全文获取类型
收费全文 | 97508篇 |
免费 | 30650篇 |
国内免费 | 56824篇 |
专业分类
测绘学 | 15754篇 |
大气科学 | 27089篇 |
地球物理 | 22990篇 |
地质学 | 69761篇 |
海洋学 | 25481篇 |
天文学 | 1612篇 |
综合类 | 9412篇 |
自然地理 | 12883篇 |
出版年
2024年 | 711篇 |
2023年 | 1807篇 |
2022年 | 4628篇 |
2021年 | 5840篇 |
2020年 | 5415篇 |
2019年 | 8501篇 |
2018年 | 7989篇 |
2017年 | 7463篇 |
2016年 | 7553篇 |
2015年 | 8172篇 |
2014年 | 7597篇 |
2013年 | 9319篇 |
2012年 | 9822篇 |
2011年 | 9958篇 |
2010年 | 9950篇 |
2009年 | 8991篇 |
2008年 | 8596篇 |
2007年 | 8299篇 |
2006年 | 7763篇 |
2005年 | 6703篇 |
2004年 | 5681篇 |
2003年 | 4588篇 |
2002年 | 4428篇 |
2001年 | 3993篇 |
2000年 | 3341篇 |
1999年 | 2334篇 |
1998年 | 1878篇 |
1997年 | 1935篇 |
1996年 | 1490篇 |
1995年 | 1372篇 |
1994年 | 1221篇 |
1993年 | 1202篇 |
1992年 | 1052篇 |
1991年 | 759篇 |
1990年 | 748篇 |
1989年 | 597篇 |
1988年 | 505篇 |
1987年 | 405篇 |
1986年 | 333篇 |
1985年 | 263篇 |
1984年 | 292篇 |
1983年 | 171篇 |
1982年 | 218篇 |
1981年 | 154篇 |
1980年 | 112篇 |
1979年 | 145篇 |
1978年 | 53篇 |
1977年 | 48篇 |
1971年 | 53篇 |
1954年 | 53篇 |
排序方式: 共有10000条查询结果,搜索用时 744 毫秒
391.
The numerical mode of nonlinear wave transformation based on both the Laplace equation for water field and the Bemoulli equation for water surface is a kind of time-domain boundary problem with initial conditions. And the basis for establishing the numerical mode of nonlinear wave in time domain is to trace the position of wave free surface and to calculale the instantaneous surface height and surface potential function. This paper firstly utilizes the ‘0-1‘ combined BEM to separate the boundary by means of discretization of Green‘ s integral equation based on the Laplace equation, then separates the free surface of wave with FEM and derives the FEM equation of wave surface that satisfies the nonlinear boundary conditions. By jointly solving the above BEM and FEM equations, the wave potential and surface height could be obtained with iteration in time domain. Thus a new kind of nonlinear numerical mode is established for calculating wave transformation. The wave test in the numerical wave tank shows that the numerical simulation with this mode is of high accuracy. 相似文献
392.
我国的渤海和黄海北部在每年冬季都出现不同程度的冰情,它直接影响结冰海区的石油平台、船舶以及港口等设施的正常作业,对海冰的观测与预报随着上述海区的开发利用越发显得重要,利用航海雷达连续准确地跟踪海冰漂移运动轨迹,是当今观测、调查、研究冰漂流移动规律的有效方法之一,我国的科研人员利用雷达成像技术对渤海的冰情进行了长期的雷达海冰观测、研究,在由雷达海冰图像对海冰的物理特征的识别、分类以及冰漂流场测量方面取得了一些进展[1-4]。 相似文献
393.
394.
395.
Water vapor budget of the Indian monsoon depression 总被引:2,自引:0,他引:2
396.
Fluxes and sources of suspended organic matter in an estuarine turbidity maximum region during low discharge conditions 总被引:6,自引:1,他引:6
Miguel A. Goni Mary W. Cathey Yong H. Kim George Voulgaris 《Estuarine, Coastal and Shelf Science》2005,63(4):683-700
Water column concentrations of total suspended solids (TSS), particulate organic carbon (POC) and particulate nitrogen (PN) were measured at three different depths in four different locations bracketing the estuarine turbidity maximum (ETM) along the main channel of a temperate riverine estuary (Winyah Bay, South Carolina, USA). Measurements were carried out over full tidal cycle (over 24 h). Salinity, temperature, current magnitude and direction were also monitored at the same time throughout the water column. Tidally averaged net fluxes of salt, TSS, POC and PN were calculated by combining the current measurements with the concentration data. Under the extreme low river discharge conditions that characterized the study period, net landward fluxes of salt were measured in the lower part of the study area, suggesting that the landward transport through the main channel of the estuary was probably balanced by export out through the sides. In contrast, the net fluxes of salt in the upper reaches of the study area were near zero, indicating a closed salt balance in this part of the estuary. In contrast to salt, the net fluxes of TSS, POC and PN in the deeper parts of the water column were consistently landward at all four sites in Winyah Bay indicating the non-conservative behavior of particulate components and their active transport up the estuary in the region around the ETM.The carbon contents (%POC), carbon:nitrogen ratios (org[C:N]a) and stable carbon isotopic compositions (δ13CPOC) of the suspended particles varied significantly with depth, location and tidal stage. Tidally averaged compositions showed a significant increase up the estuary in the %POC and org[C:N]a values of suspended particles consistent with the preferential landward transport of carbon-rich particles with higher vascular plant debris content. The combination of tidal resuspension and flood-dominated flow appeared to be responsible for the hydrodynamic sorting of particles along the estuary that resulted in denser, organic-poor particles being transported landward less efficiently. The elemental and isotopic compositions indicated that vascular C3 plants and estuarine algae were the major sources of the particulate organic matter of all the samples, without any significant contributions from salt marsh C4 vegetation (Spartina alterniflora) and/or marine phytoplankton. 相似文献
397.
海水中234Th的超低水平液闪谱仪测定 总被引:2,自引:1,他引:2
本文提出了利用超低水平液闪谱仪测定海水中234Th的方法.海水经氢氧化铁吸附共沉淀富集后,接着用阴离子交换和TBP/煤油萃取进行Th同位素的分离与纯化.对一系列测定条件进行了详细的研究,提出了测定海水中234Th的适宜程序,即在含有234Th和产额示踪剂230Th的纯化后的5mol/dm3HNO3溶液中加入TBP/煤油进行萃取,然后用0.1mol/dm3HNO3反萃取,后者是先用契伦柯夫计数法测量234Th(通过234mPa),后加入闪烁液Hisafe 3用α/β模式测量α放射体230Th.对于α和β放射体液闪谱仪的计数效率分别为100%和55.7%±2.7%.234Th的化学回收率和总探测效率分别为70%~80%和30%~45%.该法测定海水中的234Th快速、简便和高效. 相似文献
398.
399.
Lock-release gravity currents with a viscous self-similar regime are simulated by use of the renormalization group(RNG) k - ε model for Reynolds-stress closure. Besides the turbulent regime with initially a slumping phase of a conslant current front speed and later an inviseid self-similar phase of front speed decreasing as t^-1/3(where t is the time measured from release), the viseous self-similar regime is satisfactorily reproduced with front speed decreasing as t^-4/5,consistent with well known experimental observations. 相似文献
400.
A Forced System of Two Cylinders with Various Spacings 总被引:3,自引:0,他引:3
The spectrum characteristics and wake structures for a circular cylinder oscillating in a wake are investigated by use of the currently modified virtual boundary method. A forced system of two cylinders with a small spacing ( the downstream one is made to oscillate in the transverse direction) is studied and interesting flow characteristics are observed. A vortex switch and the change of vortex modes (between 2S mode and 2P mode) are observed in the “lock-in“ region. Vortex bands are formed and lost with the increasing excitation frequency. Information concerning saddle points in the flow field is obtained for different excitation frequencies. For a forced system of two cylinders with a large spacing, the upstream cylinder sheds vortexes because there is no downstream cylinder oscillating in the wake. No distinct “lock-in“ response is found for the downstream cylinder. 相似文献