<div>Strong electronic correlations in many-body systems are cradles of new physics. They give birth to novel collective states hosting emergent quasiparticles as well as intriguing geometrical charge patterns. Two-dimensional electron gas in GaAs/AlGaAs under perpendicular magnetic field is one of the most well-known hosts in condensed matter physics where a plethora of the collective states appear. In the strong magnetic field regime, strong Coulomb interactions among the electrons create emergent quasiparticles, i.e. composite fermions and Cooper-paired composite fermions. In the weak magnetic field regime, modified Coulomb interactions drive electron solid phases having geometrical charge patterns in the shape of stripes and bubbles and lower the spatial symmetry of the states.</div><div><br></div><div>The fascinating charge order in bubble geometry is the electron bubble phase predicted first by the Hartree-Fock theory. In a bubble phase, certain number of electrons cluster as an entity called bubble and the bubbles order into a crystal of triangular lattice. In addition to the Hartree-Fock theory, the density matrix renormalization group and the exact diagonalization methods further support the formation of electronic bubbles.</div><div><br></div><div>Reentrant integer quantum Hall states are commonly accepted as the manifestations of the bubble phases in transport experiment. Soon after the first prediction of the Hartree-Fock theory, the reentrant integer quantum Hall states were observed in the third and higher Landau levels. Since then, the association to the bubble phases has been tested with different experimental techniques for decades.</div><div><br></div><div>Although the experimental results from different methods support the bubble phase picture of the reentrant integer quantum Hall states, the electron confinement under the quantum well structure hindered direct scanning of bubble morphology. Thus none of the experiments could showcase the bubble morphology of the reentrant integer quantum Hall states. Meanwhile, a significant discrepancy still remained in between the bubble theories and the experiments. Even though the bubble theories predict the proliferation of bubble phases with increasing orbital index, none of the experiments could observe multiple reentrant integer quantum Hall states in a high Landau level, which signify the multiple bubble formation. Therefore, the proliferation of bubble phases with increasing Landau level index was pessimistic. </div><div><br></div><div>In this Dissertation, I present my research on solving this discrepancy. In chapter 4, we performed a magnetotransport measurement of reentrant integer quantum Hall states in the third and higher Landau levels at various different temperatures. Then, we scrutinized how each of the reentrant integer quantum Hall states develops with the gradual increase of the temperature. As a result, we observed multiple reentrant integer quantum Hall states in the fourth Landau level which are associated with the two- and three-electron bubble phases. This result strongly supports the bubble phase picture of the reentrant integer quantum Hall states by confirming the possibility of the proliferation of bubble phases in high Landau levels.</div><div><br></div><div>In chapter 5, I analyzed the energetics of newly resolved two- and three-electron bubble phases in the fourth Landau level as well as those of two-electron bubble phases in the third Landau level. Here, I first found, in the fourth Landau level, the three-electron bubbles are more stable than the two-electron bubbles indicating that the multi-electron bubbles with higher electron number are more stable within a Landau level. Secondly, I found distinct energetic features of two- and three-electron bubble phases which are independent of Landau level index throughout the third and the fourth Landau levels. These results highlight the effect of the number of electrons per bubble on the energetics of multi-electron bubble phases and are expected to contribute on improving the existing Hartree-Fock theories.</div>

Crea8tive Tea LLC is a startup business Panama, Panama City. The main objective of Crea8tive Tea is to serve the freshest Bubble Tea and desserts using the finest possible ingredients in the market by providing different flavors targeted towards the Asian and non-Asian Panamanian market. We will provide products that are natural and of high quality to cater the segment of the population that seeks a gastronomical journey. The Bubble Tea beverage & dessert industry is currently booming and there is an opportunity to leverage this moment and maximize our profits. Fortunately, Bubble Tea is not that known yet, especially for the non-Asian population. Most of the Bubble Tea Shops prepare their drinks with additives and non-natural ingredients that are harmful to our health and Crea8tive wants to target this market. An initial investment of $28,000 is required to start the business and our initial capital investment is of $50,000 funded by two business partners. Based on the financial section, starting from year 2 we will start to see our Return on Investment (ROI). It is important to keep in mind that there is still a risk that Bubble Tea might not be a drink that can be quickly accepted by the non-Asian segment because it is a niche market. On the other hand, our marketing know-how and assorted flavors targeted to the non-Asian Panamanians will be a solution to this challenge and become leaders in the industry.

A qualidade da dispersão de gás em células de flotação é comumente caracterizada através de parâmetros como velocidade superficial do gás (Jg), hold-up do gás (?g), distribuição de tamanho de bolha (db ou D3,2) e fluxo de superfície de bolha (Sb). Sendo um processo de separação de minerais que é dependente da interação (colisão + adesão) entre partículas hidrofóbicas e bolhas de ar, a flotação tem seu desempenho dependente de uma dispersão de gás apropriada na polpa de minério. Desta forma, este trabalho objetivou caracterizar o estado da dispersão de gás de duas células em um banco composto por quatro células Wemco de 42,5 m³ (subaeradas), operando em série na usina da Vale Fertilizantes (Cajati-SP). Realizaram-se três campanhas de medidas que foram conduzidas sob diferentes condições operacionais: a) Diâmetro do rotor (D) de 1,09 m e rotação (N) entre 145 RPM e 175 RPM; b) D = 0,99 m e N entre 110 RPM e 190 RPM; c) D = 0,99 m e N de 120 RPM e de 130 RPM. Observaram-se os seguintes valores de dispersão de gás: 0,7 <= Jg <= 5,4 cm/s, 7 <= ?g <= 15%, 1,6 <= D3,2 <= 2,4 mm e Sb na faixa de 24 a 162 s-1. A magnitude de Jg medida na 1ª e 2ª campanhas mostrou-se acima dos valores reportados pela literatura, indicando necessidade de modificação de condições operacionais dos equipamentos, assim como cuidadosa manutenção. Posteriormente, a 3ª campanha indicou maior conformidade dos parâmetros de dispersão de gás em relação à literatura, constatando-se uma considerável melhora de desempenho do processo de flotação. / The quality of gas dispersion within flotation cells is usually characterized by superficial gas velocity (Jg), gas hold-up (?g), bubble size distribution (db or D3,2) and bubble surface area flux (Sb). Because flotation is a separation process which depends largely on the interaction (collision + adhesion) of hydrophobic particles and air bubbles, process performance is dependent on appropriate gas dispersion within the slurry. Thus, this study aimed to characterize the gas dispersion of two flotation cells on a flotation bank composed of four 42.5 m³ Wemco cells (sub aerated), operating in series at the plant of Vale Fertilizantes (Cajati-SP). There were three measurement campaigns conducted under different operating conditions: a) Impeller diameter (D) of 1.09 m and impeller rotation speed (N) between 145 and 175 min-1; b) D = 0.99 m and 110 <= N <= 190 min-1; c) D = 0.99 m and N = 120 and 130 min-1. The following gas dispersion values were determined: 0.7 <= Jg <= 5.4 cm/s, 7 <= ?g <= 15%, 1.6 <= D3,2 <= 2.4 mm and Sb in the range of 24 to 162 s-1. The magnitude of Jg at 1st and 2nd campaigns showed to be higher than the values reported in literature, calling for modification of operational conditions of the equipments, as well as carefully maintenance. In addition, results from the 3rd campaign indicated values of gas dispersion parameters that matched those reported by current literature, accompanied by considerable mprovement in process performance.

Embora seja uma técnica tradicional, empregada largamente em processos industriais, a operação unitária de aeração pode ser otimizada ao se utilizar modelos matemáticos que descrevam seu comportamento hidrodinâmico. Diversas equações se prestam a esse fim, no entanto, avanços tecnológicos, tanto no campo instrumental como computacional, permitiram a obtenção de equações mais realísticas e abrangentes. Neste trabalho foram ajustadas e avaliadas equações para descrever três importantes parâmetros envolvidos na transferência de oxigênio em meio hídrico: coeficiente volumétrico de transferência de massa, fração de vazios e velocidade ascensional de bolhas. Para tanto, ademais da utilização de conceitos teóricos amplamente difundidos no meio acadêmico, empregou-se um conjunto de dados já existente. Essa combinação permitiu a avaliação e aproximação das equações teóricas à realidade experimental através de coeficientes de ajuste empíricos. Foram obtidos os seguintes resultados: a) revisão bibliográfica e sugestão da correlação que melhor descreve o coeficiente volumétrico de transferência de massa; b) correlação que descreve a fração de vazios, a partir da analise da concentração de bolhas segundo distribuições gaussianas; c) correlação que descreve a velocidade ascensional de bolhas em plumas. A complexidade de cada parâmetro obtido deixou clara a importância da análise isolada, precedendo a convolução dos mesmos em um único modelo, pois embora as resposta e recomendações apresentadas não forneçam conclusões definitivas sobre todos os aspectos envolvidos na transferência de oxigênio em meio hídrico, suas considerações pontuais contribuem sobremaneira ao domínio do fenômeno global, beneficiando modelagens matemáticas futuras. / Although it is a traditional technique, widely employed in industrial processes, the unit operation of aeration can be optimized when resorting to mathematical models which describe their hydrodynamic behavior. A range of equations lends itself to this purpose, however, technological advances both in the instrumental as computational field, allowed the achievement of equations more realistic and comprehensives. In this study were adjusted and evaluated equations to describe three important parameters involved in the oxygen transfer in the hydric medium: volumetric mass transfer coefficient, void fraction and upflow bubble velocity. To this end, besides the use of theoretical concepts widely disseminated in the academic literature, it was employed a set of already existing data. This combination allowed the evaluation and approximation of theoretical equations to experimental reality by means of empirical adjustment coefficients. Were obtained the following results: a) literature review and suggestion of the best correlation which describes the volumetric mass transfer coefficient, b) correlation which describes the fraction of voids, from the analysis of the concentration of bubbles by Gaussian distributions, c) correlation which describes the upflow velocity of swarms of bubbles. The complexity of each parameter obtained made clear the importance of the isolated analysis, preceding the convolution of them in a single model because although the answers and recommendations given fail to supply definitive conclusions on all aspects involved in the transfer of oxygen in hydric medium, their specific considerations contribute to the knowledge of the global phenomenon, benefiting deeply future mathematical models.

A cavitação e a dinâmica de bolhas são tópicos bastante recorrentes na literatura, devido sobretudo a seus efeitos em diversos tipos de fenômenos, como transferência de calor e escoamento em tubos. Considerando fases líquidas, sabe-se que estas estruturas de cavidade estão normalmente associadas ao equilíbrio metaestável, alcançado devido a quedas locais de pressão ou ao superaquecimento de uma substância pura (ou quase). Nestes casos é necessária a inicialização da mudança de fase através de algum mecanismo adequado, o qual gera uma sequência rápida de fenômenos. Apesar de comumente associado a danos, recentemente vários estudos vêm mostrando aplicações práticas deste tema, além de um campo ainda pouco explorado, que é o das macro cavidades. Essas cavidades podem ser geradas através do aquecimento de água a baixa pressão, sob condições específicas, criando sequências explosivas e formando movimentos como pistão para a água no interior de um invólucro convenientemente dimensionado. Este fenômeno mostra-se semelhante em diversos aspectos às micro cavidades, mais especificamente às cavidades próximas a superfícies livres, embora, sem dúvida, em escala muito maior. Os aspectos mencionados foram filmados com câmeras de alta velocidade e as características observadas foram comparadas com aquelas observadas em micro escala. Vários testes foram desenvolvidos de forma a melhor entender a dinâmica da formação e colapso dessas estruturas, sobretudo levando em conta um comportamento mais unidimensional para a evolução da bolha. Através de várias aproximações e análise de diferentes hipóteses para a variação de pressão e para a força de resistência, soluções analíticas e numéricas foram obtidas para a força exercida no fundo do contêiner e para a expansão e colapso das bolhas ao longo do tempo. As soluções propostas, em comparação com os dados experimentais, mostraram boa concordância entre si, sugerindo que os aspectos fundamentais da dinâmica da cavidade foram devidamente considerados e quantificados. / Cavitation and bubble dynamics are fairly recurring topics in literature, mostly due to their effects in various types of phenomena such as heat transfer and flow in pipes. Considering liquid phases, it is known that these cavity structures are normally associated with the metastable equilibrium, reached due to local pressure drop or overheating of a pure substance (or nearly so). In these cases, the phase change require a startup via some appropriate mechanism, which generates a fast sequence of phenomena. Although commonly associated with damage, recently several studies shown practical applications of these topics, and a still little explored field emerged, which is the field of macro cavities. These cavities can be generated by heating water at a low pressure, under specific conditions, creating an explosive sequences and forming piston like movements for the water inside a properly scaled casing. This phenomenon appears to be similar in many aspects to micro cavities, more specifically for cavities near free surfaces, although, without doubt, on a much larger scale. The mentioned aspects were filmed with high-speed cameras and the main features were compared with those observed in micro scale. Several tests have been developed to better understand the dynamics of the formation and collapse of these structures, especially taking into account a more one-dimensional behavior to the evolution of the bubble. Through various approximations, and analysis of different assumptions for the variation of pressure and the resistance force, analytical and numerical solutions were obtained for the force exerted on the bottom of the container and the expansion and collapse of bubbles over time. The proposed solutions in comparison with experimental data showed good agreement between each other suggesting that the fundamental aspects of the dynamics of the cavity were properly considered and quantified.

<p>Every tenth year a financial crisis tend to interfere with an economy. Price bubbles with an accompanying market plunge are therefore not a new phenomenon. Such market disruptions have been causing problems for centuries, as history has a tendency to repeat itself. The intention with this study is to learn more about the bubble phenomenon and increase the knowledge in this area in order to, if possible, prevent such a thing from happening again. The purpose of the essay is to identify factors that significantly affected the development of the so called IT-bubble in Sweden and Finland during the years of 1995-2000. The previous purpose can be divided into two sub-purposes, namely; to point out which financial theory/theories that succeeds the best to explain the development of the IT-bubble, and additionally; to detect factors that can help us foreseeing similar scenarios in the future.</p><p>The study concentrates on Sweden and Finland. Furthermore, only stock market bubbles are studied. As a consequence, other types of financial crises, for example bank crises, are excluded from this study. The method used to answer the first sub-purpose is an analysis of financial theories which enables us to find factors that according to theory could have caused the rise of the price bubble. In order to answer the second sub-purpose we take use of a statistical method. We have designed a statistical model based on the results of previous mentioned analysis. In this model we try the relevance of the detected factors from the theoretical analysis in order to investigate if theory manages to explain the birth of a stock market bubble.</p><p>The result of our study has generated four different factors; macro economic; institutional; psychological and asymmetric information. These four categories showed importance for the development of the IT-bubble in Sweden and Finland. Out of the four factors, the psychological factor is said to be the most important, but in the mean time the hardest one to predict. The statistical model indicates that the number of bankruptcies, the total amount of household’s borrowing and results from attitude surveys in the case of Sweden, and the number of bankruptcies, new registrations of cars and finally consumers attitude towards making a big purchase at present, in the case of Finland are variables to be aware of when looking out for a stock market bubble. The statistical model, as pointed out in the study, is not perfect. Additional studies are necessary to confirm the results presented in this report.</p> / <p>Finansiella kriser tenderar att drabba ekonomin med ungefär tio års intervaller. Prisbubblor med tillhörande djupdykning på marknaden är således inget nytt fenomen. Denna störning i marknadsharmonin har funnits under flertalet sekler och historien har en benägenhet att upprepa sig. Bakgrunden till studien är således att öka förståelsen för bubbelfenomenet och att, om det är möjligt, förhindra att det händer igen. Syftet med studien är att påvisa faktorer som har haft signifikant betydelse för den så kallade IT-bubblans utveckling i Sverige och Finland under åren 1995-2000. Det övergripande syftet kan vidare indelas i två delsyften, vars mål dels är att påvisa vilken eller vilka finansiella teorier som bäst förklarar IT-bubblans utveckling, dels hitta faktorer som kan hjälpa oss att förutse likartade scenarier i framtiden.</p><p>Studien fokuserar på länderna Sverige och Finland, och avgränsar sig därmed från övriga länder. Vidare studeras enbart börsbubblor och fall, varför övriga typer av finansiella kriser, så som exempelvis bankkriser utesluts. Metoden för att besvara det första delsyftet är att göra en analys av finansiella teorier för att lyfta fram faktorer som enligt dem kan ha haft avgörande betydelse för bubblans uppbyggnad. Metoden för det andra delsyftet är att bygga en statistisk modell med hjälp av de faktorer som resulterat av ovan nämnda analys, för att pröva deras relevans.</p><p>Resultatet av vår studie har genererat en grupp bestående av fyra olika faktorer; makroekonomiska, institutionella, psykologiska faktorer och asymmetrisk information som bäst förklarar IT-bubblans uppkomst. Av dessa är den psykologiska faktorn den viktigaste, och samtidigt också den svåraste att förutsäga. Det är framförallt teorierna inom ”behavioural finance” som fokuserar på psykologiska effekter, varför de bäst förklarar händelseförloppet under IT-bubblan. Den statistiska modellen indikerar att antalet konkurser, hushållens totala utlåning och resultatet av samhällsekonomiska attitydundersökningar i fallet Sverige, samt antalet konkurser, nyregistrering av bilar, och slutligen konsumenternas attityd till stora köp och till att ta lån för tillfället, i fallet Finland, är variabler som vi kan vara uppmärksamma på för att försöka förutse börsbubblor. Den statistiska modellen är, som poängteras i arbetet, inte fulländad utan ytterligare studier fordras för att belägga detta ytterligare.</p>

<p>Finansiella kriser tenderar att drabba ekonomin med ungefär tio års intervaller. Prisbubblor med tillhörande djupdykning på marknaden är således inget nytt fenomen. Denna störning i marknadsharmonin har funnits under flertalet sekler och historien har en benägenhet att upprepa sig. Bakgrunden till studien är således att öka förståelsen för bubbelfenomenet och att, om det är möjligt, förhindra att det händer igen. Syftet med studien är att påvisa faktorer som har haft signifikant betydelse för den så kallade IT-bubblans utveckling i Sverige och Finland under åren 1995-2000. Det övergripande syftet kan vidare indelas i två delsyften, vars mål dels är att påvisa vilken eller vilka finansiella teorier som bäst förklarar IT-bubblans utveckling, dels hitta faktorer som kan hjälpa oss att förutse likartade scenarier i framtiden.</p><p>Studien fokuserar på länderna Sverige och Finland, och avgränsar sig därmed från övriga länder. Vidare studeras enbart börsbubblor och fall, varför övriga typer av finansiella kriser, så som exempelvis bankkriser utesluts. Metoden för att besvara det första delsyftet är att göra en analys av finansiella teorier för att lyfta fram faktorer som enligt dem kan ha haft avgörande betydelse för bubblans uppbyggnad. Metoden för det andra delsyftet är att bygga en statistisk modell med hjälp av de faktorer som resulterat av ovan nämnda analys, för att pröva deras relevans.</p><p>Resultatet av vår studie har genererat en grupp bestående av fyra olika faktorer; makroekonomiska, institutionella, psykologiska faktorer och asymmetrisk information som bäst förklarar IT-bubblans uppkomst. Av dessa är den psykologiska faktorn den viktigaste, och samtidigt också den svåraste att förutsäga. Det är framförallt teorierna inom ”behavioural finance” som fokuserar på psykologiska effekter, varför de bäst förklarar händelseförloppet under IT-bubblan. Den statistiska modellen indikerar att antalet konkurser, hushållens totala utlåning och resultatet av samhällsekonomiska attitydundersökningar i fallet Sverige, samt antalet konkurser, nyregistrering av bilar, och slutligen konsumenternas attityd till stora köp och till att ta lån för tillfället, i fallet Finland, är variabler som vi kan vara uppmärksamma på för att försöka förutse börsbubblor. Den statistiska modellen är, som poängteras i arbetet, inte fulländad utan ytterligare studier fordras för att belägga detta ytterligare.</p> / <p>Every tenth year a financial crisis tend to interfere with an economy. Price bubbles with an accompanying market plunge are therefore not a new phenomenon. Such market disruptions have been causing problems for centuries, as history has a tendency to repeat itself. The intention with this study is to learn more about the bubble phenomenon and increase the knowledge in this area in order to, if possible, prevent such a thing from happening again. The purpose of the essay is to identify factors that significantly affected the development of the so called IT-bubble in Sweden and Finland during the years of 1995-2000. The previous purpose can be divided into two sub-purposes, namely; to point out which financial theory/theories that succeeds the best to explain the development of the IT-bubble, and additionally; to detect factors that can help us foreseeing similar scenarios in the future.</p><p>The study concentrates on Sweden and Finland. Furthermore, only stock market bubbles are studied. As a consequence, other types of financial crises, for example bank crises, are excluded from this study. The method used to answer the first sub-purpose is an analysis of financial theories which enables us to find factors that according to theory could have caused the rise of the price bubble. In order to answer the second sub-purpose we take use of a statistical method. We have designed a statistical model based on the results of previous mentioned analysis. In this model we try the relevance of the detected factors from the theoretical analysis in order to investigate if theory manages to explain the birth of a stock market bubble.</p><p>The result of our study has generated four different factors; macro economic; institutional; psychological and asymmetric information. These four categories showed importance for the development of the IT-bubble in Sweden and Finland. Out of the four factors, the psychological factor is said to be the most important, but in the mean time the hardest one to predict. The statistical model indicates that the number of bankruptcies, the total amount of household’s borrowing and results from attitude surveys in the case of Sweden, and the number of bankruptcies, new registrations of cars and finally consumers attitude towards making a big purchase at present, in the case of Finland are variables to be aware of when looking out for a stock market bubble. The statistical model, as pointed out in the study, is not perfect. Additional studies are necessary to confirm the results presented in this report.</p>

Carbon dioxide hydrates were synthesized in a 0.10m I.D. and 1.22m tall bubble column equipped with a cooling jacket for heat removal. Visual observations at different driving forces (pressures between 2.75 and 3.60 MPa and temperatures between 0 and 8ºC) were recorded with a digital camera through a sight glass of 118.8 by 15.6 mm. The superficial gas velocity was varied from 20 to 50 mm/s to attain different levels of turbulence in the liquid. The growth rate was found to be dependent on the sequence/method used to reach the operating temperature and pressure. A greater supersaturation was obtained when the system temperature and pressure were reached with very low or no bubble-induced mixing. As a result, hydrates nucleated and grew immediately when starting the gas flow with the reactor volume being quickly filled with hydrates. Moreover, the hydrate growth rate and solution final density were higher when operating conditions partially condensed CO2 resulting in greater interphase mass transfer rates. In parallel, since hydrate formation is an exothermic process and the reaction is often limited by the rate of heat removal, heat transfer measurements were achieved in a simulated hydrate environment. The instantaneous heat transfer coefficient and related statistics gave insight on the role of bubbles on heat transfer and hydrodynamics.

Carbon dioxide hydrates were synthesized in a 0.10m I.D. and 1.22m tall bubble column equipped with a cooling jacket for heat removal. Visual observations at different driving forces (pressures between 2.75 and 3.60 MPa and temperatures between 0 and 8ºC) were recorded with a digital camera through a sight glass of 118.8 by 15.6 mm. The superficial gas velocity was varied from 20 to 50 mm/s to attain different levels of turbulence in the liquid. The growth rate was found to be dependent on the sequence/method used to reach the operating temperature and pressure. A greater supersaturation was obtained when the system temperature and pressure were reached with very low or no bubble-induced mixing. As a result, hydrates nucleated and grew immediately when starting the gas flow with the reactor volume being quickly filled with hydrates. Moreover, the hydrate growth rate and solution final density were higher when operating conditions partially condensed CO2 resulting in greater interphase mass transfer rates. In parallel, since hydrate formation is an exothermic process and the reaction is often limited by the rate of heat removal, heat transfer measurements were achieved in a simulated hydrate environment. The instantaneous heat transfer coefficient and related statistics gave insight on the role of bubbles on heat transfer and hydrodynamics.

This is an experimental and theoretical study of a laminar separation bubble and the associated transition dynamics in its early stages. The separation of a laminar boundary layer from a solid surface is prevalent in very many flow situations such as over gas turbine blades (especially in the low-pressure turbine stage) and the wings of micro-aero-vehicles (MAVs) that operate at fairly low Reynolds numbers. Flow separation occurs in such cases due to the presence of an adverse pressure gradient. The separated shear layer becomes unstable due to the presence of an inflection point and presumably transitions to turbulence rapidly. Eventually, there is reattachment back to the solid surface further downstream, if conditions are right. The region enclosed by the shear layer is called a laminar separation bubble and has been a subject of many studies in the past. The present experiments have been conducted in a closed-circuit wind tunnel. A separation bubble was obtained on the upper surface of a flat plate by appropriately contouring the top wall of the tunnel. Four different techniques were used for qualitative and quantitative study viz. surface flow visualisation, smoke flow visualisation, surface pressure measurements and hotwire anemometry. Response of the bubble to both natural as well as artificial (impulsive excitation) disturbance environment has been studied. Linear stability analyses (both Orr-Sommerfeld and Rayleigh calculations), in the spatial framework, have been performed for the mean velocity profiles starting from an attached adverse pressure gradient boundary layer all the way up to the front portion of the separation bubble region (i.e. up to the end of the dead-air region where linear evolution of disturbances could be expected). The measured velocity profiles (both attached and separated) were fitted with analytical model profiles for doing stability calculations. A separation bubble consists of aspects of both wall-bounded and wall-free shear layers and therefore both viscous and inviscid mechanisms are expected to be at play. Most of the studies in the literature point to the inviscid instability associated with the shear layer to be the main mechanism. The main aim of the present work is to understand the exact origin of the primary instability mechanism responsible for the amplification of disturbances. We argue that at least up to the front portion of the bubble, the instability mechanism is due to the inflectional mode associated with the mean velocity profile. However, the seeds of this inviscid inflectional instability could be traced back to the attached boundary layer upstream of separation. In other words, the inviscid inflectional instability of the separated shear layer should be logically seen as an extension of the instability of the upstream attached adverse-pressure-gradient boundary layer. This modifies the traditional view that pegs the origin of the instability in a separation bubble to the free shear layer outside the bubble with its associated Kelvin-Helmholtz mechanism. Our contention is that only when the separated shear layer has moved considerably away from the wall (and this happens near the maximum height of the mean bubble) that a description by Kelvin-Helmholtz instability paradigm with its associated scaling principles could become relevant. We also propose a new scaling for the most amplified frequency for a wall-bounded shear layer in terms of the inflection point height and the vorticity thickness, and show its universality. Next, we theoretically investigate the role played by the re-circulating region of the separation bubble in the linear instability regime. In the re-circulating region near the wall, associated with the so-called wall mode, the production of disturbance kinetic energy is found to be negative. This is a very interesting observation which has been cursorily noted in earlier studies. Here we show that the near-wall negative production region exerts a stabilising influence on the downstream travelling disturbances. A theoretical support for such a mechanism to exist close to the wall is presented. It is shown that the stabilising wall-proximity effect is not a peripheral aspect but has a significant effect on the overall stability especially for the waves close to the upper neutral branch. We demonstrate the appropriateness of inviscid analysis for the stability of the separated flow velocity profile away from the wall, by comparing the numerical solutions of Rayleigh and Orr-Sommerfeld equations. Following this, the analytical consequences of the Rayleigh equation such as the inflection point criterion and the Fjortoft criterion are derived for the wall-bounded inflectional velocity profiles. Furthermore, we also discuss the relevance of the negative production region towards flow control and management for the wall-bounded flows. It appears fruitful to divide the separation bubble region into two parts with respect to the nature of disturbance dynamics: one outside the mean dividing streamline (which behaves as an amplifier) and the other inside the bubble corresponding to the re-circulating region (having oscillator type characteristics). To explore the oscillator-like behaviour of the bubble further, we have carried out spatio-temporal stability analysis of the reversed flow velocity profiles and determined the conditions for the onset of absolute instability. We contend that the presence of the negative production region for the upstream travelling waves has a restraining effect arresting the tendency of the flow (both wall-free and wall-bounded) to become absolutely unstable and thereby requiring a particular threshold of the backflow velocity to be crossed for its realisation. Moreover, the delay in the onset of absolute instability for a wall-bounded profile as compared to a free shear layer is attributed to a certain ‘negative-drag’ effect of the wall on the overall flow which increases the group velocities for the wall-bounded flows. A related theme in the literature regarding the dynamics of laminar separation bubbles is the so-called ‘bursting’ of the bubble wherein there is a sudden increase in the length and height of the bubble as some critical conditions are reached. Bubbles before bursting are termed as ‘short’ bubbles and those after bursting as ‘long’ bubbles. In this work, we provide a criterion to predict bursting which is a refinement over the existing criteria. The proposed criterion takes into account not just the length of the bubble but also the maximum height and it is shown to be more universal in differentiating short bubbles from the long ones, as compared to the other criteria. We also present a hypothesis regarding the sequence of events leading to bubble bursting by relating its onset to the instability of the re-circulating region. For this we observe that as the amount of backflow velocity is increased for a reversed flow velocity profile, the inflection point moves inside the mean dividing streamline and this happens before the onset of absolute instability. This causes a vorticity maximum to develop inside the re-circulating region which could lead to the instability of the closed streamlines with respect to two-dimensional cylindrical disturbances. The actual bursting process may be expected to involve non-linear interactions of the disturbances and the long bubble could be a nonlinearly saturated state of the instability of the re-circulating region. In order to explore the three-dimensionality associated with the bubble, extensive surface flow visualisation experiments have been performed. The surface streamline pattern is obtained for the entire span of the plate for three different freestream velocities. The patterns have been interpreted using topological ideas and various critical points have been identified. It is shown that the arrangement of critical points satisfies the ‘index theorem’ which is a topological necessity and the streamline patterns are ‘structurally stable’. An interesting observation from these patterns is the presence of three-dimensionality upstream of the separation line close to the wall even though the oncoming flow is nominally two-dimensional. Using the critical point theory, we propose a hypothesis which could be used to construct a semi-empirical model wherein the critical points are assigned with a quantity called ‘strength’ for determining the extent of upstream influence of a given separation line. Finally, we derive a necessary condition for the existence of inviscid spatial instability in plane parallel flows. It states that for spatial instability the curvature of the velocity profile should be positive in some region of the profile. This includes Rayleigh’s inflection point theorem (which was proposed and proved by Rayleigh for temporal instability) as a special case. It thus provides a rigorous basis for applying the inflection point criterion to the flows in the framework of spatial stability theory (which we have used extensively in the present thesis). Moreover, the condition derived here is more general as it also includes velocity profiles with the curvature positive everywhere which are excluded by Rayleigh’s theorem in the temporal framework. An example of such a profile is presented (Couette-Poiseuille flow with adverse pressure gradient) and it is shown that this flow is an exceptional case which is temporally stable but spatially unstable. Eigenvalue calculations as well as energy considerations suggest that the mechanism governing instability of this flow is inviscid and non-inflectional in character. This is a new result which could have important implications in understanding the instability dynamics of parallel flows.