WG-like modes selectivity in a square cavity with posts

We investigate the characteristics of Whispering-Gallery(WG)-like modes in a square cavity with posts by employing the two-dimentional (2D) finite-difference time-domain (FDTD) technique combined with the effective index method. The results indicate that the posts can result in mode selection in the WG-like modes. The WG-like modes with odd mode numbers are not much sensitive to the sizes of the posts. However, the quality factor (i.e. Q-factor) of the WG-like modes with even mode numbers decreases sharply with the increasing size of the posts. The decreasing Q-factor is attributed to mode leakage and scattering loss due to the presence of the post. The mode selection increases with the mode spacing of square cavity twice in an optimized strucure.

Elemental carbon record of paleofire history on the Chinese Loess Plateau during the last 420 ka and its response to environmental and climate changes

IEECAS SKLLQG

关中平原420―350kaBP的古土壤与环境

IEECAS SKLLQG

Elemental carbon record of paleofire history on the Chinese Loess Plateau during the last 420 ka and its response to environmental and climate changes

An elemental carbon (EC) record, covering the last 420 ka, was reconstructed using chemical oxidation method on a loess and paleosol sequence from the Lingtai section on the Chinese Loess Plateau. The EC record reveals the paleofire history and its relationship with climate and vegetation changes on the Chinese Loess Plateau. Our results show that the EC abundance is generally higher in the paleosols than in the loess layers, showing a glacial/interglacial pattern, which is coincident with biomass changes. This variation pattern indicates that paleofires were intensified when biomass accumulated and climate changed abruptly especially from wet to dry conditions. The EC abundance increases sharply at similar to 130 kaB.P., indicating a dramatic change in the vegetation and climate variation patterns. The occurrence of a peak value with the highest average EC abundance in the Holocene may reflect the occurrence of a major climate event at similar to 6 kaB.P., and may also be partly due to more frequent anthropogenic fire usages. (c) 2007 Elsevier B.V. All rights reserved.

Tensile properties of thermally exposed aluminium borate whisker reinforced 6061 aluminium alloy composite

The effect of thermal exposure on the tensile properties of aluminium borate whisker reinforced 6061 aluminium alloy composite was studied. The interfacial reaction was investigated by TEM and the mechanical properties were studied using tensile tests. The results indicated that the interfacial reaction had an influence on the mechanical properties of the composite, so that the maxima of Young’s modulus and ultimate tensile strength of the composite after exposure at 500?C for 10 h were obtained for the optimum degree of interfacial reaction. The yield strength,however, was not only affected by the interfacial state but also by many other factors.

Dynamic function of damage and its implications

Dynamic function of damage is the key to the problem of damage evolution of solids. In order to understand it, one must understand its mesoscopic mechanisms and macroscopic formulation. In terms of evolution equation of microdamage and damage moment, a dynamic function of damage is strictly defined. The mesoscopic mechanism underlying self-closed damage evolution law is investigated by means of double damage moments. Numerical results of damage evolution demonstrate some common features for various microdamage dynamics. Then, the dynamic function of damage is applied to inhomogeneous damage field. In this case, damage evolution rate is no longer equal to the dynamic function of damage. It is found that the criterion for damage localization is closely related to compound damage. Finally, an inversion of damage evolution to the dynamic function of damage is proposed.

Barium-Functionalized Multiwalled Carbon Nanotube Yarns As Low-Work-Function Thermionic Cathodes

Barium-functionalized multiwalled carbon nanotube yarns were fabricated by drawing and twisting multiwalled carbon nanotube forests through a solution containing barium nitrate. After heat activation under vacuum, the functionalized yarns were enriched in barium oxide due to the high surface-to-volume ratio of the nanotubes. The cathodes exhibited good thermionic properties, with a work function as low as 1.73-2.06 eV and thermionic current density that exceeded 185 mA/cm(2) in a field of 850 V/5 mm at 1317 K. The barium-functionalized yarns had high tensile strength of up to 420 MPa and retained strength of similar to 250 MPa after a 2 h activation process. (C) 2008 American Institute of Physics.

A micromechanical model of influence of particle fracture and particle cluster on mechanical properties of metal matrix composites

A general analytical model for a composite with an isotropic matrix and two populations of spherical inclusions is proposed. The method is based on the second order moment of stress for evaluating the homogenised effective stress in the matrix and on the secant moduli concept for the plastic deformation. With Webull's statistical law for the strength of SiCp particles, the model can quantitatively predict the influence of particle fracture on the mechanical properties of PMMCs. Application of the proposed model to the particle cluster shows that the particle cluster has neglected influence on the strain and stress curves of the composite. (C) 1998 Elsevier Science B.V.

钢板混凝土复合圆柱壳的计算

本文将钢板混凝土复合圆柱壳用于大型浮顶式油罐,探讨竖立的双层圆柱壳在液压、内外温差和混凝土收缩引起的应力分析问题.给出了计算公式和20万米~3油罐的一个算例,以显示这种复合结构的优点,供设计工作者参考.

级联激波触发的太阳耀斑理论

<正> 随着观测特征不断积累和丰富,太阳耀斑的理论模型也越来越多。空间观测的结果似乎只要求活动区磁场是双极拱形,与大量的电流片模型不一致。耀斑过程可能并不总是一种机制。本文从理论上进一步讨论太阳耀斑的级联爆发模型。对流区的波动能量转换为活动区的横向磁场能量,由于扭转不稳定性使磁能释放,并转换为激波动能,活动区激发的级联

Experimental studies on self-ignition of hydrogen/air supersonic combustion

Self-ignition tests of a model scramjet combustor were conducted by using parallel sonic injection of gaseous hydrogen from the base of a blade-like strut into a supersonic airstream, The vitiated air was produced by burning H-2, O-2, and air to a stagnation temperature of 1000-2100 K and a stagnation pressure of 0.8-1.6 MPa, The effects of different parameters on the self-ignition limits were analyzed, In addition, the effects of the combustor's different wall configurations on self-ignition limits were specifically studied. It was found that the wall configurations of the combustor had a significant effect on self-ignition limits, which might have variations of 420-840 K deg in stagnation temperature; however, the local static temperature in the recirculation zones for different wall configurations remained the same at approximately 1100 K, It was found that self-ignition could initiate at the exit of the combustor and this can be considered as a weak self-ignition characteristic.

涂层断裂韧性的声发射辅助拉伸测量方法

涂层的断裂韧性与界面结合强度是表征涂层/基体材料体系力学性能的重要指标.但如何准确地测量涂层的断裂韧性和强界面结合的涂层/基体材料体系的界面结合强度至今仍存在困难.以铬涂层/钢基体材料为对象,采用声发射和显微镜实时动态检测技术与拉伸实验相结合的方法,探索了铬涂层的断裂韧性以及铬涂层/钢基体的界面剪切强度.根据相关力学模型和实验测量结果,得到铬涂层在室温下的断裂韧性为27.41J/m~2. 同时,发现在铬涂层裂纹饱和后界面开裂都未发生,获得了该种材料体系界面剪切强度的一个下限值.

亚声速段横向射流对Coil性能影响的数值研究

在化学氧碘激光的混合喷管内发生的是一个气体动力学、化学反应动力学以及光学等相互耦合的复杂过程,每个过程都对COIL性能有着至关重要的影响.利用三维CFD技术,通过求解层流Navier-Stokes方程与组分输运方程,对简化后的化学氧碘激光RADICL模型进行数值模拟与分析,对COIL的气动和增益特性进行探讨.在不同的射流穿透条件下,计算COIL混合喷管中的混合与化学反应过程,发现穿透深度决定了增益的分布特性以及过度穿透条件下的非定常结构,

障碍物扰动对预混火焰发展的影响

研究了半开口管道中障碍物对预混火焰传播的影响．结果表明，由于障碍物引起的扰动，使火焰在传播过程中不断加速，同时管内压力上升．根据火焰速度的量级，在受限管道中的火焰传播可以分为3种状态：熄火态、雍塞态和爆轰态．在贫燃极限附近，火焰加速一段距离后自动熄灭；在雍塞态，最大火焰速度略低于燃烧产物声速，基本上不受阻塞比变化的影响；随着当量比的上升，对敏感气体而言，火焰传播由爆燃转变为爆轰，最大火焰速度随阻塞比的增加而降低；而对于非敏感气体，则不存在爆燃转爆轰现象．管内压力随障碍物阻塞比的变化并不呈现单调规律．同时用非稳态可压缩流体模型模拟了管内的火焰加速和压力发展过程，计算结果和实验结果吻合得较好．