77 research outputs found
Inter-model robustness of the forced change of the ENSO-Indian Summer Monsoon Teleconnection
No full textConcerning the robustness of predicting the forced response of the El Niño—Southern Oscillation-Indian summer monsoon (ENSO-ISM) teleconnection based on 9 CMIP-class models, we come to distinguish three time periods. (1) In the late 20th century, the trend was insignificant in most models, which does not permit a statement on robustness in this period. Thus the weakening of the teleconnection reported from the observational data might not be truly forced. (2) Furthermore, we find it typical that under global warming in the first part of the 21st century, the teleconnection is strengthening or non-decreasing. This considerable inter-model robustness is owed to an increasing ENSO variability as well as coupling strength. (3) At the end of the 21st century, however, under strong forcing, the teleconnection change is not robustly modeled: the ENSO variability change is not projected robustly across models, either with respect to the start or the rate of the ENSO variance decline, competing this time with an increase of the coupling strength. The difference in the projected coupling coefficient between models is mainly attributed to the disagreement in the projected ENSO-induced changes in the regional Hadley cell and SST patterns over the ISM domain, despite the agreement on the projected eastward shift of the Walker cell over the Pacific Ocean. © 2024, The Author(s).11Nscopu
Molecular mobility and oxygen permeability in amorphous β-lactoglobulin films
No full textEdible films and coatings are developed to extend the shelf life of food products. Our overall objective is to understand how molecular mobility modulates diffusion rates and thus chemical reactivity in films made from amorphous β-lactoglobulin. The phosphorescence emission spectra and lifetimes of the triplet probe erythrosin B embedded in the β-Lg films provide measures of the modes, rates, and distribution of molecular mobility in the film, providing the molecular detail necessary to connect food quality and stability to molecular structure and mobility. The mobility contours generated from this research provided us with information about the onset temperature and level of molecular mobility required to support permeability of atmospheric oxygen. In β-Lg -- based binary matrices, sugars (sucrose, trehalose, maltose), plasticizers (glycerol, sorbitol, maltitol and PEG-400), fatty acids (palmitic acid, caprylic acid) and protein (BSA) were selected to investigate how variations in composition influence the molecular mobility and oxygen permeability in amorphous β-Lg matrix. Further more complicated β-Lg -- based ternary matrices (maltose and maltitol) and (PEG and sucrose) were generated inorder to gain a deeper understanding of edible films.
In pure β-Lg films there was linear correlation between molecular mobility and oxygen permeability. Various additives showed different results with respect to mobility and permeability. The addition of sucrose, maltose, maltitol and trehalose greatly reduced the mobility and the permeability of the β-Lg matrix. Glycerol exhibited an anti-plasticization effect and showed decreased mobility at a molar ratio of 1:1 glycerol/ β-Lg. PEG greatly enhanced the permeability of β-Lg matrix. Fatty acids palmitic acid and caprylic acid had a rigidification effect on the matrix with no change in permeability. We were able to detect dynamic synergies in β-Lg maltose and maltitol mixtures, whereby these sugar- polyol mixtures at equal ratios anti-plasticized the β-Lg matrix and at unequal ratios plasticized the matrix. The tertiary matrix comprising of β-Lg, PEG 400 and sucrose brought about a substantial reduction in the permeability. A better understanding of the mobility in these complex matrices will help improve the effectiveness of β-Lg in barrier applications in real food systems.Ph.D.Includes bibliographical references
Measuring Access and Practice: Designing a Survey Methodology for the Hygiene, Sanitation and Water Sector
Get PDFAccess to safe water and sanitary means of excreta disposal are essential elements of
human development and poverty alleviation. It is estimated that one in four people in
the developing world lacks access to water while over half the population has no
access to sanitation. From the Alma-Ata declaration in 1978 to the recent Millennium
Development Goals, efforts to improve this situation have been hampered by the lack
of meaningful indicators to measure hygiene, sanitation and water coverage and
establish progress towards the goals and targets set out by the international
community.
This thesis aims to determine if measuring prevalence of access to water~ sanitation
and the practice of hygienic behaviour in hous~hold surveys can be.improved. With
no indicators available in current international' laws and targets, various aspects of
access and practice were examined to design indicators for field-testing. By using
- existing data sets, the research established that there is a high geographic clustering of
the measures of interest, which results in large design effects (deff) and rates of
homogeneity (roh) in cluster surveys. Based on the calculated roh optimum numbers
ofcluster and sample size were calculated for the field trials. This requires
introducing survey costs in the sample size calculations. The high clustering of water
and sanitation indicator require large sample sizes, resulting in large amounts of data
which organisations in the four field trials in Kosovo, South Africa, Kenya and Laos
found difficult to handle. Practical problems in the implementation of the survey
method resulted in non-sampling errors and could cause reluctance in adoption the
methodology. The research improved water and sanitation indicators but found that
for individual behaviour such as hygiene the household is not a suitable sampling unit.
It also showed that observation among interviewers have to be better standardised to
reduce the inter-surveyor.variation. Representative sampling is the current bottleneck
in the development of such a survey method. Current method requires a good
understanding of sampling theory as well as reliable sample frames, which are rarely
available to implementing organisations. Alternative sampling methods are
suggested, and recommendations are made for the further development ofthe survey
method designed in this research, which to date may be too complex for widespread
use
A Lattice Boltzmann Approach to Multi-Phase Surface Reactions with Heat Effects
No full textThe aim of the present research was to explore the promises and shift the limits of the numerical framework of lattice Boltzmann (LB) for studying the physics behind multi-component two-phase heterogeneous non-isothermal reactive flows under industrial conditions. An example of such an industrially relevant topic is the Fischer-Tropsch Synthesis (FTS) in the Gas-to-Liquid (GtL) conversion process of methane. The research described in this thesis was carried out in the context of a twin project supported by Shell and STW on structured reactors for Fischer-Tropsch and the meso-scale flow and transport phenomena and catalysis aspects therein. The complexity of such multi-component two-phase heterogeneous reactive flow systems with thermal effects was a good reason for splitting the topic up into a number of constitutive elements which were tackled individually. The various LB methods available in the literature dealing with all these separate elements were studied; the most promising methods were identified, improved where needed, and implemented in a three-dimensional code structure which was then validated against theory and/or experiment. At the start of the development of this numerical infrastructure, we implemented an incompressible single-phase LB based flow solver. This was used for analysing in 2-D the flow and for detecting stagnation zones in cross-flow structured packings for tubular fixed bed reactors. This study was combined with an experimental and modelling investigation comparing the heat transfer characteristics of different types of packings in such reactors. Further details of this analysis are provided in Chapter 2 of this thesis. Then, the LB approach was extended for dealing with multi-component gas-liquid flows over solid surfaces. The pseudo-potential concept due to Shan and Chen (1993) was identified as being the promising method, as it describes the interaction of components and interaction in more fundamental terms and does not require a separate equation for tracking the phase interfaces. In this concept, the various components each have their own particle distribution function governed by an own Boltzmann equation. Inter-particle forces are defined in the format of potential functions for each of the components; the interactions between the various components and phases are controlled by parameters denoted as coupling strength. These potential functions should reflect some equation of state (EOS). For the interaction of fluid components with the solid boundaries, a similar approach was considered. These component-component and component-wall pseudo potentials and the pertinent coupling strengths are responsible for phase separation, surface tension, density ratio between phases, contact angle and so on; the values of all these continuum variables follow from the selected potential functions and coupling strengths. The original Shan & Chen concept was introduced for two EOSs only (ideal gas and Van der Waals) and suffered from numerical instabilities for density ratios in excess of, say, 10. More recently, Yuan and Schaefer (2006) came up with improvements allowing the use of different EOSs (such as Carnahan-Starling and Redlich-Kwong) with a positive effect on the numerical stability at higher density ratios. Implementing such modifications resulted in successful simulations of two-phase systems with density ratios as high as 1,000 - depending on the EOS used and on the reduced temperature. The code developed so far was capable of dealing with single- and multiple-component systems with various density ratios and wettability properties. It was used for studying a variety of cases including a spontaneous phase separation process between two phases, the rise of a single bubble in a liquid pool, fully wetting (Taylor) and partially wetting segmented flows in a straight tube, and almost non-wetting/partially wetting droplet flow in inclined micro-channels. The outcome of these simulations compared favourably with available literature data or experiments. Particularly with respect to the motion of a Taylor bubble through a tube, a comprehensive quantitative analysis was performed tackling different aspects of the bubble and its motion. This validation study comprised the analysis of the variations of liquid film thickness, bubble to liquid velocity ratio, and bubble shape with the Capillary number; also the pressure field in the thin liquid film between bubble and tube wall was evaluated. Further background on this multi-component multi-phase flow solver and the details of the validation studies performed are described in Chapters 3-4 of this thesis. Solving the energy conservation equation in two-phase systems with phase change phenomena included was another step in improving the potential of the current LB approach with the view of the project objectives. In this approach, next to the ones used in the multi-component system, an additional distribution function was introduced for a pseudo-temperature scalar variable . This variable recovers the macroscopic conservation of energy in two-phase mixtures. The rest of the distribution functions take care of the mass and the momentum conservation of the multi-component system. Heats of reaction, enthalpy change associated with phase change, and diffusive transport of enthalpy are all taken into account; the dependence of enthalpy on pressure, which is usually a small effect in most non-isothermal flows encountered in chemical reaction systems, is ignored however. The energy equation was coupled with the LB equations for species transport and the pseudo-potential interaction forces through the EOS by using the local pseudo-temperature field. In order to control the noise associated with the fluctuations in the pseudo-temperature in the vicinity of the diffused interface, a robust regularized spline algorithm was introduced in the coupling between pseudo-temperature and density. The proposed scheme was used for simulating some benchmark problems the results of which were validated against available analytical solutions. An extensive explanation on this thermal two-phase model and its applications are provided in Chapter 5 of this thesis. As another important element of this research, the potential of the current multi-component LB methodology for solving the advection-diffusion equation for multiple species was assessed. This study showed - for various velocities and for various values of the ratio of the diffusivities of solvent and solute - that the accuracy of our multi-component LB methodology is high. Of course, this accuracy depends on the spatial resolution, expressed in terms of the diffusion depth per grid spacing. Furthermore, we investigated the accuracy of this model by simulating the mass transfer of a gas-like component across a gas-liquid interface at a moderate density ratio of the two phases and at a moderate value of Henry's coefficient. Further details can be found in Chapter 6 of this thesis. Finally, the current approach was used for simulating the complex interplay of diffusion and surface reaction in a multi-component gas-liquid catalytic chemical reactor under (for the time being) isothermal conditions. The current lattice Boltzmann technique was capable of reproducing quite realistically, at satisfactory temporal and spatial resolution, the combination of species transport across a phase interface, a chemical reaction at a catalytic surface, and the resulting phase change due to the surface reaction. This was observed after careful comparison of the simulation data with analytical models. We simulated a simplified isothermal 1-D Fischer-Tropsch Synthesis in which hydrogen and carbon monoxide reacted to water and paraffin at a catalytic flat surface with an educated simplification of the reaction kinetics. This four-component gas-liquid surface reaction with a liquid film covering a catalytic surface from the very beginning resulted in a gradually increasing thickness of the liquid layer as well as in quite realistic species concentration profiles in liquid and gas phases. In our simulated model, the tracked hydrocarbon was representative of all the possible carbon chain products in such a reaction. In the end, by combining all temporally resolved species concentrations with empirical Anderson-Schulz-Flory (ASF) model and by determining the chain growth probabilities, the pertinent carbon chain products were calculated for the range . The extensive description of this multi-component multi-phase surface reaction study is given in Chapter 7 of this thesis. Finally, Chapters 8 and 9 present the overall conclusions of this project and an outlook for further work.Chemical EngineeringApplied Science
A novel approach to MP-PIC: Continuum particle model for dense particle flows in fluidized beds
No full textA novel approach to Multiphase-Particle-in-Cell (MP-PIC), called Continuum Particle Model (CPM), is developed for dense gas-particle flows. CPM has high computational speed, comparable to that of MP-PIC, but a robustness and accuracy closer to that of a Discrete Element Model (DEM). The gas phase is treated as a continuum phase and particles are tracked discretely, but particle collisions are modelled by considering the divergence of the continuum particle stress tensor. Details on efficient solution to the model are presented. For comparison, a parametric study is performed for quasi-2D fluidized beds. Comparison of CFD-CPM is made with MP-PIC and CFD-DEM. The particle stress models by Harris and Crighton, and by Srivastava and Sundaresan are tested in our CFD-CPM. Results from CFD-CPM based on the Srivastava and Sundaresan particle stress model show good agreement with CFD-DEM results. We validate our model by comparison with experimental benchmark results from Gopalan et. al. (2016).Complex Fluid Processin
Comparison of experimental results with numerical simulations for pulsed thermographic NDE
Get PDFThis paper examines pulse thermographic nondestructive evaluation of flat bottom holes of isotropic materials. Different combinations of defect diameters and depths are considered. Thermographic Signal Reconstruction (TSR) method [1] is used to analyze these results. In addition, a new normalization procedure is used to remove the dependence of thermographic results on the material properties and instrumentation settings during these experiments. Hence the normalized results depend only on the geometry of the specimen and the defects. These thermographic NDE procedures were also simulated using finite element technique for a variety of defect configurations. The data obtained from numerical simulations were also processed using the normalization scheme. Excellent agreement was seen between the results obtained from experiments and numerical simulations. Therefore the scheme is extended to introduce a correlation technique by which numerical simulations are used to quantify the defect parameters.</p
Direct numerical simulations of dense suspensions: Wave instabilities in liquid-fluidized beds
No full textWe present results of direct numerical simulations of travelling waves in dense assemblies of monodisperse spherical particles fluidized by a liquid. The cases we study have been derived from the experimental work of others. In these simulations, the flow of interstitial fluid is solved by the lattice-Boltzmann method (LBM) and the particles move under the influence of gravity, hydrodynamic forces stemming from the LBM, subgrid-scale lubrication forces and hard-sphere collisions. We first show that the propagating inhomogeneous structures seen in the simulations are in agreement with those observed experimentally. We then use the detailed information contained in the simulation results to assess aspects of two-fluid model closures, namely, fluid–particle drag, and the various contributions to the effective stresses. We show that the rates of compaction and dilation of the particle phase in the travelling waves are comparable to the rate at which the microstructure relaxes, and that there is a pronounced effect of the rate of compaction on the average collisional normal stress. Although this effect can be expressed as an effective bulk viscosity term, this approach would require the use of a path-dependent bulk viscosity. We also find that the effective fluid–particle drag coefficient can be described well with the often-used closure motivated by the experiments of Richardson & Zaki (Trans. Inst. Chem. Engng vol. 32, 1954, p. 35). In this respect, the effect of the system size for determining the drag requires specific care. The shear viscosity of the particle phase manifests small, but clearly noticeable dependence on the rate of compaction/dilation of the particle phase. Our observations point to the need for higher-order closures that recognize the slow evolution of the microstructure in these flows and account for the effects of non-equilibrium microstructure on the stresses.Multi-Scale PhysicsApplied Science
Docking of underwater vehicle: Model, Autopilot design, and Guidance
No full textAllseas engineering BV is an offshore company that uses underwater vehicles for conducting subsea operations. The two primary classification of such underwater vehicles are: Remote operated vehicle (ROV) and Autonomous underwater vehicle (AUV). As the names suggests, the former requires a human operator to control and the latter is a fully autonomous vehicle. As of now, the company (and most of the offshore industries) customarily use ROVs. In the recent past, Allseas was inclined to use AUVs as a measure of reducing the operational costs and conducted tests runs with an industrial grade AUV. But the test runs were unsuccessful and the plan of using such an AUV was dropped. It was concluded that the major problem was in launch and recovery operations of the AUV, which were conducted from the deck of a ship. To be specific, the disturbances from the ship’s thrusters, ocean currents, and waves, were proved to be impossible to compensate for by the AUV during the launch and recovery operations. Therefore, this thesis aims to investigate on underwater docking capabilities of AUVs, which not only eliminates the launch and recovery issues but improves AUV’s overall operational capabilities. In this thesis, a docking problem is formulated and the solution to the problem covers the following aspects: modeling of AUV, motion control of AUV, and guidance strategies. For the motion control, an appropriate model of the vehicle is necessary. Various models used in the literature were studied which include: 6 degree of freedom (DOF) non-linear model, 3-DOF horizontal plane model, and 3-DOF vertical plane model. As an example, a 6-DOF non-linear model of ARIES AUV from literature was decoupled into the respective 3-DOF models. These models are used for controller design and docking strategies. A linear parameter varying (LPV) frame work based, gain scheduled feedback and feed forward controllers were developed for the control of vehicle heading and depth. The controller design involves the following steps: Firstly, a third order Quasi- LPV control plant for heading and depth were derived from horizontal and vertical plane models. Then, linear controllers (gains) were designed for fixed values of scheduling variables. Based on the dimension of the scheduling variables, a stability preserving interpolation of these gains were performed to obtain the final controller. A PI controller was designed for controlling the longitudinal velocity of the vehicle. The performance of the controllers were checked on 6 DOF and 3 DOF models of the ARIES AUV. Finally, two docking strategies: Three point and N-point, were developed using lookahead based path following and investigated their performance for two scenarios: stationary dock and non-stationary dock. The docking strategies addressed the docking problem in two aspects: geometric path generation and path following. Finally, the controller performance in closed loop, as in, the guidance, tracking controllers, and vehicle dynamics was compared for the two developed strategies. It was shown that the N-point docking strategy yielded better convergence to the paths than the three point docking.systems and controlDelft Center for Systems and ControlMechanical, Maritime and Materials Engineerin
A normalization procedure and determination of axisymmetric defect characters of composite panels
Get PDFIn Pulse Thermographic Nondestructive Evaluation (TNDE), the thermographic signals are heavily influenced by material directionalities, therefore comparison of thermographic results are challenging. In this study a normalization procedure is introduced which simplifies the analysis and requires only break time, equilibrium temperature, and diffusivity ratios of transverse to lateral diffusivity. The normalization for the composite panels was proved to be effective for quasi-isotropic carbon epoxy composite panels used in the study. A correlation method was utilized to estimate the defect diameter and depth of a composite panel. The correlation method utilizes several numerical models to estimate the correct defect parameter of the experimental specimen. Thermographic Signal Reconstruction (TSR) second derivatives [1] found to be effective in differentiating the defect parameters. Since this method was proved to be successful an additional advantage of this method of estimating the in-plane diffusivity of unknown composite panels also have been proposed.</p
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