The critical conditions for incipient sediment motion induced by oscillating-grid generated turbulence interacting with a sloping sediment layer were investigated experimentally. Near-spherical monodisperse sediments were used throughout with relative densities of 1.2 and 2.5 and mean diameters(d)ranging between 80 and 1087 μm. Interaction characteristics were analysed in terms of the critical Shields parameter θc, defined using the peak root mean square (r.m.s) horizontal velocity component in the near-bed region. Bed slope effects on θc were investigated by tilting the bed (and the grid) at angles between 0 and the repose limit. In all cases, the grid was aligned to be parallel to the bed surface, so that the oscillation direction is always normal to bed surface. The measured values of θc on a horizontal bed were comparable to the values reported in the traditional Shields diagram with θc seen to increase monotonically for hydraulically smooth bedforms and to be approximately constant for hydraulically rough bedforms. To account for bed slope effects, the measured values of θc were compared with a force-balance model based on the conditions for incipient grain motion on a sloping bed. For hydraulically smooth bedforms, where the bed roughness is small compared to the boundary-layer depth, the model was derived to account for how viscous stresses act to damp the drag and lift forces acting on the near surface sediment. For hydraulically rough bedforms, where this viscous-damping effect is not present, the model assumes the standard approach with the drag and lift forces scaling with the square of the near-bed (inviscid)velocity scale. In both cases the model predicts the bedforms to become more mobile as the bed slope is increased. However, the damping effect of the viscous sublayer acts as a stabilizing influenced for hydraulically smooth bedforms, to reduce the rate at which the bed mobility increases with bed slope. The measured values of θc in the hydraulically rough bedforms were in agreement with the trends predicted by this model. However, measured θc in smooth bed cases were lower than predicted, and fall on the hydraulically rough trend when bed slope is < 20. When the bed slope reaches the repose limit, θc falls between the smooth-bed and rough-bed cases. Measurements of sediment trajectories due to the turbulence interacting with the bed were obtained, for a range of impact conditions. Observations of the sediment trajectories during the interaction show the individual sediment grains to be predominantly displaced in a circular 'splash'. Data showed that the ‘splash’ feature and particle entrainment within the turbulence structure was within one eddy turnover time.

Large scale patterns in the seabed often occur in the nearshore zone of sandy beaches. A widely occurring bed pattern is the crescentic bar. These bed patterns develop under moderate wave conditions, and form a lunate shaped alongshore pattern in front of a coast. Over recent years, knowledge concerning the development, occurrence, and characteristics of these bed patterns has been significantly expanded through field studies and modelling attempts. An example of such a model is the linear stability analysis, which describes the initial development of crescentic bed patterns along an undisturbed beach. To date, comparisons between field measurements and modelling results have been general in nature. The purpose of this research is to investigate whether a linear stability analysis, which is useful for understanding the physics of emerging bed-forms, can be used to make quantitative predictions in the field. To this end a morphodynamical linear stability model (Morfo60, [Calvete et al., 2005]) is used to describe the development of crescentic bed patterns at the coast at the USACE Field Research Facility in Duck, North Carolina, USA. Wave, tide and bathymetry data recorded at Duck over a two month period in 1998 are used to model the development of these morphodynamical patterns. The model predictions are compared with field observations made at Duck, over the same two month period, reported by van Enckevort et al. [2004]. A direct comparison shows that predicted length scales of crescentic bed patterns are similar to those observed. However, the model predictions show more fluctuations than are observed in the field. This is because the model describes the development of crescentic bed patterns starting from an alongshore constant bed, whereas in reality bed patterns already exist in most situations. An algorithm is developed to overcome these fluctuations and identifies the more physically significant model predictions based on large growth rates and consistency in length scales. The moments at which physically significant model predictions occur correspond better with field observations than the original model predictions. The effects of pre-existing bed-forms on the development of crescentic bed patterns are investigated using a non-linear model (Morfo55, [Garnier, 2006]). Results show that pre-existing bed patterns can have significant effects, however, the finally dominant length scale, the linear growth and decay rates, and the migration rate can be accurately described by a linear stability model. Pre-existing length scales that exhibit significant linear growth will remain and undergo further development, whereas length scales that are outside the linear growth rate curve decay and give rise to a bed pattern with a bigger linear growth rate. The conclusions drawn from the research concerning pre-existing bed patterns are applied to improve predictions linear stability model. This results in considerable improvements in the comparison of model predictions with field observations, for certain periods of time.

An integrated horizontally two- and fully three-dimensional numerical model system has been developed based on a combined unstructured and σ-coordinate grid to simulate the flow and water quality process in large water bodies with a focus on the three dimensional behaviours at specific areas. The model is based on the time dependent Reynolds-Averaged Navier-Stokes equations with a non-hydrostatic pressure distribution and a baroclinic force being incorporated in the three dimensional (3D) model. The two sub models interact dynamically during the solution procedure with no time-step restriction due to integration. The main idea is to use a fractional step algorithm for each model and then integrate the two models fraction by fraction. Hybrid 2D-3D finite volume cells have been introduced for the link nodes which are partly in the 2D domain and partly in the 3D domain. Thus an interpolation/averaging procedure at the interface and domain overlapping is no longer needed. The 3D model uses the projection method for pressure calculation. The advection equation is solved by the semi-Lagrangian method. Other components are solved via the finite element - finite volume (FV) method. The water surface is determined implicitly through a global matrix equation created by assembling the domain's matrices. The cell integrals are calculated analytically to eliminate a common source of numerical diffusion due to the use of approximation techniques for the FV integrals. The horizontal gradients of the density and shear stresses are calculated on true horizontal planes, in order to avoid artificial velocity and diffusion in highly stratified flows. Neumann interpolation elements with virtual nodes have been introduced at Neumann type of boundaries for more accuracy. The integrated model has been verified using analytical solutions and benchmark test cases, including the Ekman velocity distribution, wind driven circulation, lock exchange and integrated 2D-3D flows in basin. The results show the model is capable of the model for accurate simulation and implicit 2D-3D integration. Keywords: integrated modelling, hydrodynamic numerical model, non-hydrostatic, unstructured mesh, hybrid finite element finite volume method.

A dam break event is considered, taking place over a uniform sediment bed. Understanding and modelling the erosion that occurs when the fluid behind the dam collapses at release has important applications in coastline morphodynamics / beach erosion modelling. A new coupled two dimensional Navier-Stokes solver and sediment transport model is presented with novel methods for dealing with non-converging solutions to the Navier-Stokes equations and a new adaptation to the Youngs [1982] volume-of-fluid reconstruction scheme. The implementation of a sediment transport model includes a new method for accounting for mass conservation for the transition of sediment between bed and flow as well as a novel method for accounting for the redistribution of material associated with the maintenance of the critical angle of repose or slope limit. The model is validated and then applied to a dam break simulation for various backwater and tailwater conditions. Classical experimental realizations of dam break events have involved the rapid removal of a barrier in a flume [e.g. [Levin, 1952; Dressler, 1954; Bell et al., 1992])]. However, early-time flow analysis encounters two problems with this method. Firstly, the removal of the barrier creates a strong vortex sheet on the face of the static fluid immediately behind the barrier that is not present in either the idealized problem, or the motivating environmental problems. Secondly, the removal of the barrier cannot take place instantaneously and so a brief jet-like flow is initially induced through the opening between the base of the barrier and the sediment layer. We partially circumvent these difficulties with the classical experiments by implementing a novel dam break barrier release, using a barrier similar to the wrapped-fabric design of [Dalziel, 1993], which minimizes the initial vortical disturbance. Three-dimensional stereoscopic Particle Image Velocimetry (PIV) measurements allow us not only to capture the velocity field in the laser-plane, but also perpendicular to it. These planar experimental results are compared to the results of the numerical study and the comparison is shown to be good while the simulation successfully converges.

No description available.

The lattice Boltzmann method (LBM) has become an effective numerical technique for computational fluid dynamics (CFD) in recent years. It has many advantages over the conventional computational methods like finite element and finite difference methods. The method is characterised by simplicity, easy treatment of boundary conditions and parallel feature in programming that makes it ideal for solving large-scale real-life problems. This thesis presents the development and applications of a lattice Boltzmann model for both steady and unsteady two-dimensional axisymmetric flows. The axisymmetric flows are described by three-dimensional (3D) Navier-Stokes equations, which can be solved by three-dimensional (3D) lattice Boltzmann method. If cylindrical coordinates are applied, such 3D equations become 2D axisymmetric flow equations. However, they cannot be solved using the 2D standard LBM. In order to study more complicated axisymmetric flow problems by 2D LBM, in this thesis, firstly, the revised axisymmetric lattice Boltzmann D2Q9 model (AxLAB®) is applied and tested for some benchmark for axisymmetric laminar flows and more complicated flows including 3D Womersley flow and forced axisymmetric cold-flow jets, and flows with swirl such as the cylindrical cavity flows and the swirling flow in a closed cylinder with rotating top and bottom. Secondly, the AxLAB® is extended to simulate turbulent flows and non-Newtonian fluid flows. A well-known power-law scheme is incorporated into the AxLAB® to simulate the non-Newtonian fluid flow: the Taylor Couette flows for Newtonian and non-Newtonian fluids are simulated and compared. The combined effects of the Reynolds number, the radius ratio, and the power-law index on the flow characteristics are analysed and compared with other literatures. All the numerical results are also compared with the existing numerical results or experimental data reported in the literature to demonstrate the accuracy of the model. Thirdly, a further developed AxLAB® is presented to simulate the turbulent flows. The turbulent flow is efficiently and naturally simulated through incorporation of the standard subgrid-scale stress (SGS) model into the axisymmetric lattice Boltzmann equation in a consistent manner with the lattice gas dynamics. The model is verified by applying it to several typical cases in engineering: (i) pipe flow through an abrupt axisymmetric constriction, (ii) axisymmetric separated and reattached flow and (iii) pulsatile flows in a stenotic vessel. All the numerical results obtained using the present methods are compared with experimental data and other available numerical solutions, indicating good agreements. This shows that the improved AxLAB® is simple and is able to predict axisymmetric turbulent, non-Newtonian complicated flows at good accuracy.

Thesis advisor: Hong Ding / High temperature superconductivity has been one of the most challenging problems in condensed matter physics since its discovery. This dissertation presents systematic studies on electronic structures of single layer high temperature supconductor Bi2Sr2CuO6+[delta] by angle-resolved photoemission spectroscopy. A high binding energy band anomaly is observed in PbxBi2−xSr2CuO6+[delta]. Comparing with LDA calculation, the band is highly remonetized by a factor of 3, the incoherent part coexists and forms a band anomaly as a non-dispersive dive structure. Systematic studies are performed on Bi2+xSr2−xCuO6+[delta] with a wide doping range, revealing a linear doping nature of Bi substitution. An unusual Coulomb gap is observed in the heavily substituted samples. Our results reveal the dual role of off-plane chemical substitution in high-TC cuprates and elucidate the nature of the quantum electronic nature due to strong correlation and disorder. High resolution ARPES on LaxBi2Sr2−xCuO6+[delta] observes a large gap and a small gap coexisting at antinodal region below TC. The small gap is d-wave like and attributed to superconductivity, while the large gap is attributed to a CDW order. A strong short-ranged correlation between the small and large gap magnitude suggesting that superconductivity and charge ordering are driven by similar physical mechanism. / Thesis (PhD) — Boston College, 2008. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

The difficulty with sludge settleability is considered one of the main drawbacks of sequencing batch reactors. The aim of this study therefore is to improve sludge settleability by introducing a novel, two-stage settling sequencing batch reactor (TSSBR) separated by an anoxic stage. The performance of the TSSBR was compared with that of a normal operating sequencing batch reactor (NOSBR), operating with the same cycle time. The results show a significant improvement in sludge settleability and nitrogen compound removal rates for the TSSBR over the NOSBR. The average removal efficiencies of ammonia-nitrogen (NH3-N), nitrate-nitrogen (NO3-N) and nitritr-nitrogen (NO2-N) have been improved from 76.6%, 86.4% and 87.3% respectively for the NOSBR to 89.2%, 95.2% and 96% respectively for the TSSBR. In addition, the average sludge volume index (SVI) for the NOSBR has been reduced from 42.04 ml/g to 31.17 ml/g for the TSSBR. After three months of operation, there was an overgrowth of filamentous bacteria inside the NOSBR reactor, while the morphological characteristics of the sludge inside the TSSBR reactor indicated a better and homogenous growth of filamentous bacteria. TSSBR system proves to be more efficient than NOSBR by improving the sludge settleability and enhancing nitrogen compounds’ removal efficiency, therefore, the TSSBR operating conditions including (mixed liquor suspended solids, hydraulic retention time, fill conditions, fill time, volumetric exchange rate, organic loading rate and hydraulic shock) have been optimised to obtain the optimal performance of the TSSBR system regarding the treatment efficiency and sludge settling performance. The results of optimising the TSSBR operating conditions are as follows: the optimal MLSS range was 3000 mg/l to 4000 mg/l; the optimal HRT was 6 h; unaerated feeding was better than the aerated feeding, and 15 minutes was the optimal feeding time; the optimal VER value was 20%; the optimal OLR ranges were 750 to 1000 mg/l glucose loading rate and 50 to 150 mg/l potassium nitrate loading rate. Finally, the TSSBR system was operated under the obtained optimal operating conditions. The results showed that the treatment efficiency of COD and NO3-N had been improved significantly. Although the removal efficiency of NH3-N and NO2-N did not improve, the removal efficiency of both is more than 90%, which is considered a good treatment efficiency for the TSSBR system. In addition, the settling performance of the TSSBR was significantly improved after operating the system under the optimal operating conditions.

This thesis is composed of three projects including the AC magnetic susceptibility study of high-temperature superconductor YBa$_2$Cu$_3$O$_{7-\delta}$, the ionic-liquid gating study of the Mott insulator Sr$_2$IrO$_4$, and the single-electron study of quantum dot device with self-assembled nanocrystal PbS. Chapter 1 covers a general introduction to all three projects. The basic background and the motivation for each project are presented. Project I is covered in Chapter 2, Chapter 3, and Chapter 4. The first part of Chapter 2 is a theoretical introduction to the Bardeen-Cooper-Schrieffer theory of superconductivity with its main conclusions presented. This chapter builds a basis for the use of high pressure technique to YBa$_2$Cu$_3$O$_{7-\delta}$ in the later chapters. The rest of Chapter 2 reviews the work in the study of high-temperature superconductors, especially on YBa$_2$Cu$_3$O$_{7-\delta}$, on both experiments and theories and the possible applications of high-temperature superconductors. Chapter 3 introduces the YBa$_2$Cu$_3$O$_{7-\delta}$ sample preparation process and the characterisation. A dry cryomagnetic equipment was employed for the measurement. The results and the discussion are presented in Chapter 4. Project II is described in Chapter 5, Chapter 6, and Chapter 7. Chapter 5 firstly introduces the background knowledge of the gated material SrTiO$_3$ and the technical details of the ionic-liquid gating technique. Then the sample growth and the characterisation are presented. The fabrication process of Sr$_2$IrO$_4$ and SrTiO$_3$ (material for a control experiment) are described in Chapter 6. Chapter 7 covers the measurement and the result of the fabricated devices and related discussion. Project III ranges from Chapter 8, and Chapter 9. A literature review of quantum-dot devices and self-assembled nanocrystals is presented in Chapter 8. The experimental design of this nanocrystal quantum dot device is also included. Following it, the fabrication process of quantum-dot devices and the techniques used for fabrication are introduced in the start of Chapter 9. Chapter 9 also gives a description of the probe-station for measurements. The results and discussion of the measurements are covered in the last section of Chapter 9. Chapter 10 summarises and concludes the three projects stated above and gives some suggestions about the directions for future work.

It is well understood that the spilled oil from damaged oil tankers poses a severe threat to the marine environment. Although great efforts have been devoted to studying the oil spilling from damaged oil tankers, especially double hull tanks (DHTs), the majority is subjected to an ideal condition (e.g., fixed tanks in still water; simple damage conditions) and adopts hydrostatic theories or quasi-steady models with over-simplified assumptions on data analysis or analytical prediction. These conditions or assumptions may not stand in the complex dynamic spilling process in the real spilling accident. This study brings a step further on the knowledge of oil spilling from a damaged tank by combining experimental and numerical investigations, with a focus on the dynamic spilling process from damaged oil tankers which is either fixed or subjected to motion, which have not been systematically investigated. In the experimental investigation, the submerged oil spilling from DHTs under different accidental scenarios including grounding and collision is studied. Two new sets of laboratory tests are carried out, where the damaged tank is fixed in still water. In the first set, the axial offset between the internal and the external holes on two hulls of the grounded DHT is considered to widen the scope of damage conditions which the tanker may suffer from during grounding accidents. Although all cases in this set are subjected to the same hydrostatic conditions, completely different dynamic spilling processes are observed. In the second set, the initial water thickness inside the ballast tank of the collided DHT is considered. This aims to represent the real scenarios that the external hull is generally damaged prior to the internal hull and, therefore the ballast space is partially filled by the water flowing from the surrounding environment before the internal hull is damaged. These experiments do not only advance the state of the art of the experimental study in this field, but also provide a reference for validating the numerical models developed in this study. Based on the experimental data, the correlation analysis for the discharge through the internal hole by using quasi-steady Bernoulli’s equation is presented, contributing to the development of an improved analytical model for predicting the oil spilling from damaged oil tankers. The numerical study is carried out using a numerical model developed in OpenFOAM framework, where the VOF is applied to deal with the air-oil-water multiphase flow. This model enables the users: (1) to consider air, oil and water three phases of fluid and their interaction with solid tanker hull using dynamic mesh technologies; (2) to model turbulence associated with the oil spilling process using various available turbulent models; and (3) to investigate the effects of the compressibility of the fluid. The oil spilling from damaged DHTs is simulated and validated by the experimental data. Intensive investigations are carried out to clarify uncertainties in existing numerical modelling of the oil spilling from damaged DHTs. These include (1) the associated turbulence behaviours and selecting an appropriate approach to turbulence modelling; (2) the role of fluid compressibility during the oil spilling; and (3) the effect of tank motion on the oil spilling process. For the turbulence modelling, various approaches to model the turbulence, including the large eddy simulation (LES), direct numerical simulation (DNS) and the Reynolds average Navier-Stokes equation (RANS) with different turbulence models are attempted. It is concluded that the oil spilling from DHTs is more sensitive to the turbulence modelling than that from SHTs. For DHT cases, the effective Reynolds number (Re) considering both oil outflow and water inflow is suggested to classify the significance of the turbulence and to correspondingly select the appropriate turbulence model. The investigation on the role of the air compressibility in the oil spilling from damaged DHTs reveals that the air compressibility may be considerable in a small temporal-spatial scale (e.g., jet-jet and jet-structure impact pressure), but plays an insignificant role in the macroscopic process of the oil spilling (e.g., spilling discharge and volume). In order to approach the spilling phenomena in the more realistic environment, a systematic numerical study is carried out to investigate the effect of the periodic ship motion on the oil spilling from the damaged tank. Different tank designs (i.e., SHTs and DHTs), accidental scenarios (i.e., grounding and collision) and tank motion parameters (i.e., types, frequencies and amplitude) are considered. The result indicates that the tank motion does not only cause a periodic oscillation of the oil/water flow through the broken hole, but also induces a second long-duration stage of spilling after a quasi-hydrostatic-equilibrium condition occurs, resulting in the more significant amount of spilled oil. By using both the experimental data and numerical results produced in this research, an improved prediction model for oil spilling from damaged DHTs in still is formulated. This model considers the case-dependent hydrodynamic interaction between the oil and water jet flows inside the ballast tank and its effect on the spilling process. The result using the improved model is compared with the numerical result indicating its superiority over the existing model.