全文获取类型
收费全文 | 1321篇 |
免费 | 4篇 |
专业分类
公路运输 | 346篇 |
综合类 | 40篇 |
水路运输 | 452篇 |
铁路运输 | 39篇 |
综合运输 | 448篇 |
出版年
2022年 | 5篇 |
2021年 | 10篇 |
2020年 | 12篇 |
2019年 | 8篇 |
2018年 | 27篇 |
2017年 | 26篇 |
2016年 | 44篇 |
2015年 | 16篇 |
2014年 | 27篇 |
2013年 | 215篇 |
2012年 | 64篇 |
2011年 | 66篇 |
2010年 | 55篇 |
2009年 | 55篇 |
2008年 | 63篇 |
2007年 | 47篇 |
2006年 | 27篇 |
2005年 | 29篇 |
2004年 | 24篇 |
2003年 | 26篇 |
2002年 | 20篇 |
2001年 | 28篇 |
2000年 | 18篇 |
1999年 | 22篇 |
1998年 | 41篇 |
1997年 | 18篇 |
1996年 | 22篇 |
1995年 | 22篇 |
1994年 | 16篇 |
1993年 | 18篇 |
1992年 | 16篇 |
1991年 | 13篇 |
1990年 | 8篇 |
1989年 | 14篇 |
1988年 | 16篇 |
1987年 | 15篇 |
1986年 | 16篇 |
1985年 | 11篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 8篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 17篇 |
1974年 | 10篇 |
1973年 | 9篇 |
排序方式: 共有1325条查询结果,搜索用时 15 毫秒
1.
Fluxes of dissolved inorganic carbon in three estuarine systems of the Cantabrian Sea (north of Spain) 总被引:3,自引:1,他引:2
The diffusive and in situ fluxes of dissolved inorganic carbon (DIC) and total alkalinity (TA) have been measured and an estimation has been made of the water–atmosphere fluxes of CO2 in three estuarine systems of the Cantabrian Sea during the spring of 1998. Each of these systems undergoes a different anthropogenic influence. The diffusive fluxes of dissolved inorganic carbon and total alkalinity obtained present values ranging between 0.54–2.65 and 0.0–2.4 mmol m−2 day−1, respectively. These ranges are in agreement with those of other coastal systems. The in situ fluxes are high and extremely variable (35–284 mmol TA m−2 day−1, 43–554 mmol DIC m−2 day−1 and 22–261 mmol dissolved oxygen (DO) m−2 day−1), because the systems studied are very heterogeneous. The values of the ratio of the in situ fluxes of TA and DIC show on average that the rate of dissolution of CaCO3 is 0.37 times that of organic carbon oxidation. Equally, the interval of variation of the relationship between the benthic fluxes of inorganic carbon and oxygen (FDIC/FDO) is very wide (0.3–13.9), which demonstrates the different contributions made by the processes of aerobic and anaerobic degradation of the organic matter, as well as by the dissolution–precipitation of CaCO3. The water–atmosphere fluxes of CO2 present a clear dependence on the salinity. The brackish water of these systems (salinity<20), where maximum fluxes of 989 mmol m−2 day−1 have been estimated, act as a source of CO2 to the atmosphere. The more saline zones of the estuary (salinity>30) act as a sink of CO2, with fluxes between −5 and −10 mmol m−2 day−1. 相似文献
2.
David A. Hensher 《Transportation Research Part A: Policy and Practice》1997,31(6):431-446
This paper presents the methodology and selective empirical results from a study of the demand for a high speed rail system serving the Sydney-Canberra corridor currently dominated by air travel for business trips and car travel for non-business trips. We outline the steps involved in the study from problem specification, data needs, development of base year trip tables, model specification and estimation to establish switching behaviour in the presence of a new mode and calculation of induced demand for current travellers. A stated choice heteroskedastic extreme value switching model is used to evaluate the choice of fare type for business and non-business travel given the current mode used in the corridor for each sampled traveller conventional train, charter coach, scheduled coach, plane or car. Starting with the current travel profile, patronage can be predicted under alternative fare regimes, taking into account diverted traffic, induced traffic and growth. Treating fare class as endogenous enhances the real choice context facing potential patrons. 相似文献
3.
J. A. Sparenberg 《Journal of Marine Science and Technology》1997,2(2):53-61
The axial force induced on a duct (duct thrust) which surrounds a propulsor is, by means of an energy conservation argument,
expressed in terms of the thrust and power of the propulsor and the kinetic energy losses per unit of time of the propulsor
and the duct. Emphasis is put on the linear theory of an axisymmetric ducted actuator disk, where it is shown that the work
done by the duct thrust is equal to the extra work done by the actuator disk when it is surrounded by the duct. This simplified
model gives more insight into the interaction between the propeller and the duct and on the limits of linearization. 相似文献
4.
The Agulhas Current with its retroflection and attendant eddy-shedding is the cause of some of the greatest mesoscale variability in the ocean. This paper considers the area to the south and east of Madagascar, which provides some of the source waters of the Agulhas Current, and examines the propagating sea surface height signals in altimetry and output from a numerical model, OCCAM. Both show bands of variability along the axis of the East Madagascar Current (EMC) and along a zonal band near 25°S. Sequences of images plus associated temperature data suggest that a number of westward-propagating eddies are present in this zonal band. The paper then focuses on the region to the south of the island, where ocean colour and infra-red imagery are evocative of an East Madagascar Retroflection. The synthesis of data analysed in this paper, however, shows that remotely observed features in this area can be explained by anticyclonic eddies moving westward through the region, and this explanation is consistent with numerical model output and the trajectories of drifting buoys. 相似文献
5.
6.
Macrobenthic surveys are an expensive, slow and labour intensive means to establish the health of benthic communities. Sediment profile imagery (SPI) is a means of rapid reconnaissance for monitoring large areas of the benthos. SPI has often been used to monitor gross anthropogenic disturbance. The aim of this study is to determine if SPI can be used as a tool to reliably map change in communities along natural estuarine gradients. Macrobenthic sampling was carried out at five stations along an established estuarine gradient. This faunal data was analysed using standard multivariate techniques and to ground truth a concurrent SPI survey. Faunal analysis showed that habitat quality in Inner Galway Bay was generally good, with some localised disturbance from the River Corrib and the sewage out flow exterior to the city dock. Four distinct groups were identified with a degree of overlap occurring between stations 3 and 4. While existing SPI indices mapped habitat quality in the same manner as the faunal data for end member stations, the level of distinction between the habitats of an intermediate staging was found to be poor. This lack of distinction amongst the stage 2 and 3 stations was overcome by developing a tailored index, the Galway Bay index of habitat quality (GBHQ). This index was derived from the 5 observed variables in the SPI data that were determined to best match the faunal distribution by permutative mantel testing. The 5 observed variables from the SPI data were the depth of the apparent redox potential discontinuity (aRPD), the depth of penetration by the prism, and the presence/absence of infauna, surface faecal pellet layer and biogenic mounding. The GBHQ was able to distinguish between the 5 stations to a greater extent than previously described indices, showing clearly the separate groupings defined by the faunal data. The index was tested on a follow up SPI survey and shown to be applicable in mapping a broader range of habitats in Galway Bay. Indices generated for localised mapping of estuarine gradients should be derived from observed features and be ground truthed using faunal data. Some aspects of the GBHQ should be generally applicable to fine grained boreal estuarine sediments (aRPD/penetration), while others may be of limited utility in other locations depending on the digging characteristics of the particular SPI camera, and local factors influencing the persistence of biogenic features in the profile. This derivation technique provides a simple way to optimise SPI to particular studies and localities. 相似文献
7.
TIMOTHY McCOY JIM ZGLICZYNSKI NILES W. JOHANSON FREDERICK A. PUHN THOMAS W. MARTIN 《Naval Engineers Journal》2007,119(2):83-91
The first of the Arleigh Burke class destroyers is nearing its mid-life. This class of ships was designed during the late 1970s through the 1980s to meet the threats that were prevalent at that time. Since entering service in 1991, these ships have shown themselves to be extremely versatile and the class now consists of nearly 60 ships in service. Their combat systems have been continually upgraded and adapted to meet the new threats the United States faces today. However, in order to keep these platforms viable throughout the first half of the 21st century, their operating costs must be reduced. Manpower, maintenance, and fuel are three of the top operating cost drivers. Most surface combatants spend very little of their underway time operating at full speed or even close to that. Over 1/3 of their underway time is spent at 12 knots and under. This is less than half of their maximum speed and only a fraction of the maximum power owing to the cubic speed–power relationship. Although the existing mechanical drive system is reasonably efficient, the main gas turbines are extremely inefficient at these very low power levels. A shaft-mounted auxiliary electric propulsion system (EPS) can take advantage of excess capacity in the ship service generators to reduce the main engine operating hours. Enabling bi-directional power flow from this auxiliary electric drive will provide additional generation capacity for ship service loads at a modest additional cost. It also provides a "cross-connect" capability from one shaft to the other. This paper will explore one prospect for reducing the operating cost of the DDG-51 class of ships by installing an auxiliary EPS that would powered by the ship service electrical plant. This additional system would serve to reduce both underway fuel usage as well as maintenance on the gas turbine main engines by reducing the number of operating hours on each engine. We will examine the technology trade-offs in this ongoing study. 相似文献
8.
对主动导向转向架的半车模型进行了多体动力学计算,并进行了滚动台试验,计算结果和试验结果基本一致。将半车模型扩展为整车模型后,进行了计算分析,研究结果表明,主动导向转向架可以大大提高转向架的曲线通过性能。 相似文献
9.
Performance indicators for transit management 总被引:1,自引:0,他引:1
Transit performance can be evaluated through quantitative indicators. As the provision of efficient and effective transit service are appropriate goals to be encouraged by federal and state governments, these goals are used to develop performance indicators.Three efficiency and four effectiveness indicators are described, together with two overall indicators. These nine indicators are analyzed for comparability utilizing operating and financial data collected from public transit agencies in California.Performance indicators selected for this study should not be viewed as final. Twenty-one performance indicators proposed by previous studies were reviewed. Theoretical considerations and unavailability or unreliability of data caused omission of several useful measures like passenger-miles. Circumstances such as improved data, emphasis upon goals other than efficiency and effectiveness, and local conditions might warrant the inclusion of indicators deleted from this research.This paper is based on work conducted for the Urban Mass Transportation Administration under University Research and Training Grant CA-11-0014, Development of Performance Indicators for Transit. The views expressed herein are those of the authors and not necessarily those of the University of California or the United States Government. We are indebted to John Feren for assistance with the statistical processing and data gathering. 相似文献
10.
Reform of trucking Vehicle Weights and Dimensions (VWD) regulations in Canada is now underway. The effect these reforms will have on the freight transportation industry are only recently the subject of research. This paper is part of this new research effort, aimed at understanding how regulatory reform in the trucking sector will affect the existing competitive relationship between trucks and the railways. The paper presents the results of study and research into modelling the relationship between mode choice and the service characteristics that are affected by VWD reform.Using several periods of data, a series of econometric models are developed which help to elucidate the relative relationships between the mode service characteristics for both of the principal interprovincial freight modes. A technique is developed and presented to model railway user costs in order to overcome the complex and often unrepresentable pricing activities of Canadian railways.The strength of the developed econometric models is presented, including their significance and statistical reliability. This is further reinforced by the similarities exhibited by all the models in the series. The selected model is applied to predict market service responses required of the railway industry in order to compete with the trucking sector now adapting to the new regulations.The impact of the newly implemented vehicle weight and dimension reform on the rail transportation industry is analyzed and railway industry improvements aimed at maintaining its market share are presented. The results predicted by the model show that railway user cost reductions should be moderate, and likely less than the level of inflation over the period of time when trucking converts its fleet to take advantage of the new regulations.Abbreviations AASHTO
American Association of State Highway and Transportation Officials
- CCMTA
Canadian Conference of Motor Transport Administrators
- EPI
End products, inedible commodity classification
- GVW
Gross Vehicle Weight
- NA
1988 VWD National Agreement
- RTAC
Roads and Transportation Association of Canada
- VWD
Vehicle Weight(s) and Dimension(s)
- WA
1988 VWD Western Agreement 相似文献