首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   865篇
  免费   6篇
公路运输   223篇
综合类   35篇
水路运输   326篇
铁路运输   32篇
综合运输   255篇
  2022年   7篇
  2021年   4篇
  2020年   9篇
  2019年   10篇
  2018年   12篇
  2017年   21篇
  2016年   21篇
  2015年   15篇
  2014年   16篇
  2013年   133篇
  2012年   32篇
  2011年   39篇
  2010年   29篇
  2009年   35篇
  2008年   38篇
  2007年   27篇
  2006年   16篇
  2005年   19篇
  2004年   16篇
  2003年   21篇
  2002年   8篇
  2001年   11篇
  2000年   20篇
  1999年   9篇
  1998年   13篇
  1997年   17篇
  1996年   21篇
  1995年   25篇
  1994年   5篇
  1993年   13篇
  1992年   17篇
  1991年   8篇
  1990年   13篇
  1989年   5篇
  1988年   14篇
  1987年   10篇
  1986年   10篇
  1985年   12篇
  1984年   6篇
  1983年   15篇
  1982年   12篇
  1981年   18篇
  1980年   9篇
  1979年   9篇
  1978年   9篇
  1977年   6篇
  1976年   6篇
  1975年   13篇
  1974年   5篇
  1973年   7篇
排序方式: 共有871条查询结果,搜索用时 0 毫秒
381.
This paper argues that the economic functions of seaports are to benefit those whose trade passes through them, i.e. through providing increments to consumers' and producers' surpluses. Whilst recent developments in the technologies of seaports (containers, bigger ships, more rapid handling of bulk cargoes) have increased technical efficiency, they have also provided such economies of scale as to reduce some opportunities for competition and, therefore, the probability that cost reductions are actually passed on in this way. For this purpose port costs need to be considered in their entirety, i.e. as generalized transport costs per tonne, comprising money, time and the risks of loss, damage and delay. Such cost reductions will lead to expanded trade in a variety of ways.

This statement of the economic function of seaports is contrasted with views that they should try to increase employment in their locality, or that they should maximize profits.  相似文献   
382.
383.
A conceptual framework of individual activity program generation   总被引:1,自引:0,他引:1  
The research in this paper attempts to better understand the process by which activities are generated at an individual level. Activity-based travel analyses have gained popularity in recent years because they recognize the complexity of activity behavior and view travel as a derivative of this behavior. Most activity-based studies have focused on the spatial and temporal linkage of trips; that is, the scheduling of activities. They consider the agenda of activities for participation, and associated attributes of the activity participation (such as mode to activity and location of activity performance), as predetermined. This paper develops a comprehensive conceptual framework of the relatively unexplored area of activity agenda generation. Such a framework will be valuable in empirical modeling of activity generation behavior. A subsequent paper focuses on translating a part of this conceptual framework into an empirical model.  相似文献   
384.
385.
文章介绍了IGBT特性模型的建模技术.通过特性模型简化了IGBT功耗的分析和计算,评估了正弦电路应用中IGBT的结温和损耗.仿真结果证实了特性模型的可行性.  相似文献   
386.
387.
388.
389.
This paper describes the development and application of a 3-dimensional model of the barotropic and baroclinic circulation on the continental shelf west of Vancouver Island, Canada. A previous study with a 2D barotropic model and field data revealed that several tidal constituents have a significant baroclinic component (the K1 in particular). Thus we embarked on another study with a 3D model to study the baroclinic effects on the residual and several selected tidal constituents.The 3D model uses a harmonic expansion in time and a finite element discretization in space. All nonlinear terms are retained, including quadratic bottom stress, advection and wave transport (continuity nonlinearity). The equations are solved as a global and a local problem, where the global problem is the solution of the wave equation formulation of the shallow water equations, and the local problem is the solution of the momentum equation for the vertical velocity profile. These equations are coupled to the advection-diffusion equation for density so that density gradient forcing is included in the momentum equations. However, the study presented here describes diagnostic calculations for the baroclinic residual circulation only.The model is sufficiently efficient that it encourages sensitivity testing with a large number of model runs. In this sense, the model is akin to an extension of analytical solutions to the domain of irregular geometry and bottom topography where this parameter space can be explored in some detail.In particular, the consequences of the sigma coordinate system used by the model are explored. Test cases using an idealized representation of the continental shelf, shelf break and shelf slope, lead to an estimation of the velocity errors caused by interpolation errors inherent in the sigma coordinate system. On the basis of these estimates, the computational grid used in the 2D model is found to have inadequate resolution. Thus a new grid is generated with increased accuracy in the region of the shelf break. However, even with increased resolution, spurious baroclinic circulation seaward of the shelf break and in the vicinity of Juan de Fuca canyon remained a significant problem when the pressure gradient terms were evaluated using the σ coordinate system and using a realistic density profile.With the new grid, diagnostic calculations of the barotropic and baroclinic residual circulation are performed using forcing from the observed σt (density) field and from the gradient of this field.  相似文献   
390.
Abstract

In this article, an overview of the status of coastal zone management (CZM) in South Africa is presented. Firstly, it provides background to the development of various initiatives to promote sustainable use of coastal resources within the context of sociopolitical changes in South Africa. Thereafter, it examines the progress made with respect to key attributes underpinning most CZM programs. Finally, it identifies obstacles to achieving effective CZM in South Africa and makes recommendations to address these shortcomings. The review reveals that whilst considerable progress has been made in certain areas of program development, such as resource conservation and pollution control, there are several gaps and inadequacies within existing efforts. These include the absence of a clear policy to guide efforts, lack of coordination amongst government departments involved in CZM, as well as inadequacies in our legal and administrative system. The promulgation of a Coastal Zone Management Act and the establishment of a Coastal Unit charged with the coordination and review of all activities impinging on coastal resources are amongst the recommendations made.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号