256,323 research outputs found
Linear instability mechanisms for sand wave formation
No full textA height- and flow-dependent model for turbulent viscosity is employed to explain the generation of sand waves in tidal seas. This new model resolves the problem of excitation of very long waves in sand wave formation, because it leads to damping of the long waves and gives a finite separation between the most excited mode and the zero mode. For parameters within their physically realistic ranges, a linear analysis of the resulting system yields a first excited mode whose wavelength is similar to the characteristic wavelength of sand waves observed in nature. The physical mechanism of sand wave formation as predicted by the new model is explained in detail. The dispersion relation obtained can be the starting point for a weakly nonlinear analysis of the system
George Sand illustré par Tony Johannot et Maurice Sand.... , Lélia / George Sand illustré par Tony Johannot et Maurice Sand. Préface et notice nouvelle...[Sur la dernière publication de M. F. La Mennais.]
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Nonlinear dynamics of sand banks and sand waves
No full textSand banks and sand waves are two types of sand structures that are commonly observed on an off-shore sea bed. We describe the formation of these features using the equations of the fluid motion coupled with the mass conservation law for the sediment transport. The bottom features are a result of an instability due to tide–bottom interactions. There are at least two mechanisms responsible for the growth of sand banks and sand waves. One is linear instability, and the other is nonlinear coupling between long sand banks and short sand waves. One novel feature of this work is the suggestion that the latter is more important for the generation of sand banks. We derive nonlinear amplitude equations governing the coupled dynamics of sand waves and sand banks. Based on these equations, we estimate characteristic features for sand banks and find that the estimates are consistent with measurements
Regeneration of sand waves after dredging
No full textSand waves are large bed waves on the seabed, being a few metres high and lying hundreds of metres apart. In some cases, these sand waves occur in navigation channels. If these sand waves reduce the water depth to an unacceptable level and hinder navigation, they need to be dredged. It has been observed in the Bisanseto Channel in Japan that the sand waves tend to regain their shape after dredging. In this paper, we address modelling of this regeneration of sand waves, aiming to predict this process. For this purpose, we combine a very simple, yet effective, amplitude-evolution model based on the Landau equation, with measurements in the Bisanseto Channel. The model parameters are tuned to the measured data using a genetic algorithm, a stochastic optimization routine. The results are good. The tuned model accurately reproduces the measured growth of the sand waves. The differences between the measured weave heights and the model results are smaller than the measurement noise. Furthermore, the resulting parameters are surprisingly consistent, given the large variations in the sediment characteristics, the water depth and the flow field. This approach was tested on its predictive capacity using a synthetic test case. The model was tuned based on constructed predredging data and the amplitude evolution as measured for over 2 years. After tuning, the predictions were accurate for about 10 years. Thus, it is shown that the approach could be a useful tool in the optimization of dredging strategies in case of dredging of sand waves
Sand transport in multiphase pipelines
No full textOver the life of an oil and gas reservoir, it is likely to encounter sand production. In offshore production fields, as there are lack of processing facilities nearby, gas, liquid and sand are often transported together in long distance pipelines. The existence of sand could accumulate in the pipelines under inappropriate operation condition and eventually will lead to a blockage. Thus, to design such systems requires knowledge on how sand is transported, when and where it will accumulate.
This thesis summarizes the experimental work undertaken using the 2 inch, 3 inch and 4 inch multiphase facilities. Generally, the main objectives of the experiments were to i) observe and enhance the understanding of sand transport characteristics in water and air-water flows; ii) investigate sand concentration effect and pipe diameter effect on sand minimum transport condition (MTC); iii) investigate the effect of pipeline orientation (0, +5, +10 and +20 degrees) and viscosity effect (Carboxy Methyl Cellulose (CMC) solution with viscosity of 7, 20cP; Oil with viscosity of 105, 250 and 340cP) on sand MTC; iv) validate the equivalent pressure drop concept for predicting sand MTC in sand-air-water flow and v) extend current MTC prediction model for sand-water flow to account for different sand concentrations .
Similar sand behaviour was observed in horizontal sand-water flow in all pipe sizes tested. At minimum transport velocity, sand particles were observed transporting in form of sand streaks. For horizontal sand-air-water flow, sand transport characteristics and MTC were strongly dependent on the air-water flow regime. Sand was found to be transported more efficiently within slug or roll wave body, where turbulence is generated intensively.
Parametric studies were conducted to investigate the factors affecting sand MTC in water and air-water flows in pipeline. It was found that the MTC will increase as sand concentration and pipe diameter increase. Pipeline orientation was found having little effect on sand behaviours and MTC in upwardly inclined water flow. However, in upwardly inclined air-water flow, although sand particles were observed sometime moving backward with the liquid film, the superficial gas and liquid velocities required to transport sand were less than those in the horizontal pipeline due to the fact that slug flow regime was found more prevailing in inclined pipeline. In addition, the liquid viscosity effect on sand MTC in single phase liquid flow was investigated due to the increase of concerns relating to solids transport in high viscosity crudes. It appeared that, in turbulent flow, sand MTC increased slightly as the fluid viscosity increased. However, when the bulk flow became laminar, the MTC decreased as the fluid viscosity increased.
After visually obtained the sand MTC in air-water flow, the measured pressure gradients were compared between MTC condition for sand-water flow for different sand concentrations, the results indicate that the equivalent pressure gradients concept is a valid approach to extend the sand MTC prediction from water flow to air-water flow conditions for the purpose of pipeline design.
Two concentration correction correlations (dual range and single range) were proposed. The modified model could account for a wider range of sand concentration (from 0.000005 to 0.3 volume fraction) in water flow. The predicted MTC velocities showed good agreement with the experimental results
Sand Creek Watershed Project / Sand Creek Watershed Management Plan
No full textThe overall goal of the Sand Creek Watershed Project is to improve and protect the designated uses of the watershed. In order to achieve this overall goal and attain compliance with the TMDL established in Sand Creek, four goals have been established and prioritized. The primary goal of the Sand Creek Watershed Project is to restore or improve the cold water fishery. The secondary goal of the project is to protect and improve the habitats of native aquatic life and wildlife. Both goals can be achieved by reducing the amount of known pollutants affecting both of these uses. Pollutant reduction can be achieved through proper storm water management that would also serve to address harmful changes in the stream’s flow regime. The third goal of the project is to improve and protect partial body contact recreational uses, such as wading and fishing, by reducing pathogen concentrations, hydrocarbons, toxic substances, and trash. These four known and suspected pollutants also affect total body contact recreation uses, such as swimming. The fourth goal of the Sand Creek Watershed Project is to improve and protect total body contact. Structural and vegetative BMPs, policy and management BMPs, and Information and Education (I&E) activities will be needed to reduce known pollutants affecting these impaired and threatened uses
Remotely sensed dune celerity and sand flux measurements of the world's fastest barchans (Bodele, Chad)
No full textQuantifying sand flux with field measurements is an expensive and time-consuming process. We here present an alternative approach using the COSI-Corr software package for Earth surface deformation detection. Using pairs of ASTER satellite images, we detected dune migration in the Bodélé depression of northern Chad over time intervals of one month to 6.5 years. The displacement map can be used to automatically distinguish dunes from interdunes, which is a crucial step towards calculating sand flux. We interpolated a surface between the interdune areas and subtracted it from a digital elevation model, thus obtaining dune heights and volumes. Multiplying height with celerity yields a pixel-by-pixel estimate of the sand flux. We applied this method to large diatomite dunes in the Bodélé, confirming that these are some of the world's fastest moving barchans. Plotting dune height against inverse celerity reveals sand flux at the dune crest of >200 m3/m/yr. Average dune sand flux values for the eastern and western Bodélé are 76 and 99 m3/m/yr, respectively. The contribution of the dunes to the total area-averaged sand flux is 24–29 m3/m/yr, which is ∼10% of the saltation flux determined by previously published field measurements
Wind Tunnel Experimental Investigation Of Sand Velocity In Aeolian Sand Transport
No full textSand velocity in aeolian sand transport was measured using the laser Doppler technique of PDPA (Phase Doppler Particle Analyzer) in a wind tunnel. The sand velocity profile, probability distribution of particle velocity, particle velocity fluctuation and particle turbulence were analyzed in detail. The experimental results verified that the sand horizontal velocity profile can be expressed by a logarithmic function above 0.01 in, while a deviation occurs below 0.01 m. The mean vertical velocity of grains generally ranges from -0.2 m/s to 0.2 m/s, and is downward at the lower height, upward at the higher height. The probability distributions of the horizontal velocity of ascending and descending particles have a typical peak and are right-skewed at a height of 4 turn in the lower part of saltation layer. The vertical profile of the horizontal RMS velocity fluctuation of particles shows a single peak. The horizontal RMS velocity fluctuation of sand particles is generally larger than the vertical RMS velocity fluctuation. The RMS velocity fluctuations of grains in both horizontal and vertical directions increase with wind velocity. The particle turbulence intensity decreases with height. The present investigation is helpful in understanding the sand movement mechanism in windblown sand transport and also provides a reference for the study of blowing sand velocity. (C) 2007 Elsevier B.V All rights reserved
Booming Sand Dunes
No full text"Booming" sand dunes are able to produce low-frequency sound that resembles a pure note from a music instrument. The sound has a dominant audible frequency (70-105 Hz) and several higher harmonics and may be heard from far distances away. A natural or induced avalanche from a slip face of the booming dune triggers the emission that may last for several minutes. There are various references in travel literature to the phenomenon, but to date no scientific explanation covered all field observations.
This thesis introduces a new physical model that describes the phenomenon of booming dunes. The waveguide model explains the selection of the booming frequency and the amplification of the sound in terms of constructive interference in a confined geometry. The frequency of the booming is a direct function of the dimensions and velocities in the waveguide. The higher harmonics are related to the higher modes of propagation in the waveguide.
The experimental validation includes quantitative field research at the booming dunes of the Mojave Desert and Death Valley National Park. Microphone and geophone recordings of the acoustic and seismic emission show a variation of booming frequency in space and time. The analysis of the sensor data quantifies wave propagation characteristics such as speed, dispersion, and nonlinear effects and allows the distinction between the source mechanism of the booming and the booming itself.
The migration of sand dunes results from a complicated interplay between dune building, wind regime, and precipitation. The morphological and morphodynamical characteristics of two field locations are analyzed with various geophysical techniques. Ground-penetrating radar images the subsurface structure of the dunes and reveal a natural, internal layering that is directly related to the history of dune migration. The seismic velocity increases abruptly with depth and gradually increases with downhill position due to compaction. Sand sampling shows local cementation of sand grains within the discrete layers that explains the increase in velocity and decrease in porosity. The subsurface layering may influence the speed of dune migration and therefore have important consequences on desertification.
The positive qualitative and quantitative correlation between the subsurface layering in the dune and the manifestation of the booming sound implies a close relation between environmental factors and the booming emission. In this thesis, the frequency of booming is correlated with the depth of the waveguide and the seismic velocities. The variability on location and season suggests that the waveguide theory successfully unravels the phenomenon
of booming sand dunes.</p
Leishmania chitinase facilitates colonization of sand fly vectors and enhances transmission to mice
No full textChitinases of trypanosomatid parasites have been proposed to fulfil various roles in their blood-feeding arthropod vectors but so far none have been directly tested using a molecular approach. We characterized the ability of Leishmania mexicana episomally transfected with LmexCht1 (the L. mexicana chitinase gene) to survive and grow within the permissive sand fly vector, Lutzomyia longipalpis. Compared with control plasmid transfectants, the overexpression of chitinase was found to increase the average number of parasites per sand fly and accelerate the escape of parasites from the peritrophic matrix-enclosed blood meal as revealed by earlier arrival at the stomodeal valve. Such flies also exhibited increased damage to the structure of the stomodeal valve, which may facilitate transmission by regurgitation. When exposed individually to BALB/c mice, those flies with chitinase-overexpressing parasites spent on average 2.4-2.5 times longer in contact with their host during feeding, compared with flies with control infections. Furthermore, the lesions that resulted from these single fly bite infections were both significantly larger and with higher final parasite burdens than controls. These data show that chitinase is a multifunctional virulence factor for L. mexicana which assists its survival in Lu. longipalpis. Specifically, this enzyme enables the parasites to colonize the anterior midgut of the sand fly more quickly, modify the sand fly stomodeal valve and affect its blood feeding, all of which combine to enhance transmission
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