全文获取类型
收费全文 | 23411篇 |
免费 | 2211篇 |
国内免费 | 1300篇 |
专业分类
化学 | 10868篇 |
晶体学 | 109篇 |
力学 | 7404篇 |
综合类 | 235篇 |
数学 | 2924篇 |
物理学 | 5382篇 |
出版年
2024年 | 31篇 |
2023年 | 238篇 |
2022年 | 507篇 |
2021年 | 606篇 |
2020年 | 794篇 |
2019年 | 626篇 |
2018年 | 662篇 |
2017年 | 697篇 |
2016年 | 875篇 |
2015年 | 843篇 |
2014年 | 1009篇 |
2013年 | 2029篇 |
2012年 | 1279篇 |
2011年 | 1407篇 |
2010年 | 1002篇 |
2009年 | 1246篇 |
2008年 | 1190篇 |
2007年 | 1315篇 |
2006年 | 1141篇 |
2005年 | 1020篇 |
2004年 | 1007篇 |
2003年 | 855篇 |
2002年 | 782篇 |
2001年 | 585篇 |
2000年 | 590篇 |
1999年 | 519篇 |
1998年 | 510篇 |
1997年 | 469篇 |
1996年 | 432篇 |
1995年 | 367篇 |
1994年 | 305篇 |
1993年 | 313篇 |
1992年 | 296篇 |
1991年 | 216篇 |
1990年 | 191篇 |
1989年 | 144篇 |
1988年 | 138篇 |
1987年 | 99篇 |
1986年 | 97篇 |
1985年 | 104篇 |
1984年 | 91篇 |
1983年 | 45篇 |
1982年 | 89篇 |
1981年 | 35篇 |
1980年 | 27篇 |
1979年 | 20篇 |
1978年 | 17篇 |
1976年 | 14篇 |
1973年 | 12篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
21.
Planar laser-induced fluorescence visualisation is used to investigate nonuniformities in the flow of a hypersonic conical nozzle. Possible causes for the nonuniformity are outlined and investigated, and the problem is shown to be due to a small step at the nozzle throat. Entrainment of cold boundary layer gas is postulated as the cause of the signal nonuniformity.
PACS 47.80.Jk, 47.40.Ki, 47.60.+i 相似文献
22.
Summary This paper is concerned with the problem of developing numerical integration algorithms for differential equations that, when
viewed as equations in some Euclidean space, naturally evolve on some embedded submanifold. It is desired to construct algorithms
whose iterates also evolve on the same manifold. These algorithms can therefore be viewed as integrating ordinary differential
equations on manifolds. The basic method “decouples” the computation of flows on the submanifold from the numerical integration
process. It is shown that two classes of single-step and multistep algorithms can be posed and analyzed theoretically, using
the concept of “freezing” the coefficients of differential operators obtained from the defining vector field. Explicit third-order
algorithms are derived, with additional equations augmenting those of their classical counterparts, obtained from “obstructions”
defined by nonvanishing Lie brackets. 相似文献
23.
24.
25.
David K Bisset 《国际流体数值方法杂志》2002,39(10):961-977
Turbulent flow simulation methods based on finite differences are attractive for their simplicity, flexibility and efficiency, but not always for accuracy or stability. This paper demonstrates that a good compromise is possible with the advected grid explicit (AGE) method. Starting from the same initial field as a previous spectral DNS, AGE method simulations of a planar turbulent wake were carried out as DNS, and then at three levels of reduced resolution. The latter cases were in a sense large‐eddy simulations (LES), although no specific sub‐grid‐scale model was used. Results for the two DNS methods, including variances and power spectra, were very similar, but the AGE simulation required much less computational effort. Small‐scale information was lost in the reduced resolution runs, but large‐scale mean and instantaneous properties were reproduced quite well, with further large reductions in computational effort. Quality of results becomes more sensitive to the value chosen for one of the AGE method parameters as resolution is reduced, from which it is inferred that the numerical stability procedure controlled by the parameter is acting in part as a sub‐grid‐scale model. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
26.
A. B. Mazo 《Fluid Dynamics》2002,37(6):913-918
Plane ideal incompressible flow in a rectangular channel partitioned by a thin permeable barrier (lattice) is considered. In flowing through the lattice the stream suddenly (jumpwise) changes direction and loses energy. The flow is assumed to be vortical; the vorticity is discontinuous on the lattice. A mathematical formulation of the problem for the stream function is proposed in the form of a nonlinear elliptic equation with coefficients discontinuous on the lattice line. A numerical solution is constructed using the finite-element iteration method. The results of the numerical simulation show how the flow velocity profile in the channel can be controlled by means of permeable barriers. 相似文献
27.
A simple scheme is developed for treatment of vertical bed topography in shallow water flows. The effect of the vertical step on flows is modelled with the shallow water equations including local energy loss terms. The bed elevation is denoted with zb‐ for the left and zb+ for the right values at each grid point, hence exactly representing a discontinuity in the bed topography. The surface gradient method (SGM) is generalized to reconstruct water depths at cell interfaces involving a vertical step so that the fluxes at the cell interfaces can accurately be calculated with a Riemann solver. The scheme is verified by predicting a surge crossing a step, a tidal flow over a step and dam‐break flows on wet/dry beds. The results have shown good agreements compared with analytical solutions and available experimental data. The scheme is efficient, robust, and may be used for practical flow calculations. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
28.
GAO Jing ZHOU Xiaohua CHEN Xun Chongqing University Chemical Engineering Department Chongqing China 《Chinese Journal of Reactive Polymers》2006,(2)
1. INTRODUCTION In the process of coking plant, about 30%~35% sulfur is transformed to H2S and some other sulfide, which form impurity in coal gas together with NH3 and HCN. Only 0.1% H2S containing in air can lead to die, so it is very important to carry on desulphurization and decyanation with coal gas [1~3]. Currently desulphurization and decyanation craft technique have Dry Oxidation Technology, Wet Oxidation Technology and Liquid Absorption Technology [2] three main kinds. The… 相似文献
29.
Most of the novel highly potent drugs, developed on the basis of modern molecular medicine, taking into account cell surface recognition techniques, show poor water solubility. A chemical modification of the drug substance enhancing the solubility often decreases the pharmacological activity. Thus, as an alternative an increase of the solubility can be obtained by the reduction of the size of the drug particles. Unfortunately, it is often difficult to obtain micro or nanosized drug particles by classical or more advanced crystallization using supercritical gases or by milling techniques. In addition, nanosized particles are often not physically stable and need to be stabilized in an appropriate matrix. Thus, it may be of interest to manufacture directly nanosized drug particles stabilized in an inert hydrophilic matrix, i.e. nanostructured and nanocomposite systems. Solid solutions and solid dispersions represent nanostructured and nanocomposite systems. In this context, the use of the vacuum-fluidized-bed technique for the spray-drying of a low water soluble drug cosolubilized with a hydrophilic excipient in a polar organic solvent is discussed. In order to avoid the use of organic solvents, a special spray-freeze-drying technique working at atmospheric pressure is presented. This process is very suitable for temperature and otherwise sensitive drugs such as pharmaproteins. 相似文献
30.
The molding processes of polymer melts involve geometrically complex dies. Such dies are usually tapered or streamlined to achieve a maximum output rate under conditions of laminar flow. The model of a generalized second-grade fluid of power-law type is used and the results obtained are illustrated by examples of convergent flows in conical and wedge-shaped dies. 相似文献