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排序方式: 共有2322条查询结果,搜索用时 15 毫秒
1.
Matthew O'Sullivan Daire
hUallachin Patrícia Oliveira Antunes Eleanor Jennings Mary Kelly‐Quinn 《河流研究与利用》2019,35(2):146-158
Cattle access to streams has been linked globally with degradation of stream water quality, driven largely by bank erosion and resultant instream, fine sediment deposition. The majority of evidence on such effects is however based in arid and semiarid regions of the United States and Australia, with few studies relating to cool temperate climates such as Northwest Europe. In this study, “Quorer” resuspendable sediment samples were taken from riffle geomorphic units upstream (control) and at two points downstream (pressure and recovery) of cattle access points in headwater streams in agricultural catchments in Ireland to assess levels of deposited stream sediment. Samples were taken in April/May (2016) prior to the grazing season and in October (2016) at the end of the grazing season. Sites in good‐high ecological status catchments and less than good ecological status catchments were included in the study. Higher levels of sediment were found downstream of cattle access points in both good‐high status and less than good status catchments; however, the impacts of access points were spatially confined to, in most cases, the area immediately downstream of the point of access. There was a strong correlation between deposited sediment mass and organic matter (OM) mass, with levels of OM increasing linearly with deposited sediment mass. Levels of measured sediment were negatively correlated with riparian habitat health (measured using a qualitative habitat assessment). The results of this study highlight the need for measures to prevent cattle access to headwater streams where access points can be many in order to manage local habitat quality and downstream water quality issues. 相似文献
2.
竖向循环荷载作用下桩土界面的作用机理是研究桩土摩擦疲劳的关键。针对循环荷载作用下桩-粉土界面的剪切性能,使用改进的剪切试验装置在恒刚度条件下进行循环剪切试验,研究循环次数、累积位移和法向刚度对其摩擦疲劳性能、循环后单调剪切性能的影响。试验结果表明,粉土在循环剪切过程中,法向应力和剪应力在初始10个循环内随循环数增加快速衰减,随着循环进行,逐渐趋于稳定;单次循环内在剪切位移方向变化时,土体呈现表现出剪缩-剪胀-剪缩交替现象,总体变形呈现剪缩的趋势;循环荷载作用下,粉土界面的法向应力和剪应力随法向刚度增大衰减速率增大,达到稳定的累积循环位移越小;粉土循环后的单调剪切、法向应力恢复的单调剪切的剪应力比小于首次单调剪切试验值,且法向应力恢复的循环后剪切试验的剪胀程度较小,表明循环剪切过程中界面处粉土颗粒棱角破碎,颗粒变得光滑。在对试验数据分析的基础上,提出了与累积位移、法向刚度和初始应力相关的无量纲累积位移,建立了法向应力和界面摩擦角随累积位移的衰减方程。 相似文献
3.
M. S. FOUND M. QUARESIMIN 《Fatigue & Fracture of Engineering Materials & Structures》2003,26(1):17-26
A brief review of the models used to predict the cumulative fatigue damage in FRP composites is presented. Two‐stage fatigue loading of a [0/90,± 452,0/90]s quasi‐ isotropic woven carbon fibre/epoxy resin laminate was evaluated at stress ratio R = 0.05 and the failure mechanisms investigated using x‐radiography after each loading stage. The results are presented in terms of fatigue strength and damage growth and are compared with those in the literature. A low‐to‐high loading sequence is more damaging than a high‐to‐low one and the Palmgren‐Miner linear damage rule may no longer be valid for this kind of material, as previously reported. 相似文献
4.
J. C. Sevy 《Calcolo》1993,30(1):41-68
Explicit error estimates are given for the iterated Boolean sum of a sequence of simultaneous approximants; the rate of convergence
is shown to be improved for smooth functions. The general results are applied in the case of the Bernstein, Durrmeyer and
Stancu operators. 相似文献
5.
6.
刘道杰 《石油工业计算机应用》2007,15(4):32-34,37
模拟退火算法是一种利用统计力学观点求解优化问题全局最优解的方法,是目前解决优化问题的一种比较先进的方法。本文应用模拟退火算法对蒸汽吞吐注采参数进行优化设计,并确定出了6个吞吐周期的最优周期产油量和周期注蒸汽量,较实际周期产油量和实际周期注汽量均有明显增加。将各周期的优化累积产油量和优化累积注汽量画在累积产油量与累积注汽量的双对数坐标中,得出累积注汽量和累积采油量关系曲线(简称注采关系曲线)。将该注采关系曲线外推估算了该井未来周期的累积产油量和累积注汽量,从而达到对蒸汽吞吐井生产能力的预测。 相似文献
7.
8.
本文总结了求幂级数和函数的四种方法。一种方法是将待求级数分解成己知和函数的级数的运算(一般是加减)表达形式,然后逐一求和新的级数;第二种方法是“先求导,再积分”或“先积分,再求导”;第三种方法是把待求级数用基本初等函数的幂级数展开式表示出来;第四种方法是列写出和函数满足的微分方程,解此微分方程得到和函数。 相似文献
9.
简支深梁用和函数法的级数解答 总被引:3,自引:1,他引:2
引用和函数法求解弹性力学平面问题, 对简支深梁受三角形荷载作用时的应力进行了分析,同时给出了该问题的付立叶级数解答,该方法不仅将求解弹性力学的偏微分方程组解耦成非耦联的几个偏微分方程,而且其结果与传统的Airy应力函数得到的解答完全一致, 证明了该方法的正确性和适用性,该方法拓展了将经典的弹性力学解析法用于解决工程实际问题的空间。 相似文献
10.
Abstract. Three linear methods for estimating parameter values of vector auto-regressive moving-average (VARMA) models which are in general at least an order of magnitude faster than maximum likelihood estimation are developed in this paper. Simulation results for different model structures with varying numbers of component series and observations suggest that the accuracy of these procedures is in most cases comparable with maximum likelihood estimation. Procedures for estimating parameter standard error are also discussed and used for identification of nonzero elements in the VARMA polynomial structures. These methods can also be used to establish the order of the VARMA structure. We note, however, that the primary purpose of these estimates is to generate initial estimates for the nonzero parameters in order to reduce subsequent computational time of more efficient estimation procedures such as exact maximum likelihood. 相似文献