34 research outputs found
Design of multicast scheduling algorithms for packet-oriented input-queued switches
No full text100 p.Technology makes sense only if it can serve some human purposes in a more efficient and reliable way. The twenty-first century has witnessed an exponential growth in the number of Internet users. A very important technology forming the backbone of the Internet as well as connecting offices and households to the Internet is switching. The ever-increasing demand for bandwidth creates an urgent need to build high-speed switched networks to boost the performance of the Internet, and there is no doubt that switches and routers will continue to play an important role in data communications.Master of Science (Communications Engineering
Ocean circulation and dynamics over the China Shelf Seas
No full textThe China Shelf Seas (CSS) has complex circulations jointly governed by wind, tides, terrestrial buoyancy and momentum influxes, exchange with the adjacent oceans, and the underlying dynamics of flow-topography interactions. Many unresolved issues about circulation dynamics remain in the CSS. By combining observations, novel numerical simulation approaches, and holistic geophysical fluid dynamics analysis, this thesis study tackled the issues in the following three parts under the wind-driven shelf circulation dynamics framework. In part 1, we investigated the responses of the upwelling circulation to oscillational tidal and persistent lateral buoyancy influx based on a newly implemented two-dimensional model that is enabled with these two forcings. We found that the tidally induced intra-tidal asymmetric friction and nonlinear advection, as well as the buoyancy influx largely alter the cross-shore transport and reshape the alongshore circulation through both baroclinic and barotropic geostrophic adjustments. In part 2, we extended the classic bottom boundary layer (BBL) dynamics by including the effects of variable wind-driven upwelling regimes. We revealed that spatially variable upslope buoyancy advection induced by geostrophic cross-isobath transport (GCT) over the meandering shelf topography greatly changes BBL dynamics by the modulation of bottom frictional shear and restratification and forms variable BBL regime over the shelf. In part 3, we expanded our study to the entire CSS and investigated the dynamics of counter-wind currents (CWCs) by combining observational study and both high-resolution realistic and process-orientated modeling. We found that the net alongshore pressure gradient force, the ensuing ageostrophic alongshore acceleration and cross-shore geostrophic adjustment, controlled by variable wind, stratification, and variable shelf topography, are the intrinsic dynamics for CWCs formation. Multi-mechanism analyses provided new understandings to the existing ambiguous CWCs dynamics. This thesis advances the knowledge of the complex shelf circulation dynamics of the CSS that is applicable to other similar coastal systems worldwide.</p
Responses of Coastal Upwelling to Tidally Induced Bottom Friction Dynamics and Plume-Modulated Geostrophy: A Process-Oriented Modeling Study
No full textWe used a high-resolution cross-shelf two-dimensional numerical model to investigate the response of coastal wind-driven upwelling circulation to barotropic tidal forcing and lateral buoyant discharge over a broad continental shelf. We found that the tidally amplified asymmetric friction effect arising from the interaction between tidal and subtidal currents modulated the upwelling structure across the shelf. The interaction weakened the water outcropping (upwelling) in the inner shelf due to tidally amplified mixing, but enhanced cross-shore velocity offshore due to tidally induced asymmetric friction effect and nonlinear advection. The enhanced mixing changed the density in the bottom boundary layer and subsequently in the upwelling front, which eventually weakened the geostrophic alongshore flow. The mass and stratification inputs of the lateral buoyant discharge weakened or even reversed geostrophic dynamics for alongshore and upslope transports. The reversed cross-shore density and elevation gradient induced by the buoyant influx weakened the along-shore current and the associated bottom friction effect. The upslope cross-shore transport was reduced due to weakened alongshore flow and the associated bottom Ekman transport. The mass of buoyant influx compensated for the wind-driven offshore transport in the upper layer. The upwelling could be reversed to downwelling when the transport of lateral influx exceeded the wind-driven offshore transport. The responses of upwelling circulation to tidal and lateral buoyancy forcing highlighted in this process-oriented study are fundamental for interpreting more complex wind-driven shelf circulation.</p
Variability of the Bottom Boundary Layer Induced by the Dynamics of the Cross-Isobath Transport Over a Variable Shelf
No full textThe bottom boundary layer (BBL) dynamics play an important role in regulating the energy, momentum balance, and circulation in the shallow shelf areas. Unlike previous studies that disconnected BBL with background variable shelf circulation, we investigate the dynamic connection between the wind-driven shelf circulation and BBL dynamics, and show the spatial characteristics of BBL dynamics in response to three-dimensional (3D) heterogeneous transport over the highly variable shelf topography in the Northern South China Sea. Our process-oriented modeling study demonstrates that the mixing dynamics and upslope buoyancy transport over varying shelf topography alter the spatial variability of BBL dynamics. Driven by southwesterly upwelling-favorable winds, the along-shelf current generated a frictional upslope Ekman transport. The along-isobath pressure gradient force ((Formula presented.)) formed by the flow-topography interactions over the meandering shelf induces the geostrophic cross-isobath transport. The downwave (upwave) (Formula presented.) enhances (offset) the frictional upslope transport over the east (west) of the shelf that has a concaving (uniform) bottom topography. Over the eastern shelf with concave isobaths, the intensified (Formula presented.) and upslope cross-isobath dense water transport strengthen stratification and weaken the effect of bottom stress-induced mixing, limiting the development of the BBL. The antithesis occurs over the western shelf, where a small bottom stress controls the BBL. River discharge and the tidal current modulate the alongshore current, upslope transport, bottom stress intensity, and BBL development. We model the trajectory of seabed particles as they respond to the BBL dynamic regimes, and find that high (low) concentration, short (long) suspension time, and strong (weak) shoreward transport occurover the eastern (western) shelf, respectively.</p
Axial wind effects on stratification and longitudinal sediment transport in a convergent estuary during wet season
No full textAuthor Posting. © American Geophysical Union, 2020. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 125(2), (2020): e2019JC015254, doi:10.1029/2019JC015254.The Coupled Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) modeling system was used to examine axial wind effects on vertical stratification and sediment transport in a convergent estuary. The model demonstrated that stratification dynamics in the upper estuary (Kelvin number <1; Ke= fB/√ g'hs) are dominated by longitudinal wind straining, whereas the dominant mechanism governing estuarine stratification in the lower estuary (Kelvin number ~1) is lateral wind straining. Barotropic advection contributes to seaward sediment transport and peaks during spring tides, whereas estuarine circulation causes landward sediment transport with a maximum during neap tides. Down‐estuary winds impose no obvious effects on longitudinal sediment flux, whereas up‐estuary winds contribute to enhanced seaward sediment flux by increasing the tidal oscillatory flux. The model also demonstrates that bottom friction is significantly influenced by vertical stratification over channel regions, which is indirectly affected by axial winds.This research was funded by the National Natural Science Foundation of China (Grants 41576089, 51761135021, and 41890851), the National Key Research and Development Program of China (2016YFC0402603) and the Guangdong Provincial Water Conservancy Science and Technology Innovation Project (Grant 201719). We thank Professor Liangwen Jia at the Sun Yat‐sen University for his kindly providing the surficial sediment samples data in 2011. We also thank graduate students Guang Zhang and Yuren Chen from the Sun Yat‐sen University for their help in data analysis. We are grateful to two anonymous reviewers for their insightful comments to help improve this manuscript. The data related to this article is available online at the Zenodo website (https://zenodo.org/record/3606471).2020-07-1
Wave Effects on Sediment Transport and Entrapment in a Channel‐Shoal Estuary: The Pearl River Estuary in the Dry Winter Season
No full textSediment dynamics have great effects on the morphology of estuaries and deltas. As riverine sediment input has decreased in many estuaries in recent years, the removal and dispersion of sediments by currents and waves inside estuaries has assumed a more dominant role; however, the overall effects of waves are less well understood. In this study, we used the Coupled Ocean Atmosphere Wave Sediment Transport system to model the effects of waves on sediment transport in China's Pearl River Estuary. The results indicate that the presence of waves increased the landward current and sediment transport at the bottom of the channel and the seaward fluxes of water and sediment at the West Shoal, which is the main shallow area that accepts riverine sediment. Relative to the case without waves, the effects of waves increased the estuary's total sediment export by 45% to 9.14 megatons in one typical year, which even exceeded the annual riverine load in the year. The dry winter season shows the highest wave effects on sediment budget, and the sediment export was increased by 86% to 2.59 megatons with waves when compared with that without waves. This increase export was mainly concentrated at the surface near the western shore. Moreover, the waves increased lateral sediment entrapment in the Southwest Shoal, which explains the local geomorphologic evolution in recent decades. This study is of implication for the study on sediment exchange between shoal and channel in estuaries and on the fates of riverine sediment from source to sink.</p
Differential Etching of Rays at Wood Surfaces Exposed to an Oxygen Glow Discharge Plasma
No full textBasswood samples were exposed to oxygen glow-discharge plasmas for 30 min, and etching of radial and tangential longitudinal surfaces was measured. It was hypothesized that there would be a positive correlation between etching and plasma energy, and differential etching of wood surfaces because of variation in the microstructure and chemical composition of different woody tissues. Etching at the surface of basswood samples was examined using profilometry. Light and scanning electron microscopy were used to examine the microstructure of samples exposed to plasma. There was a large effect of plasma energy on etching of basswood surfaces, and radial surfaces were etched to a greater extent than tangential surfaces. However, rays at radial surfaces were more resistant to etching than fibers, resulting in greater variation in the etching of radial versus tangential surfaces. The same phenomenon occurred at radial surfaces of balsa wood, jelutong and New Zealand white pine subjected to plasma etching. The possible reasons for the greater resistance of rays to plasma etching are explored, and it is suggested that such differential etching of wood surfaces may impose a limitation on the use of plasma to precisely etch functional patterns at wood surfaces (raised pillars, grooves), as has been done with other materials.Forestry, Faculty ofWood Science, Department ofReviewedFacult
Characteristics of Speciation of Heavy Metals in Municipal Sewage Sludge of Guangzhou as Fertilizer
No full textAbstractApplication of municipal sewage sludge to agricultural land especially in shallow soils, demands to validate the maximum amount of heavy metal, monitoring its effects on soil and on plants. The use of sewage sludge as a fertilizer and soil amendment has resulted in high concentrations of heavy metals in the soil and food limiting its use. Controlling the pollution of heavy metals is the key factor to realize the safe utilization of sewage sludge. In the present study, the heavy metal stabilizers were added to sludge contained in used plastic containers, through artificially watering or naturally rain falling, the nutrient components flowed out with leaching water and fertilized plants but the heavy metals retained in the sludge within container. The potential toxic risks from heavy metals of sludge depend on their chemical speciation. The contents of heavy metals in different treatments were analyzed and their speciation was determined. The pot experiments with plants (Zea mays and Laetuca satiuali) showed that the positive effects of the mixture of the sludge and K2SO4 on plant production and reduction of heavy metal contents in plants were significant. The BCR sequential extraction procedure was applied for measurement of heavy metals in the experimental sludge. The results showed that the concentrations of Zn were predominant in acid exchangeable and reducible fractions, and Cu was principally distributed in oxidizable fractions. Metals-bound sludge could be collected easily after treatment to prevent the secondary pollution, provided the heavy metals were fixed within the container and reduced obviously the leaching of heavy metals to soil
Controls on the interannual variability of hypoxia in a subtropical embayment and its adjacent waters in the Guangdong coastal upwelling system, northern South China Sea
No full textCoastal embayments located downwind of large rivers under an upwelling-favorable wind are prone to develop low-oxygen or hypoxic conditions in their bottom water. One such embayment is Mirs Bay, off the Guangdong coast, which is affected by upwelling and the Pearl River Estuary (PRE) plume during summer. The relative importance of physical and biochemical processes on the interannual variability of hypoxia in Mirs Bay and its adjacent waters was investigated using statistical analyses of monthly hydrographic and water quality monitoring data from 2001 to 2015. The results reveal that the southwesterly wind duration and the PRE river discharge together explain 49% of the interannual variability in the size of the hypoxic area, whereas inclusion of the nutrient concentrations inside Mirs Bay and phytoplankton on the shelf explains 75% of the interannual variability in the size of the hypoxic area. This finding suggests that the interannual variability of hypoxia in Mirs Bay is regulated by coupled physical and biochemical processes. Increase of the hypoxic area under a longer-lasting southwesterly wind is caused by increased stratification, extended bottom water residence time, and onshore transport of a low-oxygen water mass induced by stable upwelling. In contrast, a reduction in the size of the hypoxic area may be attributed to a decrease in the surface water residence time of the particulate organic matter outside Mirs Bay due to increased discharge from the PRE. The results also show that the effects of allochthonous particulate organic matter outside Mirs Bay on bottom hypoxia cannot be neglected.</p
