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1.
顶点覆盖问题是经典的NP完全问题,在排序、计算机网络等现实生活中有许多的应用.近几年来,许多研究者开始探究它的推广形式——顶点Pk覆盖（VCPk）问题,即寻找一个顶点子集,从拓扑结构图中删除后使得剩下的顶点导出的子图不包含Pk路,其中Pk是指包含k个顶点的路.本文简单介绍了VCPk问题的应用背景,归纳了它在近似算法、精确算法、参数化算法3个方面的主要研究进展,并分析了一些主要的方法和技巧.在此基础上,对VCPk问题及其相关问题的研究前景进行了展望.  相似文献   

2.
In the context of August 1982- July 1983 radiation balance data of the Qinghai-Xizang Plateau, detailed study is made of the relation between the active-surface temperature and surface temperature, proposing two parameterization schemes on surface long-wave radiation(SLWR)of the form U=kδσ(T0+273)4 and U=δσ[(T0+273)4+△T)]4, where k and △T are indicated by parameterization equations, separately, others being in conventional notation. Retrieved verification shows the two formulae to be of the same fitting accuracy with the mean relative error of 3.6% and suitable for computing instantaneous and mean flux density, alongside analyzed daily and annual variations of k and △T. Eventually. SLWR in the target area is investigated and its climatic characteristics examined.  相似文献   

3.
应用Leray-Schauder非线性二择一原理研究二阶动力系统x+k2x=f(t,x)+e(t)非平凡周期解的存在性,其中0<k<π/T,f∈C((R/TZ)×RN\{0},RN)在原点具有排斥的奇性.不需要任何的强制性条件,既可以处理强奇性,也可以处理弱奇性.  相似文献   

4.
1800年以来,新疆及其邻区M ≥ 7强震具有显著的可公度性和有序性,其主要有序值为(30 a)×k(k=1,2,3)、11~12 a、41~43 a、18~19 a与5~6 a等.利用翁文波信息预测理论,根据已有的研究成果,将有序网络结构分析与复杂网络技术相结合,补充新信息,不断总结优化和构建该研究区M ≥ 7强震二维平面与三维立体有序网络结构,充分揭示该区210多年来M ≥ 7强震的活动规律,并据此较为成功地预测了1996年以来该区所发生的4次M ≥ 7强震,即从1996年喀拉昆仑山7.1级强震,到2003年俄、蒙、中边境7.9级强震,再到2008、2014年于田2次7.3级强震.同时还提出新的预测意见:该研究区未来2次M ≥ 7强震将可能发生在2019-2020、2025-2026年前后.研究结果表明:强震是可以预测的.有序网络结构分析方法对于强震的中长期跨越式预测具有独特效果.  相似文献   

5.
网络虚拟化技术通过对物理资源的抽象,可以有效解决现有互联网架构中存在的网络结构僵化、可扩展性差等问题.虚拟网络映射问题是指将用户发送的所有虚网请求映射到底层物理网络中,同时还要满足虚网请求中对各个资源的限制要求（如节点计算能力、链路带宽等）.从节点负载平衡的角度出发,在基于就近原则的虚网映射算法基础上,引入节点负载平衡的反馈机制,引导各个虚网请求更均匀地映射到底层物理网络中.另外,在k短路径算法机制中引入了当前链路资源占有率作为评价参考标准,这样可以尽可能均匀地分散链路压力.同时,在检验链路资源是否满足虚网请求的过程中,由于优先选中的链路资源占有率低,所以算法映射成功率高,映射耗时更短,虚拟网络映射效率得到了有效提高.  相似文献   

6.
本文考虑了拒绝服务攻击下的网络化控制系统的H∞输出反馈控制问题.拒绝服务攻击的特征表现为能量有限和周期类型,它攻击无线网络通道进而退化系统性能.在系统状态部分未知的前提下,为了保证被控系统的稳定性和H∞性能指标,通过设计基于观测器输出反馈控制器,使得网络化控制系统在丢包和拒绝服务攻击下仍然保持稳定和预定性能.最后数值例子验证了所设计的控制器是有效的.  相似文献   

7.
本文研究了欺骗攻击环境下带有传感器故障的大规模电网分布式状态估计问题.通过引入拓扑关系来描述分布式传感器节点之间的相互关系,使用随机Bernoulli序列描述欺骗攻击模型及其随机特性.基于Lyapunov方法证明了带有传感器故障的系统在遭受欺骗攻击环境下的均方稳定及H∞稳定的充分条件,并基于LMI设计了满足H∞性能指标的分布式状态估计器.最后通过数值仿真验证了所设计估计器的有效性.  相似文献   

8.
Using the Simple Biosphere Model (SiB2), soil thermal properties (STP) were examined in a Tibetan prairie during the monsoon period to investigate ground surface temperature prediction. We improved the SiB2 model by incorporating a revised force-restore method (FRM) to take the vertical heterogeneity of soil thermal diffusivity (k) into account. The results indicate that (1) the revised FRM alleviates daytime overestimation and nighttime underestimation in modeled ground surface temperature (Tg), and (2) its role in little rainfall events is significant because the vertical gradient of k increases with increasing surface evaporation. Since the original formula of thermal conductivity (λ) in the SiB2 greatly underestimates soil thermal conductivity, we compared five algorithms of λ involving soil moisture to investigate the cause of overestimation during the day and underestimation at night on the basis of the revised FRM. The results show that (1) the five algorithms significantly improve Tg prediction, especially in daytime, and (2) taking one of these five algorithms as an example, the simulated Tg values in the daytime are closer to the field measurements than those in the nighttime. The differences between modeled Tg and field measurements are mostly within the margin of error of ±2 K during 3 August to 4 September 1998.  相似文献   

9.
利用安徽省2005—2010年的闪电定位资料,以安徽省各市的行政区域为评估单元,考虑人口密度以及经济发展因素,选取雷暴日（T）、地闪密度（N）、人口密度（L）、单位面积的GDP（D）作为评估指标,建立层次分析模型,得到安徽省的雷电灾害风险区划,然后利用历史雷灾频次对区划结果进行了校验.结果表明：黄山市、马鞍山、铜陵市、池州市、安庆市、淮南市、合肥市、阜阳市具有较高的雷电灾害风险值;宿州市、亳州市、淮北市雷电灾害风险值较低;其他城市雷电灾害风险值居中.  相似文献   

10.
根据"有效照射体"的基本概念,采用微分理论来回避椭球面近似为平面的假设,推导了双基地雷达系统有效照射体积表达式,证明双基地雷达系统有效照射体积与单基地雷达有效照射体积之比值Vb/Vm,在数值上等于双基地系统主站极轴长度Rt相对于半长轴长度a的导数.有效照射体积数值计算结果与以往相同,放宽了推导过程中的假设条件,显得更加严谨、易懂.还给出了双基地雷达系统各几何量(如Vb/Vm、Rt、主站共面方位Φ、辅站极轴长度Rr、辅站共面方位Φr、双基地角β)之间的一些关系式,分析了在双基地雷达系统探测过程中有效照射体积随有关量变化的特征,为双基地偏振雷达系统天线控制、资料处理、旁瓣影响分析、探测精度提高等提供理论依据.  相似文献   

11.
Summary k-day extreme precipitation depths (k=1,2,3, … 30) for the climatological network of Belgium (165 stations) are analysed to detect a possible evolution in the occurrence of extreme rainfall events during the 1951–1995 reference period. The calendar year and the hydrological summer and winter are considered separately. Spearman’s rank correlation coefficient shows a strong spatial correlation between extreme k-day precipitation events, depending on the time of the year (lower during summer than during winter) and increasing with k. In some cases the distances of de-correlation exceed 200 km which is comparable to the size of the country. Due to this correlation, tests for trends have been carried out on the leading principal components (PC) derived from the covariance matrix. Various PC selection rules have been applied to identify the number of components to analyse. The number of components needed to reproduce a given proportion of the total variance varies, with larger values for summer than for winter and a decrease with growing k. The Fisher test is used as a global test. It combines the individual Mann-Kendall trend tests carried out on the selected PC scores. Significant trends have been found in extreme winter k-day precipitation for all the values of k and none in extreme summer precipitation. The results for the annual k-day precipitation depths are between those for the two seasons: no trend for small k because summer events dominate and a significant trend for k larger than 7 due to the winter events. Analysis of a few stations with long-term series shows no significant trend for the period 1910–1995, these series also reproduce almost the same trends as those found for the shorter 1951–1995 period. Received April 23, 1999 Revised December 6, 1999  相似文献   

12.
Summary The principle of the group-kinetic method is elucidated. This method of renormalization serves as the basis for analyzing the spectral structure of turbulence. The spectral distributions include the Kolmogoroff lawk –5/3 for isotropic turbulence, the power lawk –1 for shear turbulence, the spectrum for stratified turbulence not in the power law form, the power lawk –3 for two-dimensional geostrophic turbulence, and the power lawsk –3,k –2 andk –5 for two-dimensional Rossby wave turbulence with uniform and differential rotations. We discuss a spectrum-dependent modeling in reference to the problems of the universal functions and parameters in the similarity theories for the atmospheric surface layer and the planetary boundary layer. A renormalization-based modeling of atmospheric turbulence is proposed.  相似文献   

13.
A theoretical study is made of a simple mixed-layer model, in the form of a well-mixed constant-depth layer, forced from above by a heat flux kT(TA−T) and salinity flux kS(SA−S), where TA and SA are two reference values and T and S the temperature and salinity of the layer. The layer has a turbulent exchange of heat and salt with underlying water, kept at constant temperature and salinity, which is small in a statically stable case; large in a statically unstable case. If kT>kS, self-sustained oscillations may occur. In one cycle, a fast temperature rise, a slower salinity increase, and a final relaxation when the layer adjusts to the conditions of the underlying water, are observed.  相似文献   

14.
林乃实  孙立潭 《气象学报》1997,55(3):306-317
本文应用Ｈｏｗａｒｄ（１９６１）所用的精巧积分方法研究了三维非静力平衡层结大气随高度变化基流上重力内波的不稳定性质。给出了不稳定产生的必要条件和相速度在相速复平面上的分布规律；发展了Ｍｉｌｅｓ定理和Ｈｏｗａｒｄ半圆定理；得到了有限深度气层的半椭圆定理。并指出在随高度变化的基流中，重力内波的不稳定范围与基流的垂向结构及波传播方向有关；半椭圆的位置和长轴分别重合于Ｈｏｗａｒｄ半圆的位置和直径，而短轴则依赖于与Ｒｉｃｈａｒｄｓｏｎ数相关的参数Ｊ０，Ｒ０和波数｜ｋ｜及流场厚度参数ｋ０  相似文献   

15.
Spectra and co-spectra of the streamwise (u) and normal or vertical (w) velocity fluctuations have been measured in the inner region of a large Reynolds number laboratory boundary layer over a rough wall. There is reasonable evidence of ak 1 –1 range in theu spectrum (wherek 1 is the streamwise wavenumber). Such a range results from an overlap between a spectral region dominated by largescale, inactive motion, which scales on the boundary-layer thickness, and a region dominated by smaller-scale, active motion which scales on the distance from the wall. Spectra ofw, anduw cospectra, scale in a manner consistent with the dominance by active motion. The present spectral data do not support local isotropy over the inertial subrange. A comparison between measuredw spectra and calculations based on isotropy indicates that the inertial subrange anisotropy is only slightly affected by the magnitude of the non-dimensional mean shear.  相似文献   

16.
应用含湍流频散效应的近地面层的运动方程求解了不同层结下的风速廓线，着重分析了湍流频散效应对近地面层平稳运动的影响。分析指出：湍流的频散效应对经典的幂律廓线一对数修正，该修正在不稳定层结时比稳定层时明显；利用相似理论也得到了该常数。  相似文献   

17.
The half-century history of the experimental evaluation of the von Karman constant in the atmospheric surface layer is reviewed, an evidence indicating that this well-known scaling factor,k, is actually a weak variable that decreases with increasing Reynolds number is discussed. A combined average ofk=0.390 with a standard error of ±0.010 is found from two field studies, a result which indicates that there is only one chance in 40 that the true value of the scaling factor in the population from which the experimental sample was drawn could have been as large as the laboratory value of 0.40. Based on experimental and theoretical results given by others, it is suggested thatk varies from a maximum of 0.41 in light winds over open water and smooth land surfaces, to a minimum near 0.37 in stronger winds over forests and cities. This range is shown to imply that a working value ofk=0.39±1% is appropriate for flows over surfaces characterized by 0.7<z 0<8.7 cm, a roughness subrange which corresponds to a wide variety of terrain types from sparsely vegetated level plains to mixed croplands with occasional hedges and trees.sponsored in part by the Global Change Post Doctoral Program, Office of Health and Environmental Research, U. S. Department of Energy.Work sponsored by the National Science Foundation under Grant No. ATM-9019682.  相似文献   

18.
Sonic anemometer path-averaging errors are determined for measurements of inertial-range velocity and temperature power spectra as a function of k 1 p, where k 1 is the streamwise wave number and p is the sonic path length. The attenuation of vertical velocity spectra is found to be quite similar for the CSAT3 and Solent anemometers and to be insensitive to wind direction. The attenuation of the horizontal wind component spectra is noticeably greater for the Solent sonics than for the CSAT3, but those for the CSAT3 have a greater dependence on wind direction. The attenuation of sonic temperature spectra is also found to be quite similar for the CSAT3 and Solent R3 sonics and to be insensitive to wind direction, while that of the Solent R2 is less than that of the other two sonics and has a minor dependence on wind direction.  相似文献   

19.
We report on a novel approach for the Reynolds-averaged Navier-Stokes (RANS) modelling of the neutral atmospheric boundary layer (ABL), using the standard k-ek-{\varepsilon} turbulence model. A new inlet condition for turbulent kinetic energy is analytically derived from the solution of the k-ek-{\varepsilon} model transport equations, resulting in a consistent set of fully developed inlet conditions for the neutral ABL. A modification of the standard k-ek-{\varepsilon} model is also employed to ensure consistency between the inlet conditions and the turbulence model. In particular, the turbulence model constant C μ is generalized as a location-dependent parameter, and a source term is introduced in the transport equation for the turbulent dissipation rate. The application of the proposed methodology to cases involving obstacles in the flow is made possible through the implementation of an algorithm, which automatically switches the turbulence model formulation when going from the region where the ABL is undisturbed to the region directly affected by the building. Finally, the model is completed with a slightly modified version of the Richards and Hoxey rough-wall boundary condition. The methodology is implemented and tested in the commercial code Ansys Fluent 12.1. Results are presented for a neutral boundary layer over flat terrain and for the flow around a single building immersed in an ABL.  相似文献   

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