View All Items

Title: Viscous Dissipation Effects On Acoustic Instabilities In Combustion Chambers.
Creator: Flores, Wilmer, Ahmed, Kareem, Kapat, Jayanta, Bhattacharya, Samik, Xu, Mengyu, University of Central Florida
Abstract / Description: Combustion chambers are naturally prone to acoustic instabilities that originate from flame propagation. Passive devices such as combustor chamber baffles, resonators, and injection liners have proven to attenuate acoustic instabilities degradate the integrity of engine components. Acoustic energy viscous dissipation effects are measured and quantified for new designs and arrangements implemented in tested suppression devices. Two passive suppression devices are introduced which exhibit new...
Show moreCombustion chambers are naturally prone to acoustic instabilities that originate from flame propagation. Passive devices such as combustor chamber baffles, resonators, and injection liners have proven to attenuate acoustic instabilities degradate the integrity of engine components. Acoustic energy viscous dissipation effects are measured and quantified for new designs and arrangements implemented in tested suppression devices. Two passive suppression devices are introduced which exhibit new baffle arrangement and combustion liner design. Audio acoustic equipment excites chamber acoustic instabilities and microphones receive acoustic pressure wave amplitudes. Using this technique viscous damping effects from acoustic sound waves are measured in un-reacting static and flow conditions. An extensive study on damping enhancements to tangential acoustic mode instabilities was explored. A baffle insert was designed with staggered offset injector baffle blades to evaluate viscous damping effects on tangential acoustic instabilities. Tangential acoustic wave energy dissipation is characterized through decay rates measurements. It was concluded that a staggered offset baffle blades with a constant outer versus inner varying injector exhibits the highest attenuation rate. Changes to baffle blades shows a 2T mode experiences the greatest damping enhancement. An empirical expression is derived from curve fitting decay rates for tangential modes and demonstrates acoustic behavior to follow a non-linear correlation. A new auxetic s-shape structure is incorporated into a combustion liner that was coupled with a Helmholtz resonator. The investigation focuses on viscous damping acoustic effects comparing circles to auxetic designs within grazing and bias flow conditions. A series of experiments were conducted that characterized flow discharge behavior, acoustic impedance, acoustic rig that couples bias and grazing flow. Auxetic designs display enhanced absorption qualities at high frequency bandwidths compared to traditional circles. S-shapes with a 60(&)deg; injection angle demonstrates superior viscous damping absorption characteristics. A higher differential pressure highlights a reduction in absorption coefficient measurements.
Show less
Date Issued: 2019
Identifier: CFE0007630, ucf:52514
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007630

Title: DISCRETE NONLINEAR WAVE PROPAGATION IN KERR NONLINEAR MEDIA.
Creator: Meier, Joachim, Stegeman, George, University of Central Florida
Abstract / Description: Discrete optical systems are a subgroup of periodic structures in which the evolution of a continuous electromagnetic field can be described by a discrete model. In this model, the total field is the sum of localized, discrete modes. Weakly coupled arrays of single mode channel waveguides have been known to fall into this class of systems since the late 1960's. Nonlinear discrete optics has received a considerable amount of interest in the last few years, triggered by the experimental...
Show moreDiscrete optical systems are a subgroup of periodic structures in which the evolution of a continuous electromagnetic field can be described by a discrete model. In this model, the total field is the sum of localized, discrete modes. Weakly coupled arrays of single mode channel waveguides have been known to fall into this class of systems since the late 1960's. Nonlinear discrete optics has received a considerable amount of interest in the last few years, triggered by the experimental realization of discrete solitons in a Kerr nonlinear AlGaAs waveguide array by H. Eisenberg and coworkers in 1998. In this work a detailed experimental investigation of discrete nonlinear wave propagation and the interactions between beams, including discrete solitons, in discrete systems is reported for the case of a strong Kerr nonlinearity. The possibility to completely overcome "discrete" diffraction and create highly localized solitons, in a scalar or vector geometry, as well as the limiting factors in the formation of such nonlinear waves is discussed. The reversal of the sign of diffraction over a range of propagation angles leads to the stability of plane waves in a material with positive nonlinearity. This behavior can not be found in continuous self-focusing materials where plane waves are unstable against perturbations. The stability of plane waves in the anomalous diffraction region, even at highest powers, has been experimentally verified. The interaction of high power beams and discrete solitons in arrays has been studied in detail. Of particular interest is the experimental verification of a theoretically predicted unique, all optical switching scheme, based on the interaction of a so called "blocker" soliton with a second beam. This switching method has been experimentally realized for both the coherent and incoherent case. Limitations of such schemes due to nonlinear losses at the required high powers are shown.
Show less
Date Issued: 2004
Identifier: CFE0000186, ucf:46176
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0000186

Title: Characterization of Acoustic Modes in Aeroengines.
Creator: Otero, Michelle, Ahmed, Kareem, Kapat, Jayanta, Bhattacharya, Samik, University of Central Florida
Abstract / Description: Acoustic instabilities remain a key design concern faced in the development of liquid rocket engines. The interaction between the acoustic modes and the occurring combustion reactions can be detrimental to the engine. The fluctuating pressure waves resulting from the flame oscillations in the system can potentially lead to engine failure. For this reason, research in acoustic instabilities and methods to minimize the influences on the engine, has maintain interest in the aerospace community....
Show moreAcoustic instabilities remain a key design concern faced in the development of liquid rocket engines. The interaction between the acoustic modes and the occurring combustion reactions can be detrimental to the engine. The fluctuating pressure waves resulting from the flame oscillations in the system can potentially lead to engine failure. For this reason, research in acoustic instabilities and methods to minimize the influences on the engine, has maintain interest in the aerospace community. The scope of this study was to design, optimize and characterize acoustic behaviors of a scaled rocket combustion chamber simulating acoustic pressure waves. Tangential and longitudinal acoustic waves of the system were extracted and validated through analytical and computational fluids dynamics models. The results of this study will assist with the process of extracting dominant oscillation frequencies of a system essential in the design of acoustic suppression devices for attenuation of critical frequencies.
Show less
Date Issued: 2018
Identifier: CFE0007354, ucf:52081
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007354

Title: OPTICAL NONLINEAR INTERACTIONS IN DIELECTRIC NANO-SUSPENSIONS.
Creator: El-Ganainy, Ramy, Christodoulides, Demetrios, University of Central Florida
Abstract / Description: This work is divided into two main parts. In the first part (chapters 2-7) we consider the nonlinear response of nano-particle colloidal systems. Starting from the Nernst-Planck and Smoluchowski equations, we demonstrate that in these arrangements the underlying nonlinearities as well as the nonlinear Rayleigh losses depend exponentially on optical intensity. Two different nonlinear regimes are identified depending on the refractive index contrast of the nanoparticles involved and the...
Show moreThis work is divided into two main parts. In the first part (chapters 2-7) we consider the nonlinear response of nano-particle colloidal systems. Starting from the Nernst-Planck and Smoluchowski equations, we demonstrate that in these arrangements the underlying nonlinearities as well as the nonlinear Rayleigh losses depend exponentially on optical intensity. Two different nonlinear regimes are identified depending on the refractive index contrast of the nanoparticles involved and the interesting prospect of self-induced transparency is demonstrated. Soliton stability is systematically analyzed for both 1D and 2D configurations and their propagation dynamics in the presence of Rayleigh losses is examined. We also investigate the modulation instability of plane waves and the transverse instabilities of soliton stripe beams propagating in nonlinear nano-suspensions. We show that in these systems, the process of modulational instability depends on the boundary conditions. On the other hand, the transverse instability of soliton stripes can exhibit new features as a result of 1D collapse caused by the exponential nonlinearity. Many-body effects on the systems' nonlinear response are also examined. Mayer cluster expansions are used in order to investigate particle-particle interactions. We show that the optical nonlinearity of these nano-suspensions can range anywhere from exponential to polynomial depending on the initial concentration and the chemistry of the electrolyte solution. The consequence of these inter-particle interactions on the soliton dynamics and their stability properties are also studied. The second part deals with linear and nonlinear properties of optical nano-wires and the coupled mode formalism of parity-time (PT) symmetric waveguides. Dispersion properties of AlGaAs nano-wires are studied and it is shown that the group velocity dispersion in such waveguides can be negative, thus enabling temporal solitons. We have also studied power flow in nano-waveguides and we have shown that under certain conditions, optical pulses propagating in such structures will exhibit power circulations. Finally PT symmetric waveguides were investigated and a suitable coupled mode theory to describe these systems was developed.
Show less
Date Issued: 2009
Identifier: CFE0002847, ucf:48538
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0002847

Title: Dynamics of a Perfectly Premixed Jet Flame Exhibiting Self-Excited High-Frequency, Transverse Thermoacoustic Instabilities.
Creator: Tran, Lucky, Kapat, Jayanta, Kassab, Alain, Vasu Sumathi, Subith, University of Central Florida
Abstract / Description: This work is an investigation of the behavior of a premixed turbulent jet flame in a cylindrical dump combustor. The degeneracy of the simple configuration in this study lends itself for a detailed study of inherent mechanisms of a self-excited thermoacoustic instability in isolation from system coupling effects, enabling detailed numerical simulations to be carried out to supplement experimental findings. Tests were done at a nominal pressure of 8 bar and inlet temperature around 450 ?C....
Show moreThis work is an investigation of the behavior of a premixed turbulent jet flame in a cylindrical dump combustor. The degeneracy of the simple configuration in this study lends itself for a detailed study of inherent mechanisms of a self-excited thermoacoustic instability in isolation from system coupling effects, enabling detailed numerical simulations to be carried out to supplement experimental findings. Tests were done at a nominal pressure of 8 bar and inlet temperature around 450 ?C. Self-excited large eddy simulations were also carried out in OpenFOAM, using a b-? flame-wrinkling model to model the combustion process. Eigenfrequency analysis in COMSOL was also done to support and explain the findings from both the numerical simulations and trends observed in the experiments. Measurements from high frequency pressure transducers were analyzed to determine the frequencies of the excited modes in the rig test and compared to the spectra from the LES simulation. The time-resolved fields from the LES simulation were phase-averaged to deduce the acoustic-flame interactions. Despite the (axis)symmetry in this configuration, the non-axisymmetric 1T and 1T1L modes were (simultaneously) excited. Two distinct behaviors are noted for the dynamic flame behavior. In the downstream region, the flame motion is well described by a bulk kinematic displacement as a result of the interaction of the flame front with the local acoustic perturbation. In the upstream region, near the combustor dump plane, large-scale wrinkles are observed in the flame front that have characteristics of a convective wave. The current findings provide additional evidence supporting and further establishing the theory of inherent acoustic-flame interactions as an excitation mechanism (distinct from acoustically-induced hydrodynamic oscillations) for high-frequency, transverse thermoacoustic instabilities.
Show less
Date Issued: 2019
Identifier: CFE0007542, ucf:52616
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007542

Title: Thermoacoustic Reimann Solver Finite Volume Method with Application to Turbulent Premixed Gas Turbine Combustion Instability.
Creator: Johnson, Perry, Kapat, Jayanta, Ilie, Marcel, Vasu Sumathi, Subith, Shivamoggi, Bhimsen, University of Central Florida
Abstract / Description: This thesis describes the development, verification, and validation of a three dimensional time domain thermoacoustic solver. The purpose of the solver is to predict the frequencies, modeshapes, linear growth rates, and limit cycle amplitudes for combustion instability modes in gas turbine combustion chambers. The linearized Euler equations with nonlinear heat release source terms are solved using the finite volume method. The treatment of mean density gradients was found to be vital to the...
Show moreThis thesis describes the development, verification, and validation of a three dimensional time domain thermoacoustic solver. The purpose of the solver is to predict the frequencies, modeshapes, linear growth rates, and limit cycle amplitudes for combustion instability modes in gas turbine combustion chambers. The linearized Euler equations with nonlinear heat release source terms are solved using the finite volume method. The treatment of mean density gradients was found to be vital to the success of frequency and modeshape predictions due to the sharp density gradients that occur across deflagration waves. In order to treat mean density gradients with physical fidelity, a non-conservative finite volume method based on the wave propagation approach to the Riemann problem is applied. For modelling unsteady heat release, user input flexibility is maximized using a virtual class hierarchy within the OpenFOAM C++ library. Unsteady heat release based on time lag models are demonstrated. The solver gives accurate solutions compared with analytical methods for one-dimensional cases involving mean density gradients, cross-sectional area changes, uniform mean flow, arbitrary impedance boundary conditions, and unsteady heat release in a one-dimensional Rijke tube. The solver predicted resonant frequencies within 1% of the analytical solution for these verification cases, with the dominant component of the error coming from the finite time interval over which the simulation is performed. The linear growth rates predicted by the solver for the Rijke tube verification were within 5% of the theoretical values, provided that numerical dissipation effects were controlled. Finally, the solver is then used to predict the frequencies and limit cycle amplitudes for two lab scale experiments in which detailed acoustics data are available for comparison. For experiments at the University of Melbourne, an empirical flame describing function was provided. The present simulation code predicted a limit cycle of 0.21 times the mean pressure, which was in close agreement with the estimate of 0.25 from the experimental data. The experiments at Purdue University do not yet have an empirical flame model, so a general vortex-shedding model is proposed on physical grounds. It is shown that the coefficients of the model can be tuned to match the limit cycle amplitude of the 2L mode from the experiment with the same accuracy as the Melbourne case. The code did not predict the excitation of the 4L mode, therefore it is concluded that the vortex-shedding model is not sufficient and must be supplemented with additional heat release models to capture the entirety of the physics for this experiment.
Show less
Date Issued: 2013
Identifier: CFE0005098, ucf:50730
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005098

Title: Combustion Instability Mechanism of a Reacting Jet in Cross Flow at Gas Turbine Operating Conditions.
Creator: Pent, Jared, Kapat, Jayanta, Deng, Weiwei, Gordon, Ali, Vasu Sumathi, Subith, Martin, Scott, University of Central Florida
Abstract / Description: Modern gas turbine designs often include lean premixed combustion for its emissions benefits; however, this type of combustion process is susceptible to self-excited combustion instabilities that can lead to damaging heat loads and system vibrations. This study focuses on identifying a mechanism of combustion instability of a reacting jet in cross flow, a flow feature that is widely used in the design of gas turbine combustion systems. Experimental results from a related study are used to...
Show moreModern gas turbine designs often include lean premixed combustion for its emissions benefits; however, this type of combustion process is susceptible to self-excited combustion instabilities that can lead to damaging heat loads and system vibrations. This study focuses on identifying a mechanism of combustion instability of a reacting jet in cross flow, a flow feature that is widely used in the design of gas turbine combustion systems. Experimental results from a related study are used to validate and complement three numerical tools that are applied in this study (-) self-excited Large Eddy Simulations, 3D thermoacoustic modeling, and 1D instability modeling. Based on the experimental and numerical results, a mechanism was identified that included a contribution from the jet in cross flow impedance as well as an overall jet flame time lag. The jet impedance is simply a function of the acoustic properties of the geometry while the flame time lag can be separated into jet velocity, equivalence ratio, and strain fluctuations, depending on the operating conditions and setup. For the specific application investigated in this study, it was found that the jet velocity and equivalence ratio fluctuations are important, however, the effect of the strain fluctuations on the heat release are minimal due to the high operating pressure. A mathematical heat release model was derived based on the proposed mechanism and implemented into a 3D thermoacoustic tool as well as a 1D instability tool. A three-point stability trend observed in the experimental data was correctly captured by the 3D thermoacoustic tool using the derived heat release model. Stability maps were generated with the 1D instability tool to demonstrate regions of stable operation that can be achieved as a function of the proposed mechanism parameters. The relative effect of the reacting jet in cross flow on the two dominant unstable modes was correctly captured in the stability maps. While additional mechanisms for a reacting jet in cross flow are possible at differing flow conditions, the mechanism proposed in this study was shown to correctly replicate the stability trends observed in the experimental tests and provides a fundamental understanding that can be applied for combustion system design.
Show less
Date Issued: 2014
Identifier: CFE0005687, ucf:50154
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005687

Title: Fiber Optimization for Operation Beyond Transverse Mode Instability Limitations.
Creator: Bradford, Joshua, Richardson, Martin, Gaume, Romain, Amezcua Correa, Rodrigo, Shah, Lawrence, University of Central Florida
Abstract / Description: Transverse Mode Instabilities (TMIs) stand as a fundamental limitation to power and brightness scaling in laser systems based upon optical fiber technologies. This work comprises experimental and theoretical investigations into fiber laser design that should minimize the effects of Stimulated Thermal Rayleigh Scattering. Theoretical discussions and simulations focus on how fiber parameters affect transverse mode coupling. These include core geometry optimization, pump geometry optimization,...
Show moreTransverse Mode Instabilities (TMIs) stand as a fundamental limitation to power and brightness scaling in laser systems based upon optical fiber technologies. This work comprises experimental and theoretical investigations into fiber laser design that should minimize the effects of Stimulated Thermal Rayleigh Scattering. Theoretical discussions and simulations focus on how fiber parameters affect transverse mode coupling. These include core geometry optimization, pump geometry optimization, in addition to the effects of HOM content and losses on the TMI threshold. Experimentally, a high-power laser facility is commissioned with beam quality diagnostics to quantify the thresholds of the onset of modal interferences and their impacts on beam quality. These diagnostics include high-resolution Fourier Transform Interferometry (FTI) and in-situ power-in-the-bucket measurements. The design and characterization capabilities developed here are crucial to the development of next-generation high-power fiber laser capabilities.
Show less
Date Issued: 2018
Identifier: CFE0006980, ucf:51646
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006980

Title: Droplet impact on deep liquid pools: secondary droplets formation from Rayleigh jet break-up and crown splash.
Creator: Castillo Orozco, Eduardo, Kumar, Ranganathan, Mansy, Hansen, Peles, Yoav, University of Central Florida
Abstract / Description: This work aims to study the impact of a droplet on liquid pools of the same fluid to understand the formation of secondary drops from the central jet and crown splash that occur after the impact. The impact of droplets on a deep pool has applications in cleaning up oil spill, spray cooling, painting, inkjet printing and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension and density. Despite the exhaustive research on different aspects of droplet...
Show moreThis work aims to study the impact of a droplet on liquid pools of the same fluid to understand the formation of secondary drops from the central jet and crown splash that occur after the impact. The impact of droplets on a deep pool has applications in cleaning up oil spill, spray cooling, painting, inkjet printing and forensic analysis, relying on the changes in properties such as viscosity, interfacial tension and density. Despite the exhaustive research on different aspects of droplet impact, it is not clear how liquid properties can affect the instabilities leading to the Rayleigh jet breakup and the number of secondary drops formed after it pinches off. In this work, through systematic experiments, the droplet impact phenomena is investigated by varying viscosity and surface tension of liquids as well as impact speeds. Further, using a Volume-of-Fluid (VOF) method, it is shown that Rayleigh-Plateau instability is influenced by these parameters, and capillary timescale is the appropriate scale to normalize the breakup time. Increase in impact velocity increases the height of the thin column of fluid that emerges from the liquid pool. Under certain fluid conditions, the dissipation of this extra kinetic energy along with the surface tension forces produces instabilities at the neck of the jet. This could result in jet breakup and formation of secondary drops. In other words, both the formation of the jet and its breakup require a balance between viscous, capillary and surface tension forces. Based on Ohnesorge number (Oh) and impact Weber number (We), a regime map for no breakup, Rayleigh jet breakup, and crown splash is suggested for 0.0033 ? Oh ? 0.136. For Weber numbers beyond the critical value and Oh ? 0.091 the jet breakup occurs (Rayleigh jet breakup regime). While for Oh (>) 0.091, the jet breakup is suppressed regardless of the Weber number. In addition, high impact velocity initiates the crown formation and if further intensified it can disintegrate it into numerous secondary drops (crown splash) and it is observed to occur at all Ohnesorge numbers and high enough Weber numbers, however, at high Oh, a large portion of kinetic energy is dissipated, thus Rayleigh jet breakup is suppressed regardless of the magnitude of the impact velocity. Moreover, a correlation is proposed for normalized time with respect to the normalized maximum height of jet.
Show less
Date Issued: 2015
Identifier: CFE0006278, ucf:51593
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006278

Title: DEGREES OF CAUSALITY: AN ASSESSMENT OF ENDOGENOUS CONTRIBUTORS TO INSTABILITY IN JORDAN, SYRIA, & TURKEY.
Creator: Willman, Gabriel, Sadri, Houman, University of Central Florida
Abstract / Description: The political instability of the Middle East is often perceived to be derived primarily from the interaction of Middle Eastern nations with external forces; with significant emphasis placed upon the disruptive effects of modern colonialism and Westernization. While this study does not seek to directly contest the catalytic primacy of exogenous factors, it does seek to establish the necessary causality of pre-existing internal factors. Rather than approaching the situation from a linear causal...
Show moreThe political instability of the Middle East is often perceived to be derived primarily from the interaction of Middle Eastern nations with external forces; with significant emphasis placed upon the disruptive effects of modern colonialism and Westernization. While this study does not seek to directly contest the catalytic primacy of exogenous factors, it does seek to establish the necessary causality of pre-existing internal factors. Rather than approaching the situation from a linear causal perspective, this assessment is oriented around an interdisciplinary examination of confluent factors. By examining the political history, ethno sociology, and economy of the region, the analysis investigates the underlying variables which have contributed to the instability of the Jordan, Syria, and Turkey. The primary conclusion of this analysis is that the interactions of multiple endogenous variables provide a basis of necessary causality which may be of equal causal import to that of modern colonialism and Westernization.
Show less
Date Issued: 2012
Identifier: CFH0004228, ucf:44965
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH0004228

Title: Theoretical And Experimental Investigation Of The Cascading Nature Of Pressure-Swirl Atomization.
Creator: Choudhury, Pretam, Kumar, Ranganathan, Deng, Weiwei, Mansy, Hansen, University of Central Florida
Abstract / Description: Pressure swirl atomizers are commonly used in IC, aero-engines, and liquid propellant rocket combustion. Understanding the atomization process is important in order to enhance vaporization, mitigate soot formation, design of combustion chambers, and improve overall combustion efficiency. This work utilizes non-invasive techniques such as ultra -speed imaging, and Phase Doppler Particle Anemometry (PDPA) in order to investigate the cascade atomization process of pressure-swirl atomizers by...
Show morePressure swirl atomizers are commonly used in IC, aero-engines, and liquid propellant rocket combustion. Understanding the atomization process is important in order to enhance vaporization, mitigate soot formation, design of combustion chambers, and improve overall combustion efficiency. This work utilizes non-invasive techniques such as ultra -speed imaging, and Phase Doppler Particle Anemometry (PDPA) in order to investigate the cascade atomization process of pressure-swirl atomizers by examining swirling liquid film dynamics and the localized droplet characteristics of the resulting hollow cone spray. Specifically, experiments were conducted to examine these effects for three different nozzles with orifice diameters .3mm, .5mm, and .97mm. The ultra-speed imaging allowed for both visualization and interface tracking of the swirling conical film which emanated from each nozzle. Moreover, this allowed for the measurement of the radial fluctuations, film length, cone angle and maximum wavelength. Radial fluctuations are found to be maximum near the breakup or rupture of a swirling film. Film length decreases as Reynolds number increases. Cone angle increases until a critical Reynolds number is reached, beyond which it remains constant. A new approach to analyze the temporally unstable waves was developed and compared with the measured maximum wavelengths. The new approach incorporates the attenuation of a film thickness, as the radius of a conical film expands, with the classical dispersion relationship for an inviscid moving liquid film. This approach produces a new long wave solution which accurately matches the measured maximum wavelength swirling conical films generated from nozzles with the smallest orifice diameter. For the nozzle with the largest orifice diameter, the new long wave solution provides the upper bound limit, while the long wave solution for a constant film thickness provides the lower bound limit. These results indicate that temporal instability is the dominating mechanism which generates long Kelvin Helmholtz waves on the surface of a swirling liquid film. The PDPA was used to measure droplet size and velocity in both the near field and far field of the spray. For a constant Reynolds number, an increase in orifice diameter is shown to increase the overall diameter distribution of the spray. In addition, it was found that the probability of breakup, near the axis, decreases for the largest orifice diameter. This is in agreement with the cascading nature of atomization.
Show less
Date Issued: 2015
Identifier: CFE0006030, ucf:51012
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006030

Title: REDUCTION OF VORTEX-DRIVEN OSCILLATIONS IN A SOLID ROCKET MOTOR COLD FLOW SIMULATION THROUGH ACTIVE CONTROL.
Creator: Ward, Jami, Leonessa, Alexander, University of Central Florida
Abstract / Description: Control of vortex-driven instabilities was demonstrated via a scaled-down, cold-flow simulation that modeled closed-end acoustics. When vortex shedding frequencies couple with the natural acoustic modes of a choked chamber, potentially damaging low-frequency instabilities may arise. Although passive solutions can be effective, an active control solution is preferable. An experiment was performed to demonstrate an active control scheme for the reduction of vortex-driven oscillations. A non...
Show moreControl of vortex-driven instabilities was demonstrated via a scaled-down, cold-flow simulation that modeled closed-end acoustics. When vortex shedding frequencies couple with the natural acoustic modes of a choked chamber, potentially damaging low-frequency instabilities may arise. Although passive solutions can be effective, an active control solution is preferable. An experiment was performed to demonstrate an active control scheme for the reduction of vortex-driven oscillations. A non-reacting experiment using a primary flow of air, where both the duct exit and inlet are choked, simulated the closed-end acoustics. Two plates, separated by 1.27 cm, produced the vortex shedding phenomenon at the chamber's first longitudinal mode. Two active control schemes, closed-loop and open-loop, were studied via a cold-flow simulation for validating the effects of reducing vortex shedding instabilities in the system. Actuation for both control schemes was produced by using a secondary injection method. The actuation system consisted of pulsing compressed air from a modifed, 2-stroke model airplane engine, controlled and powered by a DC motor. The use of open-loop only active control was not highly effective in reducing the amplitude of the first longitudinal acoustic mode, near 93 Hz, when the secondary injection was pulsed at the same modal frequency. This was due to the uncontrolled phasing of the secondary injection system. A Pulse Width Modulated (PWM) signal was added to the open-loop control scheme to correct for improper phasing of the secondary injection flow relative to the primary flow. This addition allowed the motor speed to be intermittently increased to a higher RPM before returning to the desired open-loop control state. This proved to be effective in reducing the pressure disturbance by approximately 46%. A closed-loop control scheme was then test for its effectiveness in controlling the phase of the secondary injection. Feedback of the system's state was determined by placing a dynamic pressure transducer near the chamber exit. Closed-loop active control, using the designed secondary injection system, was proven as an effective means of reducing the problematic instabilities. A 50% reduction in the FFT RMS amplitude was realized by utilizing a Proportional-Derivative controller to modify the phase of the secondary injection.
Show less
Date Issued: 2006
Identifier: CFE0000920, ucf:46728
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0000920

Title: MECHANISMS OF LEAN FLAME EXTINCTION.
Creator: Lasky, Ian M, Ahmed, Kareem, University of Central Florida
Abstract / Description: Lean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting strain rate along the flame are analyzed throughout extinction to...
Show moreLean flame blowout is investigated experimentally within a high-speed combustor to analyze the temporal extinction dynamics of turbulent premixed bluff body stabilized flames. The lean blowout process is induced through fuel flow reduction and captured temporally using simultaneous high-speed particle imaging velocimetry (PIV) and CH* chemiluminescence. The evolution of the flame structure, flow field, and the resulting strain rate along the flame are analyzed throughout extinction to distinguish the physical mechanisms of blowout. Flame-vortex dynamics are found to be the main driving mechanism of flame extinction; namely, a reduction of flame-generated vorticity coupled with an increase of downstream shear layer vorticity. The vorticity dynamics are linked to hydrodynamic instabilities that vary as a function of the decreasing equivalence ratio. Frequency analysis is performed to characterize the dynamical changes of the hydrodynamic instability modes during flame extinction. Additionally, various bluff body inflow velocity regimes are investigated to further characterize the extinction instability modes. Both equivalence ratio and flow-driven instabilities are captured through a universal definition of the Strouhal number for the reacting bluff body flow. Finally, a Karlovitz number-based criterion is developed to consistently predict the onset of global extinction for different inflow velocity regimes.
Show less
Date Issued: 2018
Identifier: CFH2000369, ucf:45710
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH2000369

Title: Mechanisms of Flame Extinction for Bluff Body Stabilized Flames with Influences of Pressure Gradient Tailoring.
Creator: Morales, Anthony, Ahmed, Kareem, Bhattacharya, Samik, Vasu Sumathi, Subith, University of Central Florida
Abstract / Description: Flame extinction continues to hinder the performance of combustion technologies used in propulsion systems and power generating turbomachinery. Within these applications, there is a crucial need to improve energy output while minimizing harmful environmental impacts. Lean combustion helps attain these goals by minimizing fuel costs and reducing NOx emissions. However, operating at lean conditions increases the likelihood of flame extinction; the flame becomes more susceptible to hydrodynamic...
Show moreFlame extinction continues to hinder the performance of combustion technologies used in propulsion systems and power generating turbomachinery. Within these applications, there is a crucial need to improve energy output while minimizing harmful environmental impacts. Lean combustion helps attain these goals by minimizing fuel costs and reducing NOx emissions. However, operating at lean conditions increases the likelihood of flame extinction; the flame becomes more susceptible to hydrodynamic instabilities which can induce global blowout and termination of the combustion process. The work in this thesis is focused on identifying the mechanisms of flame extinction and controlling these mechanisms via pressure gradient tailoring. This is accomplished within a premixed blow-down combustion facility utilizing a bluff body flame stabilizer where flame extinction is induced by removing the flow of fuel into the reactant mixture. CH* chemiluminescence imaging and high-speed particle imaging velocimetry (PIV) are used to determine the flame boundary and resolve the reacting flow field, respectively. The mechanisms of flame extinction are attributed to the changing vorticity dynamics within the flow field as the equivalence ratio is reduced, which will directly influence the strain rate experienced by the flame. To influence these vorticity dynamics, the test section walls are manipulated to alter the downstream pressure gradients. It is determined that increasing the magnitude of the downstream pressure gradient increases the growth of the strain rate and vorticity experienced by the flame.
Show less
Date Issued: 2018
Identifier: CFE0007229, ucf:52240
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007229

Title: MICROSCOPIC SURFACE TEXTURES CREATED BY INTERFACIAL FLOW INSTABILITIES.
Creator: Gu, Jing, Weiwei Deng, Dr., University of Central Florida
Abstract / Description: In nature, microscopic surface textures impact useful function, such as the drag reduction of shark skin (Dean & Bhushan, 2010) and superhydrophobicity of the lotus leaf(Pan, Kota, Mabry, & Tuteja, 2013). In this study, we explore these phenomena by re-creating microscopic surface textures via the method of interfacial flow instability in drying polyvinylidene fluoride (PVDF) acetone solutions. In general, PVDF films can be made using either spin coating or electrospray deposition with...
Show moreIn nature, microscopic surface textures impact useful function, such as the drag reduction of shark skin (Dean & Bhushan, 2010) and superhydrophobicity of the lotus leaf(Pan, Kota, Mabry, & Tuteja, 2013). In this study, we explore these phenomena by re-creating microscopic surface textures via the method of interfacial flow instability in drying polyvinylidene fluoride (PVDF) acetone solutions. In general, PVDF films can be made using either spin coating or electrospray deposition with various weight concentrations in acetone. In order to study the morphology of the porous structure of PVDF films, wet deposition samples were fabricated by spin coating or near-field electrospray. Possible theories are discussed and examined to explain the formation of these porous structures resulting in development of a well-controlled method to create porous PVDF films with various pore sizes and pore densities. All samples are characterized and found to exhibit superhydrophobicity and drag reduction. To connect porous PVDF film morphology to the established field of dry particle fabrication, PVDF particle synthesis by far-field electrospray is also reviewed and discussed. An established method to generate polymer particles of different morphologies in other polymers (Almeria-Diez, 2012) by electrospray drying is confirmed using PVDF as well. Due to the ability of scalable and re-configurable electrospray, the microscopic surface textures can be applied to areas of any size to reduce drag or impart water-repelling properties.
Show less
Date Issued: 2013
Identifier: CFH0004479, ucf:45066
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH0004479

Title: Nonlinear Dynamics in Multimode Optical Fibers.
Creator: Eftekhar, Mohammad Amin, Christodoulides, Demetrios, Amezcua Correa, Rodrigo, Li, Guifang, Kaup, David, University of Central Florida
Abstract / Description: Multimode optical fibers have recently reemerged as a viable platform for addressing a number of long-standing issues associated with information bandwidth requirements and power-handling capabilities. The complex nature of heavily multimoded systems can be effectively exploited to observe altogether novel physical effects arising from spatiotemporal and intermodal linear and nonlinear processes. Here, we have studied nonlinear dynamics in multimode optical fibers (MMFs) in both the normal...
Show moreMultimode optical fibers have recently reemerged as a viable platform for addressing a number of long-standing issues associated with information bandwidth requirements and power-handling capabilities. The complex nature of heavily multimoded systems can be effectively exploited to observe altogether novel physical effects arising from spatiotemporal and intermodal linear and nonlinear processes. Here, we have studied nonlinear dynamics in multimode optical fibers (MMFs) in both the normal and anomalous dispersion regimes. In the anomalous dispersion regime, the nonlinearity leads to a formation of spatiotemporal 3-D solitons. Unlike in single-mode fibers, these solitons are not unique and their properties can be modified through the additional degrees of freedom offered by these multimoded settings. In addition, soliton related processes such as soliton fission and dispersive wave generation will be also drastically altered in such multimode systems. Our theoretical work unravels some of the complexities of the underlying dynamics and helps us better understand these effects. The nonlinear dynamics in such multimode systems can be accelerated through a judicious fiber design. A cancelation of Raman self-frequency shifts and Blue-shifting multimode solitons were observed in such settings as a result of an acceleration of intermodal oscillations. Spatiotemporal instabilities in parabolic-index multimode fibers will also be discussed. In the normal dispersion regime, this effect can be exploited to generate an ultrabroad and uniform supercontinuum that extends more than 2.5 octaves. To do so, the unstable spectral regions are pushed away from the pump, thus sweeping the entire spectrum. Multimode parabolic pulses were also predicted and observed in passive normally dispersive tapered MMFs. These setting can obviate the harsh bandwidth limitation present in single-mode system imposed by gain medium and be effectively used for realizing high power multimode fiber lasers. Finally, an instant and efficient second-harmonic generation was observed in the multimode optical fibers. Through a modification of initial conditions, the efficiency of this process could be enhanced to a record high of %6.5.
Show less
Date Issued: 2018
Identifier: CFE0007399, ucf:52063
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0007399

Title: Three Essays on Asset Pricing in Security and Housing Markets.
Creator: Zheng, Minrong, Chen, Honghui, Turnbull, Geoffrey, Frye, Melissa, Zahirovic-Herbert, Velma, University of Central Florida
Abstract / Description: In my first essay, I investigate the relationship between IPO long-run underperformance (Ritter, 1991) and the idiosyncratic risk puzzle (Ang, Hodrick, Xing and Zhang, 2006), the phenomenon of abnormally low returns for stocks with high idiosyncratic risk. I show that IPO long-run underperformance is in fact a manifestation of the surprisingly low returns for high idiosyncratic risk stocks. IPO underperformance disappears after I control for the idiosyncratic risk. Specifically, the...
Show moreIn my first essay, I investigate the relationship between IPO long-run underperformance (Ritter, 1991) and the idiosyncratic risk puzzle (Ang, Hodrick, Xing and Zhang, 2006), the phenomenon of abnormally low returns for stocks with high idiosyncratic risk. I show that IPO long-run underperformance is in fact a manifestation of the surprisingly low returns for high idiosyncratic risk stocks. IPO underperformance disappears after I control for the idiosyncratic risk. Specifically, the underperformance of IPO firms only presents following the months in which they are classified into the highest idiosyncratic risk quintile. On the other hand, I find that the idiosyncratic risk puzzle is magnified by the IPO underperformance for two reasons. First, IPOs are over-represented in the highest volatility quintile. Second, while stocks in the highest volatility quintile underperform in general, the intra-quintile underperformance is substantially more severe for the IPO firms. My results are robust to different sample requirements. My second essay examines school quality and quality risk capitalization when school quality is uncertain, taking into account uncertainty induced by low signal content in quality measures available to parents or stochastic quality outcomes. Extending the residential bid rent theory to the uncertainty environment, the theory shows that greater school quality increases housing prices steepens the price gradient, whereas the quality risk decreases the housing prices and flattens the price gradient. The empirical models incorporate two sources of quality risk, the variance in measured school quality and school attendance zone instability. Coupling an output based measure using the over-period average of school normalized math test scores based on the Orange County public elementary school average scores with an input based measure using student/teacher ratios provides quality measures that appear to correlate sufficiently with parents' perceptions of elementary school quality, but school peer effects play important role as well. Estimates reveal capitalization of quality and uncertainty that are consistent with theory as well as systematic patterns across housing market phases and neighborhood in income level. My third essay is a meta-analysis of the body of empirical results for school quality capitalization in house prices. One puzzling aspect of the housing markets literature is that, while public school quality is a major concern of many households, empirical studies of school quality capitalization into house prices yield mixed and sometimes inconsistent results not only across studies, but also within studies when using different school quality measures and models. These differences are reflected in the capitalization coefficient value, level of significance, and even direction of capitalization effects. This paper conducts meta-analysis of the school quality capitalization estimates to identify the factors contributing to this variation. It reveals that the way the school quality is measured matters. Peer effects measures yield less significant capitalization estimates than input and output based measures and value added measures exhibit lower significance than other output based measures. Moreover, both boundary fixed effects and neighborhood fixed effect approaches can effectively and significantly control for the influence of neighborhood amenities. Adding more school quality variables reduces the capitalization significance of individual school quality variables. The most unexpected finding is that school quality capitalization significance is much less in the South than in other regions. Also surprising is that econometric methods do not appear to be driving results.
Show less
Date Issued: 2016
Identifier: CFE0006518, ucf:51358
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0006518

Title: CMOS RF CITUITS VARIABILITY AND RELIABILITY RESILIENT DESIGN, MODELING, AND SIMULATION.
Creator: Liu, Yidong, Yuan, Jiann-Shiun, University of Central Florida
Abstract / Description: The work presents a novel voltage biasing design that helps the CMOS RF circuits resilient to variability and reliability. The biasing scheme provides resilience through the threshold voltage (VT) adjustment, and at the mean time it does not degrade the PA performance. Analytical equations are established for sensitivity of the resilient biasing under various scenarios. Power Amplifier (PA) and Low Noise Amplifier (LNA) are investigated case by case through modeling and experiment. PTM 65nm...
Show moreThe work presents a novel voltage biasing design that helps the CMOS RF circuits resilient to variability and reliability. The biasing scheme provides resilience through the threshold voltage (VT) adjustment, and at the mean time it does not degrade the PA performance. Analytical equations are established for sensitivity of the resilient biasing under various scenarios. Power Amplifier (PA) and Low Noise Amplifier (LNA) are investigated case by case through modeling and experiment. PTM 65nm technology is adopted in modeling the transistors within these RF blocks. A traditional class-AB PA with resilient design is compared the same PA without such design in PTM 65nm technology. Analytical equations are established for sensitivity of the resilient biasing under various scenarios. A traditional class-AB PA with resilient design is compared the same PA without such design in PTM 65nm technology. The results show that the biasing design helps improve the robustness of the PA in terms of linear gain, P1dB, Psat, and power added efficiency (PAE). Except for post-fabrication calibration capability, the design reduces the majority performance sensitivity of PA by 50% when subjected to threshold voltage (VT) shift and 25% to electron mobility (¼n) degradation. The impact of degradation mismatches is also investigated. It is observed that the accelerated aging of MOS transistor in the biasing circuit will further reduce the sensitivity of PA. In the study of LNA, a 24 GHz narrow band cascade LNA with adaptive biasing scheme under various aging rate is compared to LNA without such biasing scheme. The modeling and simulation results show that the adaptive substrate biasing reduces the sensitivity of noise figure and minimum noise figure subject to process variation and device aging such as threshold voltage shift and electron mobility degradation. Simulation of different aging rate also shows that the sensitivity of LNA is further reduced with the accelerated aging of the biasing circuit. Thus, for majority RF transceiver circuits, the adaptive body biasing scheme provides overall performance resilience to the device reliability induced degradation. Also the tuning ability designed in RF PA and LNA provides the circuit post-process calibration capability.
Show less
Date Issued: 2011
Identifier: CFE0003595, ucf:48861
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0003595

Title: Multimaterial Fibers and Tapers A Platform for Nonlinear Photonics and Nanotechnology.
Creator: Shabahang, Soroush, Abouraddy, Ayman, Vanstryland, Eric, Dogariu, Aristide, Belfield, Kevin, University of Central Florida
Abstract / Description: The development of optical sources and components suitable for the mid-infrared is crucial for applications in this spectral range to reach the maturity level of their counterparts in the visible and near-infrared spectral regimes. The recent commercialization of quantum cascade lasers is leading to further interest in this spectral range. Wideband mid-infrared coherent sources, such as supercontinuum generation, have yet to be fully developed. A mid-infrared supercontinuum source would allow...
Show moreThe development of optical sources and components suitable for the mid-infrared is crucial for applications in this spectral range to reach the maturity level of their counterparts in the visible and near-infrared spectral regimes. The recent commercialization of quantum cascade lasers is leading to further interest in this spectral range. Wideband mid-infrared coherent sources, such as supercontinuum generation, have yet to be fully developed. A mid-infrared supercontinuum source would allow for unique applications in spectroscopy and sensing.Over the last decade, it has been shown that high-index confinement in highly nonlinear fibers pumped with high-peak-power pulses is an excellent approach to supercontinuum generation in the visible and near-infrared. Nonlinear waveguides such as fibers offer an obvious advantage in increasing the nonlinear interaction length maintained with a small cross section. In addition, fiber systems do not require optical alignment and are mechanically stable and robust with respect to the environmental changes. These properties have made fiber systems unique in applications where they are implemented in a harsh and unstable environment.In extending this approach into the mid-infrared, I have used chalcogenide glass fibers. Chalcogenide glasses have several attractive features for this application: they have high refractive indices for high optical-confinement, have a wide transparency window in the mid-infrared, and have a few orders-of-magnitude higher nonlinearity than silica glass and other mid-IR glasses. Producing chalcogenide glass fiber tapers offer, furthermore, the possibility of dispersion control and stronger field confinement and hence higher nonlinearity, desired for supercontinuum generation.
Show less
Date Issued: 2014
Identifier: CFE0005252, ucf:50594
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFE0005252

Title: THE DISSOLUTION OF THE MONASTERIES BY KING HENRY VIII AND ITS EFFECT ON THE ECONMOY, POLITICAL LANDSCAPE, AND SOCIAL INSTABILITY IN TUDOR ENGLAND THAT LED TO THE CREATION OF THE POOR LAWS.
Creator: Cooper, Casey, Bledsoe, Robert, University of Central Florida
Abstract / Description: Before the reformation and the schism of the Catholic Church, it had always been the duty of the Church and not of the state, to undertake the seven corporal works of mercy; feed the hungry, give drink to the thirsty, welcome the stranger, clothe the naked, visit the sick, visit the prisoner, and bury the dead. By dissolving these institutions, Henry had unwittingly created what would become a social disaster of biblical proportions. In essence, this act was rendering thousands of the poor...
Show moreBefore the reformation and the schism of the Catholic Church, it had always been the duty of the Church and not of the state, to undertake the seven corporal works of mercy; feed the hungry, give drink to the thirsty, welcome the stranger, clothe the naked, visit the sick, visit the prisoner, and bury the dead. By dissolving these institutions, Henry had unwittingly created what would become a social disaster of biblical proportions. In essence, this act was rendering thousands of the poor and elderly without a home or shelter, it denied the country of much of the medical aid that has been offered by the church, it denied future generations of thousands of volumes of books and scriptures from the monastic libraries, as well as denied many an education who would have otherwise never received one without the help of the Church. The ultimate goal of my thesis is to prove my hypothesis that the dissolution of the monasteries by King Henry VIII was not merely a contributory factor in the need for the creation of poor laws, but the deciding factor (in a myriad of societal issues) for their creation.
Show less
Date Issued: 2011
Identifier: CFH0003834, ucf:44770
Format: Document (PDF)
PURL: http://purl.flvc.org/ucf/fd/CFH0003834

Acoustic Instabilities (2)	+ -
Social Instability (2)	+ -
Accelerated Nonlinearities (1)	+ -
Active Control (1)	+ -
Adaptable Computer Architecture (1)	+ -

Adultery (1)	+ -
Anne Boleyn (1)	+ -
Application Specific Integrated Circuit (ASIC) (1)	+ -
Application Vulnerability Characterization (1)	+ -
Asset Pricing (1)	+ -
Bias Temperature Instability (BTI) (1)	+ -
Blowout (1)	+ -
Bluff Body (1)	+ -
CMOS (1)	+ -
Cadinal (1)	+ -
Catholic Church (1)	+ -
Church of England (1)	+ -
Cold-flow Simulation (1)	+ -
Combustion Chamber (1)	+ -
Combustion Instabilities (1)	+ -
Combustor (1)	+ -
Common Mode Failure (CMF) (1)	+ -
Compiler Optimizations (1)	+ -
Decay Rates (1)	+ -
Defense of the Seven Sacraments (1)	+ -
Design Diversity (1)	+ -
Design Guardband (1)	+ -
Digital Circuit (1)	+ -
Discrete Optics (1)	+ -
Dispersive Waves (1)	+ -

ETD Collection (17)	+ -
Honors Collection (4)	+ -

University of Central Florida (21)	+ -
Kapat, Jayanta (5)	+ -
Ahmed, Kareem (4)	+ -
Vasu Sumathi, Subith (4)	+ -
Bhattacharya, Samik (3)	+ -

Amezcua Correa, Rodrigo (2)	+ -
Christodoulides, Demetrios (2)	+ -
Deng, Weiwei (2)	+ -
Kumar, Ranganathan (2)	+ -
Mansy, Hansen (2)	+ -
Abouraddy, Ayman (1)	+ -
Ashraf, Rizwan (1)	+ -
Belfield, Kevin (1)	+ -
Bledsoe, Robert (1)	+ -
Bradford, Joshua (1)	+ -
Castillo Orozco, Eduardo (1)	+ -
Chen, Honghui (1)	+ -
Choudhury, Pretam (1)	+ -
Cooper, Casey (1)	+ -
DeMara, Ronald (1)	+ -
Dogariu, Aristide (1)	+ -
Eftekhar, Mohammad Amin (1)	+ -
El-Ganainy, Ramy (1)	+ -
Flores, Wilmer (1)	+ -
Frye, Melissa (1)	+ -
Gaume, Romain (1)	+ -
Gordon, Ali (1)	+ -
Gu, Jing (1)	+ -
Ilie, Marcel (1)	+ -
Jha, Sumit (1)	+ -

Search In

Pages

Pages

Sort Results By:

Narrow results by:

Topical Subject

Collection

Type of Resource

Language

Creator

Format

Owner