What do we know about the labour market for seafarers?: A view from the UK     

David Glen 《Marine Policy》2008

This paper considers the evidence that exists on the global demand and supply of seafarers, and their age profiles. New information on the age profiles of deck and engine officers is presented, drawn from records held by the UK Maritime and Coastguard Agency. Detailed age profiles for certain countries are constructed and compared with that of the UK, as well as with estimates published elsewhere. Using the data derived from the MCA, it is shown that there is a significant positive correlation between the age profiles of seafarer supplying countries and their level of economic development, as measured by GDP per capita. This evidence is consistent with the view that the future supply of seafaring officers will increasingly be oriented towards countries at lower levels of development, as the existing pool of highly qualified officers from traditional maritime countries continues to diminish.  相似文献   

亚临界区雷诺数下圆柱绕流场电磁力控制数值研究     

尹纪富  尤云祥  李巍 《海洋工程》2014,32(4):1-10

在亚临界区雷诺数下,采用脱体涡模拟方法对弱电解质中电磁力作用下圆柱绕流场及其升阻力特性进行了数值模拟与分析。结果表明,电磁力可以提高圆柱体边界层内的流体动能,延缓圆柱体近壁面流动分离,减弱绕流场中流向和展向大尺度涡的强度,减小圆柱体阻力及其升力脉动幅值;当电磁力作用参数大于某个临界值后,流动分离角消失,在圆柱体尾部产生射流现象,电磁力产生净推力作用,出现负阻力现象,而且升力脉动幅值显著减小且接近于零。  相似文献   

Anchor ice formation in streams: a field study     

Morten Stickler  Knut T. Alfredsen 《水文研究》2009,23(16):2307-2315

In northern steep streams anchor ice is commonly observed during winter, and plays a key role when considering in‐stream conditions. The understanding, however, of the nature of anchor ice formation is less understood, in particular, under natural conditions. In the following, observations of anchor ice formation in three stream environments with different physical characteristics are presented. Results demonstrate that anchor ice not only form in riffle areas, but also in shallow and slow running stream sections. No linkage between spatial distribution of anchor ice and calculated dimensionless numbers (Froude and Reynolds number) was found. Furthermore, analyses on growth and density showed that anchor ice may be distinguished by two main types. (1) Type I: Lower density forming on top of substrata. (2) Type II: Higher density forming between the substrata filling interstitial spaces. Distribution of anchor ice Types I and II suggests a relation between intensity of turbulence expressed by the Reynolds number, growth pattern and density. As anchor ice has both physical and biological implications on in‐stream environments, findings from the present study may be of particular interest to cold region freshwater stream management. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Combining genetic and linearized algorithms for a two-step joint inversion of Rayleigh wave dispersion and H/V spectral ratio curves     

Matteo Picozzi  Dario Albarello 《Geophysical Journal International》2007,169(1):189-200

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The Western Bohemia uppermost crust shear wave velocities from Love wave dispersion     

Petr Kolínský  Johana Brokešová 《Journal of Seismology》2007,11(1):101-120

We present a detailed study of Western Bohemia Love waves generated by blasts with an intention to estimate the uppermost crust structure for a more detailed layer distribution than previous studies have used. The use of short-period (4 s and shorter) Love waves represents a new approach in the studied region. Properties of multiple filtering as a tool of frequency–time analysis are discussed. A new method of selecting the dispersion ridges is introduced. Tests of filtering are provided by analyzing signals with analytically known dispersion. The isometric algorithm for the inversion problem is applied, the problem of non-uniqueness is discussed, and tests of reliability of the inversion are presented. During the inversion, the forward problem is solved by use of the matrix method. Six records of blasts from the Western Bohemia region are analyzed to separate the fundamental modes of Love wavegroups, and shear wave velocity distributions down to a depth of 3.0 km are inferred. Modal summation is used to compute synthetic velocigrams, which are compared to measured ones. The lateral heterogeneity of the region is discussed and the presented models are compared to those of previous studies and to the geological setting of the region.  相似文献   

应用人卫激光测距技术测定潮汐形变勒夫数   总被引：2，自引：0，他引：2  

彭碧波  吴斌  许厚泽 《测绘学报》2000,29(4):305-309

本文讨论了利用人卫激光测距（ＳＬＲ）资料确定分潮波形式的地球固体潮汐形变勒夫数的可能性,并用１１年的Ｌａｇｅｏｓ－１的人卫激光测跨资料直接解算了四个周日波Ｑ、,Ｏ１,Ｐ１,Ｋ１和四个半日波Ｓ２,Ｋ２,Ｎ２,Ｍ２的弹性和粘弹性地球状态下的地球潮汐形变勒夫数ｈ２ｓ,ｌ２ｓ,结果与ＶＬＢＩ结果相符,与ＩＥＲＳ及Ｗａｈｒ的模型值的潮汐响应趋势相符,这证明了利用ＳＬＲ技术确定潮汐形变勒失数是可行的,从而为传统技术和空间技术提供了一种新的有意义的方法和选择。  相似文献   

A review of classical dimensionless numbers for the Yusa problem based on discriminated non‐dimensionalization of the governing equations          下载免费PDF全文

M. Cánovas  I. Alhama  E. Trigueros  F. Alhama 《水文研究》2016,30(22):4101-4112

When the governing equations of a problem are known, the non‐dimensionalization of these equations (applied in their classical form) is a useful and widely used method that can be used to identify the dimensionless groups that rule the solution. However, neither this technique nor dimensional analysis necessarily provides the most precise solution in terms of the sought numbers. The use of discrimination, a qualitative rather than quantitative extension of the non‐dimensionalization method, has demonstrated notable advantages over the classical method because it invariably provides a more precise set of dimensionless groups. The basis for the correct application of discrimination depends on a deep understanding of the phenomena involved in the problem, particularly in complex (coupled) problems. In this paper, discriminated non‐dimensionalization is applied to investigate a mixed convection problem in porous media, the Yusa problem. The derived discriminate groups are compared with those already known in the literature and deduced by classical methods. A number of scenarios are numerically solved to check the reliability of the discriminated groups in contrast with classical groups. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

一些组合恒等式的概率方法证明     

孙彦飞  赵熙强 《中国海洋大学学报(自然科学版)》2011,(Z1):436-438

本文通过Stirling数等特殊组合数的概率表示,结合着已有的基本恒等式,演化得到了若干新的关于常见组合变量的恒等式及递推关系,同时对一些已有的组合恒等式及递推关系给出了新的概率的证明。  相似文献   

月球激光测距的新技术方法研究     

熊耀恒 《天文研究与技术》2002,(1)

月球激光测距是国内外所瞩目的宏伟目标 ,代表着单光子探测技术的高峰。本论文的目的是探索提高激光测月回波光子数的新方法 ,进而增加激光测月成功的几率。其思路是源于一个新的想法 :在激光测月过程中引入大气波前倾斜量实时补偿的技术。首先介绍激光测月的现状和其难度所在 :回波光子数太少 ,基本上属于亚光子探测范畴。在现有技术条件下 ,本文对影响激光测月回波光子数的因素逐一进行分析讨论 ,提出应该把激光束截面能量分布和大气湍流效应包括进去。为此分析讨论了大气湍流和大气中光场的基本统计性质、激光束在大气中传输时所受大气湍流的影响以及大气湍流对激光测距的影响 ,得出大气湍流特别对激光测月有着明显影响的结论。在此基础上对传统的激光测距方程进行了修正 ,使其应用到激光测月时更符合真实的情况 ,从而指导补偿的进行。涉及到在激光测月中对受大气湍流而畸变的激光束进行补偿 ,本文抓住重点 ,通过分析看出对大气倾斜量的实时补偿是提高激光测月回波光子数的重要因素。结合激光测月以及云南天文台现有测距系统的实际情况 ,本文独创性地提出利用激光测月目标近旁一定大小区域的扩展面源探测与计算大气倾斜量 ,然后对激光测月中的激光束进行大气倾斜量实时补偿的新技术方法。在分析比较  相似文献   

Forecasting the Time Series of Sunspot Numbers     

L. A. Aguirre  C. Letellier  J. Maquet 《Solar physics》2008,249(1):103-120

Forecasting the solar cycle is of great importance for weather prediction and environmental monitoring, and also constitutes a difficult scientific benchmark in nonlinear dynamical modeling. This paper describes the identification of a model and its use in the forecasting the time series comprised of Wolf’s sunspot numbers. A key feature of this procedure is that the original time series is first transformed into a symmetrical space where the dynamics of the solar dynamo are unfolded in a better way, thus improving the model. The nonlinear model obtained is parsimonious and has both deterministic and stochastic parts. Monte Carlo simulation of the whole model produces very consistent results with the deterministic part of the model but allows for the determination of confidence bands. The obtained model was used to predict cycles 24 and 25, although the forecast of the latter is seen as a crude approximation, given the long prediction horizon required. As for the 24th cycle, two estimates were obtained with peaks of 65±16 and of 87±13 units of sunspot numbers. The simulated results suggest that the 24th cycle will be shorter and less active than the preceding one.  相似文献