首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   668篇
  免费   108篇
  国内免费   95篇
测绘学   8篇
大气科学   251篇
地球物理   131篇
地质学   64篇
海洋学   133篇
天文学   258篇
综合类   13篇
自然地理   13篇
  2024年   8篇
  2023年   10篇
  2022年   11篇
  2021年   9篇
  2020年   13篇
  2019年   29篇
  2018年   12篇
  2017年   16篇
  2016年   12篇
  2015年   15篇
  2014年   21篇
  2013年   41篇
  2012年   23篇
  2011年   25篇
  2010年   25篇
  2009年   54篇
  2008年   55篇
  2007年   57篇
  2006年   52篇
  2005年   40篇
  2004年   42篇
  2003年   35篇
  2002年   28篇
  2001年   35篇
  2000年   30篇
  1999年   36篇
  1998年   30篇
  1997年   17篇
  1996年   15篇
  1995年   8篇
  1994年   26篇
  1993年   7篇
  1992年   8篇
  1991年   5篇
  1990年   4篇
  1989年   7篇
  1988年   1篇
  1987年   3篇
  1985年   1篇
  1983年   2篇
  1982年   1篇
  1981年   1篇
  1954年   1篇
排序方式: 共有871条查询结果,搜索用时 15 毫秒
1.
This technical communication presents some MATLAB® routines for visualizing the structure in turbulent signals based on a standard conditional averaging and thresholding approach. Up to three velocity components may be analysed and the resulting output highlights the time at which important flow events occur as well as the frequency levels that contribute the greatest energy to this particular event. The latter information is derived from a wavelet decomposition of the signal and may assist in providing a process‐based explanation of observed flow features. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   
2.
3.
Within a wave-exposed mangrove forest, novel field observations are presented, comparing millimeter-scale turbulent water velocity fluctuations with contemporaneous subtidal bed elevation changes. High-resolution velocity and bed level measurements were collected from the unvegetated mudflat, at the mangrove forest fringe, and within the forest interior over multiple tidal cycles (flood–ebb) during a 2-week period. Measurements demonstrated that the spatial variability in vegetation density is a control on sediment transport at sub-meter scales. Scour around single and dense clusters of pneumatophores was predicted by a standard hydraulic engineering equation for wave-induced scour around regular cylinders, when the cylinder diameter in the equations was replaced with the representative diameter of the dense pneumatophore clusters. Waves were dissipated as they propagated into the forest, but dissipation at infragravity periods (> 30 s) was observed to be less than dissipation at shorter periods (< 30 s), consistent with the predictions of a simple model. Cross-wavelet analysis revealed that infragravity-frequency fluctuations in the bed level were occasionally coherent with velocity, possibly indicating scour upstream of dense pneumatophore patches when infragravity waves reinforced tidal currents. Consequently, infragravity waves were a likely driver of sediment transport within the mangrove forest. Near-bed turbulent kinetic energy, estimated from the turbulent dissipation rate, was also correlated with bed level changes. Specifically, within the mangrove forest and over the unvegetated mudflat, high-energy events were associated with erosion or near-zero bed level change, whereas low-energy events were associated with accretion. In contrast, no single relationship between bed level changes and mean current velocity was applicable across both vegetated and unvegetated regions. These observations support the theory that sediment mobilization scales with turbulent energy, rather than mean velocity, a distinction that becomes important when vegetation controls the development of turbulence.  相似文献   
4.
A Hopf bifurcation describes a transition from a steady state to an oscillatory one. This report describes how the presence of white Gaussian noise in the term driving the instability (the gain) affects the character of such a transition. We find that for small noise intensities the effective critical value of the bifurcation parameter increases linearly with the noise intensity. This result is used to quantitatively describe the onset of the magneto‐rotational instability in a laboratory experiment using liquid metal with turbulent background flow. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
5.
Using three methods from nonlinear dynamics, we contrast the level of organization inthe vertical wind velocity (w) time series collected in the atmospheric surface layer(ASL) and the canopy sublayer (CSL) for a wide range of atmospheric stability ()conditions. The nonlinear methods applied include a modified Shannon entropy, waveletthresholding, and mutual information content. Time series measurements collected overa pine forest, a hardwood forest, a grass-covered forest clearing, and a bare soil, desertsurface were used for this purpose. The results from applying all three nonlinear timeseries measures suggest that w in the CSL is more organized than that in the ASL, and that as the flows in both layers evolve from near-neutral to near-convective conditions, the level of organization increases. Furthermore, we found that the degree of organization in w associated with changes in is more significant than the transition from CSL to ASL.  相似文献   
6.
利用小波变换(WT)对香港天文台飞机观测台风“妮妲”(1604)资料进行分析,研究在不稳定、不均匀的台风边界层中湍流涡旋的垂直传输作用。在0.1~5 Hz惯性子区内横风和顺风分量功率谱密度能较好符合-5/3幂律。小波分析显示:横风的小波功率谱峰值集中在1 km之下,顺风分量的小波功率谱峰值集中在1~6 km之间;眼区动量通量的主要贡献尺度为2.3 km,眼区外主要贡献尺度在1~2 km,中低层为较小尺度(< 1.0 km);湍流功能(TKE)的生成尺度主要集中在4 km之下。这项研究定量描述了南海北部台风边界层各个区域湍流结构的差异特征,讨论了对台风边界层通量参数化的可能影响。  相似文献   
7.
8.
中国西北干旱区戈壁下垫面夏季的热力输送   总被引:5,自引:1,他引:5  
以敦煌戈壁站2004年6月和2008年8月的常规观测和超声观测为例,分析了西北干旱区戈壁下垫面夏季热力输送的一般过程及特征。首先评价了湍流通量的观测质量以及仪器观测的地表能量通量闭合问题,结果表明敦煌戈壁站的观测在白天总体较好。夏季地表能量通量的平均日变化显示,潜热通量整天都很小,可以忽略,白天到达地表的短波辐射以及地表向上的长波辐射非常强,地表净辐射主要转化为感热输送(敦煌戈壁站在中午时平均分别达380W·m-2以上和250W·m-2以上);夜间土壤释放热量以平衡地表的辐射冷却,感热通量略低于0。白天时地表大气经常触发自由对流活动,影响动量通量的观测质量,并有效输送地表热力至上层大气中,有助于形成超厚大气边界层。分析了戈壁下垫面的动量粗糙度特征和热力粗糙度特征(敦煌戈壁站动量粗糙度约为0.6mm),热力粗糙度基本小于动量粗糙度一个量级,这符合目前对干旱区戈壁下垫面热力输送特征的初步认识。  相似文献   
9.
The angular momentum transport in rotating turbulent convection is simulated with the NIRVANA code for Taylor numbers up to 106. The box consists of an unstable layer embedded in two stable overshoot layers. We find the expected anisotropies in the rotating anisotropic turbulence field: 〈u′2r〉 exceeds 〈u′2ϕ〉, and 〈u′2ϕ〉 exceeds 〈u′2θ〉. The resulting radial angular momentum transport is directed inwards and peaks in the middle of the convective layer. The resulting latitudinal angular momentum transport is equatorwards, peaks at the surface and is highly concentrated to the equatorial region. Apart from a factor of 2–3 the total amplitudes of the cross‐correlations are of the same order of magnitude. In the lower overshoot region (‘tachocline’) the cross‐correlations are negative. It is argued that the concentration of the latitudinal angular momentum transport towards the surface and towards the equator does not too strongly reduce its potential to produce rotation laws with accelerated equators. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   
10.
Pebble clusters are common small‐scale morphological features in gravel‐bed rivers, occupying as much as 10 per cent of the bed surface. Important links exist between the presence of pebble clusters and the development of flow structures. These links are poorly understood at the three‐dimensional level. Particularly neglected has been the effect of clusters on the lateral flow characteristics. A laboratory study was conducted using a hydraulic flume, within which simulated pebble clusters were superimposed onto a plane bed of gravel material. High‐resolution three‐dimensional flow data were collected above the bed at two different flow depths using an acoustic Doppler velocimeter. The results present evidence of the importance of lateral flow in the development of turbulent flow structure. Narrow regions of high lateral and downstream turbulence intensity exist to both sides of clusters and in a three‐dimensional separation zone in their lee. This may indicate the presence of horseshoe‐type vortical structures analogous to those identified in less hydraulically rough environments. However, it is likely that these structures are more complicated given the mutual interference of the surrounding medium. The lateral flow was also identified as a key component in the upwelling identified by other authors in the lee of pebble clusters. The results of the vertical flow analysis confirm the hypothesis that six regions with distinct vertical flow characteristics exist above clusters: flow acceleration up the stoss‐side of the cluster; recirculation behind the cluster in the wake region; vortex shedding from the pebble crest and shear layer; flow reattachment downstream of the cluster; upwelling of flow downstream of the point of reattachment; and recovery of flow. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号