全文获取类型
收费全文 | 1901篇 |
免费 | 0篇 |
专业分类
公路运输 | 205篇 |
综合类 | 661篇 |
水路运输 | 665篇 |
铁路运输 | 3篇 |
综合运输 | 367篇 |
出版年
2022年 | 1篇 |
2018年 | 334篇 |
2017年 | 291篇 |
2016年 | 250篇 |
2015年 | 1篇 |
2014年 | 4篇 |
2013年 | 1篇 |
2012年 | 58篇 |
2011年 | 210篇 |
2010年 | 219篇 |
2009年 | 48篇 |
2008年 | 180篇 |
2007年 | 123篇 |
2006年 | 2篇 |
2005年 | 53篇 |
2004年 | 43篇 |
2003年 | 56篇 |
2002年 | 16篇 |
2001年 | 1篇 |
2000年 | 6篇 |
1998年 | 1篇 |
1997年 | 1篇 |
1996年 | 1篇 |
1991年 | 1篇 |
排序方式: 共有1901条查询结果,搜索用时 0 毫秒
791.
The presence of wrinkles in the membrane is the main factor to induce the reflector surface inaccuracy of the space inflatable
antenna. Based on the commercial finite element package ABAQUS, a numerical procedure for membrane wrinkle analysis was set
up and used to analyze a space inflatable antenna which was under inner pressure to evaluate its wrinkle characteristics.
First, the inner pressure effect on the reflector’s wrinkle pattern was studied thoroughly. As inner pressure increases, both
the number and the amplitude of the wrinkles decrease, but the total deformation of the whole reflector surface increases
much. Second, the influence of the interactions between antenna’s parts was investigated comprehensively. Any kind of unwanted
interaction deteriorates reflector’s wrinkle characteristics. The works are valuable to the development and research of the
space inflatable antenna. 相似文献
792.
Paul S. Szwed 《WMU Journal of Maritime Affairs》2011,10(2):183-208
Given the inherent complexity of the maritime transportation system, developing effective policy can be challenging. Using
risk factors identified by an elicitation and aggregation of expert judgment, a relative-risk rating scheme was developed
in the theory-building tradition of the social sciences. The model was empirically evaluated using 18 months of data from
the US small passenger vessel sector. The model identified that the top 10% of relatively highest risk vessels accounted for
50% of all marine casualties during the period of examination. A policy for deploying the model nationally is proposed. 相似文献
793.
A generalized collaborative optimization (CO) framework is proposed to the optimization design of the lines of an underwater vehicle. The resistance and maneuvering performances are concerned about and taken as the optimization objectives in the optimization framework. The resistance, lateral force and yaw moment are calculated by RANS method. To improve the optimization efficiency, an automatic integration optimization platform is constructed in which a surrogate model is adopted. A SUBOFF model is taken as the verification model. The optimal results demonstrate the validity of the optimization strategy proposed. 相似文献
794.
Seven adjustments of convergent-type Vortex Tube (VT) with different throttle angles were applied. The adjustments were made to analyze the influences of such angles on cold and hot temperature drops as well as flow structures inside the VTs. An experimental setup was designed, and tests were performed on different convergent VT configurations at injection pressures ranging from 0.45 to 0.65 MPa. The angles of the throttle valve were arranged between 30° to 90°, and the numbers of injection nozzles ranged between 2 and 6. Laboratory results indicated that the maximum hot and cold temperature drops ranged from 23.24 to 35 K and from 22.87 to 32.88 K, respectively, at four injection nozzles. Results also showed that temperature drop is a function of hot throttle valve angle with the maximum hot and cold temperature drops depending on the angle applied. We used graphs to demonstrate the changes in the cold and hot temperature drops with respect to hot throttle angle values. These values were interpreted and evaluated to determine the optimum angle, which was 60°. The CFD outputs agreed very well with the laboratory results. The proposed CFD results can help future researchers gain good insights into the complicated separation process taking place inside the VTs. 相似文献
795.
Small sample size problem is one of the main problems that heavy numerical control (NC) machine tools encounter in their reliability assessment. In order to deal with the small sample size problem, many indirect reliability data such as reliability data of similar products, expert opinion, and engineers’ experience are used in reliability assessment. However, the existing mathematical theories cannot simultaneously process the above reliability data of multiple types, and thus imprecise probability theory is introduced. Imprecise probability theory can simultaneously process multiple reliability data by quantifying multiple uncertainties (stochastic uncertainty, fuzzy uncertainty, epistemic uncertainty, etc.) together. Although imprecise probability theory has so many advantages, the existing natural extension models are complex and the computation result is imprecise. Therefore, they need some improvement for the better application of reliability engineering. This paper proposes an improved imprecise reliability assessment method by introducing empirical probability distributions to natural extension model, and the improved natural extension model is applied to the reliability assessment of heavy NC machine tool spindle to illustrate its effectiveness. 相似文献
796.
This article presents a method named pseudo-inverse to solve the optimal thrust allocation of dynamic positioning (DP) system,
proposes to optimally determine the azimuth angle of thrusters instead of man-control or semi-auto control, and combines with
the pseudo-inverse methods to get the optimal solutions for dynamic positioning control system. It is able to greatly reduce
the risk of manual mode. Three different kinds of modes are proposed and detailedly illuminated, and can be used to solve
much more complex nonlinear constraint problems, such as typical forbidden vector boundary. Several illustrative examples
are provided to demonstrate the effectiveness and correctness of the proposed thrust allocation modes. 相似文献
797.
Omer Kemal Kinaci 《船舶与海洋工程学报》2016,15(2):136-143
Aircraft flying close to the ground benefit from enhanced efficiency owing to decreased induced drag and increased lift. In this study, a mathematical model is developed to simulate the takeoff of a wing near the ground using an Iterative Boundary Element Method (IBEM) and the finite difference scheme. Two stand-alone sub-codes and a mother code, which enables communication between the sub-codes, are developed to solve for the self-excitation of the Wing-In-Ground (WIG) effect. The aerodynamic force exerted on the wing is calculated by the first sub-code using the IBEM, and the vertical displacement of the wing is calculated by the second sub-code using the finite difference scheme. The mother code commands the two sub-codes and can solve for the aerodynamics of the wing and operating height within seconds. The developed code system is used to solve for the force, velocity, and displacement of an NACA6409 wing at a 4° Angle of Attack (AoA) which has various numerical and experimental studies in the literature. The effects of thickness and AoA are then investigated and conclusions were drawn with respect to generated results. The proposed model provides a practical method for understanding the flight dynamics and it is specifically beneficial at the pre-design stages of a WIG effect craft. 相似文献
798.
The properties and electronic structure of Fe under pressures of 0–30GPa have been studied by first principles employing the
density functional theory (DFT), the ultra-soft pseudo-potentials (USPP) and the generalized gradient approximation (GGA).
The calculating results show that there is a structural transition from magnetic body-centered cubic (bcc) to nonmagnetic
hexagonal-close-packed (hcp) structure for Fe around 11GPa pressure. There is a pseudogap both in the density of states (DOS)
for bcc and hcp Fe. The pseudogap of bcc Fe is deeper and wider than that of hcp Fe. The elastic modulus is obtained by Voigt-Reuss-Hill
averaging scheme. The results indicate that the elastic properties of bcc Fe enhance with pressure except for elastic stiffness
constant C
11, shear modulus G and elastic modulus E at the transition pressure, while the elastic properties of hcp Fe increase linearly with pressure. Magnetic bcc Fe is ductile,
and hcp Fe becomes ductile from brittle around 25GPa. 相似文献
799.
Peter Muirhead 《WMU Journal of Maritime Affairs》2011,10(2):241-242
800.
杨明豪 《上海交通大学学报(英文版)》2011,16(5):586-592
This paper aims at analyzing the security issues that lie in the application layer (AL) protocols when users connect to the
Internet via a wireless local area network (WLAN) through an access point. When adversaries launch deauthentication flood
attacks cutting users’ connection, the connection managers will automatically research the last access point’s extended service
set identifier (ESSID) and then re-establish connection. However, such re-connection can lead the users to a fake access point
with the same ESSID set by attackers. As the attackers hide behind users’ access points, they can pass AL’s authentication
and security schemes, e.g. secure socket layer (SSL). We have proved that they can even spy on users’ account details, passwords,
data and privacy. 相似文献