Neste trabalho são estudados os parâmetros de verificação do estado limite de deformações excessivas (a relação flecha-altura-a/H) e da estabilidade global (o parâmetro &#945 e o coeficiente &#947z) das estruturas de edifícios. Através da utilização de exemplos de estruturas de trinta (30) edifícios usuais de concreto armado, estabelece-se uma relação entre esses parâmetros, objetivando propiciar ao projetista de estruturas condições de avaliar a eficiência e o grau de confiabilidade de cada um deles. São também discutidos e comparados alguns dos procedimentos usuais para realizar uma análise global de segunda ordem das estruturas. Nesta análise deve-se levar em conta tanto a não-linearidade física (NLF) quanto a geométrica (NLG) e, para tanto, são adotados métodos rigorosos e aproximados. Quer-se com isso, principalmente, analisar o processo simplificado como suficiente para se obter os esforços finais de segunda ordem, uma vez que se pretende incluir tal procedimento na norma brasileira NB-1, atualmente em fase de revisão. / In this work, a servicebility pararneter (displacement-height ratio - a/H) is studied, as well as the global stability parameters of building structures (&#945 and &#947z stability coefficients). By analyzing thirty (30) actual reinforced concrete building structures, a relationship among these parameters (&#945, &#947z and a/H) is achieved aiming to demonstrate to the building structure designers their level of efficiency and reliability for practical purposes. Some standard procedures for global second order structure analysis are also discussed with comparison among them. In this analysis, the material and the geometric non-linearities have to be considered and for that one can adopt approximated and accureted methods. Mainly, the interest is to verify the possibility of using the simplified method as a proper tool to compute the final second order efforts of buildings, once this procedure could be included into the Brazilian Code, NB-1, which is being revised.

Neste trabalho são estudados os parâmetros de verificação do estado limite de deformações excessivas (a relação flecha-altura-a/H) e da estabilidade global (o parâmetro &#945 e o coeficiente &#947z) das estruturas de edifícios. Através da utilização de exemplos de estruturas de trinta (30) edifícios usuais de concreto armado, estabelece-se uma relação entre esses parâmetros, objetivando propiciar ao projetista de estruturas condições de avaliar a eficiência e o grau de confiabilidade de cada um deles. São também discutidos e comparados alguns dos procedimentos usuais para realizar uma análise global de segunda ordem das estruturas. Nesta análise deve-se levar em conta tanto a não-linearidade física (NLF) quanto a geométrica (NLG) e, para tanto, são adotados métodos rigorosos e aproximados. Quer-se com isso, principalmente, analisar o processo simplificado como suficiente para se obter os esforços finais de segunda ordem, uma vez que se pretende incluir tal procedimento na norma brasileira NB-1, atualmente em fase de revisão. / In this work, a servicebility pararneter (displacement-height ratio - a/H) is studied, as well as the global stability parameters of building structures (&#945 and &#947z stability coefficients). By analyzing thirty (30) actual reinforced concrete building structures, a relationship among these parameters (&#945, &#947z and a/H) is achieved aiming to demonstrate to the building structure designers their level of efficiency and reliability for practical purposes. Some standard procedures for global second order structure analysis are also discussed with comparison among them. In this analysis, the material and the geometric non-linearities have to be considered and for that one can adopt approximated and accureted methods. Mainly, the interest is to verify the possibility of using the simplified method as a proper tool to compute the final second order efforts of buildings, once this procedure could be included into the Brazilian Code, NB-1, which is being revised.

The breaking of internal solitary waves (ISWs) of depression shoaling upon a uniformly sloping boundary in a smoothed two-layer density field was investigated using high-resolution two-dimensional simulations. The simulations were performed for a wide range of boundary slopes S∈[0.01,0.3] and wave slopes. Over steep slopes (S≥0.1), three distinct breaking processes were observed; surging, plunging and collapsing breakers which are associated with reflection, convective instability and boundary layer separation, respectively. Over mild slopes (S≤0.05), nonlinearity varies gradually and the wave fissions into a train of waves of elevation after it passes through the turning point where solitary waves reverse polarity. The dynamics of each breaker type were investigated and the predominance of a particular mechanism was associated with a relative developmental timescale. The breaker type was characterized in wave slope S_w versus S space and the reflection coefficient (R), modeled as a function of the internal Iribarren number, was in agreement with other studies. The same 2D model was applied to investigate boundary layer separation-driven global instability, which is shown to play an important role in breaking of shoaling ISWs. The simulations were conducted with waves propagating over a flat bottom and shoaling over relatively mild (S=0.05) and steep (S=0.1) slopes. Combining the results over flat and sloping boundaries, a unified criterion for vortex shedding is proposed, which depends on the momentum thickness Reynolds number and the non-dimensionalized ISW-induced pressure gradient at the point of separation. The criterion is generalized to a form that may be readily computed from field data and compared to published laboratory and field observations. During vortex shedding, the bed shear stress, vertical velocity and near-bed Reynolds stress were elevated, implying potential for sediment re-suspension. Laboratory experiments were also performed to study three-dimensionality (3D) of global instability. Our results agree with previous laboratory experiments, using the proposed pressure gradient parameter and Reynolds number. The 3D effects prevent the vortices from ascending as high as they do in 2D simulations. The instabilities were not able to re-suspend sediments with 20 µm median diameters, which suggests applying lighter sediments, as finer sediments will be cohesive and dynamically different. / Thesis (Ph.D, Civil Engineering) -- Queen's University, 2011-12-23 15:03:29.76

When entering a planet’s atmosphere, spacecraft induce a strong compression shock and must be protected from the resulting extreme heat flux by a thermal protection system made of either reusable or ablative materials. To characterise these materials, the harsh flow conditions of atmospheric entry are reproduced in plasma wind tunnels, where a jet of gas heated up to ionisation is directed at material samples for prolonged testing. Unfortunately, heated jets exhibit complex dynamic behaviours, resulting in oscillations that increase the uncertainties in the experiments.At sufficient Reynolds numbers, the dynamic behaviour of heated jets shifts from an amplifier to a self-sustained oscillator type via a Hopf bifurcation, if the centreline-to-ambient density ratio falls below a given threshold. This change is known in the literature to be related to the onset of absolute instabilities in the flow. However, this type of instability is usually studied for a simplified description of the gas, which is not suitable for the case of a plasma wind tunnel.This doctoral work investigates the nature of the instabilities responsible for the oscillations observed in a plasma jet, similar to the one in the VKI Plasmatron facility. The analysis is carried out by comparing results from different numerical methods, including linear stability analyses (both local and global) and direct numerical simulations. The thesis first describes the effect of high-temperature gas models on the stability of synthetic jets found in the literature, before analysing the case of Plasmatron.The analysis of synthetic jets with real-gas effects shows that the onset of the first dissociation reactions in the flow has a strong influence on the prevailing type of instability. Furthermore, if a sufficiently long region of absolute instability is present in the jet, the flow bifurcates to a periodic limit cycle, and steady state solutions become inadequate to describe the flow and its dynamic behaviour. In this case, a stability analysis of the time-averaged state can accurately reproduce the results of direct numerical simulations. In the case of Plasmatron, a large region of absolute instability is revealed in the plasma jet, suggesting that the observed oscillations are caused (in part) by a global non-linear mode and that the flow has entered a limit cycle. Trends of the absolute instability frequency with respect to the driving parameters of Plasmatron are in agreement with experimental observations.The present work confirms that global stability features of heated jet flows are very sensitive to subtle changes of the undisturbed or time-averaged state, which results from technological constraints in the case of Plasmatron. Furthermore, this thesis has shown the relevance of including high-temperature gas effects in the stability analysis of high-enthalpy jets. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

We use molecular mechanics (MM) simulations with the tight-binding (TB) potential to study local and global instabilities in initially defect-free finite specimens of gold crystals deformed in shear, simple shear, tension/compression, simple tension/compression, and triaxial tension/compression. The criteria used to delineate local instabilities in a system include the following: (i) a second order spatial derivative of the displacement field having large values relative to its average value in the body, (ii) the minimum eigenvalue of the Hessian of the potential energy of an atom becoming nonpositive, (iii) and structural changes represented by a high value of the common neighborhood parameter. A specimen becomes globally unstable when its potential energy decreases significantly with a small increase in its deformations. It is found that the three criteria for local instability are satisfied essentially simultaneously at the same atomic position. Deformations of a specimen are quite different when it is deformed with some bounding surfaces free from external forces as opposed to essential boundary conditions prescribed on all bounding surfaces. It is found that the initial unloaded configuration (or the reference configuration) of the minimum potential energy has significant in-plane stresses on the bounding surfaces and nonzero normal stresses at interior points. In tensile/compressive deformations of a rectangular prismatic nanobar the yield stress defined as the average axial stress when the average axial stress vs. the average axial strain curve exhibits a sharp discontinuity depends upon the specimen size; a similar result holds for simulations of shear deformations. Specimens deformed with essential boundary conditions on all bounding surfaces experience instabilities at a higher value of the average strain than identical specimens deformed similarly but with one or more pairs of opposite bounding surfaces traction free. For the former set of deformations, the response of a specimen prior to the onset of instability is the same as that of a hyperelastic body with the strain energy derived from the TB potential and deformations obeying the Cauchy-Born rule. Specimens with some traction free bounding surfaces experience local instabilities prior to the onset of a global instability but the two instabilities occur simultaneously in specimens with essential boundary conditions prescribed on all bounding surfaces. It is believed that because of residual stresses in the reference configuration, the average axial stress at yield in compression is nearly one-half of that in tension. / Ph. D.

Thunderstorms, due to their high frequency of occurrence over southern Africa, and their major contribution to summer rainfall are the primary focus of very short range forecasting and nowcasting efforts in South Africa. With a limited number of surface and upper-air observations and the limited availability of numerical model output most southern African countries are heavily reliant on satellite technology. In developing tools for the first twelve forecast hours the South African Weather Service has to address both the national and regional needs. Thus, the blending of techniques in an optimal manner is essential. This study initially describes how the Global Instability Index product derived from the European Meteosat Second Generation Satellite was adapted for South African circumstances using a different numerical model to provide background information – creating the Regional Instability Indices (RII). The focus of the study is the development of a new convection indicator, called the Combined Instability Index (CII), which calculates the probability of convection from satellite derived instability indices and moisture, as well as height above sea level early in the morning when the sky is relatively cloud free. Early morning CII values were evaluated statistically against the occurrence of lightning over South Africa, where a lightning network is available, as well as against satellite derived precipitation over southern Africa, later in the same day. It is shown that the CII not only performs well, but also outperforms the individual RII when compared to the occurrence of lightning. The CII will be beneficial to operational forecasters to focus their attention on the area which is most favourable for the development of convection later in the day. / Environmental Sciences / Ph. D. (Environmental Sciences)

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Thunderstorms, due to their high frequency of occurrence over southern Africa, and their major contribution to summer rainfall are the primary focus of very short range forecasting and nowcasting efforts in South Africa. With a limited number of surface and upper-air observations and the limited availability of numerical model output most southern African countries are heavily reliant on satellite technology. In developing tools for the first twelve forecast hours the South African Weather Service has to address both the national and regional needs. Thus, the blending of techniques in an optimal manner is essential. This study initially describes how the Global Instability Index product derived from the European Meteosat Second Generation Satellite was adapted for South African circumstances using a different numerical model to provide background information – creating the Regional Instability Indices (RII). The focus of the study is the development of a new convection indicator, called the Combined Instability Index (CII), which calculates the probability of convection from satellite derived instability indices and moisture, as well as height above sea level early in the morning when the sky is relatively cloud free. Early morning CII values were evaluated statistically against the occurrence of lightning over South Africa, where a lightning network is available, as well as against satellite derived precipitation over southern Africa, later in the same day. It is shown that the CII not only performs well, but also outperforms the individual RII when compared to the occurrence of lightning. The CII will be beneficial to operational forecasters to focus their attention on the area which is most favourable for the development of convection later in the day. / Environmental Sciences / Ph. D. (Environmental Sciences)