Species, Distributions and Conservation of Acipenseriformes

IntroductionSturgeons,fish of extremely great commercialvalue,belong to one of the mostancientgroups ofthe Osteichthyes.They are widely distributed in theNorthern Hemisphere above the 3 0 th parallel,andwithin this zone they are found everywhere alongthe coasts of the Atlantic,the Pacific,theMediterranean and the Black Sea,as well as inrivers,lakes and inland seas. Their abundance inthis huge range differs greatly:in many parts ofthe range they are found only as isolated,sparsepopulations,a…     

Fractals in Spatial Patterns of Vegetation Formations

Introduction The quantification of spatial patterns, a focal subject in ecological research, is used as a foundation for meas-urement and analysis of the quality of habitats and the biodiversity[1]. Historically, the dominant approach has been used to cho…     

基于 ANSYS 的液压油缸再制造技术方法可行性分析

为了探讨零件在结构上或技术上再制造的可行性,针对采用修理尺寸法在对进行再制造的零件作机械加工中要去除表面层厚度,以消除其表面缺陷及恢复其形状精度时,会在一定程度上削弱零件的强度或刚度的问题,本文选择了工程机械中的动臂油缸和转斗油缸作为研究对象,应用ANSYS有限元分析软件分别对其在缸壁厚度因机械加工减少0.5 mm和1.0 mm后的力学性能进行分析,通过计算缸体在原设计条件下的应力场和位移场,了解缸体结构加工后的应力应变情况,为采用修理尺寸法进行油缸再制造和确定缸体的再制造修理尺寸及其标准化提供理论依据。计算结果显示,缸体加工后零件最大应力处的安全系数分别为1.63、1.57和1.47、1.41,最小的数值仍然在1.2～1.5的设计安全系数推荐值范围内,表明再制造加工后的油缸缸体能满足强度、刚度要求,采用修理尺寸法进行油缸的再制造是可行的。     

基于最小特性值的破碎机机构参数优化设计

破碎机机构尺寸参数的设计是决定产品性能的关键.本文选择了量大面广的PE600×900鄂式破碎机为研究对象,通过建立以动颚行程特性值最小为目标函数的优化数学模型,利用MATLAB优化工具箱中的序列二次规划(SQP)算法,采用直接调用fmincon函数的方式,实现有约束条件下的破碎机机构尺寸参数优化问题的求解.优化结果显示...     

Spatial Heterogeneity of Vegetation in China

The spatial heterogeneity and ecological characteristics of 74 vegetation types and the lack of vegetation type within the 9.6 million km2 of Chinese territory were studied using data from the China vege- tation map. The analysis used 877 large quadrats 10 mm×10 mm (actual size 100 km×100 km), which cov- ered about 8.77 million km2. Each large-quadrat was divided into four small 5 mm×5 mm quadrats (actual size 50 km×50 km). The frequency of occurrence of all vegetation types was recorded in each small quadrat. The survey using the Shiyomi method of vegetation analysis based on the beta-binomial distribution was adopted to describe the frequency of occurrence and spatial heterogeneity for each kind of vegetation. The weighted average heterogeneity of all the vegetation types in the landscape provides a measure of the landscape level heterogeneity which describes the spatial intricacy of the existing vegetation composition. The maximum spatial vegetation heterogeneity was 0.8620 in the frigid-temperate coniferous forest of Larix (V1), which dominates cold moist northeast China. The minimum spatial heterogeneity with a low occurrence was the Caragana tibetica in the gravel desert in the western Yellow River Hetao area. The minimum occur- rence with a low spatial heterogeneity was the Monsoon rainforest on rock in Guangxi and Yunnan Provinces. The vegetation types on the Qing-Zang Plateau were found to be representative of Chinese vegeta- tion because of the very high occurrence and spatial heterogeneity of these vegetation types. The weighted average of the heterogeneity was 0.677, while the vegetation diversity index was 3.29.