全文获取类型
收费全文 | 31901篇 |
免费 | 6502篇 |
国内免费 | 7546篇 |
专业分类
测绘学 | 1717篇 |
大气科学 | 7174篇 |
地球物理 | 8079篇 |
地质学 | 16071篇 |
海洋学 | 3925篇 |
天文学 | 1550篇 |
综合类 | 3638篇 |
自然地理 | 3795篇 |
出版年
2024年 | 39篇 |
2023年 | 461篇 |
2022年 | 1279篇 |
2021年 | 1479篇 |
2020年 | 1327篇 |
2019年 | 1357篇 |
2018年 | 1755篇 |
2017年 | 1610篇 |
2016年 | 1847篇 |
2015年 | 1444篇 |
2014年 | 1966篇 |
2013年 | 1814篇 |
2012年 | 1721篇 |
2011年 | 1773篇 |
2010年 | 1905篇 |
2009年 | 1933篇 |
2008年 | 1658篇 |
2007年 | 1626篇 |
2006年 | 1378篇 |
2005年 | 1243篇 |
2004年 | 963篇 |
2003年 | 955篇 |
2002年 | 926篇 |
2001年 | 926篇 |
2000年 | 1120篇 |
1999年 | 1571篇 |
1998年 | 1293篇 |
1997年 | 1377篇 |
1996年 | 1163篇 |
1995年 | 1065篇 |
1994年 | 968篇 |
1993年 | 838篇 |
1992年 | 675篇 |
1991年 | 485篇 |
1990年 | 331篇 |
1989年 | 356篇 |
1988年 | 310篇 |
1987年 | 211篇 |
1986年 | 179篇 |
1985年 | 127篇 |
1984年 | 106篇 |
1983年 | 81篇 |
1982年 | 84篇 |
1981年 | 58篇 |
1980年 | 47篇 |
1979年 | 33篇 |
1978年 | 17篇 |
1977年 | 7篇 |
1976年 | 9篇 |
1958年 | 25篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
992.
993.
994.
995.
996.
The effect of expansion ratio on the critical Reynolds number in single fracture flow with sudden expansion 下载免费PDF全文
Flow in a single fracture (SF) is an important research subject in groundwater hydrology, hydraulic engineering, radioactive nuclear waste repository and geotechnical engineering. An abruptly changing aperture is a unique type of SF. This study discusses the relation between the values of the critical Reynolds number (Rec) for the onset of symmetry breaking of flow and the expansion ratio (E) of SF, which is defined as the ratio between the outlet (D) and inlet (d) apertures. This study also investigates the effect of inlet aperture d on Rec for flow in an SF with abruptly changing apertures (SF‐ACA) using the finite volume method. Earlier numerical and experimental results showed that flow is symmetric in respect to the central plane of the SF‐ACA at small Reynolds number (Re) but becomes asymmetric when Re is sufficiently large. Our simulations show that the value of Rec decreases with the increasing E, and the relationship between the logarithm of Rec and E can be described accurately using either a quadratic polynomial function or a logarithmic function. However, the relationship of Rec and d for a given E value is vague, and Rec becomes even less sensitive to d when E increases. This study also reveals that the hydraulic gradient (J) and flow velocity (v) follow a super‐linear relationship that can be fitted almost perfectly by the Forchheimer equation. The inertial component (Ji) of J increases monotonically with Re, whereas the viscous component (Jv) of J decreases monotonically with Re. The Re value corresponding to equal inertial and viscous components of J (named as the transitional point Re) decreases when E increases, and such a transitional point Re should be closely related to the critical Reynolds number Rec, although a rigorous theoretical proof is not yet available. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
997.
998.
A tree‐ring width based drought reconstruction for southeastern China: links to Pacific Ocean climate variability 下载免费PDF全文
Feng Chen Shulong Yu Yujiang Yuan Huiqin Wang Mary Gagen 《Boreas: An International Journal of Quaternary Research》2016,45(2):335-346
We present a drought reconstruction for southeastern China based on a tree‐ring width chronology of Cryptomeria fortunei developed from two sampling sites in central Fujian. A reconstruction of July–February drought variability, spanning AD 1855–2011, was developed by calibrating total tree‐ring width data with the self‐calibrating Palmer drought severity index (scPDSI). The reconstruction was verified against an independent data set, and accounts for 36% of the actual scPDSI variance during the period 1955–2011. Relatively dry intervals were reconstructed between AD 1859–1880, 1899–1911, 1927–1933, 1946–1959, 1964–1970 and 1987–1997. Relatively wet conditions prevailed during 1855–1858, 1881–1898, 1912–1926, 1934–1945, 1960–1963, 1971–1986 and 1998–2011. Comparisons between our scPDSI reconstruction and a moisture‐sensitive tree‐ring width record from Vietnam revealed consistencies between the two data sets, suggesting similar drought regimes. Spectral peaks of 2.2–6.4 years may be indicative of El Niño‐Southern Oscillation (ENSO) activity, as also suggested by the significant correlations with sea surface temperatures (SSTs) in the eastern equatorial and southeastern Pacific Ocean and an extreme event analysis. The analysis of links between our scPDSI reconstruction and the large‐scale regional climatic variation shows that there is a relationship between regional drought variation and East Asian summer monsoon (EASM) intensity. 相似文献
999.
The axisymmetric formulation of the governing equations for geomechanics in the framework of smoothed particle hydrodynamics (SPH) is presented in this study. Two forms of SPH discretization for the motion equations, which are labeled as form I and form II, are proposed, and the methods to compute the hoop stress and strain terms including hoop strain rate and the acceleration introduced by the hoop stress are compared. To avoid possible singularity problem near the axis of symmetry, a perfectly smooth contact along with ghost particles are applied to prevent the real particles from overly approaching the axis of symmetry to remove this potential singularity. In addition, the Mohr–Coulomb constitutive model is implemented into the SPH formulation in describing soil behavior. Four numerical tests are carried out to validate and compare the accuracy and stability of the proposed algorithms, and their results are compared with analytical solutions and results from FEM analysis. The performance in these comparisons suggests that SPH II with hoop terms computed through direct hoop method is more stable than the others, and the adoption of contact for the symmetric axis is efficient in eliminating the singularity problem. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
1000.
Wangcheng Zhang Dong Wang Mark F. Randolph Alexander M. Puzrin 《国际地质力学数值与分析法杂志》2016,40(17):2312-2338
Quantitative assessment of the risk of submarine landslides is an essential part of the design process for offshore oil and gas developments in deep water, beyond the continental shelf. Landslides may be triggered by a reduction in shear strength of subsea sediments over a given zone, caused for example by seismic activity. Simple criteria are then needed to identify critical conditions whereby the zone of weakness could grow catastrophically to cause a landslide. A number of such criteria have been developed over the last decade, based either on ideas drawn from fracture mechanics, or considering the equilibrium of the initial weakened zone and adjacent process zones of gradually softening material. Accounting for the history of the weak zone initiation is critical for derivation of reliable propagation criteria, in particular considering dynamic effects arising from accumulating kinetic energy of the failing material, which will allow the failure to propagate from a smaller initial zone of weakened sediments. Criteria are developed here for planar conditions, taking full account of such dynamic effects, which are shown to be capable of reducing the critical length of the softened zone by 20% or more compared with criteria based on static conditions. A numerical approach is used to solve the governing dynamic equations for the sliding material, the results from which justify assumptions that allow analytical criteria to be developed for the case where the initial softening occurs instantaneously. The effect of more gradual softening is also explored. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献