全文获取类型
收费全文 | 1841篇 |
免费 | 411篇 |
国内免费 | 323篇 |
专业分类
测绘学 | 367篇 |
大气科学 | 307篇 |
地球物理 | 588篇 |
地质学 | 508篇 |
海洋学 | 217篇 |
天文学 | 18篇 |
综合类 | 158篇 |
自然地理 | 412篇 |
出版年
2024年 | 4篇 |
2023年 | 19篇 |
2022年 | 48篇 |
2021年 | 95篇 |
2020年 | 77篇 |
2019年 | 106篇 |
2018年 | 89篇 |
2017年 | 104篇 |
2016年 | 104篇 |
2015年 | 114篇 |
2014年 | 105篇 |
2013年 | 111篇 |
2012年 | 143篇 |
2011年 | 136篇 |
2010年 | 135篇 |
2009年 | 116篇 |
2008年 | 96篇 |
2007年 | 144篇 |
2006年 | 131篇 |
2005年 | 82篇 |
2004年 | 111篇 |
2003年 | 66篇 |
2002年 | 72篇 |
2001年 | 63篇 |
2000年 | 39篇 |
1999年 | 46篇 |
1998年 | 44篇 |
1997年 | 28篇 |
1996年 | 19篇 |
1995年 | 16篇 |
1994年 | 28篇 |
1993年 | 24篇 |
1992年 | 17篇 |
1991年 | 13篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1986年 | 4篇 |
1984年 | 2篇 |
1982年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有2575条查询结果,搜索用时 15 毫秒
541.
本文根据海湾扇贝的肥满度、肉柱百分数、性腺指数及消化脓指数等指标的周年变化,并结合性腺的组织切片来确定其繁殖期。结果表明,海湾扇贝南移后的繁殖期是从每年的10月下旬开始的,直到翌年5月下旬,其中4月为繁殖盛期。繁殖期间,扇贝的性腺分批成熟,分批排放,并有消退与恢复的现象。同时,海湾扇贝引进中国北方海区并南移福建后,其繁殖规律与原产地(美国大西洋沿岸)的繁殖期有很大的不同。 相似文献
542.
Sloshing in a three-dimensional rectangular tank: Numerical simulation and experimental validation 总被引:1,自引:0,他引:1
Pressure variations and three-dimensional effects on liquid sloshing loads in a moving partially filled rectangular tank have been carried out numerically and experimentally. A numerical algorithm based on the volume of fluid (VOF) technique is used to study the non-linear behavior and damping characteristics of liquid sloshing. A moving coordinate system is used to include the non-linearity and avoid the complex boundary conditions of moving walls. The numerical model solves the complete Navier–Stokes equations in primitive variables by using of the finite difference approximations. In order to mitigate a series of discrete impacts, the signal computed is averaged over several time steps. In order to assess the accuracy of the method used, computations are compared with the experimental results. Several configurations of both baffled and unbaffled tanks are studied. Comparisons show good agreement for both impact and non- impact type slosh loads in the cases investigated. 相似文献
543.
Experimental and Theoretical Investigation of Wave Forces on A Partially-Perforated Caisson Breakwater with A Rock-Filled Core 总被引:2,自引:0,他引:2
1.IntroductionVertical breakwaters are widely used for harbor and coastline protection in coastal engineering.Recently,perforated breakwaters have been often used in practice as they can effectively reduce thewaveforces actingon,the wave reflectionfromand… 相似文献
544.
545.
Xiang Yu Chao Zhan Mengquan Wu Xueli Niu Xueyu Zhang 《Marine Georesources & Geotechnology》2020,38(8):887-895
AbstractTidal waterways perform as the paths through which seawater invades. Their spatial-temporal variation with the shift in Yellow River’s route requires the accurate measurement of their status. Visual interpretation for delineating tidal waterways is inefficient due to a substantial amount of work. Buffer analysis provides an alternative to visual interpretation. However, it is difficult to use the buffer distance threshold to identify tidal waterways from other freshwater bodies. In this study, we present a straightforward method for mapping tidal waterways. First, we adopted the normalized difference water body index (NDWI) to extract water bodies. Waterlines were subsequently delineated based on a morphological opening filter with a kernel of 9?×?9. Finally, tidal waterways were mapped based on the proposed topological relation in ArcGIS. We showed the utility of this approach by mapping tidal waterways in the Yellow River Delta, China. The proposed method enables tidal waterways be mapped precisely and was replicable and universal in other similar studies. 相似文献
546.
杭州湾的三维水流数值模拟 总被引:2,自引:1,他引:2
建立了一个基于σ变换和内外模式分裂技术三维水流数学模型,采用有限节点法(平面)和变步长差分法(σ向)对方程进行了离散;通过风生流和环岛水流两个理论模式对所建模型进行了校验,并将该模型应用到杭州湾的三维水流数值模拟中,效果良好,所建模型垂向分辨率高,简单实用,可应用于河口,海岸,湖泊等大范围水域的水流数值计算。 相似文献
547.
548.
Unsteady nonlinear wave motions on the free surface in shallow water and over slopes of various geometries are numerically simulated using a finite difference method in rectangular grid system. Two-dimensional Navier–Stokes equations and the continuity equation are used for the computations. Irregular leg lengths and stars are employed near the boundaries of body and free surface to satisfy the boundary conditions. Also, the free surface which consists of markers or segments is determined every time step with the satisfaction of kinematic and dynamic free surface conditions. Moreover, marker-density method is also adopted to allow plunging jets impinging on the free surface. Either linear or Stokes wave theory is employed for the generation of waves on the inflow boundary. For the simulation of wave breaking phenomena, the computations are carried out with various wave periods and sea bottom slopes in surf zone. The results are compared with other existing computational and experimental results. Agreement between the experimental data and the computation results is good. 相似文献
549.
550.
A method is proposed for calculating the equivalent hydraulic conductivity (EHC) within a finite difference block (FDB). Application of the constant‐flux assumption of Darcy's Law, the EHC equals to the integration of effective hydraulic conductivity (Kw) as a function of pressure head (hw) divided by the head difference at the ends of the FDB. Error analysis show that the constant‐flux (CF) EHC estimates are better than those computed by the commonly used arithmetic‐mean (AM), geometric‐mean (GM), and harmonic‐mean (HM) techniques. CF EHC results are even more superior at larger interblock head difference situations. Simulations of water infiltration experiments show that simulations using the CF EHC or AM or GM weighting technique have only slight difference while applying the Neumann type boundary condition at the ground surface. In case of the Dirichlet type boundary condition, however, the CF EHC is superior to the other two in correctly estimating the depth of infiltration while enlarging the grid size. Therefore, it is recommended to adopt the CF EHC with a larger grid size to the more stable and more efficient results. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献