首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   882篇
  免费   8篇
公路运输   307篇
综合类   36篇
水路运输   289篇
铁路运输   33篇
综合运输   225篇
  2023年   10篇
  2022年   20篇
  2021年   8篇
  2020年   8篇
  2019年   7篇
  2018年   34篇
  2017年   14篇
  2016年   30篇
  2015年   7篇
  2014年   30篇
  2013年   131篇
  2012年   33篇
  2011年   47篇
  2010年   35篇
  2009年   46篇
  2008年   47篇
  2007年   21篇
  2006年   10篇
  2005年   17篇
  2004年   18篇
  2003年   5篇
  2002年   13篇
  2001年   8篇
  2000年   14篇
  1999年   10篇
  1998年   23篇
  1997年   14篇
  1996年   19篇
  1995年   16篇
  1994年   5篇
  1993年   7篇
  1992年   12篇
  1991年   8篇
  1990年   11篇
  1989年   8篇
  1988年   5篇
  1987年   9篇
  1986年   8篇
  1985年   8篇
  1984年   9篇
  1983年   12篇
  1982年   7篇
  1981年   12篇
  1980年   8篇
  1979年   15篇
  1977年   15篇
  1976年   11篇
  1975年   10篇
  1974年   8篇
  1973年   5篇
排序方式: 共有890条查询结果,搜索用时 15 毫秒
801.
Experiments regarding the modification of the foil geometry and/or active or passive mass injection in the vortex core have been performed to investigate the possibility of inhibiting tip vortex cavitation. The ejection at very low flow rates of drag-reducing polymer solutions at the tip of hydrofoils and propeller blades has demonstrated effectiveness as a tip vortex cavitation inhibitor. This paper reports the results obtained with an elliptical hydrofoil, of 8cm maximum chord and 12cm haif-span, operating at Reynolds numbers, of =106, much larger than those previously reported in the literature. Lift coefficients and critical cavitation numbers were determined for a variety of flow and polymer solution ejection conditions. Tangential and axial components of the mean velocity as well as velocity fluctuations along the vortex path were also measured. At 12.5 m/s free stream velocity and a variety of angles of attack, the ejection of a 500 ppm aqueous solution of a drag-reducing polymer at a flow rate of about 5 cm3/s leads to a decrease of up to 30% in the cavitation number. This occurs without modification of the lift coefficient and, hence, of the midspan bound circulation of the foil. Moreover, water injection does not cause any appreciable change in the cavitation numbers. The tangential velocity profiles along the vortex path during polymer ejection indicate that the potential region remains the same, while the viscous core dimension increases, and the maximum tangential velocity decreases substantially as compared to the no ejection or water ejection experiments. Thus, the pressure coefficients at the vortex axis are smaller than for the no ejection or water ejection cases and cause the reduction of the critical cavitation numbers. It is speculated that this inhibition effect is due only to swelling of the polymer solution when exiting the ejection orifice.List of symbols a core radius (distance to the vortex axis for maximum tangential velocity) - C 1 lift coefficient - c max maximum chord - Cp pressure coefficient at the vortex axis - Cp min minimum pressure coefficient at the vortex axis - d e diameter of the ejection port - m ejection flow rate - P DDLE" BORDER="0"> reference pressure - P v vapor pressure - V DDLE" BORDER="0"> free stream velocity - V a axial velocity - V t tangential velocity - v r radial component of the velocity resulting from jet swelling - x downstream distance from the tip of the foil - y, r distance to the vortex axis - DER="0"> angle of attack - Delta" align="BASELINE" BORDER="0">r difference between the swollen jet and the ejection port radii - DER="0"> boundary layer thickness - DER="0"> tip vortex intensity - DER="0"> d (DER="0"> de ) desinent cavitation number (with ejection) - DER="0"> i (DER="0"> ie ) inception cavitation number (with ejection) - DER="0"> ii normal stresses - DDLE" BORDER="0"> viscosity - v kinematic viscosity - p specific mass  相似文献   
802.
汽车部件对污染的敏感性   总被引:1,自引:0,他引:1  
污染敏感性分析是实现对整个机械系统污染控制的重要步骤。通过对污染的分析、测试、建模及模拟分析可有助于减少污染物或微小颗粒对机械系统造成的污染,避免因污染而导致的系统故障。对有关汽车机械系统污染的问题进行了分类讨论并进行了相关性分析,对测试及计算机建模方法进行了论述。给出了污染敏感性分析流程,最后通过几个典型实例阐明了污染敏感性分析的意义。  相似文献   
803.
Net in situ production and export of dissolved organic carbon (DOC) and nitrogen (DON) have been studied in shelf waters off the Ría de Vigo (NW Spain), as part of a comprehensive hydrographic survey carried out from September 1994 to September 1995 with a fortnight periodicity. DOC and DON correlated well (r=+0.78), the slope of the regression line being 12.0±0.7 mol-C mol-N−1, about twice the Redfieldian slope of particulate organic matter, 6.5±0.2 mol-C mol-N−1 (r=+0.95). Labile DOC and DON accumulated in the upper 50 m during the upwelling season (March–September), mainly after prolonged periods of wind relaxation, when horizontal flows were reduced. This labile material represented 50% and 35% of the total (dissolved+particulate) organic carbon and nitrogen susceptible of microbial utilisation, which assert the key contribution of dissolved organic matter (DOM) to the export of new primary production in the NW Iberian upwelling system. This surface excess in shelf waters appeared to be formed into the highly productive Ría de Vigo (a large coastal indentation) at net rates of 4.4 μM-C d−1 and 1.3 μM-C d−1 in the inner and outer segments of the embayment respectively, and subsequently exported to the shelf. Once in the shelf, simple dilution with the inert DOM pool of recently upwelled Eastern North Atlantic Central Water (ENACW) occurred. Eventually, the DOM excess produced during the upwelling season is exported to the adjacent open ocean waters by the coastal circulation. Conversely, during the unproductive downwelling season (October–February), the lowest DOC and DON levels were recorded and export was prevented by the characteristic downwelling front associated to the seasonal poleward slope current.  相似文献   
804.
Road Vehicle Suspension System Design - a review   总被引:8,自引:0,他引:8  
Based mainly on English language literature, information relating to the design of automobile suspension systems for ride comfort and control of wheel load variations for frequencies below body structure resonances is reviewed. The information is interpreted in the context of vehicles which travel through a wide speed range on roads of markedly differing quality, which do so carrying different loads and which are required to possess good handling qualities.

Sections are devoted to describing road surfaces, modelling vehicles and setting up performance criteria, and to passive, active, semi-active and slow-active system types. Methods for deriving active system control laws are outlined. Strengths and weaknesses of the various systems are identified and their relative performance capabilities and equipment requirements are discussed. Attention is given to adaptation of the suspension or control system parameters to changing conditions. Remaining research needs are considered.  相似文献   
805.
806.
Creating synthetic baseline populations   总被引:1,自引:0,他引:1  
To develop activity-based travel models using microsimulation, individual travelers and households must be considered. Methods for creating baseline synthetic populations of households and persons using 1990 census data are given. Summary tables from the Census Bureau STF-3A are used in conjunction with the Public Use Microdata Sample (PUMS), and Iterative Proportional Fitting (IPF) is applied to estimate the proportion of households in a block group or census tract with a desired combination of demographics. Households are generated by selection of households from the associated PUMS according to these proportions. The tables of demographic proportions which are exploited here to make household selections from the PUMS may be used in traditional modeling. The procedures are validated by creating pseudo census tracts from PUMS samples and considering the joint distribution of the size of households and the number of vehicles in the households. It is shown that the joint distributions created by these methods do not differ substantially from the true values. Additionally the effects of small changes in the procedure, such as imputation of additional demographics and adding partial counts to the constructed demographic tables are discussed in the paper.  相似文献   
807.
A mathematical model for the steering control of an automobile is described. The structure of the model derives from linear optimal discrete time preview control theory but it is non-linear. Its parameter values are obtained by heuristic methods, using insight gained from the linear optimal control theory. The driver model is joined to a vehicle dynamics model and the path tracking performance is demonstrated, using moderate manoeuvring and racing speeds. The model is shown to be capable of excellent path following and to be robust against changes in the vehicle dynamics. Application to the simulation of manoeuvres specified by an ideal vehicle path and further development of the model to formalise the derivation of its parameter values and to put it to other uses are discussed.  相似文献   
808.
An Experimental Investigation of Preview Control   总被引:3,自引:0,他引:3  
There is mounting theoretical evidence to suggest that preview control can be of substantial benefit to a semi-active suspension for random road inputs. In this paper, the benefits of wheel-base preview control are measured experimentally, using a prototype semi-active damper in a half-car 'Hardware-in-the-loop' (HiL) rig with a planar two-axle heavy vehicle model. The benefits of preview control using the prototype semi-active damper are found to be less than theoretically possible, due to the phase lag between the demanded and achieved damping force. It is shown that the performance of the prototype damper can be improved significantly by having a theoretical simulation running ahead of the HiL vehicle. The theoretical simulation is used to predict the demanded damper force for the HiL vehicle, and thereby compensate for the phase lag in the prototype damper.  相似文献   
809.
Force Control of a Semi-Active Damper   总被引:4,自引:0,他引:4  
Two strategies are investigated for controlling a semi-active damper to track a prescribed force demand signal: (i) 'open loop' control, using a model of the damping force versus velocity characteristics; and (ii) force feedback (closed loop) control.

The damping characteristics and switching transients of a prototype damper were measured, and used to develop a mathematical model of the dynamics of the damper. The two control strategies were investigated using an idealised (constant velocity) test. Their performance was also simulated and measured under realistic operating conditions using the Hardware-in-the-Loop testing method.

Open loop damper control was generally found to give superior performance to force feedback control, due to its smaller phase lag at high frequencies.  相似文献   
810.
城市轨道交通换乘枢纽大客流换乘疏散研究   总被引:3,自引:0,他引:3  
从换乘设备设施、换乘乘客特性和运营组织方案3个方面分析了对大客流换乘疏散的影响。最后,以某地铁换乘站为例,针对列车不同的到达时间间隔对换乘疏散的影响进行仿真研究,并对大客流换乘疏散组织提出建议。  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号