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Effective simulation of flexible lateral boundaries in two-and three-dimensional DEM simulations

Particuology 2008年第6期6卷 483-500页

作者： Geraldine Cheung Catherine o'Sullivan Department of Civil and Environmental Engineering Imperial College London

Discrete element method （DEM） models to simulate laboratory element tests play an important role in advancing our understanding of the mechanics of granular material response, including bonded or cemented, particulate materials. Comparisons of the macro-scale response observed in a real physical test and a ＂virtual＂ DEM-simulated test can calibrate or validate DEM models. The detailed, particle scale information provided in the DEM simulation can then be used to develop our understanding of the material behaviour. It is important to accurately model the physical test boundary conditions in these DEM simulations. This paper specifically considers triaxial tests as these tests are commonly used in soil mechanics. In a triaxial test, the test specimen of granular material is enclosed within a flexible latex membrane that allows the material to deform freely during testing, while maintaining a specified stress condition. Triaxial tests can only be realistically simulated in 3D DEM codes, however analogue, 2D, biaxial DEM simulations are also often considered as it is easier to visualize particle interactions in two dimensions. This paper describes algorithms to simulate the lateral boundary conditions imposed by the latex membrane used in physical triaxial tests in both 2D and 3D DEM simulations. The importance of carefully considering the lateral boundary conditions in DEM simulations is illustrated by considering a 2D biaxial test on a specimen of frictional unbonded disks and a 3D triaxial test on a bonded （cemented） specimen of spheres. The comparisons indicate that the lateral boundary conditions have a more significant influence on the local, particle-scale response in comparison with the overall macro-scale observations.

关键词： Triaxial testing Discrete element simulations Boundary conditions

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The contribution of botanic gardens to ex situ conservation through seed banking

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Plant Diversity 2017年第6期39卷 373-378页

作者： Katherine o'Donnell Suzanne Sharrock Botanic Gardens Conservation International Descanso House

Target 8 of the Global Strategy for Plant Conservation calls for 'at least 75 per cent of threatened plant species in ex situ collections, preferably in the country of origin, and at least 20 per cent available for recovery and restoration programmes by 2020'.Botanic gardens make a significant contribution to ex situ conservation of wild species with more than a third of plant species represented in botanic gardens collections. These collections are a combination of living collection and seed banked material. Seed banking can provide an efficient form of conservation for wild plant genetic *** from Botanic Gardens Conservation International's(BGCI) databases(GardenSearch,PlantSearch, ThreatSearch and GlobalTreeSearch) has been analysed as well as survey data to report on global, regional and national seed banking *** from BGCI's databases indicates that there are at least 350 seed banking botanic gardens in 74 countries. In total 56,987 taxa have been banked including more than 9000 taxa that are threatened with extinction. 6881 tree species are stored in ex situ seed bank collections. More than half(3562) of these tree species are single country endemics and represent species from more than 166 *** study suggests that institutions are increasingly conserving plant species via seed *** the majority of species in collections that have a conservation assessment are not threatened with extinction. This disjunction between species that are threatened and those conserved in seed banks needs to be addressed. Data from BGCI's databases can be used to enable prioritisation of threatened plant species for collection and conservation in seed banks. Further recommendations for botanic gardens involved in seed conservation are presented.

关键词： Seed banking GSPC ex situ conservation Conservation assessments Botanic gardens

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Glaciation-induced features or sediment gravity flows——An analytic review

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Journal of Palaeogeography 2023年第4期12卷 487-545页

作者： Mats o.Molen Umea FoU AB Vallmov 61S-90352 Ume aSweden

For more than 150 years, geologic characteristics claimed to be evidence for pre-Pleistocene glaciations have been debated. Advancements in recent decades, in understanding features generated by mainly glacial and mass flow processes, are here reviewed. Detailed studies of data offered in support of prePleistocene glaciations have led to revisions that involve environments of mass movements. Similarities and differences between Quaternary glaciogenic and mass movement features are examined, to provide a more systematic methodology for analysing the origins of more ancient deposits. Analyses and evaluation of data are from a) Quaternary glaciogenic sediments, b) formations which have been assigned to pre-Pleistocene glaciations, and c) formations with comparable features associated with mass movements(and occasionally tectonics). Multiple proxies are assembled to develop correct interpretations of ancient strata. The aim is not per se to reinterpret specific formations and past climate changes, but to enable data to be evaluated using a broader and more inclusive conceptual *** occurring pre-Pleistocene features interpreted to be glaciogenic, have often been shown to have few or no Quaternary glaciogenic equivalents. These same features commonly form by sediment gravity flows or other non-glacial processes, which may have led to misinterpretations of ancient deposits. These features include, for example, environmental affinity of fossils, grading, bedding, fabrics, size and appearance of erratics, polished and striated clasts and surfaces(“pavements”), dropstones, and surface *** decades of progress in research relating to glacial and sediment gravity flow processes have resulted in proposals by geologists, based on more detailed field data, more often of an origin by mass movements and tectonism than *** most coherent data of this review, i.e., appearances of features produced by glaciation, sediment gravity flows and a fe

关键词： Diamictite Tillite Sediment gravity flow(SGF) Striation Groove Dropstone Paleoclimate Fossil vegetation Glaciogenic proxies Surface microtexture Late Paleozoic ice age

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Study on creep-fatigue damage evaluation for advanced 9%–12% chromium steels under stress controlled cycling

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Acta Metallurgica Sinica(English Letters) 2011年第2期24卷 148-154页

作者： Peng ZHA o Fuzhen XUAN Key Laboratory of Safety Science of Pressurized System Ministry of Education School of Mechanical andPower Engineering East China University of Science and Technology Shanghai 200237 China

Creep-fatigue interaction is one of the main damage mechanisms in high temperature plants and their components. Assessment of creep-fatigue properties is of practical importance for design and operation of high temperature components. However, the standard evaluation techniques, i.e. time fraction rule and ductility exhaustion one have limitations in accounting for the effects of control mode on the cyclic deformations. It was found that conventional linear cumulative damage rule failed in accurately evaluating the creep-fatigue life under stress controlled condition. The calculated creep damages by time fraction rule were excessively high, which led to overly conservative prediction of failure lives. In the present study, it was suggested that such over estimation of creep damage was mainly caused by anelastic strain upon stress loading. For precise assessment under conditions of stress control, a modified creep damage model accounting for the effect of anelastic creep was proposed. The assessments of creep fatigue data under stress controlled condition were performed with the new approach developed in this paper for a rotor material and a boiler material used in ultra supercritical power plants. It was shown that a more moderate amount of creep damage was obtained by the new model, which gave better predictions of failure life.

关键词： Creep-fatigue Creep damage 9%–12%Cr steels Stress control Time fraction rule

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Lessons from zebrafish on reprogramming the epigenetic ：ode after fertilisation

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Asian Journal of Andrology 2013年第5期15卷 582-583页

作者： Chris o'Neill Centre for Developmental and Regenerative Medicine Kolling Institute for Medical Research Sydney MedicalSchool University of Sydney NSW 2065 Australia

Methylation of cytosine bases encodes epigenetic information that governs lineage-specific patterns of gene expression. Remodelling the methylated genome accompanies major transitions during embryo development, including fertilisation, blastulation and gastrulation. The first whole genome epigenetic maps of this modification across these embryonic transitions in a metazoan has been reported in the zebrafish,

关键词： Lessons zebrafish fertilisation

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Space-time description of transverse photons: near-field aspects

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Science China(Physics,Mechanics & Astronomy) 2012年第8期55卷 1389-1393页

作者： KELLER o. Institute of Physics and Nanotechnology Aalborg University

A mean position state based on the gauge invariant transverse vector potential is used to convert single-photon states in Hilbert space to photon wave packets in direct space. The resulting photon wave-mechanical description leads to scalar products which relate to covariant integration on the light cone. A new correlation matrix displays the spatial localization problem for single photons in an explicit manner in space-time. The correlation matrix essentially is the projection of the time-ordered Feynman photon propagator onto the transverse photon subspace. The present photon wave-mechanical formalism is generalized to two-photon dynamics. In the diamagnetic limit the transverse photon becomes massive in its interaction with matter, and the correlation matrix for massivephoton interaction, which can be used in studies of evanescent-photon mediated couplings, is analyzed. on the basis of the present formalism the existence of a dynamical near-field Aharonov-Bohm effect is predicted.

关键词： one-photon wave mechanics near-field optics correlation matrix transverse photon mass

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optimal tool design in micro-milling of difficult-to-machine materials

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Advances in Manufacturing 2023年第2期11卷 222-247页

作者： Lorcan o'Toole Feng-Zhou Fang Centre of Micro/Nano Manufacturing Technology(MNMT-Dublin) University College DublinDublin 4Ireland State Key Laboratory of Precision Measuring Technology and Instruments Laboratory of Micro/Nano Manufacturing Technology(MNMT)Tianjin UniversityTianjin300072People's Republic of China

The limitations of significant tool wear and tool breakage of commercially available fluted micro-end mill tools often lead to ineffective and inefficient manufacturing,while surface quality and geometric dimensions remain unacceptably *** is especially true for machining of difficult-to-machine(DTM)materials,such as super alloys and *** conventional fluted micro-tool designs are generally down scaled from the macro-milling tool ***,simply scaling such designs from the macro to micro domain leads to inherent design flaws,such as poor tool rigidity,poor tool strength and weak cutting edges,ultimately ending in tool ***,in this article a design process is first established to determine optimal micro-end mill tool designs for machining some typical DTM materials commonly used in manufacturing orthopaedic implants and micro-feature *** design process focuses on achieving robust stiffness and mechanical strength to reduce tool wear,avoid tool chipping and tool breakage in order to efficiently machine very hard ***,static stress and deflection finite element analysis(FEA)is carried out to identify stiffness and rigidity of the tool design in relation to the maximum deformations,as well as the Von Mises stress distribution at the cutting edge of the designed *** analysis and further optimisation of the FEA results,a verified optimum tool design is established for micro-milling DTM *** experimental study is then carried out to compare the optimum tool design to commercial tools,in regards to cutting forces,tool wear and surface quality.

关键词： Precision machining Micro-milling Difficult-to-machine(DTM) Tool wear Micro-tool design Surface quality

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Synthesis and properties of novel nitro-based thermally stable energetic compounds

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Defence Technology（防务技术） 2021年第3期17卷 775-784页

作者： Łukasz Gutowski Stanisław Cudziło Military University of Technology Faculty of Advanced Technologies and ChemistryGen.Sylwestra Kaliskiego 2 Street00-908WarsawPoland

This paper reviews the achievements in the field of synthesis of new thermally resistant explosive compounds in the years 2009 through 2019. The performance characteristics of these compounds(sensitivity, thermal decomposition parameters, and detonation parameters) were compared with those of 1,3,5-triamino-2,4,6-trinitrobenzene, which still seems to be an unrivalled model of a thermally resistant and generally low-sensitivity explosive material. New thermally stable explosives(TSEs) were found among macromolecular compounds with tri-and dinitrophenyl groups, nitro and amine-nitro derivatives of azoles, and polynitro derivatives of calixarenes. Some of them match TATB in terms of thermal resistance and additionally have higher detonation parameters.

关键词： Heat resistant explosives Detonation parameters Synthesis Nitration

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Interactive influence of rainfall manipulation and livestock grazing on species diversity of the herbaceous layer community in a humid savannah in Kenya

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Plant Diversity 2019年第3期41卷 198-205页

作者： Joseph o.ondier Daniel o.okach John C.onyango Dennis o.otieno Department of Botany Maseno University Private Bag Maseno Kenya Department of Biological Sciences Jaramogi Oginga Odinga University of Science and Technology P.O Box 210-40601 Bondo Kenya Department of Plant Ecology University of Bayreuth D-95440 Bayreuth Germany

Changes in rainfall regime and grazing pressure affect vegetation composition and diversity with ecological implications for savannahs. The savannah in East Africa has experienced increased livestock grazing and rainfall variability but the impacts associated with those changes on the herbaceous layer have rarely been documented. We investigated the effect of livestock grazing, rainfall manipulation and their interaction on the composition and diversity of the herbaceous community in the savannah for two years in Lambwe, Kenya. Rainfall manipulation plots were set up for vegetation sampling;these plots received either 50% more or 50% less rainfall than control plots. Simpson’s diversity and Bergere Parker indices were used to determine diversity changes and dominance respectively. The frequency of species was used to compute their abundance and their life forms as determined from the literature. Grazing significantly increased species diversity through suppression of dominant species. Rainfall manipulation had no significant impact on plant diversity in fenced plots, but rainfall reduction significantly reduced diversity in grazed plots. In contrast, rainfall manipulation had no impact on dominance in either fenced or grazed plots. The interaction of grazing and rainfall manipulation is complex and will require additional survey campaigns to create a complete picture of the implications for savannah structure and composition.

关键词： Plant diversity Grazing Rainfall Moist savannah

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Parallel loop recovery with quiescent compensation for high performance feedback control of systems with imperfect actuators

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Control Theory and Technology 2019年第3期17卷 201-215页

作者： Y.V.o'BRIEN J.F.o'BRIEN Department of Electrical and Computer Engineering University of Wyoming Laramie WY 82073 U.S.A.

Actuator limits and imperfections introduce multiple nonlinearities in a feedback *** standard mild-feedback systems (e.g.,PID) are intrinsically robust to such nonlinearities,the stability of large feedback systems is *** eliminates from consideration the usage of imperfect actuators in large feedback applications and available disturbance rejection as a result is not *** implementation of multiple-path nonlinear dynamic compensation (NDC),however,allows the combination of large feedback and imperfect actuation and is the focus of this *** nonlinearities due to actuator imperfection are often difficult to model,the stability of the NDC systems is assessed using multivariable absolute stability *** features of a parallel loop recovery system with loop transmission modulus reduction compensation in the quiescent condition are compared to an alternative approach of strict modulus reduction via nonlinear compensation for stability *** of the loop recovery approach is illustrated using experimental data from a voice-coil actuated disturbance rejection system.

关键词： Feedback control imperfect actuators nonlinear dynamic compensation

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