全文获取类型
收费全文 | 120300篇 |
免费 | 1934篇 |
国内免费 | 751篇 |
专业分类
测绘学 | 2847篇 |
大气科学 | 8134篇 |
地球物理 | 23011篇 |
地质学 | 44782篇 |
海洋学 | 10416篇 |
天文学 | 26581篇 |
综合类 | 400篇 |
自然地理 | 6814篇 |
出版年
2022年 | 711篇 |
2021年 | 1187篇 |
2020年 | 1339篇 |
2019年 | 1479篇 |
2018年 | 4515篇 |
2017年 | 4144篇 |
2016年 | 4044篇 |
2015年 | 1778篇 |
2014年 | 3178篇 |
2013年 | 5591篇 |
2012年 | 4037篇 |
2011年 | 6051篇 |
2010年 | 5400篇 |
2009年 | 6636篇 |
2008年 | 5700篇 |
2007年 | 6133篇 |
2006年 | 4402篇 |
2005年 | 3391篇 |
2004年 | 3333篇 |
2003年 | 3237篇 |
2002年 | 3056篇 |
2001年 | 2521篇 |
2000年 | 2484篇 |
1999年 | 1977篇 |
1998年 | 2061篇 |
1997年 | 1931篇 |
1996年 | 1674篇 |
1995年 | 1627篇 |
1994年 | 1395篇 |
1993年 | 1342篇 |
1992年 | 1224篇 |
1991年 | 1276篇 |
1990年 | 1267篇 |
1989年 | 1054篇 |
1988年 | 990篇 |
1987年 | 1141篇 |
1986年 | 1050篇 |
1985年 | 1305篇 |
1984年 | 1456篇 |
1983年 | 1415篇 |
1982年 | 1311篇 |
1981年 | 1239篇 |
1980年 | 1128篇 |
1979年 | 1045篇 |
1978年 | 1001篇 |
1977年 | 920篇 |
1976年 | 871篇 |
1975年 | 871篇 |
1974年 | 846篇 |
1973年 | 941篇 |
排序方式: 共有10000条查询结果,搜索用时 703 毫秒
61.
62.
In this paper the steady lateral growth of three-dimensional turbulent inclined turbidity current is investigated. To simulate the current, an experimental setup is developed to analyze the turbidity current for different regimes in the particle laden density currents environment. The Buckingham's π theorem together with a dimensional analysis is implemented to derive the appropriate non-dimensional variables. The experimental results were normalized and plotted in the form of non-dimensional graphs from which a theoretical model is developed and analyzed. Based on the results obtained for the steady lateral growth, three different regimes, namely, inertia-viscous one as the first regime, buoyancy-viscous and gravity-viscous as the second and third regimes are distinguished within the current.In these regimes, the force balance is between the driving and resisting forces. Namely, in the first regime, the force balance is between the inertia and viscous forces, in the second regime, the buoyancy and viscous forces, and in the third regime, gravity and viscous forces are balanced. The experimental results indicate that the lateral growth rate in the first regime is smaller than that in the second and third regimes due to the magnitude and type of the forces involved in those regimes. According to the graphical results, the three different lateral growth rates appear when the normalized current length is smaller than about 3, between about 3 and 10, and larger than about 10. In those regions,the slopes of the data are different with respect to one another. 相似文献
63.
64.
Solution for a plane strain rough‐walled hydraulic fracture driven by turbulent fluid through impermeable rock 下载免费PDF全文
The impact of turbulent flow on plane strain fluid‐driven crack propagation is an important but still poorly understood consideration in hydraulic fracture modeling. The changes that hydraulic fracturing has experienced over the past decade, especially in the area of fracturing fluids, have played a major role in the transition of the typical fluid regime from laminar to turbulent flow. Motivated by the increasing preponderance of high‐rate, water‐driven hydraulic fractures with high Reynolds number, we present a semianalytical solution for the propagation of a plane strain hydraulic fracture driven by a turbulent fluid in an impermeable formation. The formulation uses a power law relationship between the Darcy‐Weisbach friction factor and the scale of the fracture roughness, where one specific manifestation of this generalized friction factor is the classical Gauckler‐Manning‐Strickler approximation for turbulent flow in a rough‐walled channel. Conservation of mass, elasticity, and crack propagation are also solved simultaneously. We obtain a semianalytical solution using an orthogonal polynomial series. An approximate closed‐form solution is enabled by a choice of orthogonal polynomials embedding the near‐tip asymptotic behavior and thus giving very rapid convergence; a precise solution is obtained with 2 terms of the series. By comparison with numerical simulations, we show that the transition region between the laminar and turbulent regimes can be relatively small so that full solutions can often be well approximated by either a fully laminar or fully turbulent solution. 相似文献
65.
66.
This study evaluated the spatial variability of streambed vertical hydraulic conductivity (Kv) in different stream morphologies in the Frenchman Creek Watershed, Western Nebraska, using different variogram models. Streambed Kv values were determined in situ using permeameter tests at 10 sites in Frenchman, Stinking Water and Spring Creeks during the dry season at baseflow conditions. Measurements were taken both in straight and meandering stream channels during a 5 day period at similar flow conditions. Each test site comprised of at least three transects and each transect comprised of at least three Kv measurements. Linear, Gaussian, exponential and spherical variogram models were used with Kriging gridding method for the 10 sites. As a goodness-of-fit statistic for the variogram models, cross-validation results showed differences in the median absolute deviation and the standard deviation of the cross-validation residuals. Results show that using the geometric means of the 10 sites for gridding performs better than using either all the Kv values from the 93 permeameter tests or 10 Kv values from the middle transects and centre permeameters. Incorporating both the spatial variability and the uncertainty involved in the measurement at a reach segment can yield more accurate grid results that can be useful in calibrating Kv at watershed or sub-watershed scales in distributed hydrological models. 相似文献
67.
Fully nonlinear features of heavy ion-acoustic solitary waves (HIASWs) have been investigated in an astrophysical degenerate relativistic quantum plasma (ADRQP) containing relativistically degenerate electrons and non-relativistically degenerate light ion species, and non-degenerate heavy ion species. The pseudo-energy balance equation is derived from the fluid dynamical equations by adopting the well-known Sagdeev-potential approach, and the properties of arbitrary amplitude HIASWs are examined. The small amplitude limit for the propagation of HIASWs is also recovered. The basic features (width, amplitude, polarity, critical Mach number, speed, etc.) of HIASWs are found to be significantly modified by the relativistic effect of the electron species, and also by the variation of the number density of electron, light ion, and heavy ion species. The basic properties of HIASWs, that may propagated in some realistic astrophysical plasma systems (e.g., in white dwarfs), are briefly discussed. 相似文献
68.
Korzhenkov A. M. Gadzhiev M. S. Ovsyuchenko A. N. Lar’kov A. S. Rogozhin E. A. 《Izvestiya Atmospheric and Oceanic Physics》2020,56(7):633-656
Izvestiya, Atmospheric and Oceanic Physics - As a result of studies conducted in the city of Derbent, it is established that its monumental fortification structures erected at the end of the 560s... 相似文献
69.
A Study on the Surface Wettability of Clastic Rocks with Potential Application for CO2 Storage Sites
Umar Bappah Adamu Gholami Raoof Raza Arshad Downey William Samuel Sarmadivaleh Mohammad Shah Afroz A. Nayak Prasanta 《Natural Resources Research》2020,29(3):2051-2061
Natural Resources Research - There have been many studies carried out in the past decades attempting to develop strategies for a safe injection of CO2 into storage sites without leakage and... 相似文献
70.
Natural Resources Research - Sand failure and production occurs when the formation stress exceeds the strength of the formation, which is derived majorly from the natural material that cements the... 相似文献