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161.
Mark Harthun 《Limnologica》1999,29(4):449-464
Due to the return of the beaver an increasing number of brooks become restored to a natural condition. It is characterized by an alternation of flowing and standing sectors. In the summer of 1995, different groups of organisms (Odonata, Mollusca, Trichoptera, Ephemeroptera, Diptera) of two brooks in the Spessart mountains (Hesse, Germany) were studied to compare the composition of invertebrates in beaver homeranges and in the non-influenced sectors above. The new living conditions resulted in the disappearance of some species (Drusus anulatus, Trich., Sericostoma personatum, Trich., Radix peregra, Gastr.). Nevertheless the beaver homeranges accommodate a significant higher number of species of dragon flies and damsel flies, molluscs and caddis flies than the brooks above the homeranges. The heterogeneity of natural river systems make a coexistence of running water and silent water organisms possible. The examinations result in a significant higher group dominance of ephemeras (i.p. Cloeon dipterum) in a beaver pond than in lenitic sectors conditioned by the gradient without beaver influence. Here the dominance of chironomids was much higher. High current sectors with their characteristic organisms are also present in the beaver homeranges. The higher number of insects is the basis of nourishment for predatory insects and fishes. The potential effects of beaver ponds on fishes are discussed. 相似文献
162.
Sarah E. Mouring 《Ocean Engineering》1999,26(8):793
The U.S. shipbuilding industry recently has started incorporating composite materials in the construction of both military and commercial ships due to the advantages of composite construction. These advantages include the reduction in total life costs, corrosion resistance, high strength- and stiffness-to-weight ratios, and improved stealth for military applications. One disadvantage is the higher costs of composites compared to steel and other conventional materials. Therefore, new higher quality materials with lower costs and new fabrication methods need to be developed before composite materials will be fully accepted for the construction of large ships. A new composite preform framing technology shows promise in the reduction of fabrication costs for large ship construction. There already has been significant cost savings using this framing technology in the construction of small recreational boats and large yachts. This framing technology involves casting a dry glass fiber-reinforced plastic (GRP) fabric into shape in a closed mold with a foam core. One unresolved issue using this framing technology is the orientation of the fiber for the frames. This paper summarizes experimental results of the testing of composite panels stiffened with preform frames under in-plane uniaxial compressive loads. Biaxial (0,90), quadaxial (0,90,+45,−45), and triaxial (+45,−45,0) laminates were used in the frames. 相似文献
163.
164.
痕量元素及其同位素采集、预处理和分析在海洋研究中具有重要的作用,对于认识全球环境变迁具有重要的价值。但其浓度在这些过程中易受沾污。本文结合海水痕量元素CTD现场采集工作环境,工作拉力等特殊要求,先后跟缆绳设计、生产制造厂家进行缆绳相关技术讨论、确定缆绳各项技术指标。设计敷塑纤维通讯缆,并委托缆绳生产厂家加工出样缆两条。通过对样缆进行静态拉力试验验证缆绳自身工作拉力,并通过成分分析、浸出试验验证最大化减少缆绳对于海中溶解态痕量金属的沾污。该研究结果可提高采样过程的可靠性,保证数据的准确率。 相似文献
165.
Spherical geometry of quaternions is employed to characterize the Bingham distribution on the 3-dimensional sphere
as being uniquely composed of a bipolar, a circular and a spherical component. A new parametrization of its dispersion parameters provides a classification of patterns of crystallographic preferred orientations (CPO, or textures). It is shown that the Bingham distribution can represent most types of ideal CPO patterns; in particular single component, fiber and surface textures are represented by rotationally invariant bipolar, circular and spherical distributions, respectively. Pole figures of given crystal directions are derived by the spherical Radon transform of the Bingham probability density function of rotations, which are displayed for general and special cases. 相似文献
166.
通过数值解非线性耦合薛定谔方程 ,研究高速光孤子通信系统中高阶色散和偏振模色散对孤子传输的影响 ,并数值模拟孤子在单模光纤中的演变。研究结果表明 :偏振模色散导致孤子脉冲展宽、峰值功率下降、峰值点随传输距离漂移 ;高阶色散和偏振模色散使孤子加速展宽、脉冲沿出现非对称的振荡结构 ,脉冲峰值点随传输距离的漂移而发生改变 相似文献
167.
168.
Model tests of floating offshore platforms replace the mooring lines by inextensible cables connected to steel springs with a linear restoring capability. With the help of fundamental investigation on the similarity laws, the present work shows that the use of very thin polyester lines in model scaling is feasible and may allow a better physical representation of the full-scale polyester lines. The proposal is to use polyester lines with length distortion in order to keep the similar restoring capability and therefore, the same non-linear behavior would also be present during the tests. The work describes the application of these ideas in a model test of an Floating Production Storage and Offloading (FPSO) submitted to regular and random waves. The tests include both the linear spring lines and the distorted equivalent polyester lines, holding the FPSO model, submitted transversally to the same waves. 相似文献
169.
FAN Haifeng WEN Hanjie HU Ruizhong CHANG Bin 《中国地球化学学报》2008,27(1):90-96
Standard reference material and different geological samples were dissolved by system A (the mixture of nitric and perchloric acids) and system B (mixture of nitric, perchloric and hydrofluoric acids), and total Se in all samples was measured by hydride-generation atomic fluorescence spectrometry (HG-AFS) after concentration with thiol cotton fiber (TCF). The analytical results obtained by the two digestion method are in good agreement (within the limit of errors) for most of the samples, particularly for those having recommended values. The Se concentrations determined by the two methods are of no difference, and the correlation coefficient is 0.9986; the relative standard deviation (RSD) for the determination of 0.04 μg/g Se is 10.2%. The recovery rates of systems A and B by the standard-addition method were 96%-106% and 99%-104%, respectively. 相似文献
170.
采用场发射扫描电镜及微型光纤光谱仪对21种双壳纲韧带中文石纤维的纳米形貌及光学反射特征进行研究,经过对比分析及统计结果表明:形貌上,文石纤维是由约30 nm~80 nm的亚颗粒沿纤维轴向排列而成,呈串珠状结构;纤维横截面常呈近六边形或浑圆形,但棕带仙女蛤、砗蚝及粒帽蚶呈长六边形.纤维直径及相邻纤维间距与生物种类有关,变化范围分别为65 nm~222 nm及88 nm~214 nm.除了砗蚝及粒帽蚶韧带无光学反射外,其他韧带均存在位于可见光范围内单反射峰,并随湿润程度增大而增加,反射强度也增大主波长还发生红移,从而导致韧带的水致变色现象,变色范围为28 nm~145 nm.韧带文石纤维独特的纳米形貌及其复杂的水致变色现象的发现对生物的矿物学及光子学研究具有重要的意义. 相似文献