排序方式： 共有11条查询结果，搜索用时 15 毫秒
1.
The influence of emergent and submerged macrophytes on flow velocity and turbulence production is demonstrated in a 140 m reach of the River Blackwater in Farnborough, Hampshire, UK. Macrophyte growth occurs in patches and is dominated by Sparganium erectum and Sparganium emersum. In May 2001, patches of S. erectum were already established and occupied 18% of the channel area. The flow adjusted to these (predominantly lateral) patches by being channelled through a narrower cross‐section. The measured velocity profiles showed a logarithmic form, with deviations attributable to topographic control. The channel bed was the main source of turbulence. In September 2001, in‐stream macrophytes occupied 27% of the channel, and overhanging bank vegetation affected 32% of the area. Overall flow resistance, described by Manning's n, showed a threefold increase that could be attributed to the growth of S. emersum in the middle of the channel. Velocity profiles showed different characteristic forms depending on their position relative to plant stems and leaves. The overall velocity field had a three‐dimensional structure. Turbulence intensities were generally higher and turbulence profiles tended to mirror the velocity profiles. Evidence for the generation of coherent eddies was provided by ratios of the root mean square velocities. Spectral analysis identified deviations from the Kolmogorov ?5/3 power law and provided statistical evidence for a spectral short‐cut, indicative of additional turbulence production. This was most marked for the submerged vegetation and, in some instances, the overhanging bank vegetation. The long strap‐like leaves of S. emersum being aligned approximately parallel to the flow and the highly variable velocity field created by the patch arrangement of macrophytes suggest that the dominant mechanism for turbulence production is vortex shedding along shear zones. Wake production around individual stems of S. emersum close to the bed may also be important locally. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
2.
J. Silvester C. Neiner H. F. Henrichs G. A. Wade V. Petit E. Alecian A.-L. Huat C. Martayan J. Power O. Thizy 《Monthly notices of the Royal Astronomical Society》2009,398(3):1505-1511
We have obtained 40 high-resolution circular spectropolarimetric measurements of 12 slowly pulsating B (SPB) stars, eight β Cephei stars and two Be stars with the Echelle Spectropolarimetric Device for the Observation of Stars at CFHT (ESPaDOnS) and Narval spectropolarimeters. The aim of these observations is to evaluate recent claims of a high incidence of magnetic field detections in stars of these types obtained using low-resolution spectropolarimetry by Hubrig et al. The precision achieved is generally comparable to or superior to that obtained by Hubrig et al., although our new observations are distinguished by their resolution of metallic and He line profiles, and their consequent sensitivity to magnetic fields of zero net longitudinal component. In the SPB stars, we confirm the detection of magnetic field in one star (16 Peg), but find no evidence of the presence of fields in the remaining 11. In the β Cep stars, we detect a field in ξ1 CMa, but not in any of the remaining seven stars. Finally, neither of the two B-type emission-line stars shows any evidence of magnetic field. Based on our results, we conclude that fields are not common in SPB, β Cep and B-type emission-line stars, consistent with the general rarity of fields in the broader population of main sequence B-type stars. A relatively small, systematic underestimation of the error bars associated with the UV Focal Reducer and Low Dispersion Spectrograph for the Very Large Telescope (FORS1) longitudinal field measurements of Hubrig et al. could in large part explain the discrepancy between their results and those presented here. 相似文献
3.
The reaction kinetics of the butadinyl radical, C4H, with various hydrocarbons detected in the atmosphere of Titan (methane, ethane, propane, acetylene, ethene and methylacetylene) are studied over the temperature range of 39-298 K using the Rennes CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus. Kinetic measurements were made using the pulsed laser photolysis—laser induced fluorescence technique. The rate coefficients, except for the reaction with methane, all show a negative temperature dependence and can be fitted with the following expressions over the temperature range of this study: ; ; , , . These expressions are not intended to be physically meaningful but rather to provide an easy way to introduce experimental results in photochemical models. They are only valid over the temperature range of the experiments. Possible channels of these reactions are discussed as well as possible consequences of these results for the production of large molecules and hazes in the atmosphere of Titan. These results should also be considered for the photochemistry of Giant Planets. 相似文献
4.
Worldwide convectively accelerated streams flowing in downstream-narrowing river sections show that riverbed vegetation growing on alluvial sediment bars gradually disappears, forming a front beyond which vegetation is absent. We revise a recently proposed analytical model able to predict the expected longitudinal position of the vegetation front. The model was developed considering the steady state approximation of 1-D ecomorphodynamics equations. While the model was tested against flume experiments, its extension and application to the field is not trivial as it requires the definition of proper scaling laws governing the observed phenomenon. In this work, we present a procedure to calculate vegetation parameters and flow magnitude governing the equilibrium at the reach scale between hydromorphological and biological components in rivers with converging boundaries. We collected from worldwide rivers data of section topography, hydrogeomorphological and riparian vegetation characteristics to perform a statistical analysis aimed to validate the proposed procedure. Results are presented in the form of scaling laws correlating biological parameters of growth and decay from different vegetation species to flood return period and duration, respectively. Such relationships demonstrate the existence of underlying selective processes determining the riparian vegetation both in terms of species and cover. We interpret the selection of vegetation species from ecomorphodynamic processes occurring in convectively accelerated streams as the orchestrated dynamic action of flow, sediment and vegetation characteristics. © 2019 John Wiley & Sons, Ltd. 相似文献
5.
Distinguishing Carbonate Reservoir Pore Facies with Nuclear Magnetic Resonance Measurements 总被引：1，自引：0，他引：1
Characterization of carbonate rocks may involve identifying the important pore types which are present. In the past, this
task has required detailed petrographic analysis of many core samples. Here, we describe a method which uses nuclear magnetic
resonance (NMR) measurements to reduce the amount of petrographic analysis needed for porosity typing of carbonate reservoir
rocks.
For a rock sample which has been measured with NMR, our method decomposes the log(T2) spectrum into at most three Gaussian-shaped components and gives a set of nine parameters. Two characteristic quantities
having geological significance are extracted from the nine parameters. Values of the two quantities are compared with a reference
set, established from samples having both NMR and petrographic evaluations of porosity types. We use a Bayesian approach to
the classification of the dominant porosity type.
Tests of our method on 103 samples show a correct prediction in 60 to 90 percent of the samples. The lower success rate was
obtained for samples with five porosity types from three fields; the higher success rate obtained with samples with three
porosity types from one well. The use of geologically significant quantities extracted from the decomposition gives comparable
success rate to those obtained using a standard, non-geological approach such as canonical variates. 相似文献
6.
Olivier Thizy Christian Buil François Cochard Coralie Neiner 《Planetary and Space Science》2008,56(14):1878-1881
Amateur spectroscopists are getting organized to contribute to professional astronomy work. We present here the work done within the AUDE association to design, manufacture and distribute the Lhires III high-resolution Littrow spectrograph, which fits usual amateur telescopes and CCD cameras. We also review how the communication is organized through the ARAS distribution list (Spectro-L), forums, and workshops to develop and promote spectroscopic work among amateurs. Last but not least, we introduce the Be stars spectra (BeSS) Virtual Observatory-compatible database and its use for the monitoring of Be stars in association with the COROT satellite. 相似文献
7.
C. Neiner D. Baade A. Fullerton C. Gry G. Hussain A. Lèbre J. Morin P. Petit J. O. Sundqvist A. ud-Doula A. A. Vidotto G. A. Wade The UVMag consortium 《Astrophysics and Space Science》2014,354(1):215-227
Important insights into the formation, structure, evolution and environment of all types of stars can be obtained through the measurement of their winds and possible magnetospheres. However, this has hardly been done up to now mainly because of the lack of UV instrumentation available for long periods of time. To reach this aim, we have designed UVMag, an M-size space mission equipped with a high-resolution spectropolarimeter working in the UV and visible spectral range. The UV domain is crucial in stellar physics as it is very rich in atomic and molecular lines and contains most of the flux of hot stars. Moreover, covering the UV and visible spectral domains at the same time will allow us to study the star and its environment simultaneously. Adding polarimetric power to the spectrograph will multiply tenfold the capabilities of extracting information on stellar magnetospheres, winds, disks, and magnetic fields. Examples of science objectives that can be reached with UVMag are presented for pre-main sequence, main sequence and evolved stars. They will cast new light onto stellar physics by addressing many exciting and important questions. UVMag is currently undergoing a Research & Technology study and will be proposed at the forthcoming ESA call for M-size missions. This spectropolarimeter could also be installed on a large UV and visible observatory (e.g. NASA’s LUVOIR project) within a suite of instruments. 相似文献
8.
There are two factors, source composition and magmatic differentiation, potentially controlling W-Sn mineralization. Which one is more important is widely debated and may need to be determined for each individual deposit. The Xitian granite batholith located in South China is a natural laboratory for investigating the above problem. It consists essentially of two separate components, formed in the Triassic at ca. 226 Ma and Jurassic at ca. 152 Ma, respectively. The Triassic and Jurassic rocks are both composed of porphyritic and fine-grained phases. The latter resulted from highly-differentiated porphyritic ones but they have similar textural characteristics and mineral assemblages, indicating that they reached a similar degree of crystal fractionation. Although both fine-grained phases are highly differentiated with elevated rare metal contents, economic W–Sn mineralization is rare in the Triassic granitoids and this can be attributed to less fertile source materials than their Jurassic counterparts, with a slightly more enriched isotopic signature and whole-rock εNd(226 Ma) of ?10.4 to ?9.2 (2σ = 0.2) compared with εNd(152 Ma) of ?9.2 to ?8.2 (2σ = 0.2) for the Jurassic rocks. The initial W-Sn enrichment was derived from the metasedimentary rocks and strongly enhanced by reworking of the continental crust, culminating in the Jurassic. 相似文献
9.
Genetic connectivity of the coral‐eating sea star Acanthaster planci during the severe outbreak of 2006–2009 in the Society Islands,French Polynesia 
下载免费PDF全文
下载免费PDF全文 Nina Yasuda Coralie Taquet Satoshi Nagai Terutoyo Yoshida Mehdi Adjeroud 《Marine Ecology》2015,36(3):668-678
Occasional population outbreaks of the crown‐of‐thorns sea star, Acanthaster planci, are a major threat to coral reefs across the Indo‐Pacific. The presumed association between the serial nature of these outbreaks and the long larval dispersal phase makes it important to estimate larval dispersal; many studies have examined the population genetic structure of A. planci for this purpose using different genetic markers. However, only a few have focused on reef‐scale as well as archipelago‐scale genetic structure and none has used a combination of different genetic markers with different effective population sizes. In our study, we used both mtDNA and microsatellite loci to examine A. planci population genetic structure at multiple spatial scales (from <2 km to almost 300 km) within and among four islands of the Society Archipelago, French Polynesia. Our analysis detected no significant genetic structure based on mtDNA (global FST = ?0.007, P = 0.997) and low levels of genetic structure using microsatellite loci (global FST = 0.006, P = 0.005). We found no significant isolation by distance patterns within the study area for either genetic marker. The overall genetically homogenized pattern found in both the mitochondrial and nuclear loci of A. planci in the Society Archipelago underscores the significant role of larval dispersal that may cause secondary outbreaks, as well as possible recent colonization in this area. 相似文献