43 resultados para scaling rules

em Chinese Academy of Sciences Institutional Repositories Grid Portal


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本文对松嫩平原羊草地的群落结构及空间格局进行了分析,主要内容包含以下三个方面: 1、本文第一部分是对碱化草地植物群落分布的空间、环境因子分离。决定碱化草地植物物种在空间分布的因素可以大致分解成4个部分:(1)通过物种的生态位起作用的环境因子;(2)决定物种、群落间竞争激烈程度的空间距离因子;(3)环境和空间因子的交叉或耦合作用;(4)其它未知因子(如生物和随机因子)。本研究在1996年野外实地采样的基础上,运用DCA消势对应分析(Detrended Correspondence Analysis)对群落主要变化趋势及其与土壤环境因子之间的关系进行了分析。运用DCCA消势典范对应分析(Detrended Canonical Correspondence Analysis)对影响松嫩平原碱化草地群落结构季节动态的空间和环境因素进行了定量的分解。结果表明: 在影响群落分布的各因子中,环境因子独立约占40%,而环境一空间耦合因子占35%,空间因子独立约占3%,其它因子约占20%。在诸多因子中,土壤的盐渍化程度在整个生长季起着决定性作用,但土壤水分和氮素的作用因季节而变化。在干旱季节,植物生长的主要制约因子表现为土壤碱化度和土壤水份,而土壤氮素的作用处于次要地位。但在降雨较多、土壤湿润度较大的季节,土壤氮素的影响明显增强,成为仅次于土经度的决定性因素。 2、第二部分对东北松嫩平原碱化草地植物群落空间格局的分形性质进行了分析,分别用边长-面积指数和Korcak指数估计了斑块边界复杂性和斑块面积分布的分形维数。结果表明: 随放牧强度增加,占优势的羊草斑块的相对斑块化加剧,而斑块的边界在中度放牧时最不规则: 在水淹地,占优势的羊草斑块和次优势的碱茅斑块、獐茅斑块的斑块边界复杂性和斑块化程度比未水淹地都低,说明水降低了群落复合体内的异质性: 水淹地优势植物斑块的边长-面积指数和Korcak指数均低于次优势种,但在重度放牧地结果好相反,说明两种样地处于不同的演替阶段; 斑块边界复杂性符合同一尺度规律,在现有的面积范围以内没有尺度转换,而斑块面积分布则存在尺度转换点。对重牧地和水淹地的尺度分析表明,较小尺度上, 群落空间格局的相对斑块化程度较低,说明较小尺度上的空间格局要相对稳定一些,而较大尺度上则相反。可能原因是放牧干扰对大尺度的斑块影响更大。 3、对1989年开始围栏封育的样地10年来的空间格局变化进行时间序列分析,研究恢复演替过程中各个主要斑块类型的分布特征及动态,以及整个样地总体格局在演替系列中的变化。得出如下结果: 羊草是该区植被的优势种,其格局动态为:1989-1993年,羊草斑块总面积增加,斑块数目减少,相对斑块化指数降低,1994年后羊草的空间格局基本稳定。 羊草的斑块面积分布曲线在20平方米左右发生转折,即空间格局在此尺度发生尺度转换。不同尺度上斑块的相对斑块化指数及其在恢复演替中的变化趋势不同:小斑块的相对斑块化指数低,在1989-1993年期间增加,而后降低;大斑块的情况正好相反。说明羊草空间格局的主要变化之一是中等大小斑块的合并和数目减少,羊草斑块生长对格局的影响比斑块合并的影响要小。 獐茅斑块的格局动态为:1989-1994,斑块总面积、斑块数目、最大斑块面积增大,1995年开始降低。相对斑块化程度则先降后增。碱蓬斑块在1989-1993年间的格局动态与羊草正好相反:总面积减少,斑块数目增加,斑块面积大小的变化范围变窄,说明1993年之前,控制碱蓬斑块的主要生态学过程是其他物种的侵入,隔离原来较大的斑块,同时占据某些小斑块。1994年之后,碱蓬斑块的变化比较随机,其格局变化受到其他物种如杂草对策种虎尾草等的影响很大。 样地总体格局的变化为:1991年前,斑块种类和总斑块数目增加,斑块化程度随着增大;1993年后,斑块种类基本稳定下来;1991-1995,斑块化指数降低。在此期间,斑块数目增加,所以斑块化程度降低意味着斑块大小频率分布趋于均匀,大斑块被分裂,小斑块在长大,群落总体空间格局逐渐稳定。总体格局在三个尺度上有不同的自相似规律,这三个尺度分别为:La(a)<=1; 15.5。小尺度上相对斑块化程度低,但年际间变化较中尺度剧烈;中尺度相对斑块化程度变化较为缓,大尺度上,斑块分布一直趋于减少最大的斑块数目。总的变化趋势为,随恢复演替的深入,最大和最小的斑块数目都减少,处于中间尺度的斑块最多。

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Using dimensional analysis and finite element calculations we derive several scaling relationships for conical indentation into elastic-perfectly plastic solids. These scaling relationships provide new insights into the shape of indentation curves and form the basis for understanding indentation measurements, including nano- and micro-indentation techniques. They are also helpful as a guide to numerical and finite element calculations of conical indentation problems. Finally, the scaling relationships are used to reveal the general relationships between hardness, contact area, initial unloading slope, and mechanical properties of solids.

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We derive, using dimensional analysis and finite element calculations, several scaling relationships for conical indentation in elastic-plastic solids with work hardening. Using these scaling relationships, we examine the relationships between hardness, contact area, initial unloading slope, and mechanical properties of solids. The scaling relationships also provide new insights into the shape of indentation curves and form the basis for understanding indentation measurements, including nano- and micro-indentation techniques. They may also be helpful as a guide to numerical and finite element calculations of indentation problems.

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The main idea of the Load-Unload Response Ratio (LURR) is that when a system is stable, its response to loading corresponds to its response to unloading, whereas when the system is approaching an unstable state, the response to loading and unloading becomes quite different. High LURR values and observations of Accelerating Moment/Energy Release (AMR/AER) prior to large earthquakes have led different research groups to suggest intermediate-term earthquake prediction is possible and imply that the LURR and AMR/AER observations may have a similar physical origin. To study this possibility, we conducted a retrospective examination of several Australian and Chinese earthquakes with magnitudes ranging from 5.0 to 7.9, including Australia's deadly Newcastle earthquake and the devastating Tangshan earthquake. Both LURR values and best-fit power-law time-to-failure functions were computed using data within a range of distances from the epicenter. Like the best-fit power-law fits in AMR/AER, the LURR value was optimal using data within a certain epicentral distance implying a critical region for LURR. Furthermore, LURR critical region size scales with mainshock magnitude and is similar to the AMR/AER critical region size. These results suggest a common physical origin for both the AMR/AER and LURR observations. Further research may provide clues that yield an understanding of this mechanism and help lead to a solid foundation for intermediate-term earthquake prediction.

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The advent of nanotechnology has necessitated a better understanding of how material microstructure changes at the atomic level would affect the macroscopic properties that control the performance. Such a challenge has uncovered many phenomena that were not previously understood and taken for granted. Among them are the basic foundation of dislocation theories which are now known to be inadequate. Simplifying assumptions invoked at the macroscale may not be applicable at the micro- and/or nanoscale. There are implications of scaling hierrachy associated with in-homegeneity and nonequilibrium. of physical systems. What is taken to be homogeneous and equilibrium at the macroscale may not be so when the physical size of the material is reduced to microns. These fundamental issues cannot be dispensed at will for the sake of convenience because they could alter the outcome of predictions. Even more unsatisfying is the lack of consistency in modeling physical systems. This could translate to the inability for identifying the relevant manufacturing parameters and rendering the end product unpractical because of high cost. Advanced composite and ceramic materials are cases in point. Discussed are potential pitfalls for applying models at both the atomic and continuum levels. No encouragement is made to unravel the truth of nature. Let it be partiuclates, a smooth continuum or a combination of both. The present trend of development in scaling tends to seek for different characteristic lengths of material microstructures with or without the influence of time effects. Much will be learned from atomistic simulation models to show how results could differ as boundary conditions and scales are changed. Quantum mechanics, continuum and cosmological models provide evidence that no general approach is in sight. Of immediate interest is perhaps the establishment of greater precision in terminology so as to better communicate results involving multiscale physical events.

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A numerical model is proposed to simulate fracture induced by the coalescence of numerous microcracks, in which the condition for coalescence between two randomly nucleated microcracks is determined in terms of a load-sharing principle. The results of the simulation show that, as the number density of nucleated microcracks increases, stochastic coalescence first occurs followed by a small fluctuation, and finally a newly nucleated microcrack triggers a cascade coalescence of microcracks resulting in catastrophic failure. The fracture profiles exhibit self-affine fractal characteristics with a universal roughness exponent, but the critical damage threshold is sensitive to details of the model. The spatiotemporal distribution of nucleated microcracks in the vicinity of critical failure follows a power-law behaviour, which implies that the microcrack system may evolve to a critical state.

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We use dimensional analysis to derive scaling relationships for self-similar indenters indenting solids that exhibit power-law creep. We identify the parameter that represents the indentation strain rate. The scaling relationships are applied to several types of indentation creep experiment with constant displacement rate, constant loading rate or constant ratio of loading rate over load. The predictions compare favourably with experimental observations reported in the literature. Finally, a connection is found between creep and 'indentation-size effect' (i.e. changing hardness with indentation depth or load).

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In order to study the failure of disordered materials, the ensemble evolution of a nonlinear chain model was examined by using a stochastic slice sampling method. The following results were obtained. (1) Sample-specific behavior, i.e. evolutions are different from sample to sample in some cases under the same macroscopic conditions, is observed for various load-sharing rules except in the globally mean field theory. The evolution according to the cluster load-sharing rule, which reflects the interaction between broken clusters, cannot be predicted by a simple criterion from the initial damage pattern and even then is most complicated. (2) A binary failure probability, its transitional region, where globally stable (GS) modes and evolution-induced catastrophic (EIC) modes coexist, and the corresponding scaling laws are fundamental to the failure. There is a sensitive zone in the vicinity of the boundary between the GS and EIC regions in phase space, where a slight stochastic increment in damage can trigger a radical transition from GS to EIC. (3) The distribution of strength is obtained from the binary failure probability. This, like sample-specificity, originates from a trans-scale sensitivity linking meso-scopic and macroscopic phenomena. (4) Strong fluctuations in stress distribution different from that of GS modes may be assumed as a precursor of evolution-induced catastrophe (EIC).

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The longitudinal fluctuating velocity of a turbulent boundary layer was measured in a water channel at a moderate Reynolds number. The extended self-similar scaling law of structure function proposed by Benzi was verified. The longitudinal fluctuating velocity, in the turbulent boundary layer was decomposed into many multi-scale eddy structures by wavelet transform. The extended self-similar scaling law of structure function for each scale eddy velocity was investigated. The conclusions are I) The statistical properties of turbulence could be self-similar not only at high Reynolds number, but also at moderate and low Reynolds number, and they could be characterized by the same set of scaling exponents xi (1)(n) = n/3 and xi (2)(n) = n/3 of the fully developed regime. 2) The range of scales where the extended self-similarity valid is much larger than the inertial range and extends far deep into the dissipation range,vith the same set of scaling exponents. 3) The extended selfsimilarity is applicable not only for homogeneous turbulence, but also for shear turbulence such as turbulent boundary layers.

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We provide an overview of the basic concepts of scaling and dimensional analysis, followed by a review of some of the recent work on applying these concepts to modeling instrumented indentation measurements. Specifically, we examine conical and pyramidal indentation in elastic-plastic solids with power-law work-hardening, in power-law creep solids, and in linear viscoelastic materials. We show that the scaling approach to indentation modeling provides new insights into several basic questions in instrumented indentation, including, what information is contained in the indentation load-displacement curves? How does hardness depend on the mechanical properties and indenter geometry? What are the factors determining piling-up and sinking-in of surface profiles around indents? Can stress-strain relationships be obtained from indentation load-displacement curves? How to measure time dependent mechanical properties from indentation? How to detect or confirm indentation size effects? The scaling approach also helps organize knowledge and provides a framework for bridging micro- and macroscales. We hope that this review will accomplish two purposes: (1) introducing the basic concepts of scaling and dimensional analysis to materials scientists and engineers, and (2) providing a better understanding of instrumented indentation measurements.

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A set of scaling criteria of a polymer flooding reservoir is derived from the governing equations, which involve gravity and capillary force, compressibility of water, oil, and rock, non-Newtonian behavior of the polymer solution, absorption, dispersion, and diffusion, etc. A numerical approach to quantify the dominance degree of each dimensionless parameter is proposed. With this approach, the sensitivity factor of each dimensionless parameter is evaluated. The results show that in polymer flooding, the order of the sensitivity factor ranges from 10(-5) to 10(0) and the dominant dimensionless parameters are generally the ratio of the oil permeability under the condition of the irreducible water saturation to water permeability under the condition of residual oil saturation, density, and viscosity ratios between water and oil, the reduced initial oleic phase saturation and the shear rate exponent of the polymer solution. It is also revealed that the dominant dimensionless parameters may be different from case to case. The effect of some physical variables, such as oil viscosity, injection rate, and permeability, on the dominance degree of the dimensionless parameters is analyzed and the dominant ones are determined for different cases.

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The finite element method was used to simulate the conical indentation of elastic-plastic solids with work hardening. The ratio of the initial yield strength to the Young's modulus Y/E ranged from 0 to 0.02. Based on the calculation results, two sets of scaling functions for non-dimensional hardness H/K and indenter penetration h are presented in the paper, which have closed simple mathematical form and can be used easily for engineering application. Using the present scaling functions, indentation hardness and indentation loading curves can be easily obtained for a given set of material properties. Meanwhile one can use these scaling functions to obtain material parameters by an instrumented indentation load-displacement curve for loading and unloading if Young's modulus E and Poisson's ratio nu are known.

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In this paper we use a simple normal form approach of scale invariant fields to investigate scaling laws of passive scalars in turbulence. The coupling equations for velocity and passive scalar moments are scale covariant. Their solution shows that passive scalars in turbulence do not generically follow a general scaling observed for velocity field because of coupling effects.