全文获取类型
收费全文 | 87932篇 |
免费 | 1396篇 |
国内免费 | 1500篇 |
专业分类
测绘学 | 2775篇 |
大气科学 | 6870篇 |
地球物理 | 17480篇 |
地质学 | 34131篇 |
海洋学 | 6577篇 |
天文学 | 15603篇 |
综合类 | 2295篇 |
自然地理 | 5097篇 |
出版年
2021年 | 552篇 |
2020年 | 652篇 |
2019年 | 744篇 |
2018年 | 5949篇 |
2017年 | 5244篇 |
2016年 | 4138篇 |
2015年 | 1178篇 |
2014年 | 1684篇 |
2013年 | 3084篇 |
2012年 | 2733篇 |
2011年 | 5012篇 |
2010年 | 4060篇 |
2009年 | 4974篇 |
2008年 | 4275篇 |
2007年 | 4684篇 |
2006年 | 2464篇 |
2005年 | 1988篇 |
2004年 | 2181篇 |
2003年 | 2216篇 |
2002年 | 1949篇 |
2001年 | 1519篇 |
2000年 | 1517篇 |
1999年 | 1271篇 |
1998年 | 1268篇 |
1997年 | 1268篇 |
1996年 | 1080篇 |
1995年 | 1041篇 |
1994年 | 916篇 |
1993年 | 833篇 |
1992年 | 766篇 |
1991年 | 793篇 |
1990年 | 810篇 |
1989年 | 736篇 |
1988年 | 693篇 |
1987年 | 807篇 |
1986年 | 799篇 |
1985年 | 904篇 |
1984年 | 1038篇 |
1983年 | 1030篇 |
1982年 | 940篇 |
1981年 | 902篇 |
1980年 | 836篇 |
1979年 | 786篇 |
1978年 | 777篇 |
1977年 | 726篇 |
1976年 | 662篇 |
1975年 | 647篇 |
1974年 | 696篇 |
1973年 | 715篇 |
1972年 | 441篇 |
排序方式: 共有10000条查询结果,搜索用时 534 毫秒
971.
972.
973.
A survey of diffraction/radiation analyses has been performed on behalf of the International Ship and Offshore Structures Congress, Committee I.2. This paper presents the results from the survey, including data from 17 organisations plotted in a common format. The variability of the computed hydrodynamic loads and responses is discussed, and it is concluded that there is a degree of uncertainty in results predicted on the basis of commonly used idealisations. Even in long period waves the variability between the computed results is considerable. To obtain reliable results at the shorter periods associated with heave, pitch and roll resonances of a deep water TLP, much finer meshes than those used by most of the participating organisations would be required. 相似文献
974.
975.
The Barra Nova inlet, in south Portugal, is known to migrate progressively southeastwards under wave action. The morphodynamics
of this system during a representative year suggests that this long-term evolution is dependent on a seasonal behavior of
the tidal inlet which can be described through a three-stage model of post-storm, transition and extended fair-weather conditions.
Processes involved in this evolution indicate that the historical migration of the Barra Nova is not dependent on the longshore
drift forcing constricting the channel on the updrift coast, but rather on the adjustment of the system to a major erosion
of the downdrift coast during short storm events. 相似文献
976.
The effects of scattering and resonance on the energy dissipation of an internal tide were investigated using a two-dimensional
model which is a reassembled version of the theoretical generation model devised by Rattray et al. (1969) for internal tide. The basic character of the scattering process at the step bottom was first investigated with a
wide shelf model. When the internal wave incited from a deep region (Region II) into the shallow shelf region (Region I),
a passing wave into the shallow region, a reflected wave into the deep region, and a beam-like wave, i.e. a scattered wave
(SW), emanated at the step bottom. The SW, which consists of the superposition of numerous internal modes, propagated upward/downward
into both regions. The general properties of the SW were well expressed around the shelf edge, even in the present model with
viscosity effect. The amplitude of the SW decreased dramatically when the depth of the velocity maximum of the incident internal
wave in Region II corresponded with the depth of the shelf edge. In the narrow shelf model, where the decay distance of the
internal wave in Region I is longer than the shelf width, the incident internal wave reflected at the coast to form a standing
wave. When the internal wave in Region I is enhanced by the resonance, the energy of the SW in Region II is also intensified.
Furthermore, the energy of the modes in Region II predominated when the velocity maximum is identical to that of the dominant
mode in Region I. These results suggest that the spatial scale of shelf region is a very important factor governing the energy
dissipation of the internal tide through reflection and scattering in a narrow shelf. 相似文献
977.
A higher order panel method based on B-spline representation for both the geometry and the solution is developed for the analysis of steady flow around marine propellers. The self-influence functions due to the normal dipole and the source are desingularized through the quadratic transformation, and then shown to be evaluated using conventional numerical quadrature. By selecting a proper order for numerical quadrature, the accuracy of the present method can be increased to the machine limit. The far- and near-field influences are shown to be evaluated based on the same far-field approximation, but the near-field solution requires subdividing the panels into smaller subpanels continuously, which can be effectively implemented due to the B-spline representation of the geometry. A null pressure jump Kutta condition at the trailing edge is found to be effective in stabilizing the solution process and in predicting the correct solution. Numerical experiments indicate that the present method is robust and predicts the pressure distribution on the blade surface, including very close to the tip and trailing edge regions, with far fewer panels than existing low-order panel methods. 相似文献
978.
V. N. Aref’ev F. V. Kashin V. K. Semenov R. M. Akimenko N. E. Kamenogradskii N. I. Sizov V. P. Sinyakov L. B. Upenek V. P. Ustinov 《Izvestiya Atmospheric and Oceanic Physics》2006,42(6):739-751
Refined data of systematic measurements of total water vapor in the atmosphere from May 1980 to April 2005 are presented. The data were obtained at the Issyk Kul atmospheric-monitoring station by the method of solar molecular-absorption spectroscopy. Over 25 years, the annual mean water-vapor content in the atmosphere increased by 4.5% at a mean rate of increase of 0.18% per year. However, the water-vapor content decreased in the last five years. The results of statistical processing of experimental data (general statistical characteristics, correlation coefficients, composite oscillations) are described. A refined model is proposed for forecasts of temporal variations in the monthly mean and annual mean water-vapor contents for the coming years. The model includes a linear trend and the sum of oscillations with periods close to the periods of a number of well-known geophysical phenomena. Regression equations are proposed to relate the water-vapor content in the atmospheric column to the surface temperature and absolute humidity. 相似文献
979.
An investigation has been made of available data on the saturation state of seawater with respect to calcium carbonate and its possible significance for scale formation on Ocean Thermal Energy Conversion (OTEC) heat exchangers. Pertinent oceanographic data is lacking at or near potential OTEC sites for the calculation of the degree of saturation of seawater with respect to calcium carbonate. Consequently, only “extrapolated” saturation values can be used. These indicate that near surface seawater is probably supersaturated, with respect to the calcium carbonate phases calcite and aragonite, at all potential OTEC sites. The deep seawater that would be brought to the surface at the potential Atlantic Ocean sites is also likely to be supersaturated with respect to calcium carbonate. The deep seawater at the potential Pacific Ocean sites may be slightly undersaturated.The fact that OTEC heat exchangers will be operating in seawater, which is supersaturated with respect to calcium carbonate, means that if nucleation of calcite or aragonite occurs on the heat exchanger surfaces, significant growth rates of calcium carbonate scale may be expected. The potential for calcium carbonate nucleation is highest at cathodic metal surface locations, which are produced as the result of aluminum corrosion in seawater. Consequently, corrosion and scale formation may be closely related. What the possible effects of biofouling may be on this process are not known. 相似文献
980.
Long-term changes in ray populations in the North Sea 总被引:2,自引:3,他引:2