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461.
The aim of the present study is twofold. First, to provide new information concerning the technical characteristics of urban bus companies on the basis of a sample of medium and large-size cities in Spain. Second, to analyze the degree of efficiency of those companies and to quantify the reasons for this efficiency. The results should be useful in evaluating possible changes in public policies relating to urban transport, specifically changes in the way the market is organized and in pricing.The analysis is carried out by estimating a cost function. The sample is made up of a panel data set consisting of observations of nine Spanish companies that operated during the period 1983–1995. The specified functional form is translogarithmic. The output unit of measure adopted is bus*kms run. The cost function includes the network length for each company, thus permitting evidence concerning economies of density and economies of scale.The use of panel data allows us to estimate the cost function, taking into account that each company is affected by the specific characteristics of each individual city, the different features of the network in question and by different levels of efficiency. The economies of scale have been calculated, taking into account that the features of the network and of the city – represented by their specific individual effect – will vary with the company's level of output.Finally, an analysis is made of the relative productive efficiency of the companies, as well as of the variables likely to influence that efficiency. 相似文献
462.
Our aim is to investigate the behavior of submerged supple nets. This work generates many problems owing to the discontinuous
and highly flexible nature of the nets. Only the action of external forces can bring an infinitely flexible structure like
a net into a definite shape. When considering supple nets immersed in a fluid, these external forces themselves depend on
the net geometry. A numerical method to solve this fluid-structure coupling problem is proposed, and is applied to fish farms.
In order to validate the calculation model of the hydrodynamic forces on the mesh sides, we measured the hydrodynamic forces
on a plane panel of netting spread across a transverse current. We thus proved that the Landweber model modified according
to the Richtmeyer formula as regards friction gives good results. The calculated shape of the fixed net cage is qualitatively
in accordance with flume tank observations. We have adapted the algorithm to the study of the dynamic behavior of floating
fish farms. 相似文献
463.
导管调距桨的定常性能预估 总被引:7,自引:6,他引:1
本文建立了一个预估导管可调螺距螺旋桨水动力性能的数值计算方法,即螺旋浆用升力面理论、导管采用面元法,通过迭代计算考虑浆和导管的相互影响。引入了一个修正的螺旋浆尾涡模型,来模拟尾涡片的扭曲变形及分离现象,对导管桨性能预估的各种影响因素分析了系统研究,并考虑了桨毂对性能的影响。对JD简易导管桨和导管调距桨(JD7704导管+JDC三叶可调螺距螺旋浆)分别进行了计算,并与实验结果进行了比较。结果表明,本文所建立的方法可较好地预估导管桨的水性能,精度比以往有较大的提高。 相似文献
464.
Dynamic characteristics of travel behavior are analyzed in this paper using weekly travel diaries from two waves of panel surveys conducted six months apart. An analysis of activity engagement indicates the presence of significant regularity in weekly activity participation between the two waves. The analysis also shows a general lack of association between regularity in activity participation and change in person and household attributes, suggesting the presence of behavioral inertia or response lags. It is further shown that observed trip rates do not exhibit patterns that would be observed if travel behavior had no response lag and no history dependence. The results point to the needs for models that are capable of representing these aspects of travel behavior. 相似文献
465.
466.
目前我国高速公路用防眩设施主要有植树、防眩网和防眩板三种形式,利用防眩板防眩,具有安装方便,无积雪、风阻小、维护费用低等优点,主要应用于桥上或其他不宜种植绿化植物的地方。文章介绍了防眩板的部分遮光设置原理,通过对金属防眩板、塑料防眩板和玻璃钢防眩板的制作工艺和性能指标的研究,发现3种防眩板的抗风荷载、抗变形量和抗冲击性都比较好,塑料防眩板和玻璃钢防眩板加入耐老化剂后,其耐候性明显提高,塑料防眩板的耐腐蚀性较好,玻璃钢防眩板和塑料防眩板在发生交通事故时,不易产生二次伤害。文章对不同条件下防眩板的选用提供了参考。 相似文献
467.
由于气垫船船型较小、空间有限,通过采用航空操控面板组件,对驾控台上各控制面板进行集成优化,使其能更合理地利用空间。在有限空间内实现驾控台各面板夜间显示、集中调光、集中试灯、集中报警等功能,并统一外形及显示方式,以达到优化各面板显示及操作方式,改善驾驶员的操纵使用界面。该面板首次在气垫船上使用,为气垫船控制面板的未来发展方向进行探索和尝试。 相似文献
468.
为对桨后自由叶轮进行综合设计研究,在前桨给定的前提下,采用螺旋桨的旋涡理论对桨后自由叶轮进行设计。前桨和自由叶轮之间的相互影响通过诱导速度来考虑,诱导速度通过以速度势为基础、采用源汇混合分布以及双曲面元的低阶面元法求得并进行周向平均,将非定常问题转化为定常问题。在实际设计过程中,通过面元法对前桨尾流场进行分析来求其收缩率,从而确定自由叶轮涡轮段直径,并用面元法分别对前桨和自由叶轮进行水动力性能预报,循环迭代直至桨后自由叶轮系统水动力性能收敛。实例设计分析表明:在较低进速条件下,设计的自由叶轮可获得较高的效益,效率最高可提升14.42%。 相似文献
469.
复合材料点阵结构以其优异的力学性能越来越受到人们的广泛关注,文中在点阵结构的基础上,提出了直柱型点阵结构填充聚氨酯材料的新型复合夹层板结构,利用有限元软件ANSYS研究其静态力学性能.分别研究了夹心层高度、面板厚度等变化对其刚度性能的影响,并将该结构与实体板、普通聚氨酯夹层板进行对比,得出对工程实际具有一定参考价值的结论. 相似文献
470.