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Modeling slow deformation of polygonal particles using DEM

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Particuology 2008年第6期6卷 506-514页

作者： Andrés A. Pea Pedro G. Lind hans j. herrmann Institute for Computational Physics Universitt Stuttgart Computational Physics IfB HIF E12 ETH Hnggerberg CH-8093 Zrich Switzerland Departamento de Física Universidade Federal do Ceará

We introduce two improvements in the numerical scheme to simulate collision and slow shearing of irregular particles. First, we propose an alternative approach based on simple relations to compute the frictional contact forces. The approach improves efficiency and accuracy of the Discrete Element Method （DEM） when modeling the dynamics of the granular packing. We determine the proper upper limit for the integration step in the standard numerical scheme using a wide range of material parameters. To this end, we study the kinetic energy decay in a stress controlled test between two particles. Second, we show that the usual way of defining the contact plane between two polygonal particles is, in general, not unique which leads to discontinuities in the direction of the contact plane while particles move. To solve this drawback, we introduce an accurate definition for the contact plane based on the shape of the overlap area between touching particles, which evolves continuously in time.

关键词： Granular media Discrete element method Slow deformation Contact forces

High-resolution Induan-Olenekian boundary sequence in Chaohu, Anhui Province

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Science China Earth Sciences 2005年第3期48卷 291-297页

作者： hans j. hansen Geological Institute Copenhagen University Oster Voldgade 10 DK-1350 Copenhage Denmark

The West Pingdingshan Section in Chaohu, Southeast China’s Anhui Province is one of the potential GSSP candidates for the Induan-Olenekian boundary. Extensive study has been done on the Induan-Olenekian boundary strata at the section to meet the requirements of defining the GSSP precisely. This paper focuses on the boundary strata at the section to present a review of the main results achieved in recent years. The Induan-Olenekian boundary strata here cover an interval of about four meters at the West Pingdingshan Section, in which the key indices of defining the boundary and the chief transitional processes of some parameters at the boundary are included. The relationship between the boundaries defined by ammonoids and by conodonts is well clarified at the section. The magnetic polarity analysis shows that the boundary interval belongs to a normal polarity zone. Both the carbonate and organic carbon isotopes com- positions experienced a significant conversion in the boundary interval.

关键词： Lower Triassic, Induan-Olenekian boundary, GSSP, candidate, Chaohu of China.

Decomposition dynamics of coarse woody debris of three important central European tree species

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Forest Ecosystems 2015年第4期2卷 339-352页

作者： Steffen herrmann Tiemo Kahl j.rgen Bauhus Institute of Forest Sciences Faculty of Environment and Natural ResourcesUniversity of Freiburg Swiss Federal Research Institute WSL

Background： Coarse woody debris （CWD） is an important element of forest structure that needs to be considered when managing forests for biodiversity, carbon storage or bioenergy. To manage it effectively dynamics of CWD decomposition should be known. Methods： Using a chronosequence approach, we assessed the decomposition rates of downed CWD of Fagus sylvatica, Picea obies and Pinus sylvestfis, which was sampled from three different years of tree fall and three different initial diameter classes （〉10 - ≤20 cm, 〉20 - ≤40 cm, 〉40 cm）. Samples originating from wind throws in 1999 were collected along a temperature and precipitation gradient. Based on the decay class and associated wood densities, log volumes were converted into CWD mass and C content. Log fragmentation was assessed over one year for log segments of intermediate diameters （〉20 - 40 cm） after 8 and 18 years of decomposition. Results： Significantly higher decomposition constants （k） were found in logs of F. sylvotica （0.054 year^-1） than in P. abies （0.033 year^-1） and P. sylvestris （0.032 year^-1）. However, mass loss of P. sylvestris occurred mainly in sapwood and hence k for the whole wood may be overestimated. Decomposition rates generally decreased with increasing log diameter class except for smaller dimensions in P. obies. About 74 % of the variation in mass remaining could be explained by decomposition time （27 %）, tree species （11%）, diameter （17 %）, the interactive effects between tree species and diameter （4 %） as well as between decomposition time and tree species （3 %） and a random factor （site and tree; 9.5 %）, whereas temperature explained only 2 %. Wood fragmentation may play a more important role than previously thought. Here, between 14 % and 30 % of the decomposition rates （for the first 18 years） were attributable to this process. Carbon （C） density （mgC· cm ^-3）, which was initially highest for F. sylvatico, followed by P. sylvestris and P. obies, decreased with increasing decay stage t

关键词： Dead wood Carbon Decay rate Beech Spruce Pine

Large-eddy simulation of wind-blown sand under unstable atmospheric boundary layer

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Science Bulletin 2022年第14期67卷 1421-1424,M0003页

作者： Panli He j.e Zhang hans j.herrmann Ning Huang Key Laboratory of Mechanics on Disaster and Environment in Western China Lanzhou UniversityLanzhou 730000China College of Civil Engineering and Mechanics Lanzhou UniversityLanzhou 730000China Department 403 Rocket Force University of EngineeringXi’an 710025China Laboratoire de Physique et Mécanique des Milieux Hétérogènes École Supérieure de Physique et de Chimie Industrielles de la Ville de ParisParis 75005France Departamento de Física Universidade do CearáFortaleza 60020-181Brazil

Atmospheric turbulence which induced by wind shear and/or convection is non-uniform anisotropic flows of high Reynolds and Rayleigh number[1].According to the mechanism of turbulence generation,atmospheric boundary-layer stratification can be classified into neutral,unstable(convective)and stable[2].Observations show that sandstorms and wind-blown sand mostly occur under unstable stratifications from afternoon to early evening[3].As the surface absorbs the solar radiation,the surface temperature increases and vertical convection develops,which enhances the momentum exchange between the upper and lower layers and hence increases the near surface wind speed and shear stress[4].

关键词：大涡模拟剪切应力风沙运动风沙流大气边界层输沙率线性关系对流速度

Evolution and stability of holographic soliton pairs

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Chinese Science Bulletin 2006年第10期51卷 1174-1179页

作者： LIU j.nsong WANG Cheng ZHANG Huilan hans j. Eichler State Key Laboratory of Laser Technology Huazhong University of Science and Technology Wuhan 430074 China Optisches Institut Technische Universitaet Berlin Strasse des 17. Juni 135 D-10623 Berlin Germany

The dynamical evolution and the stabiity property of both bright and dark holographic solton pairs have been investigated numerically. Resultshow that mutual trapping between two beams in pair is balanceable and both ... 详细信息

关键词：空间光孤子非线性光学稳定性全息光学

Anisotropic plastic deformation behavior of as-extruded ZK60 magnesium alloy at room temperature

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Science China(Technological Sciences) 2009年第1期52卷 161-165页

作者： hans j.R Department of Materials Science and Engineering Norwegian University of Science and Technology

The anisotropic plastic deformation behavior of as-extruded ZK60 magnesium alloy at room tempera-ture (RT) was investigated by compressive and tensile testing in different directions, i.e. the loading axis oriented at 0°, 45° and 90° to the extrusion direction. The relationship between texture and plastic deformation behavior were examined. The results show that the extruded ZK60 alloy exhibits a strong ring fiber texture. The mechanical properties are strongly orientation dependent. In tension testing, the 0° specimen exhibited higher yield strength and lower elongation. In compression testing, however, ZK60 alloy exhibited almost the same yield strength in three directions. The anisotropic plastic defor-mation behavior is due to strong fiber texture and the lower symmetry of the hexagonal close packed (hcp) structure of ZK60 alloy. The correlation between texture and mechanical behaviour offers the possibility to improve the mechanical properties of magnesium alloy by optimization of the material production process.

关键词： anisotropic plastic deformation behavior ZK60 Mg alloy texture mechanical property

Recycling of automotive aluminum

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Transactions of Nonferrous Metals Society of China 2010年第11期20卷 2057-2063页

作者： j.rang CUI hans j.ROVEN Department of Materials Science and Engineering Norwegian University of Science and Technology(NTNU)

With the global warming of concern,the secondary aluminum stream is becoming an even more important component of aluminum production and is attractive because of its economic and environmental *** this work,recycling of automotive aluminum is reviewed to highlight environmental benefits of aluminum recycling,use of aluminum alloys in automotive applications,automotive recycling process,and new technologies in aluminum scrap *** survey shows that newly developed techniques such as laser induced breakdown spectroscopy(LIBS) and solid state recycling provide promising alternatives in aluminum scrap *** with conventional remelting and subsequent refinement,solid state recycling utilizing compression and extrusion at room or moderate temperature can result in significant energy savings and higher metal yield.

关键词： recycling automotive aluminum sorting mechanical separation solid state recycling

Estimation of Pool Boiling Heat Transfer Coefficients of Ethanol-Water Mixtures

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Chinese j.urnal of Chemical Engineering 1999年第2期7卷 95-103页

作者：谷俊杰 hans-j.rgBart Lehrstuhl fur Thermische Verfahrenstechnik Universitat Kaiserslautern67653 KaiserslauternGermany

Based on the analysis of the principles of heat and mass transfer from the liquid bulk to bubble surface during nucleation, a new estimation method to predict the heat transfer of the nucleate pool boiling in binary mixtures is proposed in this paper by using the heat and mass transfer analogy (Colburn analogy) at the bubble surface transfer layers. With the use of Thome's concept of boiling range, an approximate method is derived to realize the calculation. The predicted results by this method are in good agreement with the experimental data from different sources.

关键词： heat transfer boiling water-ethanol mixture estimation

Nutrient retention and release in coarse woody debris of three important central European tree species and the use of NIRS to determine deadwood chemical properties

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Forest Ecosystems 2018年第3期5卷 296-310页

作者： Steffen herrmann j.rgen Bauhus Faculty of Environment and Natural Resources University of Freiburg Tennenbacherstr.4D-79106Freiburg i.Br.Germany Thunen Institute of Forest Ecosystems Alfred-M611er-Strabe 1Haus41/4216225EberswaldeGermany

Background: Coarse woody debris(CWD) is very important for forest ecosystems, particularly for biodiversity and carbon storage. Its relevance as a possible reservoir and source of nutrients is less clear, especially in central ***: Based on a chronosequence of known ages of logs, we analyzed the nutrients stored in CWD of Fagus sylvatica, Picea abies, and Pinus sylvestris at different sites in Germany. To quantify nutrient concentrations, we assessed the use of Near Infrared Reflectance Spectroscopy(NIRS) to determine the chemical properties of ***: NIRS models were suitable to predict concentrations of C, N, P, lignin and extractives. Concentrations of most nutrients increased with mass loss, with the exception of potassium, which decreased for beech and pine and remained relatively constant for spruce. The highest nutrient concentrations(N, P, S, Ca and Mn, except Mg and K) were generally observed in highly decomposed spruce logs. The net effect of decreasing CWD mass and increasing nutrient concentrations was either a decreasing(N, P and K in beech; P, Mg, K and Mn in pine), constant(S, Ca and Mg in beech; N, S and Ca in pine) or increasing amount of nutrients(N, P, S and Ca in spruce; Mn in beech) in the logsover the course of decomposition. The C/N ratio decreased for all tree species, most markedly for spruce from ca. 1000 at the beginning of the decomposition process to 180 at 36 years. The N/P ratio converged to a value of about 30 forall three species. Lignin concentrations increased for spruce and beech and remained constant for ***: Our results indicate that most nutrients remain in CWD for long periods. Nutrients may be used and cycled by microorganisms within CWD, but with the exception of P(in beech), Mg(in pine) and K(in beech and pine), there appears to be little net nutrient export until two thirds of the mass is lost. Instead, N, P, S and Ca were accumulated in spruce logs, indicating that CWD became a net sink rathe

关键词： CWD NIRS Decay Carbon F.sylvatica P.abies P.sylvestris