved in three ways: (1) extension to unstructured grids, (2) significant enhancement of computational
efficiency, and (3) extension to saturated (groundwater) as well as unsaturated systems (soil water). The novel RWPT method
is tested with numerical benchmark examples from the literature and used in two field scale applications of contaminant transport
in saturated and unsaturated ground water. To evaluate advective transport of the model, the accuracy of the velocity field
is demonstrated by comparing several published results of particle pathlines or streamlines. Given the chosen test problem,
the global node-based velocity estimation is found to be as accurate as the CK method (Cordes and Kinzelbach in Water Resour
Res 28(11):2903–2911, 1992) but less accurate than the mixed or mixed-hybrid finite element methods for flow in highly heterogeneous media. To evaluate
advective–diffusive transport, a transport problem studied by Hassan and Mohamed (J Hydrol 275(3–4):242–260, 2003) is investigated here and evaluated using different numbers of particles. The results indicate that the number of particles
required for the given problem is decreased using the proposed method by about two orders of magnitude without losing accuracy
of the concentration contours as compared to the published numbers. 相似文献
6.
In this paper, a lattice-type model to simulate the micro-mechanical behaviour of particulate/granular media is presented. In this numerical model, a particulate assembly is simulated as a lattice/truss. Nodes located at contacts between a particle and its neighbours are linked by bars to each other. Each particle is represented by a lattice within its microstructure and particle interact through load transfer at the nodes. Constraints are prescribed at the nodes to describe active, deactivated and reactivated contacts. When a particulate assembly develops into a mechanism (deformation with zero incremental load), further deformation is simulated through a framework that describes the kinematics of the particles (sliding, rolling and rotation of particles). This framework is formed by introducing nodes at the particle centroids and linking them with bars. Bars-linking particles with a non-sliding contact are assigned large stiffnesses relative to bars linking particles with a sliding contact. Numerical tests are conducted on two-dimensional assemblies of disks, arranged as very loose and very dense packing under simple shear loading conditions. The results concord with the results of numerical tests conducted using the discrete element method and with photoelastic experiments. Additionally, the model is applied to study the effects of initial imperfections caused by particles with low elastic modulus. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
7.
M. Schidlowski 《Sedimentary Geology》1975,13(3):235-236
A methodical gravel-fabric analysis has been made based upon the measurement of orientation and form of 3,300 pebbles. The study was carried out in glaciofluvial deposits in the Randers-Djursland area, Denmark.It is demonstrated that particles with axial ratios less than 0.8 are usable in gravel-fabric analysis and that shape does not essentially affect the orientation. The fabric pattern is found to be independent of particle size but an upper size limit exists above which the particles cannot obtain a preferred orientation. Further it is shown that a significant preferred orientation of the a/b planes occurs only when the particle dip is greater than about 20°. 相似文献
8.
A. Dippenaar 《International Journal of Mineral Processing》1982,9(1):1-14
The interactions of several particles of different shapes and degrees of hydrophobicity with an artificially thinned film of water were recorded by means of high-speed cinematography. On bridging a film, a particle causes it to rupture only when the two three-phase boundary lines are forced to migrate to the same point on the particle. A contact angle on the particle of θ > 90° is normally required before this migration will occur, but particles that have special shapes can rupture films even when θ < 90°. A liquid film has to be thinned to half the diameter of a particle or less before it can be ruptured by the particle. 相似文献
9.
The local pore spaces in granular materials tend to be aligned parallel to the major principal stress direction upon particle mobilization. Manifestation of this response has been numerically validated in our previous studies with the aid of discrete element method modeling and image processing techniques during creep and shearing. We now extend the modeling of pore geometry, constructed with spherical particles, to assemblies of particle clumps. Two-dimensional simulations are performed for both loose and dense assemblies of spherical particles and particle clumps. Each particle packing is bound by rigid or flexible walls and subjected to biaxial compression and the particle mobilization effect on the evolution of pore orientation is explored. Randomly shaped pores surrounded by adjacent particles are geometrically quantified by Delaunay tessellation and fitted with ellipses. Results show that localization is apparent in dense assemblies, in particular for clumped particle packing, while loose assemblies exhibit diffusive failure. Small pores within well-defined shear bands tend to align either parallel to the direction of the shear band or perpendicular to the major principal stress. On the other hand, small pores within the blocks and large pores have a tendency to become elongate towards the major principal stress direction. This study reveals for the first time that pore orientation is dependent upon particle shape, pore size, and assembly conditions on the pore and global scales. 相似文献
10.
Fractal distribution of particle size in carbonate cataclastic rocks from the core of a regional strike-slip fault zone 总被引：2，自引：0，他引：2
We present particle size data from 31 samples of carbonate cataclastic rocks collected across the 26 m thick fault core of the Mattinata Fault in the foreland of the Southern Apennines, Italy. Particle size distributions of incoherent samples were determined by a sieving-and-weighting technique. The number of weight-equivalent spherical particles by size is well fitted by a power-law function on a log–log space. Fractal dimensions (D) of particle size distributions are in the 2.091–2.932 range and cluster around the value of 2.5. High D-values pertain to gouge in shear bands reworking the bulk cataclastic rocks of the fault core. Low D-values characterise immature cataclastic breccias. Intermediate D-values are typical of the bulk fault core. Analysis of the ratio between corresponding equivalent particle numbers from differently evolved cataclastic rocks indicates that the development of particle size distributions with D>2.6–2.7 occurred by a preferential relative increase of fine particles rather than a selective decrement of coarse particles. This preferentially occurred in shear bands where intense comminution enhanced by slip localisation progressed by rolling of coarse particles whose consequent smoothing produced a large number of fine particles. Our data suggest that during the progression of cataclasis, the fragmentation mode changed from the Allègre et al.'s [Nature 297 (1982) 47] “pillar of strength” mechanism in the early evolutionary stages, to the Sammis et al.'s [Pure and Applied Geophysics 125 (1987) 777] “constrained comminution” mechanism in the subsequent stages of cataclasis. Eventually, localised shear bands developed mainly by abrasion of coarse particles. 相似文献
11.
In a small watershed underlain by relatively homogeneous (nonlayered), sandy, glacial outwash, hydraulic conductivity (K) and longitudinal dispersivity (α L) were systematically measured over a range in scale. Test methods employed in the investigation are conventional and/or based on accepted principles, measurement scales are those typically encountered in applied field problems, and the hydrogeologic setting is typical of large areas within the northern hemisphere and similar to many alluvial environments worldwide. The results show that K measured under radial flow is scale-dependent up to a radius of influence (r i) of approximately 15 m but reaches an asymptotic value above this threshold. This asymptotic value of K is consistent with results obtained at the regional scale (～104 m) for the same aquifer using non-radial three-dimensional techniques; the initial increase appears to reflect increasing dimensionality of the radial tests with larger r i. Although α L was evaluated over a much smaller interval relative to K, α L proved to be independent of scale over the full range tested, from 2 to 29 m, an unexpected result. The findings support the idea that there is, as yet, no predictable relationship between scale effects in K and α L at scales normally encountered in applied field hydrogeology. 相似文献
12.
Fu Longlong Zhou Shunhua Guo Peijun Tian Zhekan Zheng Yuexiao 《Acta Geotechnica》2021,16(5):1527-1545
Loads transfer in ballast track through contacts among randomly distributed ballast particles and have strong heterogeneity. Since the size ratio between ballast track and ballast particles is generally small, using averaged stress to describe the internal mechanical state in ballast track faces practical difficulties. For example, particle movements and high local concentration stress tend to be ignored. The inter-particle contact stress is crucial to evaluate the particle behaviors, such as abrasion, movements, and furtherly the performance of ballast track. However, the contact stress on ballast particles is hard to predict or measure. We conduct a full size model test to investigate the dynamic characteristics of longitudinal stress on ballast particles as well as different lateral regions under vertical cyclic loads with various loading magnitudes and frequencies. An obvious seesaw effect of longitudinal contact stress is observed: the stresses at some contact areas have the same phase with applied cyclic load while at other contact areas have an opposite phase. The seesaw effect of contact stress is then used to evaluate the rotational movements of ballast particles. The variation of contact area and stress of the ballast particles with loading magnitudes demonstrates that the rigid contact assumption is appropriate when analyzing the contact behavior of ballast particles. The cumulative probability distribution of contact stress with stress level can be described by an inversely proportional function, based on which the maximum contact stress can be estimated according to the longitudinal average stress. Besides, the lateral dispersion angle of the vertical loads in the ballast track is about 35°, which is independent of the given loading magnitudes and frequencies.
相似文献13.
Volume-based soil particle fractal relation with soil erodibility in a small watershed of purple soil 总被引：2，自引：0，他引：2
The particle size distribution in small watershed changes under different land uses and affects soil erodibility. The aims of this study were (1) to investigate the volume fractal dimension of particle size distribution under different land uses in a typical small watershed of purple soil, (2) to estimate soil erodibilities of various land uses utilizing the Erosion-Productivity Impact Calculator (EPIC) model and the nomogram (NOMO) model, and (3) to relate volume fractal dimension with the soil erodibility used in the Universal Soil Loss Equation (USLE) in purple soil areas. Laser diffractions and double-logarithmic model were used to measure and calculate volume fractal dimension values. The results show that soil volume fractal dimensions were well linearly fitted to the double-logarithmic model with high correlation coefficients of 0.902–0.936 under six land uses in the small watershed. The averaged volume fractal dimension values under different land uses, from high to low were in the order of Zea mays L, Ipomoea batatas, Citrus reticulata Blanco, Setaria viridis, Robinia pseudoacacia L, Pinus massoniana Lamb. The volume fractal dimension was positively correlated to clay particle fraction (R = 0.933). The average soil erodibility values under different land uses from high to low were in the order of Setaria viridis, Citrus reticulata Blanco, Pinus massoniana Lamb, Zea mays L, Ipomoea batatas, Robinia pseudoacacia L while average soil erodibilities from high to low values were in the order of Setaria viridis, Citrus reticulata Blanco, Zea mays L, Ipomoea batatas, Pinus massoniana Lamb, Robinia pseudoacacia L. The soil erodibilities calculated by the two models were similar, and positively correlated (R = 0.630–0.877). The volume fractal dimension values of six land uses were negatively correlated to both soil erodibility estimated by EPIC and by NOMO models. Moreover, the correlations of the volume fractal dimension values of Zea mays L, Ipomoea batatas and Citrus reticulata Blanco estimated by EPIC or NOMO were lower than those of Pinus massoniana Lamb, Robinia pseudoacacia L and Setaria viridis. Further research is needed to determine the influence of volume fractal dimension on the soil erodibility under different land use and managements. 相似文献
14.
G. Fischer G. Karakas M. Blaas V. Ratmeyer N. Nowald R. Schlitzer P. Helmke R. Davenport B. Donner S. Neuer G. Wefer 《International Journal of Earth Sciences》2009,98(2):281-298
The ocean off NW Africa is the second most important coastal upwelling system with a total annual primary production of 0.33 Gt
of carbon per year (Carr in Deep Sea Res II 49:59–80, 2002). Deep ocean organic carbon fluxes measured by sediment traps are also fairly high despite low biogenic opal fluxes. Due
to a low supply of dissolved silicate from subsurface waters, the ocean off NW Africa is characterized by predominantly carbonate-secreting
primary producers, i.e. coccolithophorids. These algae which are key primary producers since millions of years are found in
organic- and chlorophyll-rich zooplankton fecal pellets, which sink rapidly through the water column within a few days. Particle
flux studies in the Mauretanian upwelling area (Cape Blanc) confirm the hypothesis of Armstrong et al. (Deep Sea Res II 49:219–236,
2002) who proposed that ballast availability, e.g. of carbonate particles, is essential to predict deep ocean organic carbon fluxes.
The role of dust as ballast mineral for organic carbon, however, must be also taken into consideration in the coastal settings
off NW Africa. There, high settling rates of larger particles approach 400 m day−1, which may be due to a particular composition of mineral ballast. An assessment of particle settling rates from opal-production
systems in the Southern Ocean of the Atlantic Sector, in contrast, provides lower values, consistent with the assumptions
of Francois et al. (Global Biogeochem Cycles 16(4):1087, 2002). Satellite chlorophyll distributions, particle distributions and fluxes in the water column off NW Africa as well as modelling
studies suggest a significant lateral flux component and export of particles from coastal shelf waters into the open ocean.
These transport processes have implications for paleo-reconstructions from sediment cores retrieved at continental margin
settings. 相似文献
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16.
LeRoy Demarest 《Environmental Earth Sciences》2014,71(9):4157-4164
Chlorinated solvents are among the most common groundwater pollutants globally. A common method to remediate this groundwater pollutant is through bioremediation. One of the most important bacteria that remediation workers use in this remediation is Dehalococcoides ethenogenes, which are the only dehalogenating bacteria with the capacity to break down these contaminants to the non-chlorinated compound, ethylene. While much is known about these bacteria in the laboratory, little is known about their natural habitat. In this experiment, eight sites on Dover Air Force Base were selected based on the larger CERCLA Closeout project; 2.54 cm wells were installed and groundwater samples were collected. These sites monitored groundwater quality parameters: temperature, pH, dissolved oxygen, and oxidative reduction potential. Additionally, samples were also analyzed for the contaminants: tetrachlorethylene, trichloroethylene, and vinyl chloride. The presence of vinyl chloride is an indicator of Dehalococcoides ethenogenes. Soil samples from the aquifer level were taken from each site as well and soil particle texture was assessed using the Bouyoucos hydrometer method. The objective of this experiment was to determine if there was a relationship between clay particles at the aquifer level, groundwater quality, and the presence of Dehalococcoides ethenogenes. 相似文献
17.
A practical combined finite–discrete element method was developed to simulate the breakage of irregularly shaped particles in granular geomaterials, e.g., rockfill. Using this method, each particle is discretized into a finite element mesh. The potential fracture paths are represented by pre-inserted cohesive interface elements (CIEs) with a progressive damage model. The Mohr–Coulomb model with a tension cut-off is employed as the damage initiation criterion to rupture the predominant failure mode occurs at the particle scale. Two series of biaxial tests were simulated for both the breakable and unbreakable particle assemblies. The two assemblies have identical configurations, with the exception that the former is inserted with CIEs and is breakable. The simulated stress–strain–dilation responses obtained for both assemblies are in agreement with experimental observations. We present a comprehensive study of the role of particle breakage on the mechanical behavior of rockfill materials at both the macroscopic and microscopic scales. The underlying mechanism of particle breakage can be explained by the force chain in the assemblies. 相似文献
18.
In this paper, a new generator algorithm and a computer program PG2D is introduced for 2D numerical simulation of packing configuration in a granular material composed of elliptical particles of different a/b aspect ratios. Each elliptical particle is approximated by four connected arcs. The centre co-ordinates and radius of each arc and co-ordinates of connecting points can be determined from the formulae derived by entering the major axis length, 2a, and the eccentricity. The domain to be filled with particles can be a polygon of any shape. Given the size of the packing domain, geometrical information and numbers of particles to be generated, the packing location of each particle and the co-ordinates of contact points along with contact normal rose diagram can be generated as outputs. Simulation results show that this new algorithm can provide quite a reasonable packing model in accordance with the initial input required for the analysis of the mechanics of granular material. This generation scheme has the potential to cover packing generation and behaviour analysis of 3D sphere or ellipsoidal shaped granular materials. © 1997 by John Wiley & Sons, Ltd. 相似文献
19.
Based on the particle simulation method, a thermo-mechanical coupling particle model is proposed for simulating thermally-induced rock damage. In this model, rock material is simulated as an assembly of particles, which are connected to each other through their bonds, in the case of simulating mechanical deformation, but connected to each other through thermal pipes in the case of simulating heat conduction. The main advantages of using this model are that: (1) microscopic parameters of this model can be directly determined from the related macroscopic ones; (2) the temperature-dependent elastic modulus and strength are considered in an explicit manner, so that thermally-induced rock damage can be realistically simulated in a thermo-mechanical coupling problem. The related simulation results from an application example have demonstrated that: (1) the proposed model can produce similar behaviors to those observed in experiments; (2) the final failure is initiated from the outer surface of the testing sample and propagates toward the borehole; (3) microscopic crack initiation and propagation processes can be reasonably simulated at the cooling stage. 相似文献
20.
Soil erosion is a serious environmental and production problem in China. In particular, natural conditions and human impact have made the Chinese Loess Plateau particularly prone to intense soil erosion area. To decrease the risk on environmental impacts, there is an increasing demand for sound, and readily applicable techniques for soil conservation planning in this area. This work aims at the assessment of soil erosion and its spatial distribution in hilly Loess Plateau watershed (northwestern China) with a surface area of approximately 416.31 km2. This study was conducted at the Caijiamiao watershed to determine the erosion hazard in the area and target locations for appropriate initiation of conservation measures using the revised universal soil loss equation (RUSLE). The erosion factors of RUSLE were collected and processed through a geographic information system (GIS)-based approach. The soil erosion parameters were evaluated in different ways: The R-factor map was developed from the rainfall data, the K-factor map was obtained from the soil map, the C-factor map was generated based on Landsat-5 Thematic Mapper image and spectral mixture analysis, and a digital elevation model with a spatial resolution of 25 m was derived from topographic map at the scale of 1:50,000 to develop the LS-factor map. Support practice P factor was from terraces that exist on slopes where crops are grown. By integrating the six-factor maps in GIS through pixel-based computing, the spatial distribution of soil loss in the study area was obtained by the RUSLE model. The results showed that spatial average soil erosion at the watershed was 78.78 ton ha?1 year?1 in 2002 and 70.58 ton ha?1 year?1 in 2010, while the estimated sediment yield was found to be 327.96 × 104 and 293.85 × 104 ton, respectively. Soil erosion is serious, respectively, from 15 to 35 of slope degree, elevation area from 1,126 to 1,395 m, in the particular area of soil and water loss prevention. As far as land use is concerned, soil losses are highest in barren land and those in waste grassland areas are second. The results of the study provide useful information for decision maker and planners to take appropriate land management measures in the area. It thus indicates the RUSLE–GIS model is a useful tool for evaluating and mapping soil erosion quantitatively and spatially at a river watershed scale on a cell basis in Chinese Loess Plateau and for planning of conservation practices. 相似文献