全文获取类型
收费全文 | 585篇 |
免费 | 4篇 |
专业分类
公路运输 | 209篇 |
综合类 | 64篇 |
水路运输 | 105篇 |
铁路运输 | 115篇 |
综合运输 | 96篇 |
出版年
2023年 | 1篇 |
2022年 | 5篇 |
2021年 | 17篇 |
2020年 | 18篇 |
2019年 | 13篇 |
2018年 | 17篇 |
2017年 | 23篇 |
2016年 | 15篇 |
2015年 | 26篇 |
2014年 | 52篇 |
2013年 | 31篇 |
2012年 | 58篇 |
2011年 | 34篇 |
2010年 | 23篇 |
2009年 | 26篇 |
2008年 | 32篇 |
2007年 | 49篇 |
2006年 | 37篇 |
2005年 | 32篇 |
2004年 | 12篇 |
2003年 | 12篇 |
2002年 | 5篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1999年 | 4篇 |
1998年 | 8篇 |
1997年 | 6篇 |
1996年 | 7篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1984年 | 1篇 |
排序方式: 共有589条查询结果,搜索用时 390 毫秒
451.
452.
453.
In a deep well, under high pressure and temperature, the force, deformation, and movement of the downhole string are very complicated. In this paper, based on the analysis of the working environment and load properties of the down-hole string in deep wells, the factors of high temperature, high pressure, and the complex structure of wells have been taken into consideration. The loads of the down-hole string, stability, back buckle configuration, dynamic characteristics, mechanical strength, and deformation have been systematically studied. On this basis, mechanical models of a down-hole string under different conditions have been established and software for aiding analysis has been developed. By using this software, deep well strings under different construction parameters can be simulated and analyzed, providing a theoretical basis and method for down-hole string assembly, as well as string down-hole construction and operation. 相似文献
454.
Satisficing control remains an important concept in decision making. In this paper, a new epistemic utility satisficing control theory is proposed for a new model of complex CMMO (constrained multi-objective multi degree-of-freedom optimization) system. As well, an epistemic utility function is developed and used to adjust the feasible region of soft constraints. The theory proved in this paper indicates that the utility function not only expresses the subjectivity of the original satisfactory-degree function, but also takes the cost of searching for a solution into account. Thus, the satisfactory-degree function can be adjusted and its rationality can be validated. This theory contributes an analytical method to the inverse satisfactory optimization problem. The findings indicate that this theory has good convergence and outcomes desired for satisfactory-degree functions. 相似文献
455.
Design of conformal cooling channels for an automotive part 总被引:2,自引:0,他引:2
The cooling system plays a crucial role in determining the productivity and quality of an injection molding process. With
the current growth of Solid Freeform Fabrication (SFF) techniques, mold designers have been striving for not only improvement
in cooling system performance but also for a method to do so automatically. In this paper, a method is proposed for developing
a conformal cooling system that facilitates uniform cooling over the entire mold surface with minimum cycle time. Based on
the temperature distribution after the filling stage, the mold surface is split into zones which will be cooled by optimized
subconformal channels obtained from the optimization process. The optimization process in which the objective function is
stated as minimization of the cooling time with boundaries ensuring a realistic design will optimize the cooling system layout
in terms of cooling channel size and location. Finally, all subcooling channels are combined to generate the entire conformal
cooling system for the injection mold. 相似文献
456.
A two-level optimization method for the design of complex truss and parallel distributed implementation on a LAN is presented using parallel virtual machine (PVM) for Win 32 as message passing between PCs. The volumes of truss are minimized by decomposing the original optimization problem into a number of bar optimization problems executed concurrently and a coordinate optimization problem, subject to constraints on nodal displacements, and stresses, buckling and crippling of bars, etc. The system sensitivity analysis that derives the partial derivatives of displacements and stresses with respect to areas are also performed in parallel so as to shorten the analysis time. The convergence and the speedup performances as well as parallel computing efficiency of the method are investigated by the optimization examples of a 52-bar planar truss and a 3 126-bar three-diraensional truss. The results show that the ideal speedup is obtained in the cases of 2 PCs for the 3 126-bar space truss optimization, while no speedup is observed for the 52-bar truss. It is concluded that (1) the parallel distributed algorithm proposed is efficient on the PC-based LAN for the coarsegrained large optimization problem; (2) to get a high speedup, the problem granularity should match with the network granularity;and (3) the larger the problem size is, the higher the parallel efficiency is. 相似文献
457.
汽车覆盖件冲压方向优化系统设计 总被引:5,自引:0,他引:5
通过分析影响汽车覆盖件冲压方向选择的因素,建立了以评价函数法为基础的冲压方向优化模型。利用该模型对冲压件截面形状进行了分析,确定了截面转角的公共可行域,并建立了一组目标函数作为优化对象。在公共可行域内优化各个截面的转角,然后加权平均可得到目标函数的最优值。以典型汽车覆盖件为例,对实现覆盖件冲压方向的优化系统进行了理论验证。 相似文献
458.
459.
Diego Iannuzzi Flavio CiccarelliDavide Lauria 《Transportation Research Part C: Emerging Technologies》2012,21(1):321-337
The installation of stationary ultracapacitor storage devices, as widely recognized, allows the recovery of the braking energy for increasing the energy efficiency as well as a better pantograph voltage profile. In the paper a new methodological mean is proposed for determining the fundamental characteristics of this kind of storage device, characterized by high power density, interfaced with the railroad by a bidirectional dc-dc converter. More specifically, the parameters of the storage system can be determined by employing an optimization technique which in a quite general way is able to take contemporaneously into account several aspects in an integrated manner. Some considerations are performed for properly taking into account the stochastic aspects of the design procedure. Numerical simulations with respect to a case study are presented, pointing out the potentiality of the tailored technique. Experimental results are also reported, with reference to an electromechanical simulator, in order to put in evidence the effectiveness and the actual implementation of the proposed optimization technique. 相似文献
460.
为使道路使用者在出发前获得具有高实时性和可靠性的行程时间预测信息,提高出行效率,需提升高速公路行程时间的预测精度。鉴于此,将生物学中粒子群优化算法(Particle Swarm Optimization Algorithm, PSO)引入小波神经网络(Wavelet Neural Network, WNN)中,构建基于粒子群优化小波神经网络(Particle Swarm Optimization Wavelet Neural Network, PSO-WNN)的高速公路行程时间预测模型。首先将高速公路原始收费数据规整化,截取其中有效字段,获取研究路段一个月的行程时间数据并对其进行数据处理。然后分别基于PSO-WNN模型和WNN模型,利用Matlab进行实验。实验结果显示,PSO-WNN模型预测结果的平均绝对误差、平均相对误差和均方误差较WNN模型分别降低了83.36%, 82.20%和98.15%。PSO-WNN行程时间预测模型不仅预测精度高,而且能较准确地预测出行程时间的走向及波动情况,在收敛速度方面也呈现出一定的优势,具有较好的适应能力。 相似文献