全文获取类型
收费全文 | 74821篇 |
免费 | 6306篇 |
国内免费 | 7011篇 |
专业分类
化学 | 30096篇 |
晶体学 | 413篇 |
力学 | 6391篇 |
综合类 | 1280篇 |
数学 | 24976篇 |
物理学 | 24982篇 |
出版年
2023年 | 585篇 |
2022年 | 934篇 |
2021年 | 1298篇 |
2020年 | 1470篇 |
2019年 | 1683篇 |
2018年 | 1454篇 |
2017年 | 1769篇 |
2016年 | 2022篇 |
2015年 | 1840篇 |
2014年 | 2613篇 |
2013年 | 4944篇 |
2012年 | 3233篇 |
2011年 | 3452篇 |
2010年 | 2972篇 |
2009年 | 4195篇 |
2008年 | 4619篇 |
2007年 | 4954篇 |
2006年 | 4519篇 |
2005年 | 3896篇 |
2004年 | 3578篇 |
2003年 | 3572篇 |
2002年 | 3111篇 |
2001年 | 2659篇 |
2000年 | 2527篇 |
1999年 | 2293篇 |
1998年 | 2123篇 |
1997年 | 1808篇 |
1996年 | 1594篇 |
1995年 | 1506篇 |
1994年 | 1402篇 |
1993年 | 1196篇 |
1992年 | 1139篇 |
1991年 | 901篇 |
1990年 | 729篇 |
1989年 | 640篇 |
1988年 | 568篇 |
1987年 | 438篇 |
1986年 | 384篇 |
1985年 | 439篇 |
1984年 | 443篇 |
1983年 | 213篇 |
1982年 | 350篇 |
1981年 | 407篇 |
1980年 | 308篇 |
1979年 | 295篇 |
1978年 | 226篇 |
1977年 | 209篇 |
1976年 | 155篇 |
1974年 | 112篇 |
1973年 | 108篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
沿垂等温平板的自然对流问题的近似解析解 总被引:1,自引:0,他引:1
本文用加权残值得到了沿垂等温平板的自然对流问题的近似解析解。和已有准确解的比较表明,在平板表面附近,近似解有很好的精度,而且速度场的精度比温度场更好一些。文末还对配域法的准收敛问题作了说明。 相似文献
992.
本文应用激光多普勒测速仪对二种转盘萃取塔模型进行切向速度、轴向速度和湍流度测量。实验结果表明,在RDC是切向速度比较大,因而有较大的搅拌效果,它的内部流场比较均匀稳定,有利于传质,所以RDC是广泛应用于化学工业中的一种液-液萃取设备。 相似文献
993.
After a general review of the methods currently available for the dynamics of constrained multibody systems in the context of numerical efficiency and ability to solve the differential equations of motion in singular positions, we examine the acceleration based augmented Lagrangian formulations, and propose a new one for holonomic and non-holonomic systems that is based on the canonical equations of Hamilton. This new one proves to be more stable and accurate that the acceleration based counterpart under repetitive singular positions. The proposed algorithms are numerically efficient, can use standard conditionally stable numerical integrators and do not fail in singular positions, as the classical formulations do. The reason for the numerical efficiency and better behavior under singularities relies on the fact that the leading matrix of the resultant system of ODEs is sparse, symmetric, positive definite, and its rank is independent of that of the Jacobian of the constraint equations. The latter fact makes the proposed method particularly suitable for singular configurations. 相似文献
994.
I compare application of the method of multiple scales with reconstitution and the generalized method of averaging for determining higher-order approximations of three single-degree-of-freedom systems and a two-degree-of-freedom system. Three implementations of the method of multiple scales are considered, namely, application of the method to the system equations expressed as second-order equations, as first-order equations, and in complex-variable form. I show that all of these methods produce the same modulation equations.I address the problem of determining higher-order approximate solutions of the Duffing equation in the case of primary resonance. I show that the conclusions of Rahman and Burton that the method of multiple scales, the generalized method of averaging, and Lie series and transforms might lead to incorrect results, in that spurious solutions occur and the obtained frequency–response curves bear little resemblance to the actual response, is the result of their using parameter values for which the neglected terms are the same order as the retained terms. I show also that spurious solutions cannot be avoided, in general, in any consistent expansion and their presence does not constitute a limitation of the methods. In particular, I show that, for the Duffing equation, the second-order frequency–response equation does not possess spurious solutions for the case of hardening nonlinearity, but possesses spurious solutions for the case of softening nonlinearity. For sufficiently small nonlinearity, the spurious solutions are far removed from the actual response. But as the strength of the nonlinearity increases, these solutions move closer to the backbone and eventually distort it. This is not a drawback of the perturbation methods but an indication of an application of the analysis for parameter values outside the range of validity of the expansion.Also, I address the problem of obtaining non-Hamiltonian modulation equations in the application of the method of multiple scales to multi-degree-of-freedom Hamiltonian systems written as second-order equations in time and how this problem can be overcome by attacking the state-space form of the governing equations. Moreover, I show that application of a variation of the method of Rahman and Burton to multi-degree-of-freedom systems leads to results that do not agree with those obtained with the generalized method of averaging.Contributed by Prof. R.A. Ibrahim. 相似文献
995.
996.
997.
E. I. Romenskii 《Journal of Applied Mechanics and Technical Physics》2007,48(3):437-444
Constitutive equations that describe the experimentally observed failure waves are proposed to model inelastic strains of
brittle materials. The complete system of equations is hyperbolic, each equation of this system has divergent form. The model
is based on the assumption that continual failure is the process of transition from an intact state to a “fully damaged” state
described by the kinetics of the order parameter. The structure of stationary traveling compressive waves is analyzed using
a simplified model. It is shown that in a certain range of amplitudes, the wave splits into an elastic precursor and a failure
wave.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 3, pp. 164–172, May–June, 2007. 相似文献
998.
The main aim of this work is to develop a consistent formulation of the rheological behavior for different anisotropic polymer
systems. The unified theory of anisotropic viscoelasticity is developed based on the symmetry principles. The Maxwell rheological
equation is extended to nonsymmetric anisotropic liquids. Transitions from the most general anisotropy to particular cases
of anisotropy are established. It appears that the coupled relaxation of symmetric and antisymmetric stresses is a natural
phenomenon in nonsymmetric viscoelasticity. Within the concept of an internal state variable, a stress–order relation is derived
for a fully nonlinear case. The order tensor dynamics is also considered. A simple method of deriving the equation of the
internal rotational motion is developed for the general macroscopic anisotropy.
This paper was presented at the 3rd Annual Rheology Conference, AERC 2006, April 27–29, 2006, Crete, Greece 相似文献
999.
In this paper a rigorous dual-porosity model is formulated, which accurately represents the coupling between large-scale fractures
and the micropores within dual porosity media. The overall structure of the porous medium is conceptualized as being blocks
of diffusion dominated micropores separated by natural fractures (e.g. cleats for coal) through which Darcy’s flow occurs.
In the developed model, diffusion in the matrix blocks is fully coupled to the pressure distribution within the fracture system.
Specific assumptions on the pressure behaviour at the matrix boundary, such as step-time function employed in some earlier
studies, are not invoked. The model involves introducing an analytical solution for diffusion within a matrix block, and the
resultant combined flow equation is a nonlinear integro-(partial) differential equation. Analyses to the equation in this
text, in addition to the theoretical development of the proposed model, include: (1) discussion on the “fading memory” of
the model; (2); one-dimensional perturbation solution subject to a specific condition; and (3) asymptotic analyses of the
“long-time” and “short-time” responses of the flow. Two previous models, the Warren-Root and the modified Vermeulen models,
are compared with the proposed model. The advantages of the new model are demonstrated, particularly for early time prediction
where the approximations of these other models can lead to significant error. 相似文献
1000.
方形管内楔形障碍物对火焰结构与传播的影响 总被引:1,自引:0,他引:1
通过实验与数值模拟方法对CH4/空气预混火焰在有楔形障碍物的卧式燃烧方管内的传播进行了研究。采用多镜头Cranz Schardin高速摄像机和压力传感器等实验设备获得了高清晰度的障碍物诱导火焰失稳的分幅时序照片以及障碍物背风表面压力变化曲线。数值模拟则基于RANS方法与EDU-Arrhenius燃烧模型,计算结果与实验结果基本相符,反映了火焰在管内传播与变形的详细过程。通过综合分析实验与计算结果,得到了由楔形障碍物导致的火焰加速与变形的内在机理,揭示了火焰传播过程中由层流燃烧向湍流燃烧转捩的本质。 相似文献