Large-eddy simulation of plant canopy flows using plant-scale representation     

Wusi Yue  Marc B. Parlange  Charles Meneveau  Weihong Zhu  René van Hout  Joseph Katz 《Boundary-Layer Meteorology》2007,124(2):183-203

Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model. A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy.  相似文献   

Concentration profiles of particles settling in the neutral and stratified atmospheric boundary layer     

Marcelo Chamecki  René van Hout  Charles Meneveau  Marc B. Parlange 《Boundary-Layer Meteorology》2007,125(1):25-38

An expression for the vertical equilibrium concentration profile of heavy particles, including the effects of canopy on the eddy diffusivity as well as corrections for atmospheric stability, is proposed. This expression is validated against measurements of vertical concentration profiles of corn pollen above a corn field. The fitted theoretical profiles show that particle settling is correctly accounted for. The sensitivity to variations in the turbulent Schmidt number, settling velocity and stability corrections are explicitly characterized. The importance of independent measurements of the surface flux of pollen in future experiments is noted.  相似文献   

An 'outside-in' outburst of Aql X–1     

T. Shahbaz  R. M. Bandyopadhyay  P. A. Charles  R. M. Wagner  P. Muhli  P. Hakala  J. Casares  & J. Greenhill 《Monthly notices of the Royal Astronomical Society》1998,300(4):1035-1040

We present optical spectroscopy and optical and infrared photometry of the neutron star soft X-ray transient Aql X–1 during its X-ray outburst of 1997 August. By modelling the X-ray, optical and IR light curves, we find a 3-d delay between the IR and X-ray rise times, analogous to the UV–optical delay seen in dwarf novae outbursts and black hole X-ray transients. We interpret this delay as the signature of an 'outside-in' outburst, in which a thermal instability in the outer disc propagates inward. This outburst is the first of this type definitively identified in a neutron star X-ray transient.  相似文献   

The impact of vertebrate herbivores on wetland vegetation in Atchafalaya Bay, Louisiana     

D. Elaine Evers  Charles E. Sasser  James G. Gosselink  Deborah A. Fuller  Jenneke M. Visser 《Estuaries and Coasts》1998,21(1):1-13

Delta islands in the Atchafalaya and Wax Lake deltas in Atchafalaya Bay, Louisiana, are in an extremely dynamic successional phase. These islands initially supported large marshes dominated by the pioneering plant species Sagittaria latifolia and Sagittaria platyphylla. A general decrease in vegetated areas has occurred in the delta island marshes in the Atchafalaya Delta since about 1980, while in the Wax Lake Delta portion of the complex the vegetation still flourished. The Atchafalaya Delta provides an interesting setting for the study of herbivory because of the complex interaction of biotic and physical factors operating in this delta. We hypothesized that grazing by herbivores has a marked effect on vegetation in these developing marshes. To test this hypothesis, exclosures were erected on islands in both deltas in September 1985 and January 1986. Each set of exclosure treatments included an openly-grazed control area, an ungrazed area, an area allowing nutria grazing, and one allowing waterfowl grazing in each site. Results of the experiment, based on field sampling of vegetation, indicated decreases in plant biomass and changes in plant species composition in grazed treatments. Waterfowl and nutria reduced biomass aboul equally, but there was a more marked effect in the openly grazed areas. These findings may be extrapolated to sediment diversion areas along the Mississippi River.  相似文献   

Gamma-Ray Bursts: Where are We Now?     

Charles Meegan 《Astrophysics and Space Science》1998,261(1-4):215-224

There has been significant progress recently in our understanding of gamma-ray bursts. The long-sought counterparts at other wavelengths have finally been found for a few bursts. This breakthrough is the result of coordinated observations involving several satellites and ground-based optical and radio observatories. In one case, GRB970508, redshifted absorption lines have been detected, finally settling the debate about the distance scale. The consensus is that the burst sources lie at cosmological distances, requiring at least ∼ 10⁵¹ergs to be emitted in gamma rays in just a few seconds. The gamma radiation is thought to be produced by shocks in a highly relativistic fireball. Many mysteries remain. There is no consensus on the nature of the sources, although coalescing neutron stars are the leading candidate. There is evidence that the sources of the faintest bursts may be at redshifts above 2. If so, gamma-ray bursts may ultimately tell us something about the early Universe. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Cumberland Falls chondritic inclusions: Mineralogy/petrology of a forsterite chondrite suite     

Charles W. Neal  Michael E. Lipschutz 《Geochimica et cosmochimica acta》1981,45(11):2091-2107

As previously found for a chondritic inclusion of unknown affinity, mineralogic and petrologic properties of 9 inclusions in the Cumberland Falls enstatitc achondrite are primitive members of the forsterite (F) chondrite group, hitherto defined by 4 meteorites of similar redox state. The inclusions define a primitive suite with properties indicating 8 as F3 and one of even lower petrologic type. The abundant minerals include: low-Ca pyroxene, olivine, plagioclase, kamacite, taenite, schreibersite, troilite, ferroan alabandite and daubreelite. Diopside, oldhamite and a Ti-rich sulfide are present in one or two inclusions. Petrologic textures and jadeitic pyroxene, hitherto unidentified in meteorites, indicate substantial degree of shock. The inclusions acquired their chemical characteristics during nebular condensation and accretion over a broad redox range (metal-silicate trends in them verify Prior's Rules): their parent body later impacted the enstatite meteorite parent body. During impact, the inclusions were shocked and incorporated with enstatite achondrite host as a breccia that would become Cumberland Falls.  相似文献   

The kinetics of dissolution of muscovites at 25°C and 1 atm CO2 partial pressure     

Charles V Clemency 《Geochimica et cosmochimica acta》1981,45(4):571-576

Two different dry-ground muscovites were placed separately in deionized water saturated with 1 atm partial pressure of carbon dioxide at room temperature and pressure for about 1000 hr. The carbonic acid formed buffered the bulk solution between 4.6 and 5.1 during the whole time of the experiment. K and Na were rapidly released by a reaction with hydrogen ions in the solution, which is usually attributed to a surface exchange reaction. The cation exchange capacity is close to the amount of K and Na released to the solution. However, measurement of surface areas of the micas both before and after the rapid release of K and Na, indicate that only one third of the cation exchange capacity can be attributed to release from the outer surfaces, whereas two thirds must be attributed to removal from within deeper parts of the structure.Approximately 2.5% of total K and Na from the interlayer regions, 1.5% of Mg from the octahedral sheets, and 0.5% of Si from the tetrahedral sheets were released by the end of the experiment, indicating that dissolution was incongruent in both cases. Si showed the slowest total release rate of all the ions. In the later stages (after 400 hr), the release rate of Si becomes a linear function of time. The linear release rate of Si was found to be 2.4 × 10^?17 mol/cm²/sec for the ruby mica, and 2.7 × 10^?17 mol/cm²/sec for the green mica. The rate-controlling mechanism in the dissolution of muscovite in the later stages is probably related to the rate of destruction of the tetrahedral sheets (Si-O bonds) which in turn is reflected by the rate of release of Si into the solution.  相似文献   

Discussion of “Thermodynamic parameters of CaMgSi2O6-Mg2Si2O6 pyroxenes based on regular solution and cooperative disordering models” by Holland,Navrotsky, and Newton     

Donald H. Lindsley  Paula M. Davidson  T. J. B. Holland  A. Navrotsky  R. C. Newton 《Contributions to Mineralogy and Petrology》1981,75(3):301-306

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The impact of drilling fluid and well cuttings on polychaete feeding guilds from the US northeastern continental shelf     

Don Maurer  Wayne Leathem  Charles Menzie 《Marine pollution bulletin》1981,12(10):342-347

The effect of recent drilling operations (fluid and well cuttings) on polychaete feeding guilds from the continental shelf off Atlantic City, New Jersey, was examined. Although there were some adverse effects on macrobenthos from 2160 metric tons of cuttings and mud solids discharged into the marine environment, the composition of polychaete feeding guilds remained essentially unchanged. This key trophic relationship between polychaetes and the changing environment due to the drilling operation was apparently uninterrupted. This relationship remains to be examined in other natural and perturbed habitats.  相似文献   

Pillow talk     

Maarten J. Dewit  Charles Stern 《Journal of Volcanology and Geothermal Research》1978,4(1-2)

Three distinct types of pillows and pillow lava sequences with different modes of origin have been recognized in the extrusive sequences comprising the upper parts of ophiolite complexes that represent the mafic portion of the floor of an Early Cretaceous back-arc basin in southern Chile. One type of pillow formed by non-explosive submarine effusion. A second type formed by magmatic intrusion into pre-existing aquagene tuff formed by explosive eruption. The third type of pillow occurs within dikes, forming pillowed dikes, possibly as a result of vapor streaming within a cooling dike. Where studied in southern Chile, aquagene tuffs and intrusive pillows decrease and water-lain pillows increase in relative abundance from north to south. This variation corresponds with a north-to-south decrease in both the relative volume of extrusives to extensional dikes and the range and volume of differentiated rocks, suggesting a southward increase in rate of extension relative to rate of magma supply within the spreading ridges at which the ophiolites formed. In the northern part of the original basin where the rate of extension was small relative to the rate of magma supply, magma remained in magma chambers longer, resulting in a greater range and volume of differentiated rocks. The larger volume of more differentiated, cooler and more viscous magmas, in conjunction with the likelihood of more violent eruption of volatile-rich differentiates, may have been responsible for the large volume of aquagene tuff in the northern part of the original basin. These observations in southern Chile suggest that ophiolites which contain a great abundance of aquagene tuffs and intrusive pillow lavas formed in tectonic environments in which the rate of extension was small relative to the rate of magma supply (island arcs, embryonic marginal basins). Ophiolites with predominantly water-lain pillowed and massive lavas formed in tectonic environments in which the rate of extension was large relative to the rate of magma supply (mid-ocean ridges, mature back-arc basins). Thus geologic field data may supplement geochemical data as a tool in distinguishing the original igneous-tectonic environments in which ophiolites originate.  相似文献