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581.
A performance comparison between nonlinear similarity functions in bulk parameterization for very stable conditions 总被引:1,自引:0,他引:1
Three sets of nonlinear similarity functions for strong stability are selected to compare their performance in bulk parameterization.
To uncover their advantages and disadvantages, theoretical and measurement analyses are made with four profile metrics and
the Deacon number technique. Main disadvantages include the negligence of the different transfer efficiency between momentum
and heat, the flux cutoff due to the upper limit in gradient Richardson number (Ri) and the ignorance of limited stability range where the dimensionless gradient functions ( and ) approach constants. Accordingly, three suggestions are made for future improvement. First, the functions for wind velocity
and potential temperature should have the same function form, but with different coefficients. Second, and should approach constants only within a certain stability range. Third, the limit value in Ri should be avoided to widen their applicability in flux modeling. Furthermore, quantitative comparisons in transfer coefficients
for moment and sensible heat (C
D and C
H) are made among the similarity functions in the bulk Richardson number (Ri
B) range 0 < Ri
B < 1. Generally, significant discrepancy is found, which may approach a factor of two and three at large Ri
B in C
D and C
H, respectively. Finally, a new recommendation is made to one of the three sets, mainly because of its ability to predict C
D and C
H that decrease rather slowly in very stable conditions. 相似文献
582.
We study the spreading of contagious diseases in a population of constant size using susceptible-infective-recovered (SIR) models described in terms of ordinary differential equations (ODEs) and probabilistic cellular automata (PCA). In the PCA model, each individual (represented by a cell in the lattice) is mainly locally connected to others. We investigate how the topological properties of the random network representing contacts among individuals influence the transient behavior and the permanent regime of the epidemiological system described by ODE and PCA. Our main conclusions are: (1) the basic reproduction number (commonly called R0) related to a disease propagation in a population cannot be uniquely determined from some features of transient behavior of the infective group; (2) R0 cannot be associated to a unique combination of clustering coefficient and average shortest path length characterizing the contact network. We discuss how these results can embarrass the specification of control strategies for combating disease propagations. 相似文献
583.
Grant E. Brown Camille J. Macnaughton Chris K. Elvidge Indar Ramnarine Jean-Guy J. Godin 《Behavioral ecology and sociobiology》2009,63(5):699-706
The antipredator behaviour of prey organisms is shaped by a series of threat-sensitive trade-offs between the benefits associated
with successful predator avoidance and a suite of other fitness-related behaviours such as foraging, mating and territorial
defence. Recent research has shown that the overall intensity of antipredator response and the pattern of threat-sensitive
trade-offs are influenced by current conditions, including variability in predation risk over a period of days to weeks. In
this study, we tested the hypothesis that long-term predation pressure will likewise have shaped the nature of the threat-sensitive
antipredator behaviour of wild-caught Trinidadian guppies (Poecilia reticulata). Female guppies were collected from two populations that have evolved under high- and low-predation pressure, respectively,
in the Aripo River, Northern Mountain Range, Trinidad. Under laboratory conditions, we exposed shoals of three guppies to
varying concentrations of conspecific damage-released chemical alarm cues. Lower Aripo (high-predation) guppies exhibited
the strongest antipredator response when exposed to the highest alarm cue concentration and a graded decline in response intensity
with decreasing concentrations of alarm cue. Upper Aripo (low-predation) guppies, however, exhibited a nongraded (hypersensitive)
response pattern. Our results suggest that long-term predation pressure shapes not only the overall intensity of antipredator
responses of Trinidadian guppies but also their threat-sensitive behavioural response patterns. 相似文献
584.
巢湖塘西河河口湿地重金属污染风险不确定性评价 总被引:5,自引:2,他引:3
基于污染风险评价系统多种不确定性共存的特点,采用盲数描述和表征重金属污染风险评价模型的各项参数,在对采自巢湖塘西河河口湿地24个表层沉积物柱状样中Cu,Zn,Pb,Cd和Cr等指标分析测试的基础上,采用Matlab工具软件编制的潜在生态风险指数盲数评价模型程序,对巢湖塘西河河口湿地重金属污染风险进行定量评估. 结果表明:巢湖塘西河河口湿地处于中等污染至较高污染状态,其相应的可信度分别为0.589和0.411;潜在生态风险等级属较低至中等水平,其相应可信度分别为0.755和0.245. 5种重金属污染程度排序依次为Cu>Cd>Zn>Pb>Cr;潜在生态风险水平排序依次为Cd>Cu>Pb>Cr>Zn. 相似文献
585.
针对生产车间安全管理状况评价的多维多层次性和不确定性,运用区间数模糊综合评价方法对其进行综合评价.该方法基于层次分析和模糊综合评价的基本原理,对于单因素权重、综合评价权重、各指标评分和评价集中评价等级水平均采用区间数,以使评价更符合客观实际.研究了区间数层次分析综合排序权重的确定,建立了区间数模糊综合评价模型.在得出生产车间安全管理状况综合评价区间数得分之后,引入了区间数排序可能度和区间数符合度的概念,提出了由综合评价结果得分区间数确定评价水平等级的方法.通过实例分析,验证了该评价方法的可行性. 相似文献
586.
587.
2011年11月~2012年8月,采用WPS宽范围粒径谱仪在北京地区连续监测10nm~10μm间不同粒径大气颗粒物数浓度,并同步记录气象参数.结果表明,颗粒物数浓度均值为25014个/cm3,多呈单峰或双峰模式分布,其中冬季均值为31007个/cm3,春季23152个/cm3,夏季20882个/cm3,冬季明显高于其他季节.爱根核模态及积聚模态均呈现冬季高、春夏低的态势;核模态春季显著高于其他季节.各气象因素中,风速影响最为显著,粒径大于20nm颗粒物数浓度与风速呈反比.核模态与爱根核模态粒子数浓度在交通早高峰、正午与晚间高于其他时段,积聚模态粒子数浓度变化则相对平缓,夜间显著升高. 相似文献
588.
The traditional defence against propagating coal dust explosions is the application of dry stone dust. This proven and effective safety measure is strictly regulated based on extensive international experience. While new products, such as foamed stone dust, offer significant practical benefits, no benchmark tests currently exist to certify their dust lifting performance in comparison to dry stone dust. This paper reviews the coal dust explosion mechanism, and argues that benchmark testing should focus on dust lifting during the initial development of the explosion, prior to arrival of the flame. In a practical context, this requires the generation of shock waves with Mach numbers ranging from 1.05 to 1.4, and test times of the order of 10's to 100's of milliseconds. These proposed test times are significantly longer than previous laboratory studies, however, for certification purposes, it is argued that the dust lifting behaviour should be examined over the full timescales of an actual explosion scenario. These conditions can be accurately targeted using a shock tube at length scales of approximately 50 m. It is further proposed that useful test time can be maximised if an appropriately sized orifice plate is fitted to the tube exit, an arrangement which also offers practical advantages for testing. The paper demonstrates this operating capability with proof-of-concept experiments using The University of Queensland's X3 impulse facility. 相似文献
589.
Two-phase CFD (Computational Fluid Dynamics) model for characterising the spill-over/dispersion of peroxy-fuels is presented. The model is independent of type and burning rate of the spilled/dispersed fuel and considers only overflow Reynolds number (Re) to characterise the spill/dispersion behaviour. Additional simulations are performed for LNG (Liquified Natural Gas) dispersion and it is found that the model can be used for different fuels within a defined range of Re. Different scenarios with Re = 100 to 3 × 105 are investigated covering a wide range of mass flow rates, opening sizes and viscosities. Depending on Lower Flammability Limits (LFL) of the fuels spill/dispersion (vapour cloud) diameters (DCFD) and heights (hCFD) are predicted. A generalised correlation between DCFD and Re is established to predict the dispersion occurring at varying scales. The model is validated by: (1) conducting an extensive grid independent study; (2) comparing the results with the existing analytical methods and (3) comparing against the standard field test data on LNG dispersions. 相似文献
590.
为解决飞机结冰防御系统能力评估中不确定因素较多导致结果不准确的问题,提出基于不确定语言的评估方法。在介绍区间数和语言评估标度的基础上,给出将语言评估标度转化为区间数的方法,并阐述不确定语言变量的定义及运算法则。建立飞机结冰防御系统能力评估指标体系,采用区间数特征向量法确定各指标的区间数权重,并利用区间集结(IA)算子和区间加权矢量和法实现指标的聚合。最后,分析某飞机结冰防御系统,以区间数评估其结冰防御能力。 相似文献