全文获取类型
收费全文 | 49290篇 |
免费 | 3581篇 |
国内免费 | 2276篇 |
专业分类
化学 | 22358篇 |
晶体学 | 239篇 |
力学 | 3976篇 |
综合类 | 546篇 |
数学 | 8167篇 |
物理学 | 9642篇 |
无线电 | 10219篇 |
出版年
2024年 | 100篇 |
2023年 | 482篇 |
2022年 | 1213篇 |
2021年 | 1336篇 |
2020年 | 1333篇 |
2019年 | 1166篇 |
2018年 | 1039篇 |
2017年 | 1436篇 |
2016年 | 1946篇 |
2015年 | 1564篇 |
2014年 | 2397篇 |
2013年 | 3405篇 |
2012年 | 3037篇 |
2011年 | 2783篇 |
2010年 | 2284篇 |
2009年 | 2726篇 |
2008年 | 2825篇 |
2007年 | 3155篇 |
2006年 | 2619篇 |
2005年 | 2336篇 |
2004年 | 2229篇 |
2003年 | 1840篇 |
2002年 | 1444篇 |
2001年 | 1181篇 |
2000年 | 1133篇 |
1999年 | 996篇 |
1998年 | 949篇 |
1997年 | 841篇 |
1996年 | 753篇 |
1995年 | 712篇 |
1994年 | 633篇 |
1993年 | 497篇 |
1992年 | 476篇 |
1991年 | 405篇 |
1990年 | 355篇 |
1989年 | 240篇 |
1988年 | 237篇 |
1987年 | 142篇 |
1986年 | 121篇 |
1985年 | 153篇 |
1984年 | 122篇 |
1983年 | 66篇 |
1982年 | 103篇 |
1981年 | 64篇 |
1980年 | 54篇 |
1979年 | 80篇 |
1978年 | 36篇 |
1977年 | 21篇 |
1976年 | 26篇 |
1973年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
12.
The turbulent flow in a compound meandering channel with a rectangular cross section is one of the most complicated turbulent flows, because the flow behaviour is influenced by several kinds of forces, including centrifugal forces, pressure‐driven forces and shear stresses generated by momentum transfer between the main channel and the flood plain. Numerical analysis has been performed for the fully developed turbulent flow in a compound meandering open‐channel flow using an algebraic Reynolds stress model. The boundary‐fitted coordinate system is introduced as a method for coordinate transformation in order to set the boundary conditions along the complicated shape of the meandering open channel. The turbulence model consists of transport equations for turbulent energy and dissipation, in conjunction with an algebraic stress model based on the Reynolds stress transport equations. With reference to the pressure–strain term, we have made use of a modified pressure–strain term. The boundary condition of the fluctuating vertical velocity is set to zero not only for the free surface, but also for computational grid points next to the free surface, because experimental results have shown that the fluctuating vertical velocity approaches zero near the free surface. In order to examine the validity of the present numerical method and the turbulent model, the calculated results are compared with experimental data measured by laser Doppler anemometer. In addition, the compound meandering open channel is clarified somewhat based on the calculated results. As a result of the analysis, the present algebraic Reynolds stress model is shown to be able to reasonably predict the turbulent flow in a compound meandering open channel. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
13.
介绍了Multisim8的仿真功能的特点,并通过实例说明了用Multisim8进行仿真分析的具体方法,简单介绍了如何设置参数和进行仿真操作。 相似文献
14.
Anna L. Mazzucato 《Transactions of the American Mathematical Society》2003,355(4):1297-1364
This paper is devoted to the analysis of function spaces modeled on Besov spaces and their applications to non-linear partial differential equations, with emphasis on the incompressible, isotropic Navier-Stokes system and semi-linear heat equations. Specifically, we consider the class, introduced by Hideo Kozono and Masao Yamazaki, of Besov spaces based on Morrey spaces, which we call Besov-Morrey or BM spaces. We obtain equivalent representations in terms of the Weierstrass semigroup and wavelets, and various embeddings in classical spaces. We then establish pseudo-differential and para-differential estimates. Our results cover non-regular and exotic symbols. Although the heat semigroup is not strongly continuous on Morrey spaces, we show that its action defines an equivalent norm. In particular, homogeneous BM spaces belong to a larger class constructed by Grzegorz Karch to analyze scaling in parabolic equations. We compare Karch's results with those of Kozono and Yamazaki and generalize them by obtaining short-time existence and uniqueness of solutions for arbitrary data with subcritical regularity. We exploit pseudo-differential calculus to extend the analysis to compact, smooth, boundaryless, Riemannian manifolds. BM spaces are defined by means of partitions of unity and coordinate patches, and intrinsically in terms of functions of the Laplace operator.
15.
M. Ohsaki 《Archive of Applied Mechanics (Ingenieur Archiv)》2003,73(3-4):241-251
Summary Characteristics of optimal solutions under nonlinear buckling constraints are investigated by using a bar-spring model. It is demonstrated that optimization under buckling constraints of a symmetric system often leads to a structure with hill-top branching, where a limit point and bifurcation points coincide. A general formulation is derived for imperfection sensitivity of the critical load factor corresponding to a hill-top branching point. It is shown that the critical load is not imperfection-sensitive even for the case where an asymmetric bifurcation point exists at the limit point. 相似文献
16.
17.
18.
The ultraviolet band systemsA
1Π-X
1Σ+ of P14N and P15N were excited in an electrodeless tube containing traces of phosphorus specpure nitrogen and neon using a microwave discharge
(2450 MHz). Bands of the isotopic species, P15N, were obtained using15N2 enriched to 95.5%. Rotational analyses of eleven bands of P14N and sixteen bands of P15N were carried out. Three perturbing statese
3Σ−,d
3Δ andb
3Π, arising from the lower valence configurations were identified from the observed perturbations in thev′=0–4 levels of theA
1Π state. Deperturbation studies led to the determination of molecular constants of the perturbing states. Vibrational assignments
of the perturbing states were made from isotope shift studies. 相似文献
19.
N. Fitier 《国际流体数值方法杂志》2003,42(12):1345-1361
There is a growing interest in developing numerical tools to investigate the onset of physical instabilities observed in experiments involving viscoelastic flows, which is a difficult and challenging task as the simulations are very sensitive to numerical instabilities. Following a recent linear stability analysis carried out in order to better understand qualitatively the origin of numerical instabilities occurring in the simulation of flows viscoelastic fluids, the present paper considers a possible extension for more complex flows. This promising method could be applied to track instabilities in complex (i.e. essentially non‐parallel) flows. In addition, results related to transient growth mechanism indicate that it might be responsible for the development of numerical instabilities in the simulation of viscoelastic fluids. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
20.
Numerical simulation aspects, related to low Reynolds number free boundary viscous flows at micro and mesolevel during the resin impregnation stage of the liquid composite moulding process (LCM), are presented in this article. A free boundary program (FBP), developed by the authors, is used to track the movement of the resin front accurately by accounting for the surface tension effects at the boundary. Issues related to the global and local mass conservation (GMC and LMC) are identified and discussed. Unsuitable conditions for LMC and consequently GMC are uncovered at low capillary numbers, and hence a strategy for the numerical simulation of such flows is suggested. FBP encompasses a set of subroutines that are linked to modules in ANSYS. FBP can capture the void formation dynamics based on the analysis developed. We present resin impregnation dynamics in two dimensions. Extension to three dimensions is a subject for further research. Several examples are shown and efficiency of different stabilization techniques are compared. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献