Intersecting dilated convex polyhedra method for modeling complex particles in discrete element method     

Ben Nye  Anton V. Kulchitsky  Jerome B. Johnson 《国际地质力学数值与分析法杂志》2014,38(9):978-990

This paper describes a new method for representing concave polyhedral particles in a discrete element method as unions of convex dilated polyhedra. This method offers an efficient way to simulate systems with a large number of (generally concave) polyhedral particles. The method also allows spheres, capsules, and dilated triangles to be combined with polyhedra using the same approach. The computational efficiency of the method is tested in two different simulation setups using different efficiency metrics for seven particle types: spheres, clusters of three spheres, clusters of four spheres, tetrahedra, cubes, unions of two octahedra (concave), and a model of a computer tomography scan of a lunar simulant GRC‐3 particle. It is shown that the computational efficiency of the simulations degrades much slower than the increase in complexity of the particles in the system. The efficiency of the method is based on the time coherence of the system, and an efficient and robust distance computation method between polyhedra as particles never intersect for dilated particles. © 2014 The Authors. International Journal for Numerical and Analytical Methods in Geomechanics published by John Wiley & Sons Ltd.  相似文献   

Nursery Function Drives Temporal Patterns in Fish Assemblage Structure in Four Tropical Estuaries     

Marcus Sheaves  Ross Johnston  Ashlee Johnson  Ronald Baker  Rod M. Connolly 《Estuaries and Coasts》2013,36(5):893-905

Despite estuary-to-estuary differences in assemblage composition, fish faunas of tropical Indo-Pacific estuaries show parallel patterns of temporal change, suggesting a common set of ecological drivers. One potentially important driver is the interaction of different patterns of occupancy by functional groups that display different life-history patterns. However, most studies that have considered temporal change lack the detail needed to understand life-history utilisation. Most have focussed on changes in catch per unit effort (CPUE) or probability of encounter, with only one study going further and investigating changes in size structure and then only for a single estuary. One of the reasons for this lack of detail is the large volume of work needed to collect comprehensive data on size structures of species rich assemblages across multiple estuary systems over time. To overcome the logistical limitations on data collection, we used joint patterns of change in CPUE and mean biomass per fish (BPF) as proxies for changes in size structure. We investigated how different life-history strategies contributed to overall temporal patterns of assemblage change across four tropical Indo-Pacific estuaries. The three life-history strategies displayed characteristically different patterns in CPUE and BPF and the relationships between CPUE and BPF that reflect differences in the way that the three groups use estuaries. These different patterns interacted to produce complex assemblage patterns that are likely to be sensitive to location-specific differences in the mix of species from each group, providing at least part of the explanation for the site-specific fish assemblage structures that are characteristic of tropical estuarine fish fauna.  相似文献   

Studies on the reflectance spectral features of saline soil along the middle reaches of Tarim River: a case study in Xinjiang Autonomous Region, China     

Fei Zhang  Tashpolat Tiyip  Jianli Ding  Hsiangte Kung  Verner C. Johnson  Mamat Sawut  Nigara Tashpolat  Dongwei Gui 《Environmental Earth Sciences》2013,69(8):2743-2761

There has been growing interest in the use of reflectance spectroscopy as a rapid and inexpensive tool for soil characterization. In this study, 53 soil samples were collected from the oasis in the Weigan and Kuqa River delta along the middle reaches of Tarim River to investigate the level of soil chemical components in relation to soil spectral. An approach combining spectral technology and multi-variant statistical analysis was used to determine the reflectance spectral features of saline soil. The spectral data was first pretreated to remove noises and absorption bands from water, which eliminated influence from instrument errors and other external background factors. Several spectral absorption features were calculated for several saline soil samples to confirm that soil at the same salinity level had similar absorption spectral properties. Secondly, a correlation relationship between reflectance spectra and salinity factors was estimated by bivariate correlation method. Fourteen salinity factors including eight major ions and soil electrical conductivity (EC), soil salt content (SSC), pH, and total dissolved solid (TDS) in the saline soil were evaluated. Datasets of the salinity factors that correlated significantly with field data measurements of reflectance rate and the corresponding spectrum data were used to construct quantitative regression models. According to the multiple linear regression analysis, SSC, SO₄ ^2?, TDS, and EC had a correlation coefficient at 0.746, 0.908, 0.798, and 0.933 with the raw spectral data, respectively, which confirmed strong correlation between salinity factors and soil reflectance spectrum. Findings from this study will have significant impact on characterization of spectral features of saline soil in oasis in arid land.  相似文献   

Suspended sediment dynamics associated with snowmelt runoff in a small mountain stream of Lake Tahoe (Nevada)     

Jacques L. Langlois  Dale W. Johnson  Guy R. Mehuys 《水文研究》2005,19(18):3569-3580

The Lake Tahoe basin is experiencing an environmental decline that is partly due to sediment intakes from its tributaries. Many studies have estimated suspended sediment loads in these streams with a discrete sampling programme by collecting water samples and using a rating technique. However, the relationship between stream discharge and suspended sediment concentration (SSC) in these tributaries is known to differ during the rising and falling limbs of the snowmelt‐dominated hydrograph. Because of this hysteresis effect, sediment rating curves are poor predictors of suspended sediment dynamics in the stream. In this study, suspended sediment transport was investigated using a turbidity meter to provide a continuous record of sediment concentration during the snowmelt period. Hysteresis in suspended sediment transport was also investigated and is quantified with an H index, which is the ratio of the areas under the curve at different stages of the hydrograph. The temporal lag between the peak of SSC and the peak of stream discharge was quantified using cross‐correlation analysis. For almost all events, SSCs were higher during the rising limb of the hydrograph for a given discharge, with SSC peaks occurring before discharge peaks, resulting in clockwise hysteresis (H > 1). The H indices increased (looser hysteresis loop) as the availability of sediments increased and as the lag between peaks in SSC and discharge was larger. A restriction of the proposed H index was that it could only be computed when stream discharge increased by more than 30% during a melt event. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

Study of variable stars in the MOA data base: long-period red variables in the Large Magellanic Cloud – II. Multiplicity of the period–luminosity relation     

S. Noda  M. Takeuti  F. Abe  I. A. Bond  R. J. Dodd  J. B. Hearnshaw  M. Honda  M. Honma  J. Jugaku  Y. Kan-ya  Y. Kato  P. M. Kilmartin  Y. Matsubara  K. Masuda  Y. Muraki  T. Nakamura  K. Ohnishi  M. Reid  N. J. Rattenbury  To. Saito  Y. Saito  H. Sato  M. Sekiguchi  J. Skuljan  D. J. Sullivan  T. Sumi  R. Yamada  T. Yanagisawa  P. C. M. Yock  M. Yoshizawa 《Monthly notices of the Royal Astronomical Society》2004,348(4):1120-1134

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X-ray and γ-ray spectra and variability of the black hole candidate GX 339–4     

Grzegorz Wardzi&#;ski  rzej A. Zdziarski  Marek Gierli&#;ski  J. Eric Grove  Keith Jahoda  W. Neil Johnson 《Monthly notices of the Royal Astronomical Society》2002,337(3):829-839

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Systematic errors in cosmic microwave background polarization measurements     

Daniel O'Dea  Anthony Challinor  Bradley R. Johnson 《Monthly notices of the Royal Astronomical Society》2007,376(4):1767-1783

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Terrestrial ages,pairing, and concentration mechanism of Antarctic chondrites from Frontier Mountain,Northern Victoria Land     

K. C. Welten  K. Nishiizumi  M. W. Caffee  D. J. Hillegonds  J. A. Johnson  A. J. T. Jull  R. Wieler  L. Folco 《Meteoritics & planetary science》2006,41(7):1081-1094

Abstract— We report concentrations of cosmogenic ¹⁰Be, ²⁶Al, ³⁶Cl, and ⁴¹Ca in the metal phase of 26 ordinary chondrites from Frontier Mountain (FRO), Antarctica, as well as cosmogenic ¹⁴C in eight and noble gases in four bulk samples. Thirteen out of 14 selected H chondrites belong to two previously identified pairing groups, FRO 90001 and FRO 90174, with terrestrial ages of ?40 and ?100 kyr, respectively. The FRO 90174 shower is a heterogeneous H3–6 chondrite breccia that probably includes more than 300 individual fragments, explaining the high H/L chondrite ratio (3.8) at Frontier Mountain. The geographic distribution of 19 fragments of this shower constrains ice fluctuations over the past 50–100 kyr to less than ?40 m, supporting the stability of the meteorite trap over the last glacial cycle. The second H‐chondrite pairing group, FRO 90001, is much smaller and its geographic distribution is mainly controlled by wind‐transport. Most L‐chondrites are younger than 50 kyr, except for the FRO 93009/01172 pair, which has a terrestrial age of ?500 kyr. These two old L chondrites represent the only surviving members of a large shower with a similar preatmospheric radius (?80 cm) as the FRO 90174 shower. The find locations of these two paired L‐chondrite fragments on opposite sides of Frontier Mountain confirm the general glaciological model in which the two ice flows passing both ends of the mountain are derived from the same source area on the plateau. The 50 FRO meteorites analyzed so far represent 21 different falls. The terrestrial ages range from 6 kyr to 500 kyr, supporting the earlier proposed concentration mechanism.  相似文献   

Reduction of hydraulic conductivity and reactivity in zero-valent iron columns by oxygen and TNT   总被引：2，自引：1，他引：1  

R.L. Johnson  P.G. Tratnyek  R. Miehr  R.B. Thoms  J.Z. Bandstra 《Ground Water Monitoring & Remediation》2005,25(1):129-136

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An ATCA radio-continuum study of the Small Magellanic Cloud – II. Source identification and classification     

J. L. Payne  M. D. Filipovi&#;  W. Reid  P. A. Jones  L. Staveley-Smith  G. L. White 《Monthly notices of the Royal Astronomical Society》2004,355(1):44-50

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