全文获取类型
收费全文 | 13981篇 |
免费 | 732篇 |
专业分类
公路运输 | 4468篇 |
综合类 | 3927篇 |
水路运输 | 3030篇 |
铁路运输 | 2487篇 |
综合运输 | 801篇 |
出版年
2024年 | 31篇 |
2023年 | 109篇 |
2022年 | 257篇 |
2021年 | 462篇 |
2020年 | 547篇 |
2019年 | 274篇 |
2018年 | 201篇 |
2017年 | 292篇 |
2016年 | 358篇 |
2015年 | 524篇 |
2014年 | 1149篇 |
2013年 | 812篇 |
2012年 | 1340篇 |
2011年 | 1365篇 |
2010年 | 1012篇 |
2009年 | 872篇 |
2008年 | 887篇 |
2007年 | 1172篇 |
2006年 | 967篇 |
2005年 | 603篇 |
2004年 | 388篇 |
2003年 | 266篇 |
2002年 | 164篇 |
2001年 | 142篇 |
2000年 | 90篇 |
1999年 | 55篇 |
1998年 | 42篇 |
1997年 | 49篇 |
1996年 | 46篇 |
1995年 | 35篇 |
1994年 | 38篇 |
1993年 | 28篇 |
1992年 | 24篇 |
1991年 | 25篇 |
1990年 | 28篇 |
1989年 | 15篇 |
1988年 | 12篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
31.
This paper investigates the hydrodynamic damping of a circular cylinder with helical strakes at Keulegan-Carpenter (KC) number from 0.07 to 3 in the presence of steady currents. Experiments were performed with a straked cylinder oscillating in either in-line or cross currents over Reynolds number (based on the oscillating velocity amplitude) varying from 1260 to 54,000. With in-line current being present, the measured drag coefficients of the straked cylinder are found to depend on the ratio between the oscillating velocity amplitude and the steady current velocity. This phenomenon is further confirmed by computational fluid dynamics using large-eddy simulations. The drag coefficients obtained from the numerical simulations agree well with the experimentally determined values. Similar phenomenon is observed for the cases with cross background current. Based on the experimental data, empirical formulae are proposed to evaluate drag coefficients. These results are of importance in estimating the resonant motion and the fatigue life of risers, e.g. water intake risers, in the flow regime of low KC. Finally, recommendations are provided for fatigue analysis of risers with helical strakes from the perspective of engineering practice. 相似文献
32.
随着城市轨道交通建设快速发展,新建铁路与轨道交通交叉跨越越发频繁,为更好解决此类工程防护问题,结合京张高铁北京北动车所上跨北京市轨道交通13号线交叉工程,对铁路跨越既有轨道交通的防护刚构的进行研究。针对跨越工点,利用MIDAS等有限元软件建立防护结构有限元模型,对比选取不同基础刚度,分析研究防护刚架各设计参数,进行结构配筋并提出设计注意事项;另外,利用ANSYS软件建立全场地模型,对结构整体施工工序进行模拟,提取变形、应力等重要结果,进行跨越防护方案专项安全评估,论证防护方案的可靠度及可行性。 相似文献
33.
脐带缆作为水下生产系统进行油气资源开发的关键结构之一,在安装过程中的脐带缆受到动态的拉伸载荷和张紧器挤压载荷,保障脐带缆的安装下放过程的结构安全有着重要的工程应用意义。本文应用ABAQUS建立脐带缆有限元模型,充分考虑脐带缆内部各构件之间的相互作用,研究不同张紧器角度与不同摩擦系数下脐带缆的抗压抵抗力性能。结果表明,张紧器与脐带缆之间的摩擦力随着摩擦系数、和张紧器夹角增加而增加,基于此提出可通过优化张紧器夹角及张紧器与脐带缆间的摩擦系数来降低深水对于脐带缆安装的限制。 相似文献
34.
Fangliang Xiao 《Maritime Policy and Management》2020,47(5):687-702
ABSTRACT Even if the same two ships operate in the same encountering situation, the actual strategies and timing of operations may be different. Therefore, the fixed collision avoidance trajectory and ship movements are no longer suitable for simulating the real ship behaviour. This paper tries to develop an artificial intelligent system that fully sensitive to the local circumstances and command a ship in virtual environment. Based on AIS (Automatic Identification System) data, this study has developed artificial forces which help decision makings on-board independently under different situations and catching the stochastic nature of ship behaviours during collision avoidances manoeuvring. Actual ship tracks are helpful for ascertaining the parameters that contribute to artificial forces in collision avoidances, while the correlation coefficient analysis is helpful to distinguish the parameters. This study will help to develop a navigation traffic simulation to reflect the realistic ship behaviour and provide reliable information for port and waterway planning, risk analysis, and mitigation measures. The method can be used in developing algorithms for autonomous ships. The method introduced in this study lays a foundation for developing artificial forces at intersections or bends, although the model is only applicable in straight channels at the moment. 相似文献
35.
36.
文章分析了青岛地铁2号线蓄电池工程维护车出现车钩"低头"现象的原因,并给出了一种通过加装车钩挂梁结构以支撑车钩的解决方案。文章还对挂梁结构进行了强度校核,确保该结构满足实际工况需求。 相似文献
37.
A fracture mechanics (FM) based investigation on the mechanism of out-of-plane bending (OPB) between mooring chain links and its effects on fatigue lives of mooring chain links are conducted. Four types of OPB problems that mooring chain links laying on the chain wheel, chain links passing over the bending shoe, chain links constraint provided by the chain hawse, and chain links constraint provided by the chain stopper are considered. Tension ranges of mooring lines are calculated based on the combined loading process induced by the motions of wave frequency (WF) and low frequency (LF). Initial cracks are assumed to propagate from surfaces of chain links and stress intensity factors are calculated in terms of stress ranges determined by a finite element (FE) analysis. The results show that fatigue lives of mooring chain links are decreased significantly due to OPB effects. In addition, the increase of the number of pockets of chain wheel mitigates OPB effects on fatigue lives of mooring chain links laying on the chain wheel, and the increase of the track diameter would reduce OPB effects on fatigue lives of mooring chain links passing over the bending shoe as well. However, for chain links constraint provided by the chain hawse, the diameter of mooring chain hawse has no significant effect on fatigue lives of the mooring chain links subject to OPB if without the abrupt change of the contact conditions between chain links and chain hawse. For mooring links constraint provided by chain stoppers considering the effect of proof loading test, fatigue lives of mooring chain links would drop significantly with the increase of interlink angles and friction coefficient. 相似文献
38.
39.
40.
对轨道交通车辆多层复合结构的隔热壁进行隔热性能理论计算与仿真计算的对比,对隔热壁内部空气层的隔热性能进行分析。结论为:在车辆多层复合结构的隔热壁K值计算中,当空气层厚度大于一定厚度时,流体自然对流对隔热壁的隔热性能影响较大。利用多项式密度(Polynomial Denisity)和Boussinesq模型两种算法,对所得结论进行了验证。 相似文献