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51.
A.P. Nicholas 《地球表面变化过程与地形》2001,26(4):345-362
Results from a series of numerical simulations of two‐dimensional open‐channel flow, conducted using the computational fluid dynamics (CFD) code FLUENT, are compared with data quantifying the mean and turbulent characteristics of open‐channel flow over two contrasting gravel beds. Boundary roughness effects are represented using both the conventional wall function approach and a random elevation model that simulates the effects of supra‐grid‐scale roughness elements (e.g. particle clusters and small bedforms). Results obtained using the random elevation model are characterized by a peak in turbulent kinetic energy located well above the bed (typically at y/h = 0·1–0·3). This is consistent with the field data and in contrast to the results obtained using the wall function approach for which maximum turbulent kinetic energy levels occur at the bed. Use of the random elevation model to represent supra‐grid‐scale roughness also allows a reduction in the height of the near‐bed mesh cell and therefore offers some potential to overcome problems experienced by the wall function approach in flows characterized by high relative roughness. Despite these benefits, the results of simulations conducted using the random elevation model are sensitive to the horizontal and vertical mesh resolution. Increasing the horizontal mesh resolution results in an increase in the near‐bed velocity gradient and turbulent kinetic energy, effectively roughening the bed. Varying the vertical resolution of the mesh has little effect on simulated mean velocity profiles, but results in substantial changes to the shape of the turbulent kinetic energy profile. These findings have significant implications for the application of CFD within natural gravel‐bed channels, particularly with regard to issues of topographic data collection, roughness parameterization and the derivation of mesh‐independent solutions. Copyright © 2001 John Wiley & Sons, Ltd. 相似文献
52.
Nicholas P. Wallerstein Carlos V. Alonso Sean J. Bennett Colin R. Thorne 《地球表面变化过程与地形》2001,26(12):1265-1283
This paper presents the results of a movable‐boundary, distorted, Froude‐scaled hydraulic model based on Abiaca Creek, a sand‐bedded channel in northern Mississippi. The model was used to examine the geomorphic and hydraulic impact of simplified large woody debris (LWD) elements. The theory of physical scale models is discussed and the method used to construct the LWD test channel is developed. The channel model had bed and banks moulded from 0·8 mm sand, and flow conditions were just below the threshold of motion so that any sediment transport and channel adjustment were the result of the debris element. Dimensions and positions of LWD elements were determined using a debris jam classification model. Elements were attached to a dynamometer to measure element drag forces, and channel adjustment was determined through detailed topographic surveys. The fluid drag force on the elements decreased asymptotically over time as the channel boundary eroded around the elements due to locally increased boundary shear stress. Total time for geomorphic adjustment computed for the prototype channel at the Q2 discharge (discharge occurring once every two years on average) was as short as 45 hours. The size, depth and position of scour holes, bank erosion and bars created by flow acceleration past the elements were found to be related to element length and position within the channel cross‐section. Morphologies created by each debris element in the model channel were comparable with similar jams observed in the prototype channel. Published in 2001 John Wiley & Sons, Ltd. 相似文献
53.
Current advances in computer hardware, information technology and data collection techniques have produced very large data
sets in a wide variety of scientific and engineering disciplines. We must harness this opportunity to visualize and extract
useful information from geophysical and geological data. We have taken the task of data mining by implementing a map-like
approach over a web server for interrogating the data, using a client-server paradigm. The spatial-data is mapped onto a two-dimensional
grid from which the user (client) can quiz the data with the map-interface as a user extension. The data is stored on the
server, while the computational gateway separating the client and the server can be the front-end of an electronic publication,
electronic classroom, a survey, or an e-business. We have used a combination of Java, Java3D, and Perl for processing the
data and communicating between the client and the server. The user can interrogate the geospatial data over any particular
region with arbitrary dimensions and then receive back relevant statistical analysis, such as the histogram plots and local
statistics. We have applied this method for the following data sets: (1.) distribution of prime numbers (2.) two-dimensional
mantle convection (3.) three-dimensional mantle convection (4.) high-resolution satellite reflectance data over multiple wavelengths
(5.) molecular dynamics describing the flow of blood in narrow vessels. Using the map-interface, one can actually interrogate
this data over the Internet. 相似文献
54.
Interactions of growing folds and coeval depositional systems 总被引:7,自引:0,他引:7
Responses of both modern and ancient fluvial depositional systems to growing folds can be interpreted in terms of interactions among competing controlling variables which can be incorporated into simple conceptual models. The ratio of the rate of sediment accumulation to the rate of structural uplift determines whether a fold develops a topographic expression above local base level. The balance between (a) stream power and rates of upstream deposition vs. (b) bedrock resistance and rates of crestal uplift and of fold widening determines whether an antecedent stream maintains its course or is defeated by a growing structure. Modern drainage configurations in actively folding landscapes can often be interpreted in terms of these competing variables, and through analysis of digital topography, detailed topographic characteristics of these folds can be quantified. Modern examples of growing folds display both defeated and persistent antecedent rivers, deflected drainages and laterally propagating structures. The topography associated with a defeated antecedent river at Wheeler Ridge, California, is consistent with a model in which defeat results from forced aggradation in the piggyback basin, without the need to vary discharge or uplift rate. Reconstruction of the long-term interplay between a depositional system and evolving folds requires a stratigraphic perspective, such as that provided by syntectonic strata which are directly juxtaposed with ancient folds and faults. Analysis of Palaeogene growth strata bounding the Catalan Coastal Ranges of NE Spain demonstrates the synchronous growth and the kinematic history of multiple folds and faults in the proximal foreland basin. Although dominated by transverse rivers which crossed fold crests, palaeovalleys, interfan lows, structural re-entrants and saddles, and rising anticlines diverted flow and influenced local deposition. In the ancient record, drainage-network events, such as avulsion or defeat of a transverse stream, usually cannot be unambiguously attributed to a single cause. Examination of ancient syntectonic strata from a geomorphological perspective, however, permits successive reconstructions of synorogenic topography, landscapes and depositional systems. 相似文献
55.
During an earthquake, buildings which are vulnerable to seismic loads will be damaged, resulting in property loss and the potential for casualties. To reduce loss of life and injury, the relationship between earthquake-induced building failure and injury severity and distribution needs to be clarified. To this end, a methodology and a series of data collection forms were developed to collect pertinent data for post-event analysis and to provide a basis for structural triage in the field shortly following an earthquake for search and rescue purposes. The forms were developed in four steps: (1) identifying the variables which affect the outcome of an occupant in a damaged building; (2) classifying the variables into three levels of priority for data collection; (3) designing the forms; and (4) applying the forms to damaged buildings from past earthquakes. These forms represent a significant departure from existing forms in that they consider both casualties and building damage jointly in a consistent format. This paper describes the first two steps of the development process; a companion paper outlines the latter components. 相似文献
56.
George MillwardSteve Miller Tom StallardAlan D. Aylward Nicholas Achilleos 《Icarus》2002,160(1):95-107
Recent work has been concerned with calculating the three-dimensional ion concentrations and Pedersen and Hall conductivities within the auroral region of Jupiter for varying conditions of incident electron precipitation. Using the jovian ionospheric model, we present results that show the auroral ionospheric response to changing the incoming flux of precipitating electrons (for constant initial energy) and also the response to changing the initial energy (for both constant flux and constant energy flux). The results show that, for expected energy fluxes of precipitating particles, the average auroral integrated Pedersen conductivity attains values in excess of 1 mho. In addition, it is shown that electrons with an initial energy of around 60 keV are particularly effective at generating auroral conductivity: Particles of this energy penetrate most effectively to the layer of the jovian ionosphere at which the auroral conductivity is at a maximum. 相似文献
57.
Toshifumi Yanagisawa Yasushi Muraki Yutaka Matsubara Fumio Abe Kimiaki Masuda Sachiyo Noda Takahiro Sumi Yuji Kato Mitsuaki Fujimoto Shuji Sato Ian Bond Nicholas Rattenbury Philip Yock Pam Kilmartin John Hearnshaw Micheal Reid Denis Sullivan Brian Carter Richard Dodd Garry Nankivell Noiman Rumsey Morihiro Honda Maki Sekiguchi Masanori Yoshizawa Takashi Nakamura Humitaka Sato Seiji Kabe Makoto Kobayashi Yoshiyuki Watase Jun Jugaku Toshiharu Saito Barbel Koribalsky 《Experimental Astronomy》2000,10(4):519-535
We have constructed a large, mosaic CCD camera called MOA-cam2 which has 4096 × 6144-pixelsto search for gravitational microlensing events. MOA-cam2 has three4096 × 2048-pixel SITe CCD chips, which have a very high quantum efficiency (nearly 80% in the wave region 500 to 800 nm),and three buttable sides. We have placed the threechips side by side with 100 m dead space. MOA-cam2 has been installed on the 61 cm Boller and Chivens telescope of the MOA collaboration at the Mt. John University Observatory (MJUO) in NewZealand since July 1998. The field coverage is 0.92° × 1.38° per exposure. The technical details of MOA-cam2 and the first images obtained with the Boller and Chivens telescope are presented. MOA-cam2 introduces a second phase of research on gravitational microlensing by the MOA collaboration. 相似文献
58.
Nicholas A Walton Eduardo Gonzalez-Solarez Silvia Dalla Anita Richards Jonathon Tedds 《Astronomy& Geophysics》2006,47(3):3.22-3.24
Nicholas A Walton, Silvia Dalla, Eduardo Gonzalez-Solarez, Anita Richards and Jonathan Tedds show off the fast-developing astronomical applications of the UK's virtual observatory service. 相似文献
59.
60.
We have characterized the outer-membrane decaheme cytochromes OmcA and MtrC from Shewanella oneidensis MR-1 at the single-molecule level using scanning tunneling microscopy (STM) and tunneling spectroscopy (TS). These cytochrome proteins are of great interest because they are thought to mediate bacterial electron transfer reactions in anoxic waters that control the reductive dissolution of oxide minerals. In our study, to characterize the electron transfer properties of these proteins on a model surface, the purified cytochromes were chemically immobilized as molecular monolayers on Au(111) substrates via a recombinant tetra-cysteine sequence as verified by X-ray photoelectron spectroscopy. Atomic force microscopy images confirm the monolayer films were ∼5-8 nm thick which is consistent with the apparent lateral dimensions of individual cytochrome molecules obtained with STM. Current-voltage TS of single cytochrome molecules revealed that OmcA and MtrC have different abilities to mediate tunneling current despite having otherwise very similar molecular and biochemical properties. These observations suggest that, based on their electron tunneling properties, the two cytochromes could have specific roles during bacterial metal reduction. Additionally, this study establishes single-molecule STM/TS as an effective means for revealing insights into biogeochemical redox processes in the environment. 相似文献