4,288 research outputs found
Improving the predictive capability of the soil erosion modeling tool EROSION-3D: From observation data to validation
Ziel dieser Arbeit ist die Verbesserung der Vorhersagekraft des Bodenerosionsmodelierungs-werkzeugs EROSION-3D, welche oftmals durch die Identifizierung der werkzeugspezifischen Parameter Skinfaktor und Erosionswiderstand limitiert ist.
Als drei Betrachtungsebenen der Arbeit werden 1. Beobachtungsdaten, 2. die Fähigkeit von EROSION-3D zur Beschreibung der Beobachtungsdaten und 3. die Vorhersagekraft des Werkzeugs untersucht. Aufzeichnungen verschiedener Beregnungsversuche wurden maschinenlesbar zusammengefasst. Daran wurde EROSION-3D mit den bisher üblichen sowie Monte-Carlo Methoden kalibriert. Anhand beschreibender Daten der Beregnungsversuche wurden Vorhersagemethoden zur Schätzung der modellspezifischen Parameter entwickelt und hinsichtlich der Parameterwerte und damit modellierter Abfluss-/Abtragswerte validiert.
Die Ergebnisse zeigen, dass verbesserte Vorhersagen mit den neuen Schätzmethoden möglich sind, aber auch Möglichkeiten zur Verbesserung der Modellstruktur bestehen
Transition Physics and Boundary-Layer Stability: Computational Modeling in Compressible Flow
Laminar-to-turbulent transition of boundary layers remains a critical subject of study in aerodynamics. The differences in surface friction and heating between laminar and turbulent flows can be nearly an order of magnitude. Accurate prediction of the transition region between these two regimes is essential for design applications.
The objective of this work is to advance simplified approaches to representing the laminar
boundary layer and perturbation dynamics that usher flows to turbulence. A versatile boundary-layer solver called DEKAF including thermochemical effects has been created, and the in-house nonlinear parabolized stability equation technique called EPIC has been advanced, including an approach to reduce divergent growth associated with the inclusion of the mean-flow distortion. The simplified approaches are then applied to advance studies in improving aircraft energy efficiency.
Under the auspices of a NASA University Leadership Initiative, the transformative technology
of a swept, slotted, natural-laminar-flow wing is leveraged to maintain laminar flow over large
extents of the wing surface, thereby increasing energy efficiency. From an aircraft performance
perspective, sweep is beneficial as it reduces the experienced wave drag. From a boundary-layer transition perspective, though, sweep introduces several physical complications, spawned by the crossflow instability mechanism. As sweep is increased, the crossflow mechanism becomes increasingly unstable, and can lead to an early transition to turbulence. The overarching goal of the present analysis then is to address the question, how much sweep can be applied to this wing while maintaining the benefits of the slotted, natural-laminar-flow design? Linear and nonlinear stability analyses will be presented to assess various pathways to turbulence.
In addition, companion computations are presented to accompany the risk-reduction experiment run in the Klebanoff-Saric Wind Tunnel at Texas A&M University. Linear analyses assess a wide range of various configurations to inform experimentalists where relevant unstable content resides. A comparison between simulation and experimental measurements is presented, for which there is a good agreement
Analysis of nuclear fusion reactor discharge simulations using METIS
The Nuclear Engineering Department of UPC has the need of a nuclear fusion plasma simulator for its research activities and educational tasks. For many years, PRETOR has been the program used for this purpose, but it has become obsolete. Nowadays there are more modern simulation codes for fusion reactor’s plasma shots, like CRONOS and METIS. Due to the complexity of CRONOS, METIS was more suitable. The Nuclear Engineering Section of UPC has been authorised by ITER Organisation to use METIS for educational and research purposes after the signing of an agreement named "Agreement on Cooperation on the ITER Modelling and Analysis Suite (IMAS) and Related Repositories by Universitat Politècnica de Catalunya". This project has gotten inside METIS program, to analyse the obtained simulations on fusion plasma, trying to understand some pieces of code as well. METIS is a code based on MATLAB, combined with some modules programmed with Fortran, C and C++, developed by J.F. Artaud from the Institute for Magnetic Fusion Research, in France. Despite being METIS a very powerful tool on the simulation of fusion reactors with a great quantity of applications, as it is a restricted program only available for researchers from the same field, it does not offer enough help information for those who begin to work with it. This document includes a brief introduction on fusion physics, the principles on magnetic confinement reactors and the presentation of some different integrated modelling codes for fusion plasma. In order to complement the METIS help guide, we have used the knowledge acquired during our work to prepare a draft manual that we hope to be useful whenever it is intended to work on a teaching practices program. During this process we have tried to understand on which models the program is based, getting to analyse the source code, study and compare simulations. Furthermore, a great amount of work done in this project has been focused on the development of a setup of MATLAB functions designed to make easier the study of METIS outputs. This has permitted us to outline an example on how it would be possible to incorporate new functionalities inside METIS source codeEl Departament d’Enginyeria Nuclear de la UPC té la necessitat d’un simulador de plasmes de fusió nuclear per dur a terme activitats de recerca i educació. Durant anys, PRETOR ha estat el programa utilitzat amb aquest propòsit, però ha quedat obsolet. Actualment hi ha simuladors més moderns per les descàrregues de reactors de fusió com CRONOS i METIS. Degut a la complexitat de CRONOS, METIS era més adequat. La Secció d’Enginyeria Nuclear de la UPC està autoritzada per l’Organització ITER a poder utilitzar METIS amb propòsits educatius i de recerca després de signar un acord anomenat “Agreement on Cooperation on the ITER Modelling and Analysis Suite (IMAS) and Related Repositories by Universitat Politècnica de Catalunya". Aquest projecte s’ha endinsat dins el programa METIS, per tal d’analitzar simulacions obtingudes sobre el plasma de fusió, procurant entendre part del codi del programa en el procés. METIS és un codi basat en MATLAB, combinat amb alguns mòduls en Fortran, C i C++, elaborat per J.F. Artaud de l’Institut per la Recerca en Fusió Magnètica, a França. Tot i ser METIS una eina molt potent en la simulació de reactors de fusió amb una gran quantitat d’aplicacions, al tractar-se d’un programa restringit només a investigadors del mateix camp de recerca, no ofereix una informació suficient per a tot aquell que comença a treballar amb ell. Aquest document incorpora una breu introducció a la física de fusió, els principis de funcionament dels reactors de confinament magnètic i la presentació de diferents codis de modelat integrat per plasmes de fusió. Per intentar complementar la guia d’ajuda de METIS, hem utilitzat els coneixements adquirits durant aquest projecte per elaborar un esbós de manual d’usuari que podria ser útil a l’hora de confeccionar un manual de pràctiques acadèmiques. Durant aquest procés hem intentat entendre sobre quins models es basa el programa, arribant a analitzar el codi font, estudiar i comparar simulacions. A més a més, un dels gruixos de feina més importants d’aquest projecte, ha estat tot el desenvolupament d’un seguit de funcions de MATLAB destinades a facilitar l’estudi de les sortides de METIS. Això ens ha permès també plantejar un exemple de com es podrien incorporar noves funcionalitats dins el codi font de METISEl Departamento de Ingeniería Nuclear de la UPC tiene la necesidad de un simulador de plasmas de fusión nuclear para llevar a cabo actividades de investigación y educación. Durante años, PRETOR ha sido el programa utilizado con este propósito, pero ha quedado obsoleto. Actualmente hay simuladores más modernos para las descargas de reactores de fusión, como CRONOS y METIS. Debido a la complejidad de CRONOS, METIS era el más adecuado. La Sección de Ingeniería Nuclear de la UPC está autorizada por la Organización ITER a poder utilizar METIS con propósitos educativos y de investigación después de firmar un acuerdo llamado “Agreement on Cooperation on the ITER Modelling and Analysis Suite (IMAS) and Related Repositories by Universitat Politècnica de Catalunya”. Durante este proyecto nos hemos adentrado en el programa METIS, para analizar las simulaciones obtenidas sobre el plasma de fusión, procurando entender parte del código del programa en el proceso. METIS es un programa basado en MATLAB, combinado con algunos módulos en Fortran, C y C++, elaborado por J.F. Artaud del Instituto de Investigación en Fusión Magnética, en Francia. A pesar de ser METIS una herramienta muy potente en la simulación de reactores de fusión con una gran cantidad de aplicaciones, al tratarse de un programa restringido únicamente a investigadores del mismo campo, no ofrece información suficiente para todo aquel que empieza a trabajar con él. Este documento incorpora una breve introducción a la física de fusión, los principios de funcionamiento de los reactores de confinamiento magnético y la presentación de diferentes códigos de modelado integrado para plasmas de fusión. Para intentar complementar la guía de ayuda de METIS, hemos usado los conocimientos adquiridos durante este proyecto para elaborar un esbozo de manual de usuario que podría ser útil a la hora de confeccionar un manual de prácticas académicas. Durante este proceso hemos intentado entender sobre qué modelos se basa el programa, llegando a analizar el código fuente, estudiar y comparar simulaciones. Además, uno de los principales trabajos en este proyecto ha sido el desarrollo de un conjunto de funciones de MATLAB destinadas a facilitar el estudio de las salidas de METIS. Esto nos ha permitido también plantear un ejemplo de cómo se podrían incorporar nuevas funcionalidades dentro del código fuente de METI
Inkjet printing digital image generation and compensation for surface chemistry effects
Additive manufacturing (AM) of electronic materials using digital inkjet printing (DIJP) is of research interests nowadays because of its potential benefits in the semiconductor industry. Current trends in manufacturing electronics feature DIJP as a key technology to enable the production of customised and microscale functional devices. However, the fabrication of microelectronic components at large scale demands fast printing of tight features with high dimensional accuracy on substrates with varied surface topography which push inkjet printing process to its limits. To understand the DIJP droplet deposition on such substrates, generally requires computational fluid dynamics modelling which is limited in its physics approximation of surface interactions. Otherwise, a kind of “trial and error” approach to determining how the ink spreads, coalesce and solidifies over the substrate is used, often a very time-consuming process. Consequently, this thesis aims to develop new modelling techniques to predict fast and accurately the surface morphology of inkjet-printed features, enabling the optimisation of DIJP control parameters and the compensation of images for better dimensional accuracy of printed electronics devices.
This investigation explored three categories of modelling techniques to predict the surface morphology of inkjet-printed features: physics-based, data-driven and hybrid physics-based and data-driven. Two physics-based numerical models were developed to reproduce the inkjet printing droplet deposition and solidification processes using a lattice Boltzmann (LB) multiphase flow model and a finite element (FE) chemo-mechanical model, respectively. The LB model was limited to the simulation of single tracks and small square films and the FE model was mainly employed for the distortion prediction of multilayer objects. Alternatively, two data-driven models were implemented to reconstruct the surface morphology of single tracks and free-form films using images from experiments: image analysis (IA) and shape from shading (SFS). IA assumed volume conservation and minimal energy drop shape to reconstruct the surface while SFS resolved the height of the image using a reflection model. Finally, a hybrid physics-based and data-driven approach was generated which incorporates the uncertainty of droplet landing position and footprint, hydrostatic analytical models, empirical correlations derived from experiments, and relationships derived from physics-based models to predict fast and accurately any free-form layer pattern as a function of physical properties, printing parameters and wetting characteristics.
Depending on the selection of the modelling technique to predict the deformed geometry, further considerations were required. For the purely physics-based and data-driven models, a surrogate model using response surface equations was employed to create a transfer function between printing parameters, substrate wetting characteristics and the resulting surface morphology. The development of a transfer function significantly decreased the computational time required by purely physics-based models and enabled the parameter optimisation using a multi-objective genetic algorithm approach to attain the best film dimensional accuracy. Additionally, for multilayer printing applications, a layer compensation approach was achieved utilizing a convolutional neural network trained by the predicted (deformed) geometry to reduce the out of plane error to target shape. The optimal combination of printing parameters and input image compensation helped with the generation of fine features that are traditionally difficult for inkjet, improved resolution of edges and corners by reducing the amount of overflow from material, accounted for varied topography and capillary effects thereof on the substrate surface and considered the effect of multiple layers built up on each other.
This study revealed for the first time to the best of our knowledge the role of the droplet location and footprint diameter uncertainty in the stability and uniformity of printed features. Using a droplet overlap map which was proposed as a universal technique to assess the effect of printing parameters on pattern geometry, it was shown that reliable limits for break-up and bulging of printed features were obtained. Considering droplet position and diameter size uncertainties, predicted optimal printing parameters improved the quality of printed films on substrates with different wettability. Finally, a stability diagram illustrating the onset of bulging and separation for lines and films as well as the optimal drop spacing, printing frequency and stand-off distance was generated to inform visually the results.
This investigation has developed a predictive physics-based model of the surface morphology of DIJP features on heterogeneous substrates and a methodology to find the printing parameters and compensate the layer geometry required for optimum part dimensional accuracy. The simplicity of the proposed technique makes it a promising tool for model driven inkjet printing process optimization, including real time process control and paves the way for better quality devices in the printed electronics industry
Drift instabilities, anomalous transport, and heating in low-temperature plasmas
Plasma is an ideal gas of charged particles (ions and electrons) in addition to neutral particles. The
presence of charged particles results in the generation of electric and magnetic fields that serve as the primary
mechanism of the interaction and coupling of particles. As a result, various nonlinear collective phenomena
occur in the plasma, the understanding of many of which remains elusive today. On the other hand, plasmas
have many applications in different branches of science and technology. Different kinds of plasmas are
studied in the atmospheric and space sciences. In the semiconductor industry, the fabrication of electronic
chips relies heavily on plasma etching. Plasma is used in modern electrical thrusters for producing the
driving force of satellites and spacecrafts. It is also used in future fusion reactors for producing abundant
clean energy. Therefore, understanding the complicated phenomena in plasma is important for predicting
and controlling its behaviours in various conditions. In this regard, nonlinear phenomena, such as turbulence,
are formidable barriers to understanding plasma behaviours. These phenomena are described by nonlinear
differential equations that can be barely understood by analytical means and are usually investigated by
numerical simulations. Because of this, it is also important to understand the effect of numerical artifacts on
simulations.
In this thesis, we investigate the nonlinear characteristics of drift instabilities and the role of numerical
methods in our understanding of these instabilities. The drift instabilities are driven by excess free energy
that exists due to the average (drift) velocities of electron and ion components in plasmas. As a result of
these instabilities, the amplitude of fluctuations grows while the drift energy converts into electrostatic energy.
This growth continues until the nonlinear effects, such as turbulence, trapping, and wave-wave interactions,
become active. As a result of these nonlinear effects, the growth of the fluctuations saturates.
In this thesis, our focus will be on two particular types of drift instabilities, namely the Buneman instability
and electron-cyclotron drift instability (ECDI). The Buneman instability is driven when a beam of electrons is
injected into the stationary ions, while both electrons and ions are unmagnetized. In the ECDI, however, the
electrons are magnetized and are also influenced by an external electric field, perpendicular to the magnetic
field. This configuration of fields leads to the E × B drift of the electrons that drives the ECDI. Many kinetic
simulations are performed, and several nonlinear phenomena such as trapping, heating, anomalous transport,
backward waves, and transition of magnetized plasmas to the unmagnetized regime are studied with regard
to both instabilities. For the study of the nonlinear effects of drift instabilities, a grid-based Vlasov code is
developed and used. The numerical method used in this code is the “semi-Lagrangian” method, which is
among the most popular methods for continuum simulations of plasma. In the study of the drift instabilities,
we compare the results of the semi-Lagrangian Vlasov simulations with the more traditional particle-in-cell
(PIC) method. The results of these benchmarking studies reveal several similarities and discrepancies between
Vlasov and particle-in-cell simulations, showing how the numerical methods can interfere with the physics of
the problems
Endogenous measures for contextualising large-scale social phenomena: a corpus-based method for mediated public discourse
This work presents an interdisciplinary methodology for developing endogenous measures of group membership through analysis of pervasive linguistic patterns in public discourse. Focusing on political discourse, this work critiques the conventional approach to the study of political participation, which is premised on decontextualised, exogenous measures to characterise groups. Considering the theoretical and empirical weaknesses of decontextualised approaches to large-scale social phenomena, this work suggests that contextualisation using endogenous measures might provide a complementary perspective to mitigate such weaknesses.
This work develops a sociomaterial perspective on political participation in mediated discourse as affiliatory action performed through language. While the affiliatory function of language is often performed consciously (such as statements of identity), this work is concerned with unconscious features (such as patterns in lexis and grammar). This work argues that pervasive patterns in such features that emerge through socialisation are resistant to change and manipulation, and thus might serve as endogenous measures of sociopolitical contexts, and thus of groups.
In terms of method, the work takes a corpus-based approach to the analysis of data from the Twitter messaging service whereby patterns in users’ speech are examined statistically in order to trace potential community membership. The method is applied in the US state of Michigan during the second half of 2018—6 November having been the date of midterm (i.e. non-Presidential) elections in the United States. The corpus is assembled from the original posts of 5,889 users, who are nominally geolocalised to 417 municipalities. These users are clustered according to pervasive language features. Comparing the linguistic clusters according to the municipalities they represent finds that there are regular sociodemographic differentials across clusters. This is understood as an indication of social structure, suggesting that endogenous measures derived from pervasive patterns in language may indeed offer a complementary, contextualised perspective on large-scale social phenomena
Computational modelling of large-scale fire spread through informal settlements
Informal settlements are a global phenomenon and are characterised by low quality construction
and dense layouts, generally as a result of a lack of application of formal building regulations.
They may be known by another name, such as slums or shantytowns, or exist in more novel
contexts, for example refugee camps and homeless tented communities. However, a consistent
feature of informal settlements in any context is their exposure to fire risk. Fire risk presents
both as risk of fire ignition – how often fires occur – and risk of fire spread. Where there is a
high risk of fire spread, a localised fire in an individual home may swiftly develop to the scale
of tens or hundreds of homes. In some cases this may result in injury and loss of life, but the
predominant issue is the extensive scale of property loss to communities that already exist in
impoverished and precarious circumstances. There is vital need to understand and quantify
fire spread risk factors to develop mitigation measures that may be used to protect informal
settlements from large-scale fires.
In recent years, a growing set of experiments have contributed to knowledge around
how fires in informal structures, particularly of the type found in Cape Town’s expansive
informal settlements, grow and spread. This knowledge is rooted in fundamental concepts
used in fire science and engineering such as compartment fire dynamics, heat transfer
and material ignition. However, it is practically challenging and cost-prohibited to scale
experimentation to settlement-scale fire spread where multiple adjacent structures may
be involved in the fire. As such, it is proposed that computational tools are developed to
simulate fire spread through settlements, such that mitigation measures can be quantitatively
tested and refined at settlement-scale, thus reducing the need for costly experimentation.
Large-scale urban fire spread models have existed since the 1950s but have largely focussed on
the phenomenon of post-earthquake fire spread. Additionally, it is only in the last 20 years that
modellers have moved away from simplistic empirical models towards dynamic ‘physics-based’
models that attempt to explicitly define the fire behaviour. Previous models have achieved
this to varying degrees of success, with many failing to conceptualise the underlying fire
behaviour in a physically realistic manner even if at settlement-scale they may visually appear
to be representative of fire spread. Intrinsic to this failure is a distinct lack of robust model
validation. However, one model – of post-earthquake fire spread in Japan – sets itself apart
with a clear underpinning of well-reasoned fire behaviour at the key unit of analysis for any
urban fire model, the compartment fire. This thesis presents the adoption and integration of
this model for application in informal settlements, both contextualising it to a new physical
domain and proposing improved methods for modelling key aspects of fire behaviour. The
thesis is underpinned by particularly strong focus on local-level validation, showing that the
model accurately captures key fire characteristics at the level of individual compartments
before even considering outcomes at settlement scale.
The first area of focus is on the compartment fire, the fundamental unit of analysis of the
model. It is contextualised for informal settlements by adapting fuel loads and boundary
conditions to align with values from experiments. Additionally, a new fuel-oxygen mixing model
is implemented where previously over-efficient combustion was being assumed within the
compartment. In combination with timestep refinement, this also contributed to stabilisation
of the compartment fire, where previously the model destabilised due to its inability to
resolve gas flows and the neutral plane for ventilation-controlled fires. The result is a
stable compartment fire model that can sustain ventilation-controlled conditions and which
compares well to key metrics – temperature, heat release rate and oxygen concentration
– when compared to experimental results. New phenomena of cardboard lining ‘flashover’
and externally-heated accelerated fire growth are also implemented in the model for the first time.
Subsequently, the external thermal environment – external flaming, radiative heat transfer and
ignition – is investigated. Prior external flame models are found to be rigid, context-specific and
insensitive to wind. Given informal settlement fires are known to be sensitive to wind and highly
driven by flame impingement and high local levels of radiation, it is crucial to model external
venting flame as accurately as possible. Computational fluid dynamics (CFD) models are utilised
to derive new correlations for external venting flame dimensions. Ignition is then modelled as
a dependence on both radiation and flame impingement, where previously these have been
treated as two independent mechanisms. The model is also adapted to reflect the dynamic and
intermittent nature of external flames, such that probability is built into flame dimensions,
based on data from experiments and CFD models. The new method is appropriately sensitive
to wind speed and direction, to a degree not yet achieved in prior urban fire spread models.
Development of both the compartment fire and external flame submodels was conducted at the
local scale, modelling only one or two compartments at a time. Finally, the model is examined
in a domain of 20 structures (as per a previous experiment) to assess how newly implemented
submodels affect fire spread at the multi-structure level. This first verifies the efficacy of the
new model in capturing the characteristics of fire spread in informal settlements compared
to the original model. It also shows the dynamic response of the model to wind-driven
conditions. Additionally, this process helps to uncover where there are still errors or inconsistent
conceptualisation of fire spread behaviours, particularly associated with the model’s reading
and mapping of the spatial environment. Thus, it provides the basis and scope for continued
adaptation of the model to increase its robustness.
Overall, interrogation of previous ‘physics-based’ models uncovered a lack of robust validation
which inspired the approach taken in this research of developing the model at high resolution.
Specific submodels had to be carefully examined and developed to ensure they were first stable
and physically reasonable and, secondly, contextually appropriate for informal settlements.
Though the research stopped short of application to entire informal settlement domains, the
underlying model functionality has been updated with significantly more detailed and robust
representation of real fire behaviour than has previously been applied in any large urban
fire spread context. Future application to informal settlements or adaptation for other urban
environments should be much expedited as a result
Development of nonorthogonal wavefunction theories and application to multistate reaction processes.
Many prominent areas of technological development rely on exploiting the photochemical response of molecules. An application of particular interest is the control of molecular switches through a combination of different external stimuli. However, despite significant advances in theoretical approaches and numerous cases of successful application of theory, simulating photochemical reactions remains a computational challenge. Theoretical methods for describing excited states can be broadly divided into single-reference response methods and multireference methods. Single reference methods provide reliable semiquantitative results for single excitations. However, these methods cannot describe double-excited states, systems with strongly correlated ground states, or regions of degeneracy on the potential energy surface. The alternative, multireference methods, can provide more accurate results. However, multireference methods require significant technical and chemical insight and become computationally costly as the system size increases. I will discuss my work applying newly developed and well-known methods for understanding multistate processes. I will highlight the limitations and extent of current methodologies that prevent researchers from studying larger and more complex systems. I will also discuss new methodological developments using spin projection, which seeks to overcome several problems of single reference excited state models. I will illustrate the motivation and its performance compared to more established theories. Despite its success, the new method cannot account for ‘multiple correlation mechanisms’. As a result, I will introduce how multiple correlation mechanisms can be exploited to perform nonorthogonal active space decomposition, along with applications and paths for future improvements
Phase Change Materials as thermal energy storage and management in solar energy-based systems
Le risorse energetiche rinnovabili hanno una natura intermittente e i sistemi di accumulo dell'energia sono fondamentali per aumentare la loro affidabilità. Tra le alternative, i materiali a cambiamento di fase (PCM) sfruttano il passaggio di fase solido-liquido (a temperatura quasi costante) per accumulare energia. I PCM mantengono più bassa la temperatura dei sistemi dove applicati, riducendo le dispersioni di calore verso l'esterno e per auto scarica. Questo lavoro si concentra su applicazioni basate su energia solare, in particolare pannelli fotovoltaici e laghi solari a gradienti salini.Renewable energy sources suffer from an intermittent nature and energy storage systems are a crucial step to increase their reliability. Among the alternatives, Phase Change Materials (PCM) belong to the latent thermal storage group, and exploit the solid-liquid phase change (at nearly constant temperature) to store energy. PCMs allows to reduce the heat loss outwards and the auto-discharge thermal losses, storing energy at lower the temperatures. This work investigates their applications in solar-based system to improve the energy performance, focusing on photovoltaic panels and solar ponds
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