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31.
Numerical simulation and experimental research on hydrodynamic performance of propeller with varying shaft depths 总被引:2,自引:0,他引:2
In order to study hydrodynamic performance of a propeller in the free surface, the numerical simulation and open-water experiments are carried out with varying shaft depths of propeller. The influences of shaft depths of a propeller on thrust and torque coefficient in calm water are mainly studied. Meanwhile, this paper also studies the propeller air-ingestion under special working conditions by experiment and theoretical calculation method, and compares the calculation results and experimental results. The results prove that the theoretical calculation model used in this paper can imitate the propeller air-ingestion successfully. The successful phenomenon simulation provides an essential theoretical basis to understand the physical essence of the propeller air-ingestion. 相似文献
32.
33.
Combined data of physical property, benthic foraminifera, and stable isotopes from ODP Sites 1148, 1146, and 1143 are used to discuss deep water evolution in the South China Sea (SCS) since the Early Miocene. The results indicate that 3 lithostratigraphic units, respectively corresponding to 21-17 Ma, 15-10 Ma, and 10-5 Ma with positive red parameter (a*) marking the red brown sediment color represent 3 periods of deep water ventilation. The first 2 periods show a closer link to contemporary production of the Antarctic Bottom Water (AABW) and Northern Component Water (NCW), indicating a free connection of deep waters between the SCS and the open ocean before 10 Ma. After 10 Ma, red parameter dropped but stayed higher than the modern value (a*=0), the CaCO3 percentage difference between Site 1148 from a lower deepwater setting and Site 1146 from an upper deepwater setting enlarged significantly, and benthic species which prefer oxygen-rich bottom conditions dramatically decreased. Coupled with a major negative excursion of benthic δ13C at ~10 Ma, these parameters may denote a weakening in the control of the SCS deep water by the open ocean. Probably they mark the birth of a local deep water due to shallow waterways or rise of sill depths during the course of sea basin closing from south to east by the west-moving Philippine Arc after the end of SCS seafloor spreading at 16-15 Ma. However, it took another 5 Ma before the dissolved oxygen approached close to the modern level. Although the oxygen level continued to stabilize, several Pacific Bottom Water (PBW) and Pacific Deep Water (PDW) marker species rapidly increased since ~6 Ma, followed by a dramatic escalation in planktonic fragmentation which indicates high dissolution especially after ~5 Ma. The period of 5-3 Ma saw the strongest stratified deepwater in the then SCS, as indicated by up to 40% CaCO3 difference between Sites 1148 and 1146. Apart from a strengthening PDW as a result of global cooling and ice cap buildup on northern high latitudes, a deepening sea basin due to stronger subduction eastward may also have triggered the influx of more corrosive waters from the deep western Pacific. Since 3 Ma, the evolution of the SCS deep water entered a modern phase, as characterized by relative stable 10% CaCO3 difference between the two sites and increase in infaunal benthic species which prefer a low oxygenated environment. Thesubsequent reduction of PBW and PDW marker species at about 1.2 Ma and 0.9 Ma and another significant negative excursion of benthic δ13C to a Neogene minimum at ~0.9 Ma together convey a clear message that the PBW largely disappeared and the PDW considerably weakened in the Mid-Pleistocene SCS. Therefore, the true modern mode SCS deep water started to form only during the "Mid-Pleistocene climatic transition" probably due to the rise of sill depths under the Bashi Strait. 相似文献
34.
In reliability analysis, the crude Monte Carlo method is known to be computationally demanding. To improve computational efficiency, this paper presents an importance sampling based algorithm that can be applied to conduct efficient reliability evaluation for axially loaded piles. The spatial variability of soil properties along the pile length is considered by random field modeling, in which a mean, a variance, and a correlation length are used to statistically characterize a random field. The local averaging subdivision technique is employed to generate random fields. In each realization, the random fields are used as inputs to the well-established load transfer method to evaluate the load–displacement behavior of an axially loaded pile. Failure is defined as the event where the vertical movement at the pile top exceeds the allowable displacement. By sampling more heavily from the region of interest and then scaling the indicator function back by a ratio of probability densities, a faster rate of convergence can be achieved in the proposed importance sampling algorithm while maintaining the same accuracy as in the crude Monte Carlo method. Two examples are given to demonstrate the accuracy and the efficiency of the proposed method. It is shown that the estimate based on the proposed importance sampling method is unbiased. Furthermore, the size of samples can be greatly reduced in the developed method. 相似文献
35.
�ܹ�����������ϵ�������ݴ������о� 总被引:3,自引:0,他引:3
????????????﹫????????????ζ??????????????????????????????????????????????λ????????μ?????????衢????????????????????裬??????????????????????????????????????????????????????????????????????????????????????????????????????侫??????ó???????????????????????ε????????????????????????????????????????????淶???????????????????????????????????????? 相似文献
36.
A 3-year study of indoor radon in more than 1000 homes in northern Virginia and southern Maryland was conducted using 3-month
exposure alpha-track monitors. In a study set of 200 homes, first-floor indoor radon concentrations, which most closely approximates
home exposure levels, averaged slightly more than 3 pCi/l. In a study set of 100 homes, sub-slab ventilation was used to reduce
indoor radon concentrations. Interest in remediation was related to public perception of the hazardous nature of radon; people
living in homes with indoor radon measurements of more than 4 pCi/l were more likely to participate in the remediation phase
of the project. Sub-slab ventilation was successful in more than 90% of the homes in reducing indoor radon from concentrations
as high as 30 pCi/ to less than 4 pCi/l, at least for the entire year of post-remediation radon measurements.
Received: 29 February 1996 · Accepted: 29 May 1996 相似文献
37.
对以褶皱构造为主要构造型式的突出矿片来说,用褶皱变形系数这一定量指标,通过控制煤和瓦斯突出褶皱构造的研究,在一定程度上能提高突出预测的准确率。本文针对平顶山东矿区煤层褶皱变形的特点,分别用剖面变形系数法和平面变形系数法进行了突出危险性预测研究,取得了较可靠的预测结果。 相似文献
38.
介绍了斜井延伸相向掘进贯通的实例,从测量方法、实测偏差的比较到误差预计都作了详细说明,目的是为了与各位同行共同探讨斜井贯通方面的有关问题。 相似文献
39.
结合基桩承载过程中的桩周土体应力状态分析和桩土界面摩擦特性分析,推导出基于土体应力状态的沿桩土界面的土体抗剪强度计算模型和基于界面摩擦特性的界面抗剪强度计算模型。通过对比土体抗剪强度和界面抗剪强度,推导出受该两种强度耦合作用影响的极限侧阻力计算模型,并用上海某工程算例验证该模型的合理性和可行性,同时利用该模型分析不同类型的土的极限侧阻力随埋深的分布规律。结果显示:因内摩擦角和侧压力系数之间的不同关系,极限侧阻力随埋深表现出不同的分布特点,在特定关系下,存在一个临界深度,超过该深度极限侧阻力维持在一个稳定值甚至不断减小直至为零。 相似文献
40.