基于原子体积场拉普拉斯算子对金属玻璃剪切转变区的预测

剪切转变区(shear transformation zone, STZ)作为金属玻璃塑性事件的一个基本特征单元, 已被研究者们逐渐接受,但STZ产生的机制和来源仍具争议. 本文采用分子模拟方法对 Cumetallic glasses,shear transformation zones,nonaffine displacements,atomic volume,molecular simulation,1)国家自然科学基金(11790292);国家自然科学基金(11672298);国家自然科学基金(11432014);中国科学院战略性先导科技专项(B类)(XDB22040501)2019-12-242020-04-10剪切转变区(shear transformation zone, STZ)作为金属玻璃塑性事件的一个基本特征单元, 已被研究者们逐渐接受,但STZ产生的机制和来源仍具争议. 本文采用分子模拟方法对 Cu$_{64}$Zr$_{36}$金属玻璃在受简单剪切加载时的变形行为展开了研究. 结果表明,体系的初始构型与加载后STZ的产生是相关的. 虽然原子体积场及其梯度可以用来有效表征金属玻璃中局部原子构型的非均匀性, 但它们与STZ产生的区域没有直接明显的对应关系. 基于此, 提出一个新的局域结构参数 $\xi $ 来用于金属玻璃中STZ产生区域的预测,它由两部分构成: 原子体积场的拉普拉斯算子和体积场梯度分量的绝对差值. 原子体积场的拉普拉斯算子为负且绝对值较大时, 体积场梯度向量呈现向内指的分布特征, 代表体系中的局域软区; 而体积场梯度分量的绝对差值则用于遴选体积场梯度不同的分布模式. 进一步地,建立了该结构参数与非仿射位移和剪切局部化三者关系, 发现特定的体积场梯度向量分布模式, 将导致局部剪切增强的非仿射位移场, 从而更容易诱发STZ的形成. 相关性分析表明,该参数与STZ区域平均相关性高于78%, 因此, 该参数能有效用于金属玻璃剪切转变区的预测,且运用拉普拉斯算子的思想有望应用于金属玻璃力学行为的理论分析.     

Randomization-and-relaxation model revisited

Abstract

The interatomic potentials of Stillinger-Weber and Tersoff were incorporated into the randomization-and-relaxation model, which was originally developed for modelling amorphous silicon by using the Keating interatomic potential. The inclusion of more recent and more complicated interatomic potentials resulted in a more sophisticated set of bond switching rules which form the basis for the randomization-and-relaxation algorithm. This improved model was then used to model small isolated amorphous zones which are produced by individual heavy ions during ion implantation in silicon. The temperature evolution during zone creation was calculated by using idealized thermal spike model. The structure and stability of these amorphous zones was examined with respect to the energy of incoming ion and with respect to the interatomic potential employed. It was established that significantly lower spike energy is required to create a stable amorphous region than in the simulation where the Keating potential was employed.     

MODIS多光谱研究攀西地区植被对气候变化响应

遥感方法定量监测植被与气候变化的关系已经成为全球变化研究的一个重要领域。攀西地区是我国长江上游重要的生态屏障,对维持四川乃至我国整体生态环境的稳定具有重要作用。基于攀西地区多年MODIS多光谱数据和气象数据对植被动态变化以及该变化与气候因子的关系进行了探讨。结果显示,NDVI变化和气温、降水呈正相关,其中降水是影响攀西地区植被生长的主要气候因子,NDVI变化与秋季降水量变化呈现出良好的一致性;研究区植被长势与气温和降水存在一定的时滞性,主要体现在前1个月气温和降水对植被长势的影响上。     

Analysis of supersonic flow around two bodies of revolution near a surface

The results of experimental and numerical investigations of the peculiarities of flow around two identical cylindrical bodies of revolution of diameter D = 50 mm and the body aspect ratio λ = 5 with conical forebodies whose apex angles are θ = 40° and 60°, which are located above a horizontal surface in parallel with one another and with the flow, are presented for the Mach numbers M_∞ = 4.03, Reynolds numbers Re₁ ≈ 55·10⁶ m⁻¹, fixed distance from the surface Y = Δy/D = 0.96, and the gaps between their axes Z = Δz/D = 1.06−2.4. The peculiarities of three-dimensional turbulent separated flows realizing on the bodies and on the plate as well as the possibilities of predicting the aerodynamic forces and moments acting on the bodies on the basis of numerical computations within the framework of the Euler equations are considered.     

Long-term memory dynamics of continental and oceanic monthly temperatures in the recent 125 years

For both Northern and Southern hemispheres, the long-term memory dynamics for continent and ocean temperature records in the recent 125 years is studied in this paper. It is found that the records exhibit long-range memory and multifractality characteristics where large temperature anomalies display a more random behavior than the overall time series. A 256-month moving window was used to compute the time evolution of the fractal scaling exponent, giving the following results: (i) Ocean temperatures are more persistent than land temperatures, a result already reported in recent publications, (ii) All records show multifractality features, reflecting the nonlinear behavior of the temperature dynamics. Continent temperatures present sharper multifractal spectra than ocean temperatures, (iii) The persistency, as revealed by the scaling exponent, for ocean temperatures displays a cyclic behavior around a nearly constant average value of 22 years, (iv) The persistency for the Northern Hemisphere land temperature is also cyclical but with an increasing trend, and (v) The time at which the Northern Hemisphere continent temperature persistency will converge into the Northern Hemisphere ocean behavior was estimated with linear and exponential extrapolation functions, showing hitting dates around 2050±20 A.D. Potential implications of these results concerning the nature of climate change are also discussed.     

Monatomic Optical Near–Field Microscope Based on Photoinduced Dimensional Resonances of Interacting Atoms

The operating principle of the optical near–field microscope is suggested; it is based on narrow photoinduced dimensional resonances in the system atom on the tip of a needle -- atom of the sample. It is shown that this microscope has high sensitivity and spatial resolution of the order of 1 nm.     

Rock deformation problems

Studies into the stress-strain state of rock mass are reviewed. The solutions of a wide range of applied problems are analyzed and methods used to solve them are generalized. Important areas of further research in rock mechanics are pointed out.__________Translated from Prikladnaya Mekhanika, Vol. 40, No. 12, pp. 3–17, December 2004.     

Ideal gas flows with separation zones and time-dependent contact discontinuities of complicated shape

Some examples of flows with separation zones andmovable contact discontinuities obtained as a result of the numerical integration of the time-dependent equations for an ideal gas are presented. The examples concern a steady annular separation zone on the blunt nose of a body in a supersonic flow, periodic shedding of unsteady discontinuities from a cylinder in a steady uniform subsonic flow with a supercritical Mach number, and the complicated deformation of a contact (tangential) discontinuity, namely, the boundaries of a two-dimensional jet, either subsonic or supersonic, flowing into a cocurrent subsonic low-velocity flow. A multiple increase in the difference grid capacity in the numerical integration of the Euler equations indicates the absence of a noticeable scheme viscosity effect in the examples calculated. The inviscid nature of the separation flows obtained is also confirmed by their well-known counterparts constructed in the ideal incompressible fluid approximation. The time-average velocity fields of the two-dimensional jet and the intensity of its sound field are in reasonable agreement with the available data.     

A novel implicit method for coastal hydrodynamics modeling: application to the Arcachon lagoon

The present Note reports on numerical modeling of shallow flows in coastal areas. Successful numerical simulations of such flows should be able to cope with strong irregularities of the bathymetry and to reproduce the covering/uncovering (wetting/drying) of tidal flats due to the tidal oscillations of the free surface. Also, adoption of large time steps is necessary to simulate phenomena which last actually several days or months. In the present study, a new numerical model based on an implicit resolution of the shallow water equations is proposed. A penalty method has been employed for numerical treatment of dry zones emerging during the wetting and drying processes. The capability of the present model has been verified by comparison with standard test cases. Further applications and comparisons have been also carried out to simulate the tidal propagation in the Arcachon lagoon. To cite this article: A. Le Dissez et al., C. R. Mecanique 333 (2005).     

Phytochemical Profile and Antioxidant Activities of Coleus amboinicus Lour. Cultivated in Indonesia and Poland

Coleus amboinicus Lour., Lamiaceae, is a perennial herb that is native to Indonesia and also cultivated in Africa, Asia and Australia. The major phytochemicals responsible for its bioactivity are rosmarinic acid (RA) and its analogues, flavonoids and abietane diterpenoids. The possibility of cultivation in a colder climate would extend the use of this herb and provide new opportunities to herb growers and livestock farmers. Our study to compare feed value and phytochemical composition of C. amboinicus plants cultivated in its original region, Indonesia, and in Poland. The crude protein content was significantly higher in plants cultivated in Indonesia compared to those cultivated in Poland—21% and 13% of dry matter, respectively. The higher ADF contents were detected in C. amboinicus cultivated in Indonesia, 38–41%, in comparison to 34% in plants cultivated in Poland. The phytochemical composition was also significantly influenced by the cultivation location. Polish samples were higher in polyphenols (RA and its analogues), and also had 1.5–2-fold higher antioxidant potential, as measured by DPPH scavenging, phosphomolybdenum reduction and Fenton reaction driven lipid peroxidation. The Indonesian samples contained more diterpenoid compounds, such as dihydroxyroyleanone, and the sum of terpenoids was ca. 10 times higher than in samples from Poland (15.59–23.64 vs. 1.87 µg/g of extracts). In conclusion, C. amboinicus is suitable for cultivation in non-optimal climatic conditions but some nutritional properties and bioactivity are significantly affected.