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   118篇
  免费   4篇
  国内免费   1篇
测绘学   4篇
大气科学   4篇
地球物理   29篇
地质学   22篇
海洋学   10篇
天文学   40篇
自然地理   14篇
  2022年   2篇
  2021年   1篇
  2020年   4篇
  2019年   4篇
  2018年   5篇
  2017年   4篇
  2016年   3篇
  2015年   1篇
  2014年   2篇
  2013年   8篇
  2012年   2篇
  2011年   6篇
  2010年   7篇
  2009年   5篇
  2008年   3篇
  2007年   10篇
  2006年   7篇
  2005年   4篇
  2004年   12篇
  2003年   4篇
  2002年   5篇
  2001年   6篇
  2000年   3篇
  1999年   2篇
  1998年   3篇
  1997年   1篇
  1992年   2篇
  1989年   1篇
  1984年   1篇
  1981年   1篇
  1977年   2篇
  1973年   1篇
  1970年   1篇
排序方式: 共有123条查询结果,搜索用时 0 毫秒
61.
62.
Solitary wave evolution over a shelf including porous damping is investigated using Volume-Averaged Reynolds Averaged Navier–Stokes equations. Porous media induced damping is determined based on empirical formulations for relevant parameters, and numerical results are compared with experimental information available in the literature. The aim of this work is to investigate the effect of wave damping on soliton disintegration and evolution along the step for both breaking and non-breaking solitary waves. The influence of several parameters such as geometrical configuration (step height and still water level), porous media properties (porosity and nominal diameter) or solitary wave characteristics (wave height) is analyzed. Numerical simulations show the porous bed induced wave damping is able to modify wave evolution along the step. Step height is observed as a relevant parameter to influence wave evolution. Depth ratio upstream and downstream of the edge appears to be the more relevant parameter in the transmission and reflection coefficients than porosity or the ratio of wave height–water depth. Porous step also modifies the fission and the solitary wave disintegration process although the number of solitons is observed to be the same in both porous and impermeable steps. In the absence of breaking, porous bed triggers a faster fission of the incident wave into a second and a third soliton, and the leading and the second soliton reduces their amplitude while propagating. This decrement is observed to increase with porosity. Moreover, the second soliton is released before on an impermeable step. Breaking process is observed to dominate over the wave dissipation at the porous bottom. Fission is first produced on a porous bed revealing a clear influence of the bottom characteristics on the soliton generation. The amplitude of the second and third solitons is very similar in both impermeable and porous steps but they evolved differently due to the effect of bed damping.  相似文献   
63.
The objective of this study was to determine if tamoxifen or an aromatase inhibitor (4-hydroxyandrostenedione; 4-OHA) affected plasma concentrations of o,p-DDT and its metabolites, o,p-DDD and o,p-DDE, in mature tilapia. Male and female tilapia were injected 6 times intraperitoneally with o,p-DDT (40 mg/kg) alone or in combination with 4-OHA (2 mg/kg) over a 12 day period. An additional group of male fish was injected with tamoxifen (5 mg/kg) plus o,p-DDT. At the end of the treatment period, plasma samples were extracted and analyzed by GC/ECD. Females injected only with o,p-DDT had significantly higher levels of o,p-DDT compared with males. Interestingly, females and males treated concomitantly with o,p-DDT and 4-OHA or tamoxifen had significantly lower concentrations of plasma o,p-DDT (about 50%) compared with fish treated with only o,p-DDT. These initial results suggest that an interaction between endocrine-active compounds occurs in vivo in tilapia and may involve alterations in metabolism of o,p-DDT.  相似文献   
64.
New light curves and photometric solutions of FP Lyn, FV CVn and V354 UMa are presented.We found that these three systems are W-subtype shallow contact binaries. In addition, it is obvious that the light curves of FP Lyn and V354 UMa are asymmetric. Therefore, a hot spot was added on the primary star of FP Lyn and a dark spot was added on the secondary star of V354 UMa. At the same time, we added a third light to the photometric solution of FP Lyn for the final result. The obtained mass ratios and fill-out factors are q = 1.153 and f = 13.4% for FP Lyn, q = 1.075 and f = 4.6% for FV CVn, and q = 3.623 and f = 10.7% for V354 UMa respectively. The investigations of orbital period for these three systems indicate that the periods are variable. FP Lyn and V354 UMa were discovered to have secularly increasing components with rates of dp/dt = 4.19 × 10~(-7) d yr~(-1) and dp/dt = 7.70 × 10~(-7) d yr~(-1) respectively,which are feasibly caused by conservative mass transfer from the less massive component to the more massive component. In addition, some variable components were discovered for FV CVn, including a rate of dp/dt =~(-1).13 × 10~(-6) d yr~(-1) accompanied by a cyclic oscillation with amplitude and period of 0.0069 d and 10.65 yr respectively. The most likely explanation for the long-term decrease is angular momentum loss.The existence of an additional star is the most plausible explanation for the periodic variation.  相似文献   
65.
66.
67.
The star formation histories of galaxies in the Sloan Digital Sky Survey   总被引:1,自引:0,他引:1  
We present the results of a moped analysis of  ∼3 × 105  galaxy spectra from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3), with a number of improvements in data, modelling and analysis compared with our previous analysis of DR1. The improvements include: modelling the galaxies with theoretical models at a higher spectral resolution of 3 Å, better calibrated data, an extended list of excluded emission lines and a wider range of dust models. We present new estimates of the cosmic star formation rate (SFR), the evolution of stellar mass density and the stellar mass function from the fossil record. In contrast to our earlier work the results show no conclusive peak in the SFR out to a redshift around 2 but continue to show conclusive evidence for 'downsizing' in the SDSS fossil record. The star formation history is now in good agreement with more traditional instantaneous measures. The galaxy stellar mass function is determined over five decades of mass, and an updated estimate of the current stellar mass density is presented. We also investigate the systematic effects of changes in the stellar population modelling, the spectral resolution, dust modelling, sky lines, spectral resolution and the change of data set. We find that the main changes in the results are due to the improvements in the calibration of the SDSS data, changes in the initial mass function and the theoretical models used.  相似文献   
68.
We use the Hipparcos colour–magnitude diagram of field stars with Tycho colours to make a new minimum age estimate for the Galactic disc. The method is based on fits to the red envelope of subgiants in the Hipparcos colour–magnitude diagram with synthetic isochrones covering the range of disc metal abundance. The colours and luminosities of the isochrones as a function of abundance are checked using new techniques involving 'red-clump' stars in the giant branch region and on the main sequence using G and K dwarfs. We derive a minimum disc age of 8 Gyr, in good agreement with other methods.  相似文献   
69.
In 1998 the EXPORT team monitored microlensing event light curves using a charge-coupled device (CCD) camera on the IAC 0.8-m telescope on Tenerife to evaluate the prospect of using northern telescopes to find microlens anomalies that reveal planets orbiting the lens stars. The high airmass and more limited time available for observations of Galactic bulge sources make a northern site less favourable for microlensing planet searches. However, there are potentially a large number of northern 1-m class telescopes that could devote a few hours per night to monitor ongoing microlensing events. Our IAC observations indicate that accuracies sufficient to detect planets can be achieved despite the higher airmass.  相似文献   
70.
We derive physical parameters of galaxies from their observed spectra using MOPED, the optimized data compression algorithm of Heavens, Jimenez & Lahav. Here we concentrate on parametrizing galaxy properties, and apply the method to the NGC galaxies in Kennicutt's spectral atlas. We focus on deriving the star formation history, metallicity and dust content of galaxies. The method is very fast, taking a few seconds of CPU time to estimate ∼17 parameters, and is therefore specially suited to studying large data sets, such as the Anglo-Australian two-degree-field (2dF) galaxy survey and the Sloan Digital Sky Survey (SDSS). Without the power of MOPED, the recovery of star formation histories in these surveys would be impractical. In Kennicutt's atlas, we find that for the spheroidals a small recent burst of star formation is required to provide the best fit to the spectrum. There is clearly a need for theoretical stellar atmospheric models with spectral resolution better than 1 Å if we are to extract all the rich information that large redshift surveys contain in their galaxy spectra.  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