Turbulent flows affected by buoyancy lie at the basis of many applications, both within engineering and the atmospheric sciences. A prominent example of such an application is the atmospheric boundary layer, the lowest layer of the atmosphere, in which many physical processes are heavily influenced by both stably stratified and convective turbulent transport. Modelling these turbulent flows correctly, especially in the presence of stable stratification, has proven to be a great challenge and forms an important problem in the context of climate models. In this thesis, we address this issue considering an advanced class of turbulence models, the so-called explicit algebraic models.In the presence of buoyancy forces, a mutual coupling between the Reynolds stresses and the turbulent heat flux exists, which makes it difficult to derive a fully explicit turbulence model. A method to overcome this problem is presented based on earlier studies for cases without buoyancy. Fully explicit and robust models are derived for turbulence in two-dimensional mean flows with buoyancy and shown to give good predictions compared with various data from direct numerical simulations (DNS), most notably in the case of stably stratified turbulent channel flow. Special attention is given to the problem of determining the production-to-dissipation ratio of turbulent kinetic energy, for which the exact equation cannot be solved analytically. A robust approximative method is presented to calculate this quantity, which is important for obtaining a consistent formulation of the model.The turbulence model derived in this way is applied to the atmospheric boundary layer in the form of two idealized test cases. First, we consider a purely stably stratified boundary layer in the context of the well-known GABLS1 study. The model is shown to give good predictions in this case compared to data from large-eddy simulation (LES). The second test case represents a full diurnal cycle containing both stable stratification and convective motions. In this case, the current model yields interesting dynamical features that cannot be captured by simpler models. These results are meant as a first step towards a more thorough investigation of the pros and cons of explicit algebraic models in the context of the atmospheric boundary layer, for which additional LES data are required. / <p>QC 20150522</p>

<p>A large number of studies show that educators are exposed to workloads that result particularly in stress. According to Olivier and Venter (2003), stress is considered to be the main factor contributing to job-related illnesses, early retirement, job dissatisfaction and absenteeism. Teaching, has without a doubt become a more demanding and stressful job. The lack of discipline in schools, abolishment of corporal punishment, large pupilteacher ratios and a new curriculum approach all contribute to raising the stress levels of teachers (Ngidi &amp / Sibaya, 2002). Stanton, Bachiochi, Robie, Perez and Smith (2002 cited in Brewer &amp / McMahan-Landers, 2003) is of the view that job stress is an antecedent of job satisfaction but the two constructs have been treated as related, yet they are different. The aim of this study is to determine the relationship between stress levels and job satisfaction amongst high school educators in De Aar as well as to&nbsp / determine whether biographical variables (namely, gender, age, tenure, marital status and number of children) have an impact on job satisfaction and stress levels. The researcher hopes that&nbsp / the findings would give the Department of Education in the Northern Cape an idea of the stress levels and job satisfaction experienced by high school educators in De Aar so that interventions can be implemented to issues proactively. The sample consisted of 86 educators employed at four high schools in De Aar. A biographical questionnaire, the Job Satisfaction Survey (JSS) and Stress Diagnostic Survey (SDS) were administered to assess respondents&rsquo / stress and job satisfaction levels. Informed consent was obtained from the various participants and anonymity of participation was ensured. Statistical analyses included inferential (Pearson&rsquo / s Product Moment Correlation Coefficient, Analysis of Variance &amp / T-Tests) as well as descriptive statistics. The results of the study&nbsp / indicate that there is a statistically significant, inverse relationship between stress levels and job satisfaction amongst high school educators in De Aar. Furthermore, the results indicate that there&nbsp / are statistically significant differences in stress levels as well as job satisfaction of high school educators on the basis of their biographical characteristics. The chapter concludes with presenting limitations of the study and putting forth recommendations for future research.</p>

<p>A large number of studies show that educators are exposed to workloads that result particularly in stress. According to Olivier and Venter (2003), stress is considered to be the main factor contributing to job-related illnesses, early retirement, job dissatisfaction and absenteeism. Teaching, has without a doubt become a more demanding and stressful job. The lack of discipline in schools, abolishment of corporal punishment, large pupilteacher ratios and a new curriculum approach all contribute to raising the stress levels of teachers (Ngidi &amp / Sibaya, 2002). Stanton, Bachiochi, Robie, Perez and Smith (2002 cited in Brewer &amp / McMahan-Landers, 2003) is of the view that job stress is an antecedent of job satisfaction but the two constructs have been treated as related, yet they are different. The aim of this study is to determine the relationship between stress levels and job satisfaction amongst high school educators in De Aar as well as to&nbsp / determine whether biographical variables (namely, gender, age, tenure, marital status and number of children) have an impact on job satisfaction and stress levels. The researcher hopes that&nbsp / the findings would give the Department of Education in the Northern Cape an idea of the stress levels and job satisfaction experienced by high school educators in De Aar so that interventions can be implemented to issues proactively. The sample consisted of 86 educators employed at four high schools in De Aar. A biographical questionnaire, the Job Satisfaction Survey (JSS) and Stress Diagnostic Survey (SDS) were administered to assess respondents&rsquo / stress and job satisfaction levels. Informed consent was obtained from the various participants and anonymity of participation was ensured. Statistical analyses included inferential (Pearson&rsquo / s Product Moment Correlation Coefficient, Analysis of Variance &amp / T-Tests) as well as descriptive statistics. The results of the study&nbsp / indicate that there is a statistically significant, inverse relationship between stress levels and job satisfaction amongst high school educators in De Aar. Furthermore, the results indicate that there&nbsp / are statistically significant differences in stress levels as well as job satisfaction of high school educators on the basis of their biographical characteristics. The chapter concludes with presenting limitations of the study and putting forth recommendations for future research.</p>

This study investigates the cyclic deformation, fatigue behavior, and failure mechanisms for Ti-6Al-4V ELI (extra low interstitial) with and without mean strain/stress. Mean stress effects on fatigue behavior were studied using four strain ratios. Fatigue data generated was used to assess mean stress fatigue life prediction approaches, including stress-based methods such as Goodman, Gerber, Morrow, Walker and Kwofie; as well as strain-based models, such as Morrow, Smith-Watson-Topper, Walker, Kwofie, Ince-Glinka and a modified version of the Smith-Watson-Topper. The stress-based models did not yield reasonable results and data scatter was observed. The strain-based models offered better results, specifically the Morrow approach which provided more accurate fatigue life predictions. Fractography analysis determined that the influence of material defects on fatigue life had no major differences across all the strain ratios considered. Overall observations indicate that inclusions near the surface had great influence on the fatigue behavior.

Thermal Barrier Coating systems (TBCs) serve as a key component in gas turbines in aerospace engines, isolating the metallic substrate from severe heat flux of the environment. The durability of TBCs has been considered to be a critical issue to determine the service lifespan of hot section components. Comprehensive studies of failure mechanisms benefit the gas turbine industry to develop TBCs with better material properties and stable microstructures, thus potentially enhancing their durability. To date, many failure mechanism analyses have been conducted based on the understanding of critical residual stress developed under different thermal tests. For the present study, using the Finite Element (FE) method with temperature-process-dependent model parameters, the maximum residual stress is calculated with evolution of the localized/global interfacial roughness profile based on Electron Beam-Physical Vapour Deposition Thermal Barrier Coating system (EB-PVD TBCs). With studies of cracking routes from past research, qualitative failure mechanism analysis is conducted for EB-PVD TBCs. In addition, the estimated energy release rates are compared to reveal the effect of different thermal profiles on the crack driving forces for Atmospheric Plasma Sprayed Thermal Barrier Coating systems (APS-TBCs). Using previously observed cracking routes from different thermal cycling experiments, a quantitative failure mechanism analysis is conducted for APS-TBCs with modified analytical expressions. In addition, literature works revealed that physics and mechanics-based models were proposed to evaluate environment induced damage. For the last part of my research, erosion of EB-PVD TBCs is estimated using a modified solid particle erosion model. A stochastic approach is applied to study the erosion of EB-PVD topcoat (TC) under real engine service conditions. The durability of TBCs is affected by both oxidation-induced degradation and environment-induced damage. The combination of “internal” crack driving forces (generated from residual stresses developed upon different stages of thermal cycles) and “external” erosion damage (from temperature-process dependent brittle/ductile erosion) lead to complexity of evaluating durability under different service conditions.

Using experimentally measured temperature-process-dependent model parameters, the failure analysis and life prediction were conducted for Atmospheric Plasma Sprayed Thermal Barrier Coatings (APS-TBCs) and electron beam physical vapor deposition thermal barrier coatings (EB-PVD TBCs) with Pt-modified -NiAl bond coats deposited on Ni-base single crystal superalloys. For APS-TBC system, a residual stress model for the top coat of APS-TBC was proposed and then applied to life prediction. The capability of the life model was demonstrated using temperature-dependent model parameters. Using existing life data, a comparison of fitting approaches of life model parameters was performed. The role of the residual stresses distributed at each individual coating layer was explored and their interplay on the coating’s delamination was analyzed. For EB-PVD TBCs, based on failure mechanism analysis, two newly analytical stress models from the valley position of top coat and ridge of bond coat were proposed describing stress levels generated as consequence of the coefficient of thermal expansion (CTE) mismatch between each layers. The thermal stress within TGO was evaluated based on composite material theory, where effective parameters were calculated. The lifetime prediction of EB-PVD TBCs was conducted given that the failure analysis and life model were applied to two failure modes A and B identified experimentally for thermal cyclic process. The global wavelength related to interface rumpling and its radius curvature were identified as essential parameters in life evaluation, and the life results for failure mode A were verified by existing burner rig test data. For failure mode B, the crack growth rate along the topcoat/TGO interface was calculated using the experimentally measured average interfacial fracture toughness.

Researchers conduct life testing on objects of interest in an attempt to determine their life distribution as a means of studying their reliability (or survivability). Determining the life distribution of the objects under study helps manufacturers to identify potential faults, and to improve quality. Researchers sometimes conduct accelerated life tests (ALTs) to ensure that failure among the tested units is earlier than what could result under normal operating (or environmental) conditions. Moreover, such experiments allow the experimenters to examine the effects of high levels of one or more stress factors on the lifetimes of experimental units. Examples of stress factors include, but not limited to, cycling rate, dosage, humidity, load, pressure, temperature, vibration, voltage, etc. A special class of ALT is step-stress accelerated life testing. In this type of experiments, the study sample is tested at initial stresses for a given period of time. Afterwards, the levels of the stress factors are increased in agreement with prefixed points of time called stress-change times. In practice, time and resources are limited; thus, any experiment is expected to be constrained to a deadline which is called a termination time. Hence, the observed information may be subjected to Type-I censoring. This study discusses maximum likelihood inferential methods for the parameters of multiple step-stress models from a generalized Birnbaum-Saunders distribution under time constraint alongside other inference-related problems. A couple of general inference frameworks are studied; namely, the observed likelihood (OL) framework, and the expectation-maximization (EM) framework. The last-mentioned framework is considered since there is a possibility that Type-I censored data are obtained. In the first framework, the scoring algorithm is used to get the maximum likelihood estimators (MLEs) for the model parameters. In the second framework, EM-based algorithms are utilized to determine the required MLEs. Obtaining observed information matrices under both frameworks is also discussed. Accordingly, asymptotic and bootstrap-based interval estimators for the model parameters are derived. Model discrimination within the considered generalized Birnbaum-Saunders distribution is carried out by likelihood ratio test as well as by information-based criteria. The discussed step-stress models are illustrated by analyzing three real-life datasets. Accordingly, establishing optimal multiple step-stress test plans based on cost considerations and three optimality criteria is discussed. Since maximum likelihood estimators are obtained by numerical optimization that involves maximizing some objective functions, optimization methods used, and their software implementations in R are discussed. Because of the computational aspects are in focus in this study, the benefits of parallel computing in R, as a high-performance computational approach, are briefly addressed. Numerical examples and Monte Carlo simulations are used to illustrate and to evaluate the methods presented in this thesis. / Thesis / Doctor of Science (PhD)