全文获取类型
收费全文 | 12192篇 |
免费 | 2525篇 |
国内免费 | 4143篇 |
专业分类
测绘学 | 785篇 |
大气科学 | 1963篇 |
地球物理 | 3506篇 |
地质学 | 8261篇 |
海洋学 | 1465篇 |
天文学 | 463篇 |
综合类 | 1419篇 |
自然地理 | 998篇 |
出版年
2024年 | 22篇 |
2023年 | 101篇 |
2022年 | 279篇 |
2021年 | 388篇 |
2020年 | 264篇 |
2019年 | 318篇 |
2018年 | 343篇 |
2017年 | 355篇 |
2016年 | 396篇 |
2015年 | 361篇 |
2014年 | 405篇 |
2013年 | 421篇 |
2012年 | 419篇 |
2011年 | 898篇 |
2010年 | 706篇 |
2009年 | 880篇 |
2008年 | 564篇 |
2007年 | 688篇 |
2006年 | 835篇 |
2005年 | 833篇 |
2004年 | 1392篇 |
2003年 | 1018篇 |
2002年 | 793篇 |
2001年 | 659篇 |
2000年 | 558篇 |
1999年 | 579篇 |
1998年 | 572篇 |
1997年 | 574篇 |
1996年 | 458篇 |
1995年 | 369篇 |
1994年 | 344篇 |
1993年 | 523篇 |
1992年 | 417篇 |
1991年 | 240篇 |
1990年 | 150篇 |
1989年 | 150篇 |
1988年 | 102篇 |
1987年 | 101篇 |
1986年 | 69篇 |
1985年 | 32篇 |
1984年 | 27篇 |
1983年 | 23篇 |
1982年 | 27篇 |
1981年 | 17篇 |
1978年 | 11篇 |
1977年 | 18篇 |
1976年 | 14篇 |
1950年 | 10篇 |
1937年 | 15篇 |
1933年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Morphodynamics of a bar-trough surf zone 总被引:3,自引:0,他引:3
A field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar-trough topography (the “longshore-bar-trough” and “rhytmic-bar-and-beach” states as described by Wright and Short, 1984). Characteristic features of the morphology include a shallow bar with a steep shoreward face, a deep trough, and a steep beach face. This morphology, which is favored by moderate breaker heights and small tidal ranges, strongly controls the coupled suite of hydrodynamic processes. In contrast to fully dissipative surf zones, the bar-trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach. The degree of incident wave groupiness does not change appreciably across the surf zone. Infragravity standing waves which, in dissipative surf zones, dominate the inshore energy, remain energetically secondary and occur at higher frequencies in the bar trough surf zone. Analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal—antinodal positions over observed surf-zone profiles, indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough. Standing waves which would have nodes over the bar are suppressed. Sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress. In addition, the extra near-bottom eddy viscosity provided by the reformed, non-breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current. Whereas the bar is highly mobile in terms of onshore—offshore migration rates, the beach face and inner regions of the trough are remarkably stable over time. 相似文献
102.
Temperature data from 18 measurement series obtained during logging of the Oseberg field in the northern North Sea are presented. Because the measurement series are taken at approximately the same depth, they should give identical temperatures after depth correction, and are suitable for assessing the performance of different models used to determine virgin rock temperatures from well log information.We have used this data set to test the properties of the different models given by Shen and Beck (1986). Although these models were built to simulate closely the thermal recovery of a well and are unbiased, the uncertainties in the temperature estimates when applied to real data are found to be no less than those from simpler (biased) models. This fact confirms the conclusion of Hermanrud (1989a) who showed that physical factors other than those presently accounted for significantly influence the thermal recovery of a borehole. 相似文献
103.
104.
Li Yucheng He Ming Professor Dalian University of Technology Dalian Former master student Dalian University of Technology Dalian 《中国海洋工程》1994,(3)
-Based on the extended Morison Equation and model tests, the in-line forces on small square cylinders caused by waves (regular and irregular) and currents are analyzed in detail in this paper. The hydrodynamic coefficient CD and Cu related to KC number and the effect of direction of wave incidence are also given, which can be used in engineering practice. 相似文献
105.
南黄海陆架沉积学研究 总被引:4,自引:0,他引:4
黄海中部、济州岛西南,存在厚层的泥质沉积,它们分别与上述地区的冷涡活动区相对应。在两个泥质沉积区之间又出现连接它们的通道,形成别有特征的“冷涡-通道”沉积体系。经分析发现,这个沉积体系,完全受控于黄海暖流、冷涡及其相关的环流体系。此外,在黄海西南部及东海北部海底发现大面积砂岩及其砾石,揭示了黄、东海陆架沉积物的新源地。 相似文献
106.
提出波浪作用下岸坡和海底动态和静态平衡条件的数学模型。在已建立的推移质泥沙体积输沙率基本关系式的基础上,根据连续方程,计算出底坡、泥沙、波浪三要素在动态和静态平衡情况下的关系式,得出反映这种关系的底坡平衡函数曲线图。用实际资料对这一函数曲线进行了验证,并对实际资料相对模型的某些差异作出解释。 相似文献
107.
Li Dashan Shen Ying Ren Rushu Chen Yao
Senior Engineer River Harbour Department Nanjing Hydraulic Research Institute Nanjing Engineer River Harbour Department Nanjing Hydraulic Research Institute Nanjing Associate Professor Hohai University Nanjing Master Hohai University Nanjing 《中国海洋工程》1997,(1)
In this paper,the characteristics of density current under the action of waves are describedwith the help of flume experiment and theoretical analysis.The study shows that turbid water under the ac-tion of the waves can present three types of motion,i.e.significant stratification,fragile stratification andstrong mixing.The motion of turbid water presents significant stratification when(H/D)/△ρ/ρ~(1/2)≤4.5,generally this state is known as density current.The formulas of motionvelocity,thickness,and discharge of density current moving on horizontal bottom are derived by use of ba-sic equations such as momemtum equation,equation of energy conservation and continuity equation offluid.The time-average velocity and the thickness of density current under the action of waves have a rela-tionship with such parameters as relative density(△ρ/ρ),wave height(H),and water depth(D).Whenthese parameters are determined,the time-average thickness and motion velocity of density current are al-so determined.The relat 相似文献
108.
109.
Li Runpei Chen Weigang Gu Yongning
Professor Shanghai Jiao Tong University Shanghai Assistant Shanghai Jiao Tong University Shanghai 《中国海洋工程》1996,(1)
In this paper a numerical analysis method combining FEM incemental technique with limit analysis concept is proposed for the study of the static strength of offshore platform in collision. Large deformation and plasticity are accounted for and the limit yield surface expressed by generalized stress for a tubular section is derived. The modified stiffness matrix of space beam element is formulated by Plastic Node Method. The buckling behavior of beam columns can also be taken into account. It can trace the generation of plastic hinges during loading and finally the ultimate strength of offshore platform against collision is obtained. 相似文献
110.
Zhang Jianguo Yan Chi Yan Shuwang
Lecturer Dept. of Hydraulic Eng. Tianjin University Tianjin Associate Professor Dept. of Hydraulic Eng. Tianjin University Tianjin Professor Dept. of Hydraulic Eng. Tianjin University Tianjin 《中国海洋工程》1996,(1)
This paper describes the model tests for determining the axial friction and the lateral resistance of sand to pipeline by using fine sand and prototype pipeline, and the calculation method based on limit analysis theory is verified. The effect of cyclic loading is considered in the test. 相似文献