全文获取类型
收费全文 | 52583篇 |
免费 | 12898篇 |
国内免费 | 18771篇 |
专业分类
测绘学 | 9862篇 |
大气科学 | 7073篇 |
地球物理 | 9736篇 |
地质学 | 34260篇 |
海洋学 | 9788篇 |
天文学 | 874篇 |
综合类 | 4510篇 |
自然地理 | 8149篇 |
出版年
2024年 | 418篇 |
2023年 | 1211篇 |
2022年 | 3281篇 |
2021年 | 3994篇 |
2020年 | 3169篇 |
2019年 | 3834篇 |
2018年 | 3287篇 |
2017年 | 3133篇 |
2016年 | 3037篇 |
2015年 | 3788篇 |
2014年 | 3563篇 |
2013年 | 4494篇 |
2012年 | 4810篇 |
2011年 | 4841篇 |
2010年 | 4760篇 |
2009年 | 4392篇 |
2008年 | 4485篇 |
2007年 | 4257篇 |
2006年 | 3999篇 |
2005年 | 3379篇 |
2004年 | 2587篇 |
2003年 | 1869篇 |
2002年 | 1894篇 |
2001年 | 1734篇 |
2000年 | 1421篇 |
1999年 | 689篇 |
1998年 | 327篇 |
1997年 | 288篇 |
1996年 | 188篇 |
1995年 | 155篇 |
1994年 | 111篇 |
1993年 | 102篇 |
1992年 | 123篇 |
1991年 | 53篇 |
1990年 | 83篇 |
1989年 | 47篇 |
1988年 | 51篇 |
1987年 | 41篇 |
1986年 | 51篇 |
1985年 | 43篇 |
1984年 | 41篇 |
1983年 | 24篇 |
1982年 | 23篇 |
1981年 | 17篇 |
1980年 | 10篇 |
1979年 | 30篇 |
1978年 | 11篇 |
1977年 | 10篇 |
1957年 | 20篇 |
1954年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Near Wall Flow over Urban-like Roughness 总被引:3,自引:35,他引:3
In this study, comprehensive measurements over a number of urban-type surfaces with the same area density of 25% have been performed in a wind tunnel. The experiments were conducted at a free stream velocity of 10 m s-1 and the main instrumentation was 120 ° x-wire anemometry, but measurement accuracy was checked using laser Doppler anemometry.The results haveconfirmed the strong three-dimensionalityof the turbulent flow inthe roughness sublayer and the depths of the inertial sublayer (log-law region) and roughness sublayer for each surface have been determined. Spatial averaging has been used to remove the variability of the flow in the roughness sublayer due to individual obstacles and it is shown that the spatially averaged mean velocity in the inertial sublayer and roughness sublayer can,together, be described by a single log-law with a mean zero-plane displacement and roughness length for the surface, provided that the proper surface stress is known. The spatially averaged shear stresses in the inertial sublayer and roughness sublayer are compared with the surface stress deduced from form drag measurements on the roughness elements themselves.The dispersive stress arising from the spatial inhomogeneity in the mean flow profiles was deduced from the data and is shown to be negligible compared with the usual Reynolds stresses in the roughness sublayer. Comparisons have been made between a homogeneous (regular element array) surface and one consisting of random height elements of the same total volume. Although the upper limits of the inertial sublayer for both surfaces were almost identical at equivalent fetch, the roughness sublayer was much thicker for the random surface than for the uniform surface, the friction velocity and the roughness length were significantly larger and the `roughness efficiency' was greater. It is argued that the inertial sublayer may not exist at all in some of the more extreme rough urban areas. These results will provide fundamental information for modelling urban air quality and forecasting urban wind climates. 相似文献
962.
简述了GPS的组成及发展概况,以及如何利用地面GPS观测探测大气可降水汽量(PWV),从而可以得到高精度,几乎连续和实时的水汽序列,骤报降水量以及监测恶劣天气,为天气预报和气候分析作出贡献。 相似文献
963.
应用统计检验和回归分析两种数理统计方法,对吐鲁番1997-2000年山区人工增水作业效果进行了分析。结果得出作业影响区内作业年份5-6月平均流量显著增加。 相似文献
964.
模式所需要的参数被合理地设置之后, 根据GAME/ Tibet (GEWEX亚洲季风试验/青藏高原试验) 那曲近地层观测站的资料, 将大气强迫变量代入SiB2(Simple Biosphere model version2), 文章模拟了该观测站地表能量收支。结果表明:SiB2能够较好地模拟青藏高原的能量收支情况, 净辐射、潜热通量和土壤热通量的模拟值和观测值吻合, 它们的相对误差分别为8% (低估)、6% (低估) 和3 %(低估)。同时, SiB2高估感热通量达40%。文章还给出了能量各分量的详细比较分析。 相似文献
965.
An experimental study of the initial flow field downstream of a step change in surface roughness is presented. The roughness length of the downstream surface was approximately tenfold that of the upstream roughness and, unlike all previous studies, attention was concentrated on the roughness sublayer region beneath the inertial (log-law) region. The experiments were conducted at a boundary layer Reynolds number of about 6 × 104 (based on layer thickness andfree-stream velocity) and around a longitudinal location where the (downstream) roughness length, zo2, was about 1% of the boundary-layer thickness atthe roughness change point.The thickness of the roughness sublayer was found for the two roughness. It was observed that the vertical profiles of mean velocity and turbulence characteristics started to show similarity after about 160z02 downstream of the roughness change. The presence of a shear stress overshoot is shown to depend strongly on the precise location (with respect to the roughness elements) at which the measurements are made and the thickness of the equilibrium layer is shown to be very sensitive to the way it is defined. It is demonstrated that the growing equilibrium layer has first to encompass the roughness sublayer before any thickness of inertial sublayer can be developed. It follows that, in somepractical cases, like flows across some urban environments, the latter(log-law) region may never exist at all. 相似文献
966.
In the context of tower measured radiation datasets.following the correction principle meetinga diagnostic equation in data quality control and in terms of a technique for model construction ondata and ANN(artificial neural network)retrieval for BP correction of radiation measurementswith rough errors available,a BP model is presented.Evidence suggests that the developed modelworks well and is superior to a convenient multivariate linear regression model,indicating its wideapplications. 相似文献
967.
地震作用下的灰坝液化特征及其动力稳定性分析--以安阳电厂为例 总被引:1,自引:0,他引:1
根据安阳电厂灰坝岩土工程性质及其动力学特性,建立了适合灰坝工程的动力分析模型并采用有限元法进行了动力分析.在此基础上,在不同工况条件下对灰坝进行了抗液化安全评价及抗震稳定性分析.研究表明,在不设碎石桩、无排渗体条件下,粉煤灰子坝的抗液化安全系数Ks<1.25,将发生液化;在设碎石桩、有排渗体条件下,粉煤灰子坝的抗液化安全系数明显提高,Ks≥1.25,不会发生液化.抗震稳定性分析表明,在上述两种工况条件下灰坝是稳定的. 相似文献
968.
969.
大别造山带地壳泊松比结构与超高压变质带—来自宽角反射与近垂直反射剖面的启示 总被引:3,自引:0,他引:3
综合宽角反射、近垂直反射的探测结果和有关地质资料,对大别造山带地壳结构和超高压变质带研究显示:大别造山带地壳具有层块结构特征。沿安义-庄墓剖面,上地壳有7个弹性块体,中地壳有5个,下地壳有4个。扬子与华北地块的主缝合带是超高压变质带,扬子地块与大别造山带的现今分界线是与郯庐断裂相交的太湖-马庙断裂,磨子潭-晓天断裂是大别造山带的北界,北淮阳构造带呈楔状向下插入,它与华北地块的分界是肥中断裂。郯庐断裂在中、上地壳近于直立,下地壳向西倾斜。超高压变质带的厚度为5-7km,产状向北插入到北大别块体之下,折返过程是构造就位,不是大别山的均匀抬升,折返的主运动面是水吼-五河高温剪切带。 相似文献