Combustor blowout is a very serious concern in modern land-based and aircraft engine combustors. The ability to sense blowout precursors can provide significant payoffs in engine reliability and life. The objective of this work is to characterize the blowout phenomenon and develop a sensing methodology which can detect and assess the proximity of a combustor to blowout by monitoring its acoustic signature, thus providing early warning before the actual blowout of the combustor. The first part of the work examines the blowout phenomenon in a piloted jet burner. As blowout was approached, the flame detached from one side of the burner and showed increased flame tip fluctuations, resulting in an increase in low frequency acoustics. Work was then focused on swirling combustion systems. Close to blowout, localized extinction/re-ignition events were observed, which manifested as bursts in the acoustic signal. These events increased in number and duration as the combustor approached blowout, resulting an increase in low frequency acoustics. A variety of spectral, wavelet and thresholding based approaches were developed to detect precursors to blowout. The third part of the study focused on a bluff body burner. It characterized the underlying flame dynamics near blowout in greater detail and related it to the observed acoustic emissions. Vorticity was found to play a significant role in the flame dynamics. The flame passed through two distinct stages prior to blowout. The first was associated with momentary strain levels that exceed the flames extinction strain rate, leading to flame holes. The second was due to large scale alteration of the fluid dynamics in the bluff body wake, leading to violent flapping of the flame front and even larger straining of the flame. This led to low frequency acoustic oscillations, of the order of von Karman vortex shedding. This manifested as an abrupt increase in combustion noise spectra at 40-100 Hz very close to blowout. Finally, work was also done to improve the robustness of lean blowout detection by developing integration techniques that combined data from acoustic and optical sensors.

Interactions between the aerodynamics and dynamics of bluff bodies are important in many engineering applications, including suspension bridges, tall buildings, oil platforms, wind turbine towers, air drops, and construction with cranes. In the rotorcraft field, bluff bodies are commonly suspended underneath the vehicle by tethers. This approach is often the only practical way to deliver a payload in a reasonable amount of time in disaster relief efforts, search-and-rescue operations, and military operations. However, currently a fundamental understanding of the aerodynamics of these bluff bodies is lacking, and accurate dynamic simulation models for predicting the safe flying speed are not available. In order to address these shortcomings, two main advancements are presented in this thesis. The aerodynamics of several three-dimensional canonical bluff bodies are examined over a range of Reynolds numbers representative of wind-tunnel-scale to full-scale models. Numerical experiments are utilized, with a focus on uncertainty analysis and validation of the computations. Mean and unsteady forces and moments for these bluff bodies have been evaluated, and empirical models of the shear layer characteristics have been extracted to quantify the behaviors and provide predictive capability. In addition, a physics-based reduced-order simulation model has been developed for bluff bodies. The physics-based approach is necessary to ensure that the predicted behavior of new configurations is accurate, and it is made possible by the breakthroughs in three-dimensional bluff body aerodynamics presented in this thesis. The integrated aerodynamic forces and moments and dynamic behavior predicted by model are extensively validated with data from wind tunnels, flight tests, and high-fidelity computations. Furthermore, successful stability predictions for tethered loads are demonstrated. The model is applicable to the simulation of any generic bluff body configuration, is readily extensible, and has low computational cost.

Combustion instability plagues the combustion community in a wide range of applications. This un-solved problem is especially prevalent and expensive in aerospace propulsion and ground power generation. The challenges associated with understanding and predicting combustion instability lie in the flame response to the acoustic field. One of the more complicated flame response mechanisms is the velocity coupled flame response, where the flame responds dynamically to the acoustic velocity as well as the vortically induced velocity field excited by the acoustics. This vortically induced, or hydrodynamic, velocity field holds critical importance to the flame response but is computationally expensive to predict, often requiring high fidelity CFD computations. Furthermore, its behavior can be a strong function of the numerous flow parameters that change over the operability map of a combustor. This research focuses on a nominally two dimensional bluff body combustor, which has rich hydrodynamic stability behavior with a manageable number of stability parameters. The work focuses first on experimentally characterizing the dynamical flow and flame behavior. Next, the research shifts focus toward hydrodynamic stability theory, using it to explain the physical phenomena observed in the experimental work. Additionally, the hydrodynamic stability work shows how the use of simple, model analysis can identify the important stability parameters and elucidate their governing physical roles. Finally, the research explores the forced response of the flow and flame while systematically varying the underlying hydrodynamic stability characteristics. In the case of longitudinal combustion instability of highly preheated bluff body combustors, it shows that conditions where an acoustic mode frequency equals the hydrodynamic global mode frequency are not especially dangerous from a combustion instability standpoint, and may actually have a reduced heat release response. This demonstrates the very non-intuitive role that the natural hydrodynamic flow stability plays in the forced heat release response of the flame. For the fluid mechanics community, this work contributes to the detailed understanding of both unforced and forced bluff body combustor dynamics, and shows how each is influenced by the underlying hydrodynamics. In particular, it emphasizes the role of the density-shear layer offset, and shows how its extreme sensitivity leads to complicated flow dynamics. For the flow-combustor community as a whole, the work reviews a pre-existing method to obtain the important flow stability parameters, and demonstrates a novel way to link those parameters to the governing flow physics. For the combustion instability community, this thesis emphasizes the importance of the hydrodynamic stability characteristics of the flow, and concludes by offering a paradigm for consideration of the hydrodynamics in a combustion instability problem.

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A new experimental setup for studies on wake flow instability and its control that successfully has been designed and manufactured, is introduced and de- scribed. The main body is a dual-sided flat plate with an elliptic leading edge and a blunt trailing edge. Permeable surfaces enable boundary layer suction and/or blowing that introduce the unique feature of adjusting the inlet condition of the wake created behind the plate. This, in combination with a trailing edge that is easily modified, makes it an ideal experiment for studies of different control methods for the wake flow instability as well as extensive parameter studies. Experimental validation of the setup has been performed by means of measurements of the wake symmetry and boundary layer velocity profiles at the trailing edge. Some preliminary results on the Strouhal number versus different inlet conditions are reported. Additionally, an in-house vortex detection (VD) program has been developed in order to detect, analyse and compare small-scale vortical structures in instantaneous velocity fields from flow measurements. This will be a powerful tool for comparison of wake characteristics for varying inlet conditions and control methods in the new experimental setup. Measurements from three completely separate experimental setups with different geometries and flow cases, have been analysed by the VD-program.          i.     In order to obtain improved ventilation we have studied the effect of pulsating inflow into a closed volume compared to having the inflow at a constant flow rate. We show that the number of small-scale eddies is significantly increased and that the stagnation zones are reduced in size, which enhances the mixing.         ii.     Instantaneous velocity fields in the wake behind a porous cylinder subjected to suction or blowing through the entire cylinder surface have also been analysed using the VD-program. The results show that the major change for different levels of blowing or suction is the location of vortices while the most common vortex size and strength are essentially unchanged.        iii.     Another study on how the geometry of a V-shaped mixer in a pipe flow affects the mixing have also been examined, where no general differences were found between different thicknesses, why a thickness that is favourable from an acoustic point of view can be chosen.   We also propose a new method, using global mode analysis on experimental data, showing that randomly ordered snapshots of the velocity field behind the porous cylinder can be re-ordered and phase-averaged. / QC 20111108 / Active control of vortex shedding behind bluff bodies

Ministry research project (D. Min.)--Midwestern Baptist Theological Seminary, 2004. / Submitted in partial fulfillment of the requirements for the Doctor of Ministry Degree. Includes bibliographical references (leaves 139-142).

Ministry research project (D. Min.)--Midwestern Baptist Theological Seminary, 2004. / Submitted in partial fulfillment of the requirements for the Doctor of Ministry Degree. Includes bibliographical references (leaves 139-142).

Thesis (D. Min.)--Midwestern Baptist Theological Seminary, 2004. / Abstract. Includes bibliographical references (leaves 139-142).

A presente pesquisa apresenta um estudo do escoamento turbulento ocasionado pela interação entre “bluff-bodies” - corpos geométricos que podem ser instalados em turbinas aeronaves como estabilizadores de chamas que geram zonas de recirculação e, consequentemente, aumentam a eficácia da queima do combustível. Este trabalho utilizou o software livre de Computational Fluid Dynamics (CFD) OpenFoam para realizar simulações computacionais que permitiram a análise do comportamento do ar em relação às zonas de recirculação geradas pela inserção dos corpos em forma de cone e disco. Foram realizadas simulações envolvendo diferentes valores de espessura (em mm) destes corpos e velocidades do escoamento (em m/s). Para analisar o comportamento do escoamento turbulento foram utilizados alguns dos modelos matemáticos mais comuns em estudos envolvendo o fenômeno da turbulência, k-E, k-W, k-ERN G, k-WS ST , fato que permitiu que fosse feita a comparação entre os dados obtidos computacionalmente e os experimentais previamente obtidos na literatura. Pode-se dizer que, entre os modelos estudados, o que mostrou maior conformidade com os dados experimentais foi o k-W SST. / 85 f.

Bluffenomenet är en inre upplevelse av att känna sig som en bedragare och en rädsla att bli avslöjad som otillräcklig i sociala sammanhang. Personer som upplever bluffenomenet tror sig ha övertygat sin omgivning om att de besitter mer kunskap och mer kompetens än de verkligen har och känner en rädsla för att deras brist på förmåga ska avslöjas av omgivningen. Syftet med uppsatsen var att undersöka på vilket sätt och i vilka sammanhang bluffenomenet påverkar socionomen i sin professionella yrkesroll i socialt arbete. Uppsatsens syfte har undersökts genom fyra intervjuer med yrkesverksamma socionomer genom en metodtriangulering där en kvantitativ enkät kombinerades med kvalitativa djupintervjuer. Innehållet i uppsatsen har utgångspunkt i symbolisk interaktionism som vetenskapsteoretisk tradition och den insamlade datan har analyserats utifrån teoretiska begrepp bestående av Bluffenomenet och Yrkesroll samt ett teoretiskt perspektiv i form av det dramaturgiska perspektivet. Uppsatsens huvudsakliga resultat visar att bluffenomenet framförallt uppkommer hos socionomer som är nya i sin yrkesroll eller befinner sig i ett nytt sammanhang arbetsmässigt. Bluffenomenet påverkar informanternas yrkesroll genom att arbetsuppgifter skjuts upp, osäkerhet gentemot klienter och en ödmjukhet inför den egna prestationen i arbetet. Uppsatsens diskussion lyfter bland annat komplexiteten i socionomens yrkesroll i form av andras och egna förväntningar. Uppsatsen kan genom de upplevelser som informanterna delat med sig av användas för att sprida kunskap om bluffenomenets uppkomst och sammanhang hos yrkesverksamma socionomer. / The impostor phenomenon is an inner experience of feeling like a fraud and a fear of being revealed as inadequate in social contexts. People who experience the impostor phenomenon believe they have convinced their surroundings to possess more knowledge and more skills than they really have and fear that their lack of ability will be revealed by the environment. The purpose of the essay was to investigate in what way and in what context the impostor phenomenon affects the social worker in its profession in social work. The purpose of the essay has been answered through four interviews with professional social workers in the form of a method triangulation where a quantitative form was combined with qualitative deep interviews. The content of the essay has been interpreted by symbolic interactionism as a science-theoretic tradition, and the collected data has been analyzed from the theoretical concepts consisting of Impostor phenomenon and Profession and a theoretical perspective in the form of the dramaturgical perspective. The main findings of the essay show that the impostor phenomenon occurs primarily in social workers who are new in their occupational role or in a new context in work. The impostor phenomenon affects the informants in their profession in the way by postponing some duties, insecurity in client meetings and humility for their own performance at work. The discussion discusses the complexity of social workers professional role in the form of others and their own expectations. Through the experiences that the informants shared, the essay can be used to disseminate knowledge about the conception of the impostor phenomenon and context in professional social workers.