全文获取类型
收费全文 | 58149篇 |
免费 | 14574篇 |
国内免费 | 21515篇 |
专业分类
测绘学 | 10768篇 |
大气科学 | 8140篇 |
地球物理 | 10636篇 |
地质学 | 39041篇 |
海洋学 | 11183篇 |
天文学 | 762篇 |
综合类 | 5118篇 |
自然地理 | 8590篇 |
出版年
2024年 | 484篇 |
2023年 | 1321篇 |
2022年 | 3505篇 |
2021年 | 4253篇 |
2020年 | 3455篇 |
2019年 | 4196篇 |
2018年 | 3723篇 |
2017年 | 3553篇 |
2016年 | 3447篇 |
2015年 | 4154篇 |
2014年 | 4040篇 |
2013年 | 4997篇 |
2012年 | 5466篇 |
2011年 | 5532篇 |
2010年 | 5453篇 |
2009年 | 5020篇 |
2008年 | 5212篇 |
2007年 | 4894篇 |
2006年 | 4643篇 |
2005年 | 3950篇 |
2004年 | 2993篇 |
2003年 | 2089篇 |
2002年 | 2113篇 |
2001年 | 1949篇 |
2000年 | 1578篇 |
1999年 | 748篇 |
1998年 | 275篇 |
1997年 | 182篇 |
1996年 | 147篇 |
1995年 | 113篇 |
1994年 | 80篇 |
1993年 | 84篇 |
1992年 | 86篇 |
1991年 | 53篇 |
1990年 | 71篇 |
1989年 | 28篇 |
1988年 | 14篇 |
1987年 | 26篇 |
1986年 | 30篇 |
1985年 | 27篇 |
1984年 | 30篇 |
1983年 | 19篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1979年 | 27篇 |
1978年 | 11篇 |
1965年 | 8篇 |
1957年 | 21篇 |
1954年 | 32篇 |
1933年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
161.
162.
A method combining boundary and finite element approaches as well as modal analysis to solve the acoustical scattering from elastic obstacle in infinite fluid is described. An elastic cylinder with finite length immersed in water is chosen as an example for numerical demonstration. The computed frequency response and direction pattern of the backscattering target strength of the cylinder are conformable to those measured. 相似文献
163.
Up to now, available method of numerical forecasting and suitable wind field model for the Bohai Sea storm surge have been few. In this paper, through the analysis of the weather situation is presented a mathematical model for the wind fields involved mainly the deformation field of a high pressure matched with a low pressure, the temperate cyclone, the cold wave and the northword typhoon. Meanwhile, numerical computations combined with the nonlinear storm surge models are made by using "ADI" method. The computed results are generally coincident with the practical observations. It has showed a success in the simulated wind field and the feasibility of using "ADI" method to forecast the Bohai Sea storm surge. 相似文献
164.
太湖表面定振波的数值计算和最大熵谱分析 总被引:1,自引:0,他引:1
利用水动力学方程对太湖表面的定振波进行计算,算得定振波周期约为452min,另外,利用1992年8月29-31日在太湖西山观测到的水位资料,采用最大熵谱法,分析太湖表面的定振波,得周期值约为450min。计算和分析的周期值基本吻合,取熵谱分析结果得太湖表面的单节点定振波周期值为450min。 相似文献
165.
-Nonlinear tidal waves in a kind of estuary are studied in the paper using one-dimensional nonlinear hydrody-namic equations with friction. The estuary has exponentially varying width B=B0 e-bx and uniform depth h. The one-dimensional hydrodynamic equations are solved by perturbation method. It was found that our solution included two special cases, Pelisenpeki's solution and Airy's solution. The former can be got by letting b=0 in our solutions, and the latter by setting 6 = 0 and f= 0 (f is linear frictional coefficient). In terms of the second-order solution, the physical mechanism of nonlinear tidal waves in estuaries with gradually varying cross-section is explored. It is shown that, under the assumption of linear friction coefficient, shallow water constituent waves consist of two parts, one is produced by shallow water nonlinear effect outside the estuary, the other is generated by shallow water nonlinear effect inside estuary. In addition, the physical mechanism of the residual tidal current and 相似文献
166.
Using 203HgCl2 as a tracer, uptake and combination of inorganic mercury in Tilapia mossambica (petres) and its transfer between different tissues of the fish were studied. The results demonstrate that gill is the main tissue uptaking mercury directly from seawater and that the mercury in gill, skin, digestive tube and fin would be transferred to the other tissues. The result of gel chromatography with Sephadex G-75 shows that most of mercury in gill filaments, liver, and digestive tube are bound in high molecules (MW-70000) and not in metallothionein, but in kidney and bile mercury binding proteins are similar to metallothionein and are accounted for up to 60% and 46% respectively. 相似文献
167.
Based on the shear effect of internal wave currents in stratified fluid, the necessary condition for the possible existence of an internal wave and the sufficient condition under which there will be no internal wave are deduced starting from the continuity equation of incompressible fluid. The above two conditions are verified by the measured results from vertical current meter arraies at some observation stations in the Bohai Sea and the South Huanghai Sea. In the treatment of measured results, first of all, the tidal period components are separated from the measured currents by Fourier-analysis method, and then the internal wave currents of tidal period from the tidal period components in the light of some considerations. The analytical results of observed currents are well consistent with theoretical analysis. Furthermore, one can also deduce some features of internal waves therefrom. Consequently, it is concluded that the observation from a vertical current meter array is an important means to verif 相似文献
168.
A new approach to high-order Boussinesq-type equations with ambient currents is presented. The current velocity is assumed to be uniform over depth and of the same magnitude as the shallow water wave celerity. The wave velocity field is expressed in terms of the horizontal and vertical wave velocity components at an arbitrary water depth level. Linear operators are introduced to improve the accuracy of the kinematic condition at the sea bottom. The dynamic and kinematic conditions at the free surface are expressed in terms of wave velocity variables defined directly on the free surface. The new equations provide high accuracy of linear properties as well as nonlinear properties from shallow to deep water, and extend the applicable range of relative water depth in the case of opposing currents. 相似文献
169.
170.