排序方式： 共有13条查询结果，搜索用时 31 毫秒
1.
We analytically relate hydrostatic stress to strain in a random dense pack of identical spheres cemented at their contacts. The spheres are elastic and the cement is perfectly plastic. This solution for the sphere pack is based on a solution for the normal interaction of two cemented spheres. Initially, the two spheres touch each other at a point. We show that, as loading increases and cement becomes plastic, a finite (Hertzian) direct-contact area between the spheres necessarily has to develop and progress. The stress-strain behavior of the pack depends on the cement's yield limit and on the amount of cement. At the same hydrostatic stress, the deformation of the cemented aggregate is smaller than that of the uncemented one. This difference becomes large as the yield limit increases. We calculate the bulk modulus of an aggregate from the stress-strain curve. In the plasticity domain, the bulk modulus of the cemented aggregate is smaller than that of the uncemented one. The difference between the two may easily reach 50%. Of course, as the cement's yield limit decreases, the aggregate's stress-strain curve and the bulk modulus approach those of the uncemented sphere pack. This theoretical conclusion is qualitatively supported by experiments on epoxy-cemented glass beads. The maximum contact stress in the cemented aggregate may be less than a half of that in the uncemented one. This result explains an experiment where an uncemented glass bead sample failed at a hydrostatic stress of 50 MPa, whereas an epoxy-cemented sample stayed intact. 相似文献
2.
3.
The conditional nonlinear optimal perturbation (CNOP), which is a nonlinear generalization
of the linear singular vector (LSV), is applied in important problems of atmospheric and oceanic
sciences, including ENSO predictability, targeted observations, and ensemble forecast. In this study,
we investigate the computational cost of obtaining the CNOP by several methods. Differences and
similarities, in terms of the computational error and cost in obtaining the CNOP, are compared among
the sequential quadratic programming (SQP) algorithm, the limited memory Broyden-Fletcher-Goldfarb-Shanno
(L-BFGS) algorithm, and the spectral projected gradients (SPG2) algorithm. A theoretical grassland
ecosystem model and the classical Lorenz model are used as examples.
Numerical results demonstrate that the computational error is acceptable with all three algorithms.
The computational cost to obtain the CNOP is reduced by using the SQP algorithm. The experimental
results also reveal that the L-BFGS algorithm is the most effective algorithm among the three
optimization algorithms for obtaining the CNOP. The numerical results suggest a new approach and
algorithm for obtaining the CNOP for a large-scale optimization problem. 相似文献
4.
利用常规高空、地面观测资料、FY-2E卫星云图和NCEP再分析场,分析了2013年11月25日发生在中国东北东部的暴雪天气过程,并用HYSPLIT模式模拟了暴雪区上空气块的轨迹。结果表明:卫星云图显示暴雪发生在锢囚气旋的钩状云区中,且具有中尺度特征。钩状云区不同代表点、不同层次25日08时120 h气块的后向轨迹计算结果表明,在每个代表点的6条轨迹中,只有1条轨迹来自新疆以西地区的对流层上层,其他5条轨迹均来自蒙古国或我国北方地区的对流层中低层。几乎每条轨迹对应的气块在东移或南移时先以水平运动为主,伴有弱的下沉;中低层气块在72～24 h经过渤海湾和日本海;而中上层气块则主要经过黄海或东海,到达降雪区前几小时气块移速快并有明显的上升运动。对钩状云区不同代表点1500和3000 m上空120 h后向轨迹中气块湿度分析表明,来自东亚大陆的气块水汽含量并不大、相对湿度也小于60%,但在经过渤海湾和日本海时,海气交换使得气块的含水量和相对湿度均呈增加的趋势;特别是气块途径日本海的时间和距离越长,水汽含量越多。因此暴雪区的水汽主要来自日本海,其次是渤海湾。在降雪发生前几小时,气块随偏南风或东南风快速北移,相对湿度接近饱和并伴有上升运动,从而引起降雪。 相似文献
5.
Gregory Mintz 《The Australian geographer》2014,45(4):541-558
Modern industrial systems of food production cause a wide range of detrimental impacts to the environment, human health and society. Integrating community gardens into local food systems is one way to counteract some of those impacts. This research looks at the elements that community garden policies should contain in order to best promote community gardens and attain their benefits. Focusing on Sydney, the research evaluates and compares the quality of the community garden policies with the distribution of community gardens in local councils and across broader geographic regions. Councils or regions that might benefit from greater access to community gardens are identified and ways in which councils, and the Sydney Region as a whole, might improve their community garden policies are discussed. 相似文献
6.
Richard A. Davis Jr. Kristin E. Yale John M. Pekala Megan V. Hamilton 《Marine Geology》2003,200(1-4):103-123
Although the morphology of the barrier-inlet system along the west-central Florida coast is quite complicated, the stratigraphy of these barriers is rather simple. The basal Holocene unit in most cores is an organic-rich, muddy sand that represents a vegetated, paralic marine, coastal environment similar to that which is north and south of the present barrier system. Above that unit is a muddy, bioturbated sand that displays a marine fauna at most locations but also contains Crassostrea virginica in a few places. These sediments accumulated in a low-energy marine setting that may or may not have been protected by a barrier island. Much of this facies also represents sediment that was delivered as washover deposits in an intertidal or subtidal setting and was subsequently bioturbated. The facies that can be attributed to a barrier island with some certainty are no more than 3000 years old, and on most islands, are much younger. These are the shelly sand and sorted sand facies. The shelly strata represent deposition in nearshore, beach, supratidal washover or intertidal spillover environments, and tidal inlet and tidal delta channels, whereas the sorted sand is typical of eolian deposition in dunes or the backbeach and some tidal delta elements. The presence of Holocene oyster beds offshore of a present barrier suggests that some of these islands formed significantly offshore and moved to their present position through washover. It is likely that most of these barriers initially formed through upward shoaling by waves. Although there is significant morphologic difference between the wave-dominated and mixed-energy, drumstick barrier islands, their stratigraphy is quite similar. The only significant difference is the presence of extensive progradation on at least part of the drumstick islands and a relatively high amount of former washover deposits on the wave-dominated type. 相似文献
7.
Volatile organic compounds (VOCs) were measured at two sites in a highly industrialized zone in western Canada from September
2004 to March 2006. Principal component analysis (PCA) with varimax rotation was performed on 30 VOCs to identify the pollution
sources. Aliphatics, aromatics, and halogenated aliphatics were studied. The two monitoring sites were 11 km apart, with site
1 closer to the city of Fort Saskatchewan and site 2 predominantly down wind from the industrial sources. PCA results provided
the basis for interpreting the relationship between the ambient 24-h integrated VOC samples and the emission sources in the
region. Challenges existed in interpreting the PCA results in such a highly industrialized region; however a unique feature
to this study was the fact that the region was home to the only 1,2-dichloroethane emitting facility in Canada. Other specific
industry related VOCs in the region were vinyl chloride, styrene and HCFC-22. Making use of these specific VOCs in the PCA
allowed for easy identification of an industrial contribution. For factors that were not easily distinguishable, further PCA
tests were conducted using carbon monoxide concentrations, wind direction data and seasonal splitting of the samples. The
analysis found that five factors accounted for 82% of the variance at site 1 and five factors accounted for 81% of the variance
at site 2. The factor accounting for the highest variability (∼40%) at the two sites was the most difficult to interpret,
but showed contributions from both industry and vehicle related emissions. Specific industrial sources were identified using
1,2-dichloroethane as a chemical tracer or by corroborating wind speed with known industry VOC emissions. Both sites had two
factors identified as specific industry sources and these factors totaled to over 20% of the variance. Long range transport
of stable halogenated compounds accounted for greater than 10% of the variance, and seasonal effects accounted for 5% of the
variance. 相似文献
8.
四个耦合模式ENSO后报试验的“春季预报障碍” 总被引：2，自引：0，他引：2
用CliPAS计划中3个气候模式和中国科学院大气物理研究所耦合模式FGOALS-g短期气候异常回报试验结果,将动力和统计方法相结合,考察了1982—2003年厄尔尼诺/拉尼娜事件发展期和衰减期海表温度春季可预报性障碍现象。结果表明,所考察的耦合模式对ENSO事件预报的误差发展存在明显的季节依赖性,最大误差增长通常发生在春季,发生显著的可预报性障碍现象。进一步分析发现厄尔尼诺事件和拉尼娜事件在发展期的季节预报障碍现象比衰减期明显,以厄尔尼诺事件发展期春季可预报性障碍现象最为显著,拉尼娜事件衰减期季节预报障碍现象不显著。研究还发现,预报误差的增长在ENSO事件冷暖位相具有显著的非对称性,发展期暖位相预报误差强于冷位相,而衰减期冷位相的预报误差比暖位相大。通过回归分析,诊断了海-气相互作用的强度,发现耦合系统在春季最不稳定,使预报误差最易在春季发展,从而导致可预报性障碍。 相似文献
9.
A global monthly climatology of soil moisture and water balance 总被引：4，自引：0，他引：4
Global monthly climatology of available soil moisture content is derived on a 4° by 5° grid from observed precipitation and air surface temperature by use of a simple water budget model. The governing equations and methods of calculation for deriving these fields, which follow the formulation of Thornthwaite, are first described and the importance of the various assumptions and simplifications of this approach are discussed. The derived global fields are then presented. A comparison of some of the derived fields with other calculations is also made in order to permit an evaluation of the results: For example, our indirect estimate of the river run-off is generally in good agreement with more direct estimates, except for high latitude regions where the freezing of the soil may play an important role.Yale Mintz died on 27 April 1991. This work was carried out jointly over a number of years preceding his death 相似文献
10.
本文选用中国科学院大气物理研究所大气科学和地球流体力学数值模拟国家重点实验室(LASG/IAP)发展的全球海洋—大气—陆面气候系统模式(FGOALS)的4个版本g2.0、s2.0、g1.1和g1,利用模式的长时间积分结果,结合观测、再分析资料比较、评估模式对太平洋年代际变率的模拟能力,并通过对海气相互作用及其海洋动力过程分析,探讨了模式中太平洋年代际振荡形成机制.研究发现,FGOALS 模式g2.0和s2.0版本对太平洋年代际振荡(PDO/IPO)的模拟能力优于 g1.1和g1.模式中太平洋年代际变率的正反馈过程与Bjerknes(1969)提出的海气相互作用正反馈机制有关,其负反馈则主要与海洋内部动力过程有关.太平洋异常经向热量输送将热带与中纬度海洋联系在一起,可以抑制正反馈作用,但无法使得年代际振荡变化位相发生反转;FGOALS模式中,热带海表温度(SST)暖距平信号通过大气桥影响热带外大气环流,在海气作用下,热带与热带外海洋次表层分别以Kelvin 波和Rossby 波的形式传播,使得冷暖位相反转,4个版本均能再现这种负反馈机制.但不同版本Rossby波所处的纬度不同,太平洋SST异常年代际变化信号最明显的范围越宽,则由此激发的Rossby 波便更为偏北,纬度越高Rossby 波西传的时间也越长,PDO/IPO的周期与其SST异常的经向尺度密切相关. 相似文献