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761.
目的 研究并提出一种基于序贯截尾检验方法的任务可靠度评定方法.方法 将序贯截尾检验方法引入一维修正弹任务可靠度评定过程中,建立检验方法数学描述模型,并结合工程实例,综合应用Monte-Carlo法,给出数值算例和任务可靠度检验方案,验证方法的有效性和可行性.结果 与二项式经典假设检验方法(所需试验样本量N=54)相比,... 相似文献
762.
为研究建筑职业风险和新冠疫情风险下的统计生命价值(VOSL),以成都市建筑从业者为对象采集数据,基于生存分析处理删失数据的特性,运用二分式条件价值法(CVM)和Kaplan-Meier法测算不同背景下的VOSL,运用COX回归对VOSL影响因素进行分析。研究结果表明:成都市建筑从业者在职业风险和新冠疫情风险下的VOSL分别为429.472,459.079万元,生命价值评估具有情境依赖性且存在“引导悖论”;2种风险背景下受访者学历、月收入及安全重视程度的提高均会增强支付意愿,而安全满意程度的提高则会降低支付意愿;职业风险下影响支付意愿的关键因素为安全设备保障满意程度、重视程度和学历,新冠疫情风险下影响支付意愿的关键因素为卫生安全重视程度、学历和月收入。 相似文献
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766.
Wusi Yue Charles Meneveau Marc B. Parlange Weihong Zhu Hyung Suk Kang Joseph Katz 《Environmental Fluid Mechanics》2008,8(1):73-95
A comparative study of turbulence in a wind-tunnel model canopy is performed, using Large eddy simulation (LES) and experimental
data from PIV and hot-wire anemometry measurements. The model canopy is composed of thin cylindrical stalks. In the LES, these
are represented using a plant-scale approach, while the scale-dependent Lagrangian dynamic model is used as subgrid-scale
model. LES predictions of turbulence statistics and energy spectra are found to be in good agreement with the experimental
data. Turbulent kinetic energy (TKE) budgets from the LES simulation are analyzed to provide more information absent in the
measurements. Results confirm that sloshing motions at the low levels of the canopy are mainly driven by pressure fluctuations.
A difference between the energy flux obtained from the energy spectrum and the SGS dissipation rate is observed, consistent
with a spectral bypass mechanism in which the real spectral flux due to cascade is smaller than that implied by the energy-spectrum
level, due to direct drain by the canopy. 相似文献
767.
Trait-mediated effects on flowers: artificial spiders deceive pollinators and decrease plant fitness 总被引:1,自引:0,他引:1
Although predators can affect foraging behaviors of floral visitors, rarely is it known if these top-down effects of predators may cascade to plant fitness through trait-mediated interactions. In this study we manipulated artificial crab spiders on flowers of Rubus rosifolius to test the effects of predation risk on flower-visiting insects and strength of trait-mediated indirect effects to plant fitness. In addition, we tested which predator traits (e.g., forelimbs, abdomen) are recognized and avoided by pollinators. Total visitation rate was higher for control flowers than for flowers with an artificial crab spider. In addition, flowers with a sphere (simulating a spider abdomen) were more frequently visited than those with forelimbs or the entire spider model. Furthermore, the presence of artificial spiders decreased individual seed set by 42% and fruit biomass by 50%. Our findings indicate that pollinators, mostly bees, recognize and avoid flowers with predation risk; forelimbs seem to be the predator trait recognized and avoided by hymenopterans. Additionally, predator avoidance by pollinators resulted in pollen limitation, thereby affecting some components of plant fitness (fruit biomass and seed number). Because most pollinator species that recognized predation risk visited many other plant species, trait-mediated indirect effects of spiders cascading down to plant fitness may be a common phenomenon in the Atlantic rainforest ecosystem. 相似文献
768.
Chang-Keun Kang Eun Jung Choy Yongsoo Son Jae-Young Lee Jong Kyu Kim Youngdae Kim Kun-Seop Lee 《Marine Biology》2008,153(6):1181-1198
Loss of macroalgae habitats has been widespread on rocky marine coastlines of the eastern Korean peninsula, and efforts for
restoration and creation of macroalgal beds have increasingly been made to mitigate these habitat losses. Deploying artificial
reefs of concrete pyramids with kelps attached has been commonly used and applied in this study. As a part of an effort to
evaluate structural and functional recovery of created and restored habitat, the macroalgal community and food web structure
were studied about a year after the establishment of the artificial macroalgal bed, making comparisons with nearby natural
counterparts and barren ground communities. Dominant species, total abundance, and community structure of macroalgal assemblage
at the restored macroalgal bed recovered to the neighboring natural bed levels during the study period. The main primary producers
(phytoplankton and macroalgae) were isotopically well separated. δ13C and δ15N values of consumers were very similar between restored and natural beds but varied greatly among functional feeding groups.
The range of consumer δ13C was as wide as that of primary producers, indicating the trophic importance of both producers. There was a stepwise trophic
enrichment in δ15N with increasing trophic level. A comparison of isotope signatures between primary producers and consumers showed that, while
suspension feeders are highly dependent on pelagic sources, invertebrates of other feeding guilds and fishes mainly use macroalgae-derived
organic matter as their ultimate nutritional sources in both macroalgal beds, emphasizing the high equivalency of trophic
structure between both beds. Isotopic signatures of a few mollusks and sea urchins showed that they use different dietary
items in macroalgal-barren grounds compared with macroalgal beds, probably reflecting their feeding plasticity according to
the low macroalgal biomass. However, isotopic signatures of most of the consumers at the barren ground were consistent with
those at the macroalgal beds, supporting the important trophic role of drifting algae. Our results revealed the recoveries
of the macroalgal community and trophic structure at the restored habitat. Further studies on colonization of early settlers
and the following succession progress are needed to better understand the process and recovery rate in the developing benthic
community. 相似文献
769.
全球气候变化已经影响到人类生存环境的方方面面.全球升温及其幅度的区域差异,必然引起降水在区域分布的变化.文章利用160站点的夏季降水资料,分别计算出各个站点1951-2002年的变化斜率,以展示我国各地在这半个世纪中的降水总趋势变化的区域差异.通过分析发现,降水量总的变化趋势存在着随纬度而变化的规律:31°N以北区域近半个世纪降水量总的变化趋势是减少,31°N以南地区降水量以增加为主.40°N以北地区的降水变化幅度相对要小得多.区域上,中国西北和东南存在一个降水增多的趋势.中国降水的区域变化,在季风区受季风强弱变化影响;在非季风区,受西风强弱变化影响. 相似文献
770.
总需氧量能完全反映水中还原性物质的污染程度。该方法具有灵敏度高、精密度好、检测速度快等优点,且与COD具有良好的相关性。因此在环境监测领域具有广泛的应用前景,特别是对COD总量控制监测是一种快速、有效的方法。 相似文献