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171.
We make a statistical analysis of the periodsP and period-derivativesP of pulsars using a model independent theory of pulsar flow in theP-P diagram. Using the available sample ofP andP values, we estimate the current of pulsars flowing unidirectionally along theP-axis, which is related to the pulsar birthrate. Because of radio luminosity selection effects, the observed pulsar sample
is biased towards lowP and highP. We allow for this by weighting each pulsar by a suitable scale factor. We obtain the number of pulsars in our galaxy to
be 6.05−2.80
+3.32 × 105 and the birthrate to be 0.048−0.011
+0.014 pulsars yr−1 galaxy−1. The quoted errors refer to 95 per cent confidence limits corresponding to fluctuations arising from sampling, but make no
allowance for other systematic and random errors which could be substantial. The birthrate estimated here is consistent with
the supernova rate. We further conclude that a large majority of pulsars make their first appearance at periods greater than
0.5 s. This ‘injection’, which runs counter to present thinking, is probably connected with the physics of pulsar radio emission.
Using a variant of our theory, where we compute the current as a function of pulsar ‘age’ (1/2P/P), we find support for the
dipole braking model of pulsar evolution upto 6 × 106 yr of age. We estimate the mean pulsar braking index to be 3.7−0.8
+0.8. 相似文献
172.
《Mathematical Geology》1995,27(6):731-748
Laser Doppler Anemometry (LDA) has proved a powerful tool for quantifying fluid turbulence and is increasingly being applied
in fields such as fluvial sedimentology and geomorphology. When operated in the burst-signal processing mode, high-frequency
velocity fluctuations are measured at irregular time internals. In many situations, there is a need to transform these data
to obtain evenly spaced velocity values but at a lower frequency. However, clear guidelines for this type of data processing
are lacking. Three steps are necessary in order to transform the original files into evenly spaced data: (1) resampling at
the average sampling rate, (2) low-pass filtering with half-power frequency adjusted to the final sampling frequency, and
(3) decimating at the desired frequency. The decision taken at each step will affect the resulting signal and may cause, if
not assessed carefully, severe problems in the signal such as aliasing errors. This paper examines each stage of data processing
and details the advantages and drawbacks of different techniques in relation to the effects on turbulence statistics (variance,
instantaneous shear stress, etc.). A standard method and specific guidelines are finally proposed.
An erratum to this article is available at . 相似文献
173.
174.
LIU Xuhua WANG Jinfeng LIU Mingliang & MENG Bin . Institute of Geographic Sciences Natural Resources Research Chinese Academy of Sciences Beijing China . Graduate School of the Chinese Academy of Sciences Beijing China . College of Arts Sciences of Beijing Union University Beijing China . School of Forestry Wildlife Sciences Auburn University Auburn Alabama AL USA 《中国科学D辑(英文版)》2005,48(12):2231-2240
Along with the increasing problems of rapid popu-lation increase, resources scarcity and environment deterioration, the interaction between human devel-opment and natural environment changes, especially the Land-Use/Land-Cover, LUCC issue is becoming a frontier and hot field[1], in which investigation on the mechanisms of land use change is one of the three core issues. Deficient natural resources, in particular the shortage of cropland resource in China, are the important constraints to Ch… 相似文献
175.
A chemical analysis gives a solid base for a study of the distribution of elements in magmatic systems. Development of groups of constituent elements leads to the appearance of regularities which are translated by such close correlations that values for average rock compositions may be calculated using regression lines. These lines are divided into three segments corresponding to granitoids, gabbroids, and dunitoids. This normal geochemical law is not only valid for the average values obtained statistically but also for those obtained from petrographical families resulting from mineralogical classifications. The law also is true for the correlations: valencies/volumes and valencies/theoretical densities. The existence of such a law governing the fundamental geochemical equilibria enables deductions and comparisons to be made with the composition of regional series. 相似文献
176.
Groundwater chemistry and mass transfers in the Independence aquifer,central Mexico,by using multivariate statistics and mass-balance models 总被引:2,自引:0,他引:2
In the light of progressive depletion of groundwater reservoir and water quality deterioration of the Independence aquifer, an investigation on chemical data of dissolved major and minor constituents in 246 recent groundwater samples was performed. The main objective was the detection of processes responsible for the geochemical evolution and mineralization throughout the area. Multivariate techniques revealed different sources of solutes (a) dissolution of calcium and magnesium carbonate minerals, (b) weathering of acid volcanic minerals, (c) alteration of manganese containing alkaline silicates, (d) leaching of halite deposits of meteoric origin, (e) contamination from agricultural and urban wastewaters, and (f) evaporative effects due to intensive irrigation. Although nitrate contamination is associated with pollution from intensive cultivated areas, natural contamination plays an important role in the study area. The investigation reveals that weathering of acid volcanic rocks (rhyolite) and oxidation of arsenic bearing sulfide minerals are the responsible processes for high fluorine (up to 16 mg/l) and arsenic (up to 0.12 mg/l) contents, respectively, exceeding the Mexican maximum admissible concentration for drinking water. Except for kaolinite, all recharge processes are dissolution oriented (CO2, calcite, dolomite, K-feldspar, plagioclase). Silicate precipitation (amorphous silica and chalcedony) is of growing importance in discharge zones. Cation exchange is not an important issue in the whole study area. 相似文献
177.
Spatial statistics of clustered data 总被引:7,自引:7,他引:0
J. L.?KovitzEmail author G.?Christakos 《Stochastic Environmental Research and Risk Assessment (SERRA)》2004,18(3):147-166
Modern spatial statistics techniques are widely used to make predictions for natural processes that are continuously distributed over some convex domain. Implementation of these techniques often relies on the adequate estimation of certain spatial correlation functions such as the covariance and the variogram from the data sets available. This work studies the practical estimation of such spatial correlation functions in the case of clustered data. The coefficient of variation of the dimensionless spatial density of the point pattern of sample locations is suggested as a useful metric for degree of clusteredness of the clustered data set. We show that the common variogram estimator becomes increasingly unreliable with increasing coefficient of variation of the dimensionless spatial density of the point pattern of sample locations. Moreover, we present a modified form of the variogram estimator that incorporates declustering weights, and propose a scheme for estimating the declustering weights based on zones of proximity. Finally, insight is gained in terms of a numerical application of the common and modified methods on piezometric head data collected over an irregular network.Acknowledgments. This work has been supported by grants from the National Institute of Environmental Health Sciences (P42 ES05948-02), the Army Research Office (DAAG55-98-1-0289), and the National Aeronautics and Space Administration (60-00RFQ041). Some of the calculations conducted in support of this work were done on the SGI Origin 2400 at the North Carolina Supercomputing Center, RTP, NC. 相似文献
178.
传统的地质统计学所形成的算法理论和方法(如克里格算法)过分依赖样品的数据,变异函数参数众多,给地质模拟造成很大困难。基于马尔科夫链的地质属性建模采用转移概率描述样品区域的各种参数变量,通过转移概率矩阵直接推导地质属性分布比例、平均长度,其简化了地质空间各向异性处理过程,克服了传统地质统计学中参数众多且复杂难以计算和地质体分布过程中存在不对称性等缺陷,使得整个地质属性建模的过程更简洁、清晰,容易理解,且建模的结果可以很好地反映地质体空间分布的复杂性。该文利用马尔科夫链对南京市河西地区的新近地质层进行了地质属性建模,实例应用表明,使用该模型进行地质属性建模可为进一步的数值模拟提供支持。 相似文献
179.
Large-Eddy Simulation of Flows over Random Urban-like Obstacles 总被引:2,自引:2,他引:0
Further to our previous large-eddy simulation (LES) of flow over a staggered array of uniform cubes, a simulation of flow
over random urban-like obstacles is presented. To gain a deeper insight into the effects of randomness in the obstacle topology,
the current results, e.g. spatially-averaged mean velocity, Reynolds stresses, turbulence kinetic energy and dispersive stresses,
are compared with our previous LES data and direct numerical simulation data of flow over uniform cubes. Significantly different
features in the turbulence statistics are observed within and immediately above the canopy, although there are some similarities
in the spatially-averaged statistics. It is also found that the relatively high pressures on the tallest buildings generate
contributions to the total surface drag that are far in excess of their proportionate frontal area within the array. Details
of the turbulence characteristics (like the stress anisotropy) are compared with those in regular roughness arrays and attempts
to find some generality in the turbulence statistics within the canopy region are discussed. 相似文献
180.
核电站因它不排放如二氧化碳这样的温室气体,核电对降低能源部门的碳密集作出了贡献,因此愈益受到人们的重视.在对核电站地区的天气、气候环境进行分析的基础上,对影响靖宇核电站地区的温带气旋作了研究.结果表明:1949-2007年期间影响靖宇地区的温带气旋计111个;受温带气旋影响的年数有41年(约2/3).影响靖宇地区且产生强降雨的温带气旋,主要发生在6、7、8月这3个月,并且一半以上出现在7月.影响靖宇地区的温带气旋大部分是东北气旋,占总数的65%;蒙古气旋与华北气旋次之.此外,对造成严重影响的温带气旋的个例也从天气学角度进行了分析. 相似文献