全文获取类型
收费全文 | 2167篇 |
免费 | 369篇 |
国内免费 | 67篇 |
专业分类
化学 | 52篇 |
晶体学 | 3篇 |
力学 | 1038篇 |
综合类 | 8篇 |
数学 | 151篇 |
物理学 | 1351篇 |
出版年
2023年 | 13篇 |
2022年 | 21篇 |
2021年 | 31篇 |
2020年 | 46篇 |
2019年 | 29篇 |
2018年 | 49篇 |
2017年 | 72篇 |
2016年 | 68篇 |
2015年 | 62篇 |
2014年 | 116篇 |
2013年 | 218篇 |
2012年 | 112篇 |
2011年 | 137篇 |
2010年 | 109篇 |
2009年 | 97篇 |
2008年 | 118篇 |
2007年 | 112篇 |
2006年 | 94篇 |
2005年 | 125篇 |
2004年 | 92篇 |
2003年 | 82篇 |
2002年 | 94篇 |
2001年 | 74篇 |
2000年 | 86篇 |
1999年 | 59篇 |
1998年 | 59篇 |
1997年 | 61篇 |
1996年 | 55篇 |
1995年 | 43篇 |
1994年 | 37篇 |
1993年 | 68篇 |
1992年 | 23篇 |
1991年 | 30篇 |
1990年 | 15篇 |
1989年 | 20篇 |
1988年 | 5篇 |
1987年 | 13篇 |
1986年 | 4篇 |
1985年 | 16篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1975年 | 1篇 |
1972年 | 1篇 |
1957年 | 4篇 |
排序方式: 共有2603条查询结果,搜索用时 312 毫秒
991.
部分相干光通过强湍流对通信系统误码率的影响 总被引:1,自引:0,他引:1
为研究部分相干光通过强湍流对系统误码率的影响,借助对激光在大气湍流场中的传输方程进行解析求解(忽略系统中其他噪声,仅考虑由大气湍流引起的系统误码率),得到不同湍流内尺度、传输激光波长和光源相干参数条件下,系统误码率和传输距离的关系.结果表明:在强湍流条件下,当发射天线数目达到一定时,随着传输距离的增加,系统误码率逐渐增大,但增大到一定程度后趋于饱和;光源相干参数越大,系统误码率越低;湍流内尺度越大,系统误码率越高;传输激光波长的变化对系统误码率无明显影响. 相似文献
992.
993.
本文首先通过理论推导,建立了计其本身质量的旋转体弹簧所满足的波动方程,运用能量法给出振动弹簧的有效弹性常数。然后导出了弹簧本身质量忽略不计的极限情况下弹簧有效弹性常数的表达式。最后运用泛函极值原理简单讨论了在固定材料以及外形下弹簧圈的疏密如何分布,使弹簧达到最大有效弹性常数的最优化模型。 相似文献
994.
We primarily deal with homogeneous isotropic turbulence and use a closure model for the von Kármán-Howarth equation to study
several geometric properties of turbulent fluid dynamics. We focus our attention on the application of Riemannian geometry
methods in turbulence. Some advantage of this approach consists in exploring the specific form of a closure model for the
von Kármán-Howarth equation that enables to equip a model manifold (a cylindrical domain in the correlation space) by a family
of inner metrics (length scales of turbulent motion) which depends on time. We show that for large Reynolds numbers (in the
limit of large Reynolds numbers) the radius of this manifold can be evaluated in terms of the second-order structure function
and the correlation distance. This model manifold presents a shrinking cylindrical domain as time evolves. This result is
derived by using a selfsimilar solution of the closure model for the von Kármán-Howarth equation under consideration. We demonstrate
that in the new variables the selfsimilar solution obtained coincides with the element of Beltrami surface (or pseudo-sphere):
a canonical surface of the constant sectional curvature equals − 1.
相似文献
995.
Based on Reynolds-averaged Navier-Stokes approach,a laminar-turbulence transition model is proposed in this study that takes into account the effects of different instability modes associated with the variations in Mach numbers of compressible boundary layer flows.The model is based on k-ω-γ three-equation eddy-viscosity concept with k representing the fluctuating kinetic energy,ωthe specific dissipation rate and the intermittency factorγ.The particular features of the model are that:1)k includes the non-tu... 相似文献
996.
A three-dimensional k-ε-Ap solid-liquid two-phase two-fluid model with the effect of vegetation is solved numerically with a finite-volume method on an adaptive grid to study water-sediment movements and bed evolution in vegetated channels. The additional drag force and additional turbulence generation due to vegetation are added to the relevant control equations for simulating the interaction between vegetation and flow. The flow structure and the bed-topography changes in a 60° partly vegetated channel be... 相似文献
997.
C. E. Ozdemir X. Yu S. Sororian L. Zhu M. Tyagi S. Haddadian 《Journal of Turbulence》2019,20(6):360-380
In wall-bounded time-periodic flows, nonlinearity, associated with higher harmonic term(s) in velocity and/or acceleration outside the boundary layer, can significantly change the wall turbulence compared with that in the linear Stokes Boundary Layer. A significant feature of a nonlinear wall-bounded turbulent time-periodic flow is the formation of a net current which has not yet been mechanistically explained. This study investigates the effects of asymmetric velocity outside the boundary layer on wall turbulence and net current formation through Direct Numerical Simulations of a smooth-walled planar channel driven by the Second-order Stokes Wave. Simulation results suggest that net current characteristics depend on whether developed turbulence is present. When turbulence is developed, asymmetric viscous length scale is found to be the primary reason of the net current whereby a vertical offset between negative and positive Reynolds shear stress profiles, associated with forward and reverse flows, respectively, is created in a cycle. After averaging over a cycle, residual Reynolds shear stress, which drives the net current, is observed to be within the offset layer. 相似文献
998.
Mitsuru Tanaka 《Journal of Turbulence》2017,18(12):1144-1179
Fully resolved simulations of homogeneous shear turbulence (HST) laden with sedimenting spherical particles of finite size have been performed to clarify the effects of gravity on the development of particle-laden turbulent shear flows. We consider turbulence in a horizontal flow subjected to vertical or horizontal shear. Numerical results show that the development of HST laden with finite-size particles are significantly altered by gravity. The effects of gravity lead to a slower increase in the Taylor-microscale Reynolds number, whose value is found to be well correlated with the average particle Reynolds number. The gravity also causes a slower increase in the turbulence kinetic energy (TKE) through the enhancement of energy dissipation. The change in the Reynolds shear stress (RSS) due to particles also significantly contributes to the relative change in TKE. In vertically sheared cases, RSS has high values between counter-rotating trailing vortices behind the particles, which causes a transient relative increase in TKE. In horizontally sheared cases, on the other hand, RSS is reduced in the wakes of particles, which contributes to a significant relative reduction in TKE. 相似文献
999.
In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k?3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k?3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k?11/5 form at large scales, to a steeper approximate k?3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k?1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k?2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k?1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux. 相似文献
1000.
Turbulence modulation model for gas–particle flow based on probability density function approach 下载免费PDF全文
The paper focuses on the turbulence modulation problem in gas–particle flow with the use of probability density function(PDF) approach. By means of the PDF method, a general statistical moment turbulence modulation model without considering the trajectory difference between two phases is derived from the Navier–Stokes equations. A new turbulence production term induced by the dispersed-phase is analyzed and considered. Furthermore, the trajectory difference between two media is taken into account. Subsequently, a new k–ε turbulence modulation model in dilute particle-laden flow is successfully set up. Then, the changes to several terms, including the turbulence production, dissipation, and diffusion terms, are well described consequently. The promoted model provides a more probable explanation for the modification of particles on the turbulence. Finally, we applied the model to simulate a gas–particle turbulence flow case in a wall jet, and found that the simulation results agree well with the experimental data. 相似文献