排序方式： 共有16条查询结果，搜索用时 15 毫秒
1.
Natural Hazards - Collapse of residential buildings is the major cause of death during earthquakes. Seismic retrofitting of residential buildings is a cost-effective way to reduce injury and death.... 相似文献
2.
M. Akashi Noam Soker Ehud Behar John Blondin 《Monthly notices of the Royal Astronomical Society》2007,375(1):137-144
We calculate the X-ray emission from both constant and time-evolving shocked fast winds blown by the central stars of planetary nebulae (PNe) and compare our calculations with observations. Using spherically symmetric numerical simulations with radiative cooling, we calculate the flow structure and the X-ray temperature and luminosity of the hot bubble formed by the shocked fast wind. We find that a constant fast wind gives results that are very close to those obtained from the self-similar solution. We show that in order for a fast shocked wind to explain the observed X-ray properties of PNe, rapid evolution of the wind is essential. More specifically, the mass-loss rate of the fast wind should be high early on when the speed is ∼300–700 km s−1 , and then it needs to drop drastically by the time the PN age reaches ∼1000 yr. This implies that the central star has a very short pre-PN (post-asymptotic giant branch) phase. 相似文献
3.
We describe concepts for small space telescopes that are able to provide significant UV science and can be realized with small (but realistic) budgets. The concepts are based on nano-satellites carrying small optics, with no redundancy, without producing intermediate models prior to flight model, and using COTS (custom off-the-shelf) components. We describe a few concepts of deployable optics that could provide large collecting areas and high angular resolution while packaged in the small volume of a nano-satellite. We point out areas where technological development is still required. 相似文献
4.
5.
6.
The geometrical properties of the armor layer of rubble mound breakwaters were investigated by observations made on two cases. The first case is a laboratory-scaled model of natural rock, designed with a composite slope. The second is a breakwater in the sea, constructed with a uniform slope of tetrapods. In both cases the cross-section underwent changes, resulting in an apparently stable profile of composite slope. This implied that for stability the optimal armour layer should be of composite slope. The general nature of this profile was implied by the geometrical similarity between the two cases which were basically so different. 相似文献
7.
Muhammad Akashi Noam Soker Ehud Behar 《Monthly notices of the Royal Astronomical Society》2006,368(4):1706-1716
We calculate the X-ray emission from the shocked fast wind blown by the central stars of planetary nebulae (PNe) and compare with observations. Using spherically symmetric self-similar solutions, we calculate the flow structure and X-ray temperature for a fast wind slamming into a previously ejected slow wind. We find that the observed X-ray emission of six PNe can be accounted for by shocked wind segments that were expelled during the early-PN phase, if the fast wind speed is moderate, v 2 ∼ 400–600 km s−1 , and the mass-loss rate is a few times 10−7 M⊙ yr−1 . We find, as proposed previously, that the morphology of the X-ray emission is in the form of a narrow ring inner to the optical bright part of the nebula. The bipolar X-ray morphology of several observed PNe, which indicates an important role of jets, rather than a spherical fast wind, cannot be explained by the flow studied here. 相似文献
8.
We present a semi-analytical model for the dispersion of passive scalars from continuous ground sources up to distances of a few hundred metres. We attempt to cope with problems typical of analytical models, such as the correct representation of the near-ground concentration and lateral dispersion, while avoiding the use of any empirical parameters. A previous analysis of Prairie Grass Project (PGP) data has shown that the near-ground, cross-wind integrated concentration decreases as some power of the distance from the source that is, itself, distance dependent. As the conventional power-law model is incapable of reproducing this behaviour, we propose a model in which the vertical diffusivity depends on both the height as a power law, and on the distance from the source. For this equation, we construct an infinite-series formal solution, with the first term used as an approximation. A set of equations based on this approximation and on Monin–Obukhov similarity theory is proposed for the the vertical diffusivity, from which the cross-wind integrated concentration is derived analytically. We further construct a simple empirical model for the distance-dependent vertical diffusivity. For the plume lateral width, a Langevin stochastic model depending on the plume height is proposed, whose formal analytical solution is used to derive a set of equations for the cloud width, which are easily solved numerically, with the results validated against PGP data. We apply four statistical measures to evaluate the performance of the model, including the computation of the 95% confidence intervals, for which we find very good agreement. Implementation of this model is extremely simple and computationally efficient. 相似文献
9.
10.
Several researchers have investigated morphological changes on the south-eastern Mediterranean coast during the late Holocene.
However, very few of these studies include quantitative data covering the last 200 years. In this study, topographical maps,
nautical charts and aerial photographs are used to estimate the shoreline migrations and beach–nearshore sand balance over
the last 200 years in Haifa Bay, Israel, the northernmost final depositional sink of the Nile littoral cell. The findings
reflect two main periods. During the first period, between 1799 and 1928, human intervention along the bay’s coast was negligible,
a significant coastal expansion of ∼50 to 150 m (averages of 0.4–1.2 m/year) was measured, and sand accumulation was estimated
at ∼70,000 m3 annually in the beach–nearshore area. A dramatic change in the sedimentological pattern was observed during the second period,
between 1928 and 2006, following the completion of Haifa Port’s main breakwater (1929–1933). During this period, most of the
bay’s coast was in a steady state, with seasonal fluctuations of less than about ±20 m, and slight erosion of ∼7,000 m3 annually. These findings are consistent with previous studies which conclude that from approximately 4,000 years ago until
the construction of Haifa Port, sea level remained relatively stable, and a continuous accumulation of Nile-derived sand dried
up the Zevulun Plain and shifted the Haifa Bay shoreline westwards to its present location. This long-term trend ceased after
completion of the Haifa Port main breakwater. 相似文献