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Computational simulations of a perturbed vortex ring are performed to recreate and understand the instability seen in impacting water droplet experiments. Three initial conditions are tried to respectively trigger a Widnall instability, a Rayleigh centrifugal instability, and a vortex breakdown instability. Simulations with a perturbed solitary ring result in an instability similar to that seen experimentally. Waviness of the core which would be expected from a Widnall instability is not visible. Adding an opposite-signed secondary vortex ring or an image vortex ring to the initial conditions does not appear to significantly change the instability from what is seen with a solitary ring. This suggests that a Rayleigh centrifugal instability or a vortex breakdown instability are not likely at work, though tests are not conclusive. Elliptical streamlines are visible in the core of the solitary ring at early times, suggesting that an elliptic instability may be the source of the experimental instability.

The concept of a front plays a decisive role in various elds in physics and beyond. In the present thesis we study key aspects of front dynamics and stability in the context of laser plasmas, organic semiconductors and quantum media. In laser plasmas, we investigate the hydrodynamic instabilities developing at the fronts of laser deagration (ablation). Using direct numerical simulations, we nd noticeable velocity increase of the Rayleigh-Taylor bubble at a deagration front in comparison with that arising at an inert interface. We study the Darrieus-Landau instability of laser deagration accounting for the specific features of the fusion plasmas: strong temperature dependence of the thermal conduction and sonic velocities of the plasma flow. We find that these features of the laser plasmas make the Darrieus-Landau instability stronger by a factor of 3 in comparison with the well-known case of slow flames. We clarify the experimental conditions required for observations of the Darrieus-Landau instability in laser plasmas. In quantum plasmas, we study interplay of the classical and quantum eects for shock waves and for the pseudo-ferrouid instability. For shocks in quantum plasmas, we demonstrate transition from a monotonic Burgers classical shock structure to the train of oscillations (solitons) in the quantum limit. We obtain also a counterpart of the ferrouid instability in quantum magnetized plasmas due to collective spin-dynamics in an external magnetic eld. We discuss importance of the instability for thermonuclear explosions of white dwarfs in the Supernovae Ia events. In organic semiconductors, we develop the theoretical and numerical model of the electrochemical doping fronts. The study is based on the modifed mobilitydiffusion approach to the complex semiconductor plasmas consisting of holes, electrons, positive and negative ions. The m odel describes the doping front structure and predicts the front velocity in a very good agreement with the experiments. We discover a new fundamental instability, which distorts the doping fronts and speeds-up the process considerably. We demonstrate how the instability may be controlled and used to improve performance of optoelectronic devices. Finally, we study avalanches of spin-switching in crystals of nanomagnets, which may be described as magnetic deagration and detonation due to striking resemblance to the respective combustion phenomena. We find that magnetic deflagration becomes unstable and propagates in a pulsating regime when potential barrier of the spin-switching is sufficiently high in comparison with the energy release in the process. We also demonstrate the possibility of magnetic detonation in the crystals, which explains the astounding effect of ultra-fast spin-avalanches encountered in recent experiments. We find that magnetic detonation does not destroy the unique properties of the crystals, a very important conclusion in view of possible applications of nanomagnets in quantum computing. / Konceptet med en utbredningsfront spelar en avgörande roll inom många olika områden i fysik. I denna avhandling studeras centrala aspekter av utbredningsfronters dynamik och deras stabilitet i för laser-plasmaväxelverkan, organiska halvledare samt kvantmedier.  För laser-plasmaväxelverkan har vi undersökt de hydrodynamiska instabiliteter, som t ex Rayleigh-Taylor-instabiliteten, vilka utvecklas vid deflagrationsfronter (under så kallad laserablation). Med hjälp av direkta numeriska simuleringar har vi hittat en märkbar hastighetökning av Rayleigh-Taylor bubblan i en deaflgrationsfront jämfört med det som kan ses vid ett inert gränssnitt. Vi har även studerat Darrieus-Landau-instabiliteten vid laserdeflagration, speciellt hur denna påverkas av de specifika egenskaperna hos ett fusionsplasma: ett starkt temperaturberoende hos värmeledningen samt plasmaflödet som uppnår ljudhastighet. Vi har funnit att dessa egenskaper hos laser-plasmasystem gör Darrieus-Landau instabilitet starkare jämfört med det vanliga fallet av långsamma flammor. Vi har även klargjort de experimentella förutsättningar som krävs för observationer av Darrieus-Landau instabilitet i laser-plasmasystem.  Vi har studerat samspelet mellan klassiska och kvantmekaniska aspekter i kvantplasmor. Specifikt har vi undersökt chockvågors utbredning och dynamik samt instabiliteter i pseudo-ferrofluider. För chocker i kvantplasmor har vi visat att en övergång från Burgers klassiska monotona chockstruktur till ett vågtåg av solitoner sker i kvantgränsen. Vi för också en motsvarighet till ferrofluidinstabiliter i magnetiserade plasmor  på grund av dess kollektiva spinn-dynamik i ett yttre magnetfält. Vi har diskuterar instabilitetens roll för termonukleära explosioner hos vita dvärgar i supernovor av typ Ia.   I organiska halvledare har vi utvecklat en teoretisk och numerisk modell av elektrokemiska dopningsfronter. Studien är baserad på en modifierade drift-diffusiondmodell för komplexa dopade halvledare, vilka består av hål, elektroner, positiva och negativa joner. Modellen beskriver dopningsstrukturen och gör det möjligt att beräkna dopningsfrontens hastighet med värden som överensstämmer mycket väl med experimenten. Vi har även upptäckt en ny grundläggande instabilitet, vilket gör dopningsfronter anisotrop och snabbar upp processen betydligt. Vi visar hur instabilitet kan styras och användas för att förbättra optoelektronisk utrustning.   Slutligen studerar vi laviner av spin-växlingar i kristaller bestående av nanomagneter. Denna process kan beskrivas i termer av ett nytt analogt koncept, så kallad magnetiska deflagration och detonation, på grund av de slående likheter dessa har till motsvarande förbränningsfenomen. Vi har funnit att magnetiska deflagration blir instabil och propagerar i en pulserande regim när potentialbarriären för spin-växling är tillräckligt hög i jämförelse med frigörelsen av energi i processen. Vi visar också möjlighet till magnetisk detonation i dessa  kristaller, vilket förklarar de ultrasnabba spin-laviner man stött på under vid experiment helt nyligen. Denna magnetiska detonation förstör inte de unika egenskaperna hos kristallerna, en mycket viktig slutsats med tanke på möjliga tillämpningar av nanomagneter i kvantdatorer.

The use of lean, premixed combustion in gas turbines is now widespread due to their low NOx emissions. Such systems are, however, susceptible to a phenomenon called thermoacoustic instability, which occurs as a result of unstable coupling between the combustion chamber acoustics and the flame. It can lead to large amplitude pressure oscillations within a combustor at frequencies in the hundreds of hertz. These pressure oscillations can result in unacceptably large noise levels, flame blow-out, reduced performance and fatigue failure of the combustor walls. This thesis investigates two problems of particular relevance to thermoacoustic instability. (For complete abstract open document)

Objective: The objective of this study was to investigate active shoulder stiffness and strength in recurrent shoulder instability. Additionally, this study sought to investigate the relationship between active stiffness, and quality of life, functional outcome, and perceived instability. The relationship between strength and quality of life, functional outcome and perceived instability was also investigated. Study Design: A cross sectional study of a cohort of subjects with unilateral recurrent anterior shoulder instability was undertaken. Background: Recurrent shoulder instability affects up to 94% of young athletes following a dislocation (Rowe & Skallerides, 1961; Rowe & Zairns, 1956). Active stiffness is possibly an important factor in protecting the joint from episodes of instability (Myers, 2001; Riemann & Lephart, 2002). While studies have examined passive stiffness at the shoulder, there is little that has examined active stiffness. Method: Maximal Voluntary Strength (MVS) of the muscles involved in horizontal flexion and their active stiffness at 30%, 50% and 70% MVS was tested in 16 male subjects, with unilateral traumatic anterior shoulder instability. Additionally, quality of life, function and perceived instability were measured using the Western Ontario Stability Index (WOSI), American Shoulder and Elbow Surgeons Questionnaire (ASES) and the Single Alpha Numeric Evaluation score (SANE) respectively. Results: There was a significant decrease in horizontal flexion strength in the recurrently unstable shoulder. Stiffness was also decreased significantly at 30% and 50% MVS. No statistical difference was demonstrated in stiffness values between limbs at 70% MVS. No significant correlation was shown between active stiffness controlling for strength, and quality of life, function or perceived instability. Additionally, no significant association was shown between strength and quality of life, function or perceived instability. Conclusion: The observed reduction in stiffness in the unstable shoulder warrants the inclusion of exercises in the rehabilitation program to enhance this parameter in an effort to protect the joint from perturbations that might lead to dislocation. Deficits in strength in horizontal flexion indicate that strengthening exercises may also be valuable to enhance performance in activities that incorporate horizontal flexion. The lack of an association between active stiffness and quality of life and overall function may indicate that stiffness investigated in one plane of motion does not adequately reflect tissue stiffness during functional activities. Further examination into stiffness in the unstable shoulder is necessary, utilising methodology that examines stiffness in all three dimensions simultaneously.

Objective: The objective of this study was to investigate active shoulder stiffness and strength in recurrent shoulder instability. Additionally, this study sought to investigate the relationship between active stiffness, and quality of life, functional outcome, and perceived instability. The relationship between strength and quality of life, functional outcome and perceived instability was also investigated. Study Design: A cross sectional study of a cohort of subjects with unilateral recurrent anterior shoulder instability was undertaken. Background: Recurrent shoulder instability affects up to 94% of young athletes following a dislocation (Rowe & Skallerides, 1961; Rowe & Zairns, 1956). Active stiffness is possibly an important factor in protecting the joint from episodes of instability (Myers, 2001; Riemann & Lephart, 2002). While studies have examined passive stiffness at the shoulder, there is little that has examined active stiffness. Method: Maximal Voluntary Strength (MVS) of the muscles involved in horizontal flexion and their active stiffness at 30%, 50% and 70% MVS was tested in 16 male subjects, with unilateral traumatic anterior shoulder instability. Additionally, quality of life, function and perceived instability were measured using the Western Ontario Stability Index (WOSI), American Shoulder and Elbow Surgeons Questionnaire (ASES) and the Single Alpha Numeric Evaluation score (SANE) respectively. Results: There was a significant decrease in horizontal flexion strength in the recurrently unstable shoulder. Stiffness was also decreased significantly at 30% and 50% MVS. No statistical difference was demonstrated in stiffness values between limbs at 70% MVS. No significant correlation was shown between active stiffness controlling for strength, and quality of life, function or perceived instability. Additionally, no significant association was shown between strength and quality of life, function or perceived instability. Conclusion: The observed reduction in stiffness in the unstable shoulder warrants the inclusion of exercises in the rehabilitation program to enhance this parameter in an effort to protect the joint from perturbations that might lead to dislocation. Deficits in strength in horizontal flexion indicate that strengthening exercises may also be valuable to enhance performance in activities that incorporate horizontal flexion. The lack of an association between active stiffness and quality of life and overall function may indicate that stiffness investigated in one plane of motion does not adequately reflect tissue stiffness during functional activities. Further examination into stiffness in the unstable shoulder is necessary, utilising methodology that examines stiffness in all three dimensions simultaneously.

Dissertation (Ph.D.).--University of Texas Graduate School of Biomedical Sciences at San Antonio, 2006. / Vita. Includes bibliographical references.

This study examined the perceptions of housing transitions of individuals with histories of residential instability. Participants were recruited from the Health and Housing in Transition study (HHiT), a longitudinal multi-city Canadian study tracking the health and housing of 1,192 participants over a 5-year period. Twenty-two participants from Ottawa were recruited from the larger study to participate in the present study. Participants were interviewed using a semi-structured interview guide about their experiences of housing transitions (moves from one housing situation to another). A general inductive method for the analysis of qualitative data was used to examine participants’ perceptions of their transitions. Findings suggest that participants experienced feelings of disconnection from their housing situations. Their transitions were characterized as chaotic and initiated by factors outside of their control. Participants attributed their transitions to factors such as interpersonal issues, substance use, financial issues, safety concerns, and pests. The present study emphasizes the complexity of participants’ precarious housing situations and the multifactorial nature of their residential instability.