Bioactive dammarane-type triterpenoids derived from the acid hydrolysate of Gynostemma pentaphyllum saponins

From an acid hydrolysate of the crude saponins of Gynostemma pentaphyllum, three triterpenes, including a new compound (23S)-3β-hydroxydammar-20,24-dien-21-oic acid 21,23-lactone (1) and two known aglycons (20S, 23R)-3β,20β-dihydroxydamma-24-dien-21-oic acid 21,23-lactone (2) and (20S, 24S)-20,24-epoxydammarane-3β,12β,25-triol (3), were isolated. Their structures were established on the basis of extensive spectral evidence (HR-ESI-MS, IR, 1D and 2D-NMR experiments). In bioactive assays in vitro, compound 1 was found to have potent cytotoxicity against the human breast cancer cells MDA-MB-435, whereas compounds 2 and 3 exhibited modest inhibitory activity toward porcine pancreatic lipase. The results indicated that acid treatment of G. pentaphyllum extract could produce diverse structures with interesting bioactivity.     

芯模参数对3A21矩形管绕弯截面变形的影响研究

为解决截面变形对铝合金薄壁矩形波导管通信信号传输质量的影响,基于ABAQUS/Explicit平台建立了3A21铝合金薄壁矩形管绕弯成形三维有限元模型,并试验验证了该模型的可靠性,在此基础上模拟研究了芯模参数对截面变形的影响规律.研究结果表明:增加芯头个数、增大芯头厚度以及芯头间间距,截面变化率减小,而增大芯头圆弧角度,截面变化率增大;当芯模支撑角度不变时,芯头个数和芯头厚度的改变仍会对截面变形产生很大影响,芯头厚度越大,管坯的截面变形程度越小.     

传输法模拟滤波器型输出回路的间隙阻抗频率特性

利用传输法把滤波器型输出回路等效为双端口微波网络问题,通过理论分析,数值模拟和冷测实验证明了S21平方后的曲线能定性反映滤波器型输出回路的间隙阻抗频率特性.从而在仿真设计阶段,可将S21设为优化目标,利用三维电磁仿真软件的参数优化功能实现精确设计滤波器结构,比相位法,场分析法更加快捷准确.传输法同样适用于矢量网络分析仪冷测滤波器的实验,可消除系统拆装及波导元件引起的误差,使测量更加方便准确.     

糙率改变对城市河道水位及堤防防洪能力的影响

以海城河为例,针对现代城市发展过程中城市河道由自然渠化生态的过程导致河槽糙率变化问题,分析其对河道行洪能力的影响,并利用一维河网模型MIKE11建立了海城河城区段水动力模型,分析生态河道规划前后不同糙率情况下海城河现状堤防的防洪能力,为海城河防洪规划提供理论依据。     

电流种类对Cr-Mo-V钢埋弧焊焊缝冲击韧性的影响

为寻求提高焊缝金属韧性的途径,以21/4Cr-1Mo-1/4V钢焊接为例,采用埋弧焊及相应的焊接材料,分别用直流反接与交流两种焊接电流进行焊接,通过对焊缝金属的性能试验,结果表明焊缝金属的化学成分和抗拉强度没有明显差别,但交流焊接的焊缝金属冲击韧性较直流有明显提高。这主要是因为焊缝金属的一次组织形态和多层焊再热作用使细晶区扩大,改善了交流焊接的焊缝韧性。     

Asymptotic bootstrap corrections of AIC for linear regression models

The Akaike information criterion, AIC, and its corrected version, AIC_c are two methods for selecting normal linear regression models. Both criteria were designed as estimators of the expected Kullback–Leibler information between the model generating the data and the approximating candidate model. In this paper, two new corrected variants of AIC are derived for the purpose of small sample linear regression model selection. The proposed variants of AIC are based on asymptotic approximation of bootstrap type estimates of Kullback–Leibler information. These new variants are of particular interest when the use of bootstrap is not really justified in terms of the required calculations. As its the case for AIC_c, these new variants are asymptotically equivalent to AIC. Simulation results which illustrate better performance of the proposed AIC corrections when applied to polynomial regression in comparison to AIC, AIC_c and other criteria are presented. Asymptotic justifications for the proposed criteria are provided in the Appendix.     

风生流对贡湖湾水环境的影响过程模拟研究

为分析风生流对贡湖湾水环境的影响,基于MIKE21构建贡湖湾二维水动力和水质模型,模拟分析TN、TP、NH3-N迁移变化与风生环流的关系.结果 表明,该模型误差符合要求,能较为准确地模拟风生流影响下贡湖湾污染物的迁移扩散过程;风生流对贡湖湾湖流结构影响明显;夏季湖流流速全年最低,大部分介于0.8～1.2 cm/s,水体...     

Hopkins Architects' Kroon Hall,Yale University

Stone-walled, barrel-vaulted Kroon Hall, the hulking new home of Yale University's pioneering School of Forestry and Environmental Studies, is a living laboratory of energy-efficient design as well as a sympathetic neighbour. Jayne Merkel describes the numerous problems the architects had to solve to make it compatible with the historic campus and actively experimental at the same time. Its pioneering green agenda even determined the character of its interior space warmly and subtly. Copyright © 2009 John Wiley & Sons, Ltd.     

基于模式匹配法的波导滤波器分析

采用模式匹配法对波导滤波器的不连续性进行了分析,给出了模式匹配法分析波导滤波器的基本步骤,并用Matlab编写了用于计算波导滤波器S11和S21参数的模式分析程序,最后对波导对称H面膜片滤波器的S11和S21参数特性进行了计算,并对仿真结果进行了分析,总结了应用模式匹配法分析波导滤波器时应注意的一些问题。     

Static globularization and grain morphology evolution of α and β phases during annealing of hot-rolled TC21 titanium alloy

A lamellar-structure TC21 titanium alloy was hot-rolled and subsequently annealed at 820, 880 and 940 °C for 1 and 6 h, and the effects of annealing parameters on static globularization and morphology evolution of both α and β phases were studied. The results show that α globularization process is sluggish due to the limited boundary splitting at 820 °C. With increasing temperature to 880 °C, the accelerated boundary splitting and termination migration promote the α globularization. At 820 and 880 °C, the static recovery (SRV) and recrystallization (SRX) induce the grain refinement of interlamellar β phase. However, the excessively high temperature of 940 °C results in the coarsening of α grains due to the assistance of Ostwald ripening, and produces coarse β grains mainly due to the absence of SRX in interlamellar β phases. Conclusively, 880 °C is an appropriate annealing temperature to produce a homogeneous microstructure in which globularized α and refined β grains distribute homogeneously.