全文获取类型
收费全文 | 3962篇 |
免费 | 427篇 |
国内免费 | 267篇 |
专业分类
化学 | 332篇 |
晶体学 | 25篇 |
力学 | 2242篇 |
综合类 | 32篇 |
数学 | 466篇 |
物理学 | 1559篇 |
出版年
2024年 | 7篇 |
2023年 | 43篇 |
2022年 | 87篇 |
2021年 | 92篇 |
2020年 | 136篇 |
2019年 | 96篇 |
2018年 | 90篇 |
2017年 | 154篇 |
2016年 | 167篇 |
2015年 | 142篇 |
2014年 | 197篇 |
2013年 | 208篇 |
2012年 | 181篇 |
2011年 | 234篇 |
2010年 | 168篇 |
2009年 | 195篇 |
2008年 | 176篇 |
2007年 | 222篇 |
2006年 | 225篇 |
2005年 | 207篇 |
2004年 | 206篇 |
2003年 | 202篇 |
2002年 | 137篇 |
2001年 | 132篇 |
2000年 | 124篇 |
1999年 | 114篇 |
1998年 | 128篇 |
1997年 | 73篇 |
1996年 | 77篇 |
1995年 | 55篇 |
1994年 | 62篇 |
1993年 | 49篇 |
1992年 | 45篇 |
1991年 | 51篇 |
1990年 | 36篇 |
1989年 | 30篇 |
1988年 | 26篇 |
1987年 | 8篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 3篇 |
1983年 | 2篇 |
1982年 | 13篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1971年 | 4篇 |
1957年 | 9篇 |
排序方式: 共有4656条查询结果,搜索用时 46 毫秒
151.
This communication brings forward the concept of an effective drag for circular cylinders undergoing controlled or self-excited oscillation transverse to a free stream. A relationship between the effective drag and the apparent drag customarily measured by force transducers, is derived on the basis of the average rate of energy dissipation in the fluid. The effective drag is employed to gain insight into the fluid dynamics of vortex-induced vibrations using available data from the published literature. It is shown that the effective drag varies almost continuously as a function of the true reduced velocity except for a sudden decrease near conditions of maximum amplitude. The variation displays similarities but also important differences among independent experiments, which highlight the importance of various influencing parameters whose effect on the free response is currently not well understood. Some evidence is found that self-excited oscillations occur at points in the frequency–amplitude space where the effective drag is comparatively low avoiding the region where energy transfer from the fluid to the structure is most efficient. A total energy balance is employed to reveal the effect of mechanical damping on the free response which indicates that the amplitude scales with the inverse of the square root of damping in conformity with experimental data. The dimensionless damping parameters proposed by Vandiver are examined in light of the present analysis, and alternative empirical formulas are suggested for organizing the free vibration response. 相似文献
152.
Image restoration is a fundamental problem in image processing. Blind image restoration has a great value in its practical application. However, it is not an easy problem to solve due to its complexity and difficulty. In this paper, we combine our robust algorithm for known blur operator with an alternating minimization implicit iterative scheme to deal with blind deconvolution problem, recover the image and identify the point spread function(PSF). The only assumption needed is satisfy the practical physical sense. Numerical experiments demonstrate that this minimization algorithm is efficient and robust over a wide range of PSF and have almost the same results compared with known PSF algorithm. 相似文献
153.
A four-node discrete singular convolution (DSC) method is developed for free vibration analysis of arbitrary straight-sided quadrilateral plates. The straight-sided quadrilateral domain is mapped into a square domain in the computational space using a four-node element. By using the geometric transformation, the governing equations and boundary conditions of the plate are transformed from the physical domain into a square computational domain. Numerical examples illustrating the accuracy and convergence of the DSC method for skew, trapezoidal, rhombic and arbitrary quadrilateral plates are presented. The results obtained by DSC method were compared with those obtained by the other numerical methods. 相似文献
154.
155.
156.
Dynamics of a Cubic Nonlinear Vibration Absorber 总被引:1,自引:0,他引:1
We study the dynamics of a nonlinear active vibration absorber. We consider a plant model possessing curvature and inertia nonlinearities and introduce a second-order absorber that is coupled with the plant through user-defined cubic nonlinearities. When the plant is excited at primary resonance and the absorber frequency is approximately equal to the plant natural frequency, we show the existence of a saturation phenomenon. As the forcing amplitude is increased beyond a certain threshold, the response amplitude of the directly excited mode (plant) remains constant, while the response amplitude of the indirectly excited mode (absorber) increases. We obtain an approximate solution to the governing equations using the method of multiple scales and show that the system possesses two possible saturation values. Using numerical techniques, we perform stability analyses and demonstrate that the system exhibits complicated dynamics, such as Hopf bifurcations, intermittency, and chaotic responses. 相似文献
157.
The free vibration of a functionally graded material hollow sphere submerged in a compressible fluid medium is exactly analyzed.
The sphere is assumed to be spherically isotropic with material constants being inhomogeneous along the radial direction.
By employing a separation technique as well as the spherical harmonics expansion method, the governing equations are simplified
to an uncoupled second-order ordinary differential equation, and a coupled system of two such equations. Solutions to these
equations are given when the elastic constants and the mass density are power functions of the radial coordinate. Numerical
examples are finally given to show the effect of the material gradient on the natural frequencies.
The project is supported by the National Natural Sciences Foundation of China(No. 19872060). 相似文献
158.
M. Rahman 《Applied Mathematical Modelling》1986,10(6):401-408
This paper considers the wave loads on large monolithic offshore structures. The second order wave force formulae developed by Rahman and Heaps, applicable to large circular cylinders in waves, are extended to evaluate the overturning moments on large circular cylinders. The theory is then applied to square section caissons in waves, to predict the wave loads on these structures. These calculations are performed using the exact form of the second order velocity potential, φ2, with arbitrary wave number, k2, and the approximate form of φ2, with twice the value of the wave number of the first order velocity potential. The second order analytical predictions are compared with available experimental data for various ranges of wave parameters for both circular and square caissons in large amplitude waves. 相似文献
159.
Linas Vepštas 《Numerical Algorithms》2008,47(3):211-252
This paper sketches a technique for improving the rate of convergence of a general oscillatory sequence, and then applies
this series acceleration algorithm to the polylogarithm and the Hurwitz zeta function. As such, it may be taken as an extension
of the techniques given by Borwein’s “An efficient algorithm for computing the Riemann zeta function” by Borwein for computing
the Riemann zeta function, to more general series. The algorithm provides a rapid means of evaluating Li
s
(z) for general values of complex s and a kidney-shaped region of complex z values given by ∣z
2/(z–1)∣<4. By using the duplication formula and the inversion formula, the range of convergence for the polylogarithm may be
extended to the entire complex z-plane, and so the algorithms described here allow for the evaluation of the polylogarithm for all complex s and z values. Alternatively, the Hurwitz zeta can be very rapidly evaluated by means of an Euler–Maclaurin series. The polylogarithm
and the Hurwitz zeta are related, in that two evaluations of the one can be used to obtain a value of the other; thus, either
algorithm can be used to evaluate either function. The Euler–Maclaurin series is a clear performance winner for the Hurwitz
zeta, while the Borwein algorithm is superior for evaluating the polylogarithm in the kidney-shaped region. Both algorithms
are superior to the simple Taylor’s series or direct summation. The primary, concrete result of this paper is an algorithm
allows the exploration of the Hurwitz zeta in the critical strip, where fast algorithms are otherwise unavailable. A discussion
of the monodromy group of the polylogarithm is included.
相似文献
160.
As a first endeavor, a mixed differential quadrature (DQ) and finite element (FE) method for boundary value structural problems in the context of free vibration and buckling analysis of thick beams supported on two-parameter elastic foundations is presented. The formulations are based on the two-dimensional theory of elasticity. The problem domain along axial direction is discretized using finite elements. The resulting system of equations and the related boundary conditions are discretized in the thickness direction and in strong-form using DQM. The method benefits from low computational efforts of the DQ in conjunction with the effectiveness of the FE method in general geometry and systematic boundary treatment resulting in highly accurate and fast convergence behavior solution. The boundary conditions at the top and bottom surface of the beams are implemented accurately. The presented formulations provide an effective analysis tool for beams free of shear locking. Comparisons are made with results from elasticity solutions as well as higher-order beam theory. 相似文献