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291.
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
292.
Andreas Sterl Geert Jan van Oldenborgh Wilco Hazeleger Gerrit Burgers 《Climate Dynamics》2007,29(5):469-485
Using observations covering the last 128 years we show that apparent changes in El Niño-Southern Oscillation (ENSO) teleconnections can be explained by chance and stem from sampling variability. This result is backed by experiments in which an atmosphere model is driven by 123 years of observed sea surface temperature. The possibility of ENSO teleconnection changes in a warming climate is further investigated using coupled GCMs driven by past and projected future greenhouse gas concentrations. These runs do not exclude physical changes in the teleconnection strength but do not agree on their magnitude and location. If existing at all, changes in the strength of ENSO teleconnection, other than obtained by chance, are small and will only be detectable on centennial time scales. 相似文献
293.
Particle image velocimetry (PIV) data obtained in a wind-tunnel model of a canopy boundary layer is used to examine the characteristics
of mean flow and turbulence. The vector spacing varies between 1.7 and 2.5 times the Kolmogorov scales. Conditional sampling
based on quadrants, i.e. based on the signs of velocity fluctuations, reveals fundamental differences in flow structure, especially
between sweep and ejection events, which dominate the flow. During sweeps, the downward flow generates a narrow, highly turbulent,
shear layer containing multiple small-scale vortices just below canopy height. During ejections, the upward flow expands this
shear layer and the associated small-scale flow structures to a broad region located above the canopy. Consequently, during
sweeps the turbulent kinetic energy (TKE), Reynolds stresses, as well as production and dissipation rates, have distinct narrow
peaks just below canopy height, whereas during ejections these variables have broad maxima well above the canopy. Three methods
to estimate the dissipation rate are compared, including spectral fits, measured subgrid-scale (SGS) energy fluxes at different
scales, and direct measurements of slightly underresolved instantaneous velocity gradients. The SGS energy flux is 40–60%
of the gradient-based (direct) estimates for filter sizes inside the inertial range, while decreasing with scale, as expected,
within the dissipation range. The spectral fits are within 5–30% of the direct estimates. The spectral fits exceed the direct
estimates near canopy height, but are lower well above and below canopy height. The dissipation rate below canopy height increases
with velocity magnitude, i.e. it has the highest values during sweep and quadrant 1 events, and is significantly lower during
ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent transport during sweep
events acts as a source below the narrow shear layer within the canopy and as a sink above it. Transport during ejection events
is a source only well above the canopy. The residual term in the TKE transport equation, representing mostly the effect of
pressure–velocity correlations, is substantial only within the canopy, and is dominated by sweeps. 相似文献
294.
Laura Brakenhoff Maarten Kleinhans Gerben Ruessink Maarten van der Vegt 《地球表面变化过程与地形》2020,45(5):1248-1261
Ebb-tidal deltas are highly dynamic environments affected by both waves and currents that approach the coast under various angles. Among other bedforms of various scales, these hydrodynamics create small-scale bedforms (ripples), which increase the bed roughness and will therefore affect hydrodynamics and sediment transport. In morphodynamic models, sediment transport predictions depend on the roughness height, but the accuracy of these predictors has not been tested for field conditions with strongly mixed (wave–current dominated) forcing. In this study, small-scale bedforms were observed in the field with a 3D Profiling Sonar at five locations on the Ameland ebb-tidal delta, the Netherlands. Hydrodynamic conditions ranged from wave dominated to current dominated, but were mixed most of the time. Small-scale ripples were found on all studied parts of the delta, superimposed on megaripples. Even though a large range of hydrodynamic conditions was encountered, the spatio-temporal variations in small-scale ripple dimensions were relatively small (height 0.015 m, length 0.11 m). Also, the ripples were always highly three-dimensional. These small dimensions are probably caused by the fact that the bed consists of relatively fine sediment. Five bedform height predictors were tested, but they all overestimated the ripple heights, partly because they were not created for small grain sizes. Furthermore, the predictors all have a strong dependence on wave- and current-related velocities, whereas the ripple heights measured here were only related to the near-bed orbital velocity. Therefore, ripple heights and lengths in wave–current-dominated, fine-grained coastal areas ( mm) may be best estimated by constant values rather than values dependent on the hydrodynamics. In the case of the Ameland ebb-tidal delta, these values were found to be m and m. ©2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd. 相似文献
295.
The power spectra for line intensities of several lines formed in the upper transition region around 100000 to 250000 K are presented. A period of 5 min is clearly present in lines due to Oiii, Oiv, and Ov. In one dataset a period approaching 10 min is present for 40 min. The size of the emitting features is limited to 7 arc sec squared. In all datasets examined, there is excess power below 4 mHz everywhere along the slit, although the observed periods do not always come from the most intense regions. In 40% of instances clear periods are observable in the 2–5 mHz range with the largest power peak at 3.0 mHz. In all regions, the 5.0 mHz power peak is smaller. For the frequencies investigated there are no significant time delays in any of the datasets examined. This finding may not be entirely unexpected as the formation temperatures of Oiii (100000 K) and Ov (250000 K) may be too close in order to result in an observable phase shift.The observations are discussed in terms of trapped magnetic modes below the transition region and resonant absorption of MHD waves. For resonant absorption we derive from the observed period of 5 min and the observed extent of the structure a typical magnetic field strength of about 2 G. This value is in good agreement with results from MDI for quiet-Sun regions. Our results seem to imply that resonant waves can play a role in the heating of the quiet Sun. We discuss the effect of observing without tracking on the power spectrum and show that the effect is small. 相似文献
296.
Conflicts arise increasingly in Tanzania which involve claims in land located in conservation areas. These conflicts arise,
in many cases, between members of the local communities and the state authorities in charge of the conservation areas. They
concern customary land rights both of pastoral and of agricultural communities, a topic which also touches upon their identities.
The article investigates the legal dimension of these disputes by discussing the law governing conservation areas in the wider
context of land tenure legislation. Within this context, the legal framework of conservation areas is discussed from both
historical and contemporary perspectives. Nature conservation must respect the needs of the local population affected. It
is therefore argued that concepts of community-based conservation should be developed further to work towards the goal of
integrating nature conservation and the sustainable use of natural resources.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
297.
R. Swennen J. van der Sluys R. Hindel A. Brusselmans 《Journal of Geochemical Exploration》1998,62(1-3)
A geochemical survey of Belgium and Luxembourg was carried out as part of an international research project entitled ‘Regional geochemical mapping of Western Europe towards the year 2000'. The aim of this research was to map regional background geochemical patterns based on pristine or at least pre-industrial overbank samples and to deduce regional information on the degree of environmental pollution of floodplain and present-day river sediments. Over the entire study area (about 33,000 km2), 66 overbank sites have been sampled. Catchment areas range between 60 and 600 km2. At each site an overbank profile has been dug out in the immediate vicinity of the river and described in detail. A first composite sample was taken 5–25 cm below the surface. This sample is supposed to represent deposition over the last centuries. Human interferences in this interval are often inferable based on changed sedimentary characteristics and the presence of anthropogenic particles such as charcoal, slags and brick fragments. A second composite sample was taken at depth, usually >1.5 m below the surface over an interval of about 20 cm. In most sites, the profile characteristics allowed to assume pre-industrial or even pristine conditions for this lower overbank sample. In some profiles this was confirmed by 14C-dating and/or by the absence of anthropogenic particles. Finally, a present-day stream sediment was sampled on the site to infer the actual pollution status. After drying at 80°C, disaggregation and sieving, the <125 μm fractions of the three sediment samples were analysed by XRF for major elements and several trace elements. Lower overbank samples generally show a direct link with the geological substrate and allow to assess natural background concentrations. Results from the mapping exercise as well as from the statistical analyses display a clear contrast between the northern part of Belgium where Cenozoic unconsolidated sandy and silty formations dominate which are especially vulnerable for erosion, and the southern part of Belgium and Luxembourg where Paleozoic and Mesozoic sandstones, carbonates, marls and shales are the prevalent lithologies. Here the shales are the most intensively eroded lithologies. This is especially reflected in the element patterns of Al2O3, MgO, K2O, Ga, Ni, Rb, Sc and V which negatively correlate with SiO2. Despite the human related pollution, the geological contrast between north and south Belgium is still recognisable in the geochemical pattern of the upper overbank and present-day stream sediment samples for the above-mentioned elements. Furthermore there is a clear increase in heavy metal contents (Zn, Cd, Pb, Cu), As and in certain locations in Ba from the lower to the upper overbank sediment, as well as to the present-day stream sediment. The relative increase in element content allows to assess the degree of pollution and helps to define those drainage areas where more detailed research is needed. 相似文献
298.
Autoclaves are closed vessels in which liquids can be heated above their ‘normal' boiling points. As a consequence the pressure will rise. This equipment has permitted a variety of processes like the syntheses of organics and the leaching of metal ores. Pressure technologies can be used for accelerating reactions, for specific syntheses or they offer an environmental or economical alternative for existing processes. Wet pressure oxidation of sewage sludge, an organic waste product, has become a proven technology. If less oxygen is added than stoichiometrically required for a complete oxidation, cellulose will break down by a different reaction mechanism, releasing electrons. Under these circumstances reduction of inorganic compounds is possible. 相似文献
299.
Pit crater structure and processes governing persistent activity at Masaya Volcano, Nicaragua 总被引:1,自引:1,他引:0
Hazel Rymer Benjamin van Wyk de Vries John Stix Glyn Williams-Jones 《Bulletin of Volcanology》1998,59(5):345-355
Persistent activity at Masaya Volcano, Nicaragua, is characterised by cycles of intense degassing, lava lake development
and pit crater formation. It provides a useful site to study the processes which govern such activity, because of its easy
accessibility and relatively short cycles (years to decades). An understanding of the present activity is important because
Masaya is visited by large numbers of tourists, is located close to major cities and has produced voluminous lavas, plinian
eruptions and ignimbrites in the recent past. We provide structural and geophysical data that characterise the "normal" present
state of activity. These indicate that the ongoing degassing phase (1993 to present) was not caused by fresh magma intrusion.
It was associated with shallow density changes within the active Santiago pit crater. The activity appears to be associated
predominantly with shallow changes in the pit crater structure. More hazardous activity will occur only if there are significant
departures from the present gravity, deformation and seismic signatures.
Received: 16 May 1997 / Accepted: 29 October 1997 相似文献
300.
David M. Pyle Tjeerd H. van Andel Panayiotis Paschos Paul van den Bogaard 《Quaternary Research》1998,49(3):280-286
A newly recognized 2-m-thick trachytic volcanic ash deposit from northwestern Greece is dated at 374,000 ± 7000 yr and correlated with the Middle Pleistocene volcanic activity of central Italy. The deposit represents ash fallout from one of the largest volcanic eruptions in Europe of the past 400,000 yr and should provide an important stratigraphic marker within the poorly dated Middle Pleistocene deposits of Italy and Greece. 相似文献