With the high costs of flight testing, especially at hypersonic speeds, ground based facility testing of scramjets becomes an attractive option. The expansion tube is the only facility currently that can offer full flight property duplication at the total pressures and total enthalpies required, while maintaining correct chemical composition. Due to difficulties with short test times and unsteady flow phenomena, scramjet testing in these facilities has not been thoroughly investigated. This study examines these issues, in order to explore the practicality of testing a full ’tip to tail’ scramjet engine at a true flight replication condition in an expansion tube facility. An investigation was initially undertaken on the large X3 expansion tunnel facility to maximise test time and core flow, aimed at producing a 30 km altitude, Mach 10 flow condition. This was identified as the limitation point of the T4 reflected shock tunnel, which has generally been accepted to produce reliable scramjet data for propulsion tests. Using a condition that is also able to be produced in the T4 facility, will permit direct comparison of data between the two facilities in the future, providing confidence in results from expansion tube facilities. Both experimental measurements and numerical calculations showed that the limitation of the test time was due to large boundary layer growth after transition, which engulfed the entire core flow 200 μs into the test time. This phenomenon is likely to affect all scramjet duplication conditions in expansion tubes, as the flow properties are conducive to boundary layer transition occurring. Two solutions where proposed and investigated in order to overcome the flow disruption caused by boundary layer transition; the use of a steady expansion nozzle at the acceleration tube exit; the use of hydrogen as an accelerator gas. Since the smaller X2 facility had a Mach 10 steady expansion nozzle and X3 was decommissioned for the free piston driver to be upgraded, the investigation was shifted to X2. Due to a restricted test time of 550 μs, the static pressure of the flow condition was increased to allow a reduction in the length of the scramjet (pressure-length scaling). A combination of experimental and numerical calculations of the facility was used to define the flow properties. With the confidence of overcoming the phenomenon associated with boundary transition in the X2 facility, numerical modelling of the X3 facility with a steady expansion nozzle was then undertaken to show a 1 ms condition could be produced. Although initially promising, the hydrogen accelerator gas solution requires further investigation. A two dimensional scramjet was designed with upstream injection for testing in X2. This was a three shock inlet with a constant area combustor and a planar thrust surface. Since the flow condition involved changes in flow properties during the test time, aninvestigation of the appropriateness of a quasi steady analysis was undertaken. Using a fuel off simulation of the scramjet duct with the transient inflow properties from the X2 facility nozzle exit, the convective terms for pressure were shown to be two orders of magnitude larger than local terms indicating the dominance of the convective terms change in flow properties at any location allowing quasi-steady flow to be assumed. A normalisation procedure was developed to deal with the transient nature of the data and to accurately represent the axial progression of the gas through the duct. The numerical simulations were also used to show that both flow establishment was achieved and that impulsive starting of the intake would occur. Experimentation with the scramjet using static pressure measurements throughout the body side of the engine provided verification of supersonic combustion. This was verified by the doubling of the static pressure from the start to the end of the combustor for an air test gas, whereas with a nitrogen test gas no significant change in pressure occurred. Effects of fuel equivalence ratio, injector size and cowl position were also investigated. A net inviscid thrust was predicted, using the quasi-steady flow analysis, indicating a specific impulse of 183 s. This work provides evidence to validate the use of expansion tube facilities for experimental testing of scramjets at flight duplication conditions. Limitations due to boundary layer transition flow effects has been shown to be avoidable. Numerical simulations of the facilities showed good agreement with experimental measurements, allowing definition of freestream properties and can now be applied to further scramjet conditions with confidence. Stable, supersonic combustion was shown to be produced for these expansion tube conditions. Coupling the transient simulation of the flow condition with a numerical calculation of the fuel off experimental scramjet has been useful in both verification of the design and performance predictions. Appropriate techniques have been presented to analyse scramjet pressure and thrust measurements where transient effects are present in the freestream.

No description available.

Buoyant plumes in cold fresh water are of interest because of the possibility of buoyancy reversal due to the nonlinear relation between temperature and density in water. Thus an initially rising plume may become a fountain. This project aims to mathematically model such plumes and fountains using numerical simulation by the means of a commercial software, Comsol Multiphysics. Both turbulent and lam- inar cases were investigated in different geometries, and with the assumption that density is a quadratic function of temperature. The turbulent flow cases as con- sidered here in this thesis are relevant to practical applications such as industrial discharge in cold lakes: whereas, the laminar flow case relates to laboratory experi- ments which are typically at scales too small for the flow to be turbulent. Previous investigation on warm discharge placed more attention on the biological implications of the spread along the lake bed, and not interested in analysing the dynamics of such flow, which turns out to be our focus. Furthermore, investigations on buoyant plumes that become negatively buoyant at later time (fountain flow) as considered previously, are based on the assumption that density is a linear function of tem- perature: where entrainment always reduces buoyancy. Whereas, the consideration of the temperature of maximum density is crucial and realistic in many practical situations, especially the power station warm discharge. Mixing is then bound to produce a mixture that is denser than both the discharge and the ambient water if receiving water is less than Tm: where this situation differs from plumes with linear mixing properties. Therefore, our focus is to better fathom the behaviour of warm discharge so as to give a detailed description of the flow, and also to observe buoyancy reversal whenever water that is denser than both the discharge and the receiving water is produced. The simulations were carried out for Prandtl number Pr = 7 & 11.4 and over the ranges of Froude number 0.1 ≤ Fr ≤ 5 and Reynolds numbers 50 ≤ Re ≤ 106, with source temperatures that are assumed to be higher than the temperature of maximum density Tm, and the ambient water below the Tm. Our results show some distinct behaviours from those experimental investigations by Bukreev, who also considered warm discharge where water that has temperature above the temperature Tm is initiated into a medium below Tm. The results here also showed some differences from those investigations with the linear dependence relation assumption.

[pt] O presente trabalho trata de modelagem e simulação numérica de carregamentos cíclicos em estruturas elasto-plásticas. Uma decomposição aditiva do problema contínuo é usada para o desenvolvimento de técnicas por elemento finitos para a aproximação da solução com boas propriedades de estabilidade e que: -consideram a matriz de rigidez constante (exatamente igual á obtida num problema elástico equivalente). Esta matriz de rigidez é montada e triangularizada uma única vez ao longo do processo de solução. -integram as leis de evolução elemento por elemento. A eficiência e utilidade das técnicas numéricas propostas é verificada analisando-se o comportamento cíclico de vasos de pressão e treliças de alumínio AU4G e aço inoxidável 316L. / [en] This work is concerned with the modeling and numerical simulation of cyclic loadings in elastro-plastic structures. Na aditive decomposition of the continuum problem is used to develop stable and robust finite element techniques of solution: -that consider a Constant stiffness matrix (exatly the same obtained in na equivalent elastic problem). This matrix is assembled and composed only once along the process of solution. -in which the integration of the evolution Law is performed element-by-element. The effectiveness and usefullness of the proposed numerical technique is verified by analysing the cyclic behaviour of AU4G aluminium alloy and 316 L stainless steel pressure vessels and trusses.

In the present thesis, the sensitivity of flame propagation to the turbulent motion of burning gases is investigated. The long-standing issue of the 'bending effect' is focused upon, which refers to the experimentally-observed inhibition of flame propagation velocity at high intensities of turbulence. Plausible mechanisms for the bending effect are investigated by isolating systematically the effects of turbulence intensity. By providing a novel perspective on this topic, the thesis addresses the fundamental limits of turbulent burning. The investigation employs Direct Numerical Simulation (DNS), which enables the basic conditions of burning to be controlled directly. A parametric DNS dataset is designed and generated by increasing turbulence intensity over five separate simulations. Effects of turbulent motion are isolated in this manner, such that the bending effect is reproduced in the variation of flame propagation velocity recorded. Subsequently, the validity of Damköhler's hypotheses is investigated to ascertain the mechanism of bending. Analysis of the DNS dataset highlights the significance of kinematic flame response in determining turbulent flame propagation. Damköhler's first hypothesis is found to be valid throughout the dataset, suggesting that the bending effect may be a consequence of self-regulation of the flame surface. This contradicts the dominant belief that bending occurs as a result of flame surface disruption by the action of turbulence. Damköhler's second hypothesis is found to be valid in a relatively limited regime within the dataset, its validity governed by flame-induced effects on the prescribed turbulent flow field. Therefore, this thesis presents turbulent flame propagation and the bending effect as emergent from the dynamics of a flame surface that retains its internal thermo-chemical structure. Finally, experimental validation is sought for the proposed mechanisms of bending. Comparisons have been initiated with measurements in the Leeds explosion vessel, based on which the widely accepted mechanism of bending was hypothesized twenty-five years ago. Modifications to the DNS framework warranted by this comparison have aided the development of novel computationally-efficient algorithms. The ongoing work may yield insights into the key mechanism of the bending effect in turbulent flame propagation.

abstract: Development of renewable energy solutions has become a major interest among environmental organizations and governments around the world due to an increase in energy consumption and global warming. One fast growing renewable energy solution is the application of wind energy in cities. To qualitative and quantitative predict wind turbine performance in urban areas, CFD simulation is performed on real-life urban geometry and wind velocity profiles are evaluated. Two geometries in Arizona is selected in this thesis to demonstrate the influence of building heights; one of the simulation models, ASU campus, is relatively low rise and without significant tall buildings; the other model, the downtown phoenix model, are high-rise and with greater building height difference. The content of this thesis focuses on using RANS computational fluid dynamics approach to simulate wind acceleration phenomenon in two complex geometries, ASU campus and Phoenix downtown model. Additionally, acceleration ratio and locations are predicted, the results are then used to calculate the best location for small wind turbine installments. / Dissertation/Thesis / Masters Thesis Mechanical Engineering 2015

abstract: Thin-film modules of all technologies often suffer from performance degradation over time. Some of the performance changes are reversible and some are not, which makes deployment, testing, and energy-yield prediction more challenging. The most commonly alleged causes of instability in CdTe device, such as “migration of Cu,” have been investigated rigorously over the past fifteen years. As all defects, intrinsic or extrinsic, interact with the electrical potential and free carriers so that charged defects may drift in the electric field and changing ionization state with excess free carriers. Such complexity of interactions in CdTe makes understanding of temporal changes in device performance even more challenging. The goal of the work in this dissertation is, thus, to eliminate the ambiguity between the observed performance changes under stress and their physical root cause by enabling a depth of modeling that takes account of diffusion and drift at the atomistic level coupled to the electronic subsystem responsible for a PV device’s function. The 1D Unified Solver, developed as part of this effort, enables us to analyze PV devices at a greater depth. In this dissertation, the implementation of a drift-diffusion model defect migration simulator, development of an implicit reaction scheme for total mass conservation, and a couple of other numerical schemes to improve the overall flexibility and robustness of this coupled Unified Solver is discussed. Preliminary results on Cu (with or without Cl-treatment) annealing simulations in both single-crystal CdTe wafer and poly-crystalline CdTe devices show promising agreement to experimental findings, providing a new perspective in the research of improving doping concentration hence the open-circuit voltage of CdTe technology. Furthermore, on the reliability side, in agreement of previous experimental reports, simulation results suggest possibility of Cu depletion in short-circuited cells stressed at elevated temperature. The developed solver also successfully demonstrated that mobile donor migration can be used to explain solar cell performance changes under different stress conditions. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

abstract: ABSTRACT A vortex tube is a device of a simple structure with no moving parts that can be used to separate a compressed gas into a hot stream and a cold stream. Many studies have been carried out to find the mechanisms of the energy separation in the vortex tube. Recent rapid development in computational fluid dynamics is providing a powerful tool to investigate the complex flow in the vortex tube. However various issues in these numerical simulations remain, such as choosing the most suitable turbulent model, as well as the lack of systematic comparative analysis. LES model for the vortex tube simulation is hardly used in the present literatures, and the influence of parameters on the performance of the vortex tube has scarcely been studied. This study is aimed to find the influence of various parameters on the performance of the vortex tube, the best geometric value of vortex tube and the realizable method to reach the required cold out flow rate 40 kg/s . First of all, setting up an original 3-D simulation vortex tube model. By comparing experiment results reported in the literature and our simulation results, a most suitable model for the simulation of the vortex tube is obtained. Secondly, we perform simulations to optimize parameters that can deliver a set of desired output, such as cold stream pressure, temperature and flow-rate. We also discuss the use of the cold air flow for petroleum engineering applications. / Dissertation/Thesis / M.S. Mechanical Engineering 2013

La propagation des incendies à l’échelle de locaux et de villes est un enjeu majeur. Elle est notamment conditionnée par l’inflammation des matériaux dans les locaux attenants au sinistre. Cette dernière résulte de l’allumage du mélange gazeux combustible issu de la décomposition thermique de la phase condensée.Deux types d’inflammation sont définis dans la littérature : l’inflammation pilotée par la présence d’une source d’allumage, et l’auto-inflammation, résultant de l’emballement de la réaction dans la phase gazeuse. L’auto-inflammation joue un rôle majeur dans le contexte d’une propagation de local à local. Toutefois, ce processus n’a été que très peu étudié expérimentalement du fait de sa complexité et seules des analyses théoriques sont aujourd’hui disponibles concernant les phénomènes en jeu.L’enjeu de la présente étude est de caractériser les régimes d’autoallumage en fonction de différentes typologies de solide (comportement thermique et cinétique), afin de mieux comprendre leurs processus et leurs conditions d’occurrence. Cette compréhension fine permet alors de développer des modèles plus globaux de propagation pour une considération déterministe du risque incendie à l’échelle urbaine.Le caractère bref et local de l’auto-inflammation impose le choix d’une méthode de résolution complète des écoulements, des transferts et de la chimie. La Simulation Numérique Directe (DNS) a donc été sélectionnée afin de capter ces phénomènes, avec l’introduction d’une cinétique fine et non infiniment rapide de la décomposition thermique et de la combustion. / Fire propagation on the scale of buildings and cities is a major stake. It is conditioned by the ignition of solid fuels in rooms adjacent to the one where the disaster originally takes place. The ignition is so piloted by the initiation of the combustion reaction of the gaseous mixture stemming from the thermal decomposition of the condensed phase induced by heat transfer.Two types of ignition are defined in the literature: piloted-ignition related to the presence of a hot spot and auto-ignition resulting from the thermal runaway within the gas phase. The auto-ignition plays a major role in the context of fire spread between rooms. However, this process has been very little experimentally studied, because of its complexity, and only theoretical analyses were lead concerning the phenomena which take place during solid fuels auto ignition.The aim of the present study is to characterize auto-ignition regimes according to various solid typologies (regarding to thermal and kinetic behaviour) in order to understand better their processes and their occurrence conditions. Thereby, this fine understanding allows to develop global models of fire spread for a deterministic consideration of the fire hazards at urban scale.The brief and local character of the auto-ignition requires the choice of a complete resolution for flows, transfers and chemistry. Thus, the Direct Numerical Simulation (DNS) was selected to capture the phenomena, with the introduction of a fine and non-infinitely fast chemistry of thermal decomposition and combustion.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14589号 / 工博第3057号 / 新制||工||1455(附属図書館) / 26941 / UT51-2009-D301 / 京都大学大学院工学研究科化学工学専攻 / (主査)教授 山本 量一, 教授 宮原 稔, 教授 大嶋 正裕 / 学位規則第4条第1項該当