全文获取类型
收费全文 | 28020篇 |
免费 | 1631篇 |
专业分类
公路运输 | 8224篇 |
综合类 | 8173篇 |
水路运输 | 5875篇 |
铁路运输 | 5803篇 |
综合运输 | 1576篇 |
出版年
2024年 | 148篇 |
2023年 | 221篇 |
2022年 | 566篇 |
2021年 | 1010篇 |
2020年 | 1052篇 |
2019年 | 621篇 |
2018年 | 469篇 |
2017年 | 617篇 |
2016年 | 666篇 |
2015年 | 945篇 |
2014年 | 2250篇 |
2013年 | 1637篇 |
2012年 | 2555篇 |
2011年 | 2572篇 |
2010年 | 2038篇 |
2009年 | 1781篇 |
2008年 | 1834篇 |
2007年 | 2417篇 |
2006年 | 2091篇 |
2005年 | 1255篇 |
2004年 | 799篇 |
2003年 | 518篇 |
2002年 | 360篇 |
2001年 | 309篇 |
2000年 | 195篇 |
1999年 | 113篇 |
1998年 | 79篇 |
1997年 | 78篇 |
1996年 | 76篇 |
1995年 | 64篇 |
1994年 | 61篇 |
1993年 | 46篇 |
1992年 | 38篇 |
1991年 | 40篇 |
1990年 | 32篇 |
1989年 | 25篇 |
1988年 | 17篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 19篇 |
1984年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
992.
深水SPAR风机系统全耦合动力响应分析研究 总被引:1,自引:0,他引:1
文章采用联合开发的计算程序对深水SPAR风机的浮体、锚泊和风机各子系统进行了水—气动力的全耦合数值分析,研究了深水浮式风机系统的动力响应特点。浮体水动力计算采用基于二阶精度的混合波浪模型(Hybrid Wave Model)的MORISON公式,锚泊系统采用细长杆理论通过非线性有限元方法实现,风机系统的空气动力分析采用基于多体气动弹性理论的FAST模块。以浮体控制方程为主体,通过模块间的载荷与位移传递在每个时间步上迭代求解,形成完全耦合的时域分析方法。通过对NREL的5MW SPAR风机系统在随机海况下的水动力响应分析,验证了该方法的有效性,并分析了浮式风机子系统间的混合动力作用。 相似文献
993.
超声冲击处理(UIT)是一种有效的焊后改善焊接接头疲劳性能的工艺措施,其借助机械撞击和超声振动的共同作用,使焊趾表面产生塑性变形从而引入有益的压缩残余应力。为评价UIT技术对焊接接头残余应力的影响,该文提出了一套新的数值分析方法,包括焊接数值模拟及随后的超声冲击处理过程的动态弹塑性有限元分析。在有限元模型中考虑了实际的工艺参数和超声促成的材料软化效应。以船用高强钢AH36非承载十字焊接接头为研究对象,将预测的超声冲击处理前后的残余应力分布与实验结果进行对比,两者有较好的吻合。在此基础上,探讨了静态预载荷对超声冲击处理态残余应力再分布的影响。 相似文献
994.
995.
996.
以线性和非线性需求结合方式建立了以满足任务需要为条件的海警大型舰艇基本需求组合模型,预测计算了多个样本国海警大型舰艇基本需求数,结果表明该模型有一定的实用性.据此对中国海警大型舰艇基本需求进行预测,预测结果可对中国海警大型舰艇发展规划提供参考. 相似文献
997.
Transport models are used to evaluate new infrastructure and public transport services, varied levels of demand, and new ideas for demand management. Exploring these proposals virtually is easier than implementation and testing in situ. However, existing models are based around traditional forms of transportation. As part of a feature analysis using a case study approach, three different simulation packages (a simple custom-developed package, traffic microsimulation, and agent-based simulation) are used to develop and demonstrate simulations of demand-responsive transportation (DRT) and analyze the advantages and disadvantages of each simulation approach for evaluating DRT. While the simulations display some relational replication (meaning they produce similar relational patterns with respect to certain variables), they do not show distributional replication (that is, the value of the results is not statistically similar), meaning that under- or over-estimation of predicted travel could occur. Recommendations for the application of each modeling approach are made. 相似文献
998.
Due to the difficulty of obtaining accurate real-time visibility and vehicle based traffic data at the same time, there are only few research studies that addressed the impact of reduced visibility on traffic crash risk. This research was conducted based on a new visibility detection system by mounting visibility sensor arrays combined with adaptive learning modules to provide more accurate visibility detections. The vehicle-based detector, Wavetronix SmartSensor HD, was installed at the same place to collect traffic data. Reduced visibility due to fog were selected and analyzed by comparing them with clear cases to identify the differences based on several surrogate measures of safety under different visibility classes. Moreover, vehicles were divided into different types and the vehicles in different lanes were compared in order to identify whether the impact of reduced visibility due to fog on traffic crash risk varies depending on vehicle types and lanes. Log-Inverse Gaussian regression modeling was then applied to explore the relationship between time to collision and visibility together with other traffic parameters. Based on the accurate visibility and traffic data collected by the new visibility and traffic detection system, it was concluded that reduced visibility would significantly increase the traffic crash risk especially rear-end crashes and the impact on crash risk was different for different vehicle types and for different lanes. The results would be helpful to understand the change in traffic crash risk and crash contributing factors under fog conditions. We suggest implementing the algorithms in real-time and augmenting it with ITS measures such as VSL and DMS to reduce crash risk. 相似文献
999.
Land use can influence walking (measured by the number of steps) and so the health of people. This paper presents the result of empirical research on the impact of regional population densities (inhabitants per inhabitable area) on the number of steps (all steps, both outdoors and indoors). With data collected from almost 11,000 respondents in 148 Japanese regions, we estimate polynomial regression models, the total number of steps being the dependent variable and densities being the main independent variable. Regional population density significantly affects the number of steps after controlling for individual and household attributes. The estimated population density that maximizes the number of steps is around 11,000?persons/km2. Increasing densities, up to levels of around 11,000?inhabitants/km2, could increase walking and consequently the health of inhabitants. The population density elasticity of the number of steps is 0.046–0.049 in a simple log linear regression model without a peak. 相似文献
1000.