Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
Browse
6 results
Search Results
Item Effects of Vibrational Nonequilibrium on the Acoustic Noise Radiated by a Compressible Boundary Layer(2023) Gillespie, Graeme Ivry; Laurence, Stuart J; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Turbulence-generated acoustic noise is of critical concern in the nozzle flows of conventional high-speed wind tunnels, where the disturbance environment encountered by models in the freestream is substantially stronger than that experienced in atmospheric flight and leads to much reduced transition Reynolds numbers. To obtain more accurate comparisons of experimental, computational, and free-flight data, a new control mechanism is needed to reduce freestream disturbance levels. Therefore, the aim of the present work is to investigate the ability of vibrational nonequilibrium processes to attenuate acoustic radiation emitted by turbulent boundary layers in high-speed facilities. Predicting the attenuation from vibrational nonequilibrium processes remains a challenge, and there exist limited experimental data for model validation, particularly at elevated temperatures. To better understand the absorption properties of various gas mixtures, a heated acoustic chamber is developed to measure the attenuation of CO2, N2O, and mixtures of CO2/He, CO2/N2,and N2O/He at temperatures up to 529 K. In mixtures of CO2/He at room temperature, an increase in helium is found to decrease the peak attenuation modestly, but increase the peak attenuation frequency. At higher temperatures, the peak attenuation increased substantially, but as the helium fraction increased, the rate of increase in peak attenuation drops and the values asymptote at lower temperatures. These results illustrate that varying the fraction of helium in mixtures of CO2/He can shift the attenuation to a desired frequency range, providing a method to control acoustic radiation. The effects of vibrational nonequilibrium processes on turbulence-generated acoustic noise are investigated in a Mach-2.8 shock-tunnel facility at the University of Maryland. CO2, N2, He, and He/CO2 mixtures are injected into the lower boundary layer of the flow through a porous plate located in the upstream region of the test section. A four-point Focused Laser Differential Interferometer (FLDI) positioned above the turbulent boundary layer is used to obtain freestream fluctuation measurements assumed to be representative of entropic fluctuations propagating along streamlines and acoustic disturbances along Mach lines. Compared to a boundary layer of pure air, the injection of 30%, 35%, and 40% He/CO2 mixtures resulted in reduced fluctuation powers correlated along a Mach line in the frequency range of 200−800 kHz. Minimal reductions in fluctuation power were measured along corresponding streamlines; therefore, it could be concluded that the vibrationally active gas species in the boundary layer primarily affected acoustic radiation and not entropic disturbances. As measurements are affected by noise radiated from the boundary layers on all four walls of the facility, a mathematical disturbance model is created to examine the sensitivity of the measured attenuation to acoustic disturbances propagating from the lower boundary layer only. Disturbances are modeled as Gaussian wave packets propagating along Mach lines from the four test section walls and along streamlines. Modeling the acoustic disturbances from the lower boundary layer with a 15−30% amplitude reduction resulted in amplitude spectral densities and cross power spectral densities that agreed well with the FLDI measurements. Thus, the injection of a vibrationally active gas into a turbulent boundary layer has the potential to significantly reduce acoustic-disturbance amplitudes in the freestream, greatly expanding the utility of conventional high-speed facilities to study flows in which transition plays an important role.Item SPECTRAL CONTRASTS PRODUCED BY CHILDREN WITH COCHLEAR IMPLANTS: INVESTIGATING THE IMPACT OF SIGNAL DEGRADATION ON SPEECH ACQUISITION(2022) Johnson, Allison Ann; Edwards, Jan; Hearing and Speech Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The primary objective of this dissertation was to assess four consonants, /t/, /k/, /s/, and /ʃ/, produced by young children with cochlear implants (CIs). These consonants were chosen because they comprise two place-of-articulation contrasts, which are cued auditorily by spectral information in English, and they cover both early-acquired (/t/, /k/) and late-acquired (/s/, /ʃ/) manners of articulation. Thus, the auditory-perceptual limitations imposed by CIs is likely to impact acquisition of these sounds: because spectral information is particularly distorted, children have limited access to the cues that differentiate these sounds.Twenty-eight children with CIs and a group of peers with normal hearing (NH) who were matched in terms of age, sex, and maternal education levels participated in this project. The experiment required children to repeat familiar words with initial /t/, /k/, /s/, or /ʃ/ following an auditory model and picture prompt. To create in-depth speech profiles and examine variability both within and across children, target consonants were elicited many times in front-vowel and back-vowel contexts. Patterns of accuracy and errors were analyzed based on transcriptions. Acoustic robustness of contrast was analyzed based on correct productions. Centroid frequencies were calculated from the release-burst spectra for /t/ and /k/ and the fricative noise spectra for /s/ and /ʃ/. Results showed that children with CIs demonstrated patterns not observed in children with NH. Findings provide evidence that for children with CIs, speech acquisition is not simply delayed due to a period of auditory deprivation prior to implantation. Idiosyncratic patterns in speech production are explained in-part by the limitations of CI’s speech-processing algorithms. The first chapter of this dissertation provides a general introduction. The second chapter includes a validation study for a measure to differentiate /t/ and /k/ in adults’ productions. The third chapter analyzes accuracy, errors, and spectral features of /t/ and /k/ across groups of children with and without CIs. The fourth chapter analyzes /s/ and /ʃ/ across groups of children, as well as the spectral robustness of both the /t/-/k/ and the /s/-/ʃ/ contrasts across adults and children. The final chapter discusses future directions for research and clinical applications for speech-language pathologists.Item Echolocation, high frequency hearing, and gene expression in the inner ear of bats(2017) Mao, Beatrice; Wilkinson, Gerald S; Moss, Cynthia F; Behavior, Ecology, Evolution and Systematics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Bats are the only mammals capable of true flight, and are the second-most speciose mammalian radiation, represented by over 1200 extant species. Key to their evolutionary success was echolocation, which is a complex trait requiring specializations for vocalization, hearing, and echo processing. Because they rely on detecting and analyzing echoes that may return greatly attenuated relative to their outgoing calls, interference from non-target ‘clutter’ echoes poses a challenge for echolocating bats. Here, I demonstrate that the echolocating bat Eptesicus fuscus alters its echolocation behavior to ameliorate the impact of clutter echoes when tracking a moving target, and that the magnitude of its behavioral adjustments depended on the distance and angular offset of two symmetrically placed ‘distracter’ objects. Furthermore, I found that individual bats make different adjustments to their calls, call timing, or head movements, suggesting that multiple strategies for echolocating in clutter may exist. In my second chapter, I examined the expression patterns of hearing-related genes in juvenile bats. Biomedical research establishing the functional roles of hearing genes rarely examines gene expression beyond the early post-natal stage, even though high frequency hearing does not mature until late in development. I show that several key hearing genes implicated in human deafness are upregulated in juvenile bats relative to adults, or exhibit sustained upregulation through the developmental period corresponding to the maturation of echolocation behavior. In my third chapter, I review the evolution of high frequency hearing in mammals, focusing on echolocating bats and whales, which have independently evolved this complex trait. I provide an overview of recent studies that have reported molecular convergence in hearing genes among distantly related echolocators, and assert that the contribution of gene expression to hearing deserves further investigation. Finally, I argue that echolocators provide a unique opportunity to investigate the basis of high frequency amplification, and may possess mechanisms of hearing protection which enable them to prolong the use of echolocation throughout their long lives.Item Magnetic and Acoustic Investigations of Turbulent Spherical Couette Flow(2016) Adams, Matthew Michael; Lathrop, Daniel P; Physics; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Title of dissertation: MAGNETIC AND ACOUSTIC INVESTIGATIONS OF TURBULENT SPHERICAL COUETTE FLOW Matthew M. Adams, Doctor of Philosophy, 2016 Dissertation directed by: Professor Daniel Lathrop Department of Physics This dissertation describes experiments in spherical Couette devices, using both gas and liquid sodium. The experimental geometry is motivated by the Earth's outer core, the seat of the geodynamo, and consists of an outer spherical shell and an inner sphere, both of which can be rotated independently to drive a shear flow in the fluid lying between them. In the case of experiments with liquid sodium, we apply DC axial magnetic fields, with a dominant dipole or quadrupole component, to the system. We measure the magnetic field induced by the flow of liquid sodium using an external array of Hall effect magnetic field probes, as well as two probes inserted into the fluid volume. This gives information about possible velocity patterns present, and we extend previous work categorizing flow states, noting further information that can be extracted from the induced field measurements. The limitations due to a lack of direct velocity measurements prompted us to work on developing the technique of using acoustic modes to measure zonal flows. Using gas as the working fluid in our 60~cm diameter spherical Couette experiment, we identified acoustic modes of the container, and obtained excellent agreement with theoretical predictions. For the case of uniform rotation of the system, we compared the acoustic mode frequency splittings with theoretical predictions for solid body flow, and obtained excellent agreement. This gave us confidence in extending this work to the case of differential rotation, with a turbulent flow state. Using the measured splittings for this case, our colleagues performed an inversion to infer the pattern of zonal velocities within the flow, the first such inversion in a rotating laboratory experiment. This technique holds promise for use in liquid sodium experiments, for which zonal flow measurements have historically been challenging.Item NUMERICAL SIMULATION AND VALIDATION OF HELICOPTER BLADE-VORTEX INTERACTION USING COUPLED CFD/CSD AND THREE LEVELS OF AERODYNAMIC MODELING(2014) Amiraux, Mathieu; Baeder, James D; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rotorcraft Blade-Vortex Interaction (BVI) remains one of the most challenging flow phenomenon to simulate numerically. Over the past decade, the HART-II rotor test and its extensive experimental dataset has been a major database for validation of CFD codes. Its strong BVI signature, with high levels of intrusive noise and vibrations, makes it a difficult test for computational methods. The main challenge is to accurately capture and preserve the vortices which interact with the rotor, while predicting correct blade deformations and loading. This doctoral dissertation presents the application of a coupled CFD/CSD methodology to the problem of helicopter BVI and compares three levels of fidelity for aerodynamic modeling: a hybrid lifting-line/free-wake (wake coupling) method, with modified compressible unsteady model; a hybrid URANS/free-wake method; and a URANS-based wake capturing method, using multiple overset meshes to capture the entire flow field. To further increase numerical correlation, three helicopter fuselage models are implemented in the framework. The first is a high resolution 3D GPU panel code; the second is an immersed boundary based method, with 3D elliptic grid adaption; the last one uses a body-fitted, curvilinear fuselage mesh. The main contribution of this work is the implementation and systematic comparison of multiple numerical methods to perform BVI modeling. The trade-offs between solution accuracy and computational cost are highlighted for the different approaches. Various improvements have been made to each code to enhance physical fidelity, while advanced technologies, such as GPU computing, have been employed to increase efficiency. The resulting numerical setup covers all aspects of the simulation creating a truly multi-fidelity and multi-physics framework. Overall, the wake capturing approach showed the best BVI phasing correlation and good blade deflection predictions, with slightly under-predicted aerodynamic loading magnitudes. However, it proved to be much more expensive than the other two methods. Wake coupling with RANS solver had very good loading magnitude predictions, and therefore good acoustic intensities, with acceptable computational cost. The lifting-line based technique often had over-predicted aerodynamic levels, due to the degree of empiricism of the model, but its very short run-times, thanks to GPU technology, makes it a very attractive approach.Item The Role of Density Gradient in Liquid Rocket Engine Combustion Instability(2008-12-01) Ghosh, Amardip; Yu, Kenneth H; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Experimental and analytical studies were conducted to investigate key physical mechanisms responsible for flame-acoustic coupling during the onset of acoustically driven combustion instabilities in liquid rocket engines (LREs). Controlled experiments were conducted in which a turbulent hydrogen-oxygen (GH2-GO2) diffusion flame, established downstream of a two-dimensional model shear coaxial injector was acoustically forced by a compression driver unit mounted in a transverse direction and excited through a broad range of frequencies (200Hz-2000Hz) and amplitudes. Characteristic interactions between flame and acoustics visualized through OH* and CH* chemiluminescence imaging and dynamic pressure measurements obtained using high frequency dynamic pressure transducers indicated that small acoustic disturbances could be amplified by flame-acoustic coupling under certain conditions leading to substantial modulation in spatial heat release fluctuations. Density gradient between fuel and oxidizer was found to significantly affect the way acoustic waves interacted with density stratified flame fronts. The particular case of an asymmetric flame front oscillation under transverse acoustic forcing indicated that baroclinic vorticity, generated by the interactions between misaligned pressure gradient (across the acoustic wave) and density gradient (across the fuel oxidizer interface) could further amplify flame front distortions. Asymmetric interaction between flame and acoustics is shown to occur preferentially on flame fronts where controlled waves from the compression driver travel from lighter fluid to denser fluid and the amount of interaction between flame and acoustics is shown to depend strongly on the density ratio between the fluids on either sides of the flame front. This observation is in agreement with the baroclinic vorticity mechanism and a variant of the classical Rayleigh-Taylor instability mechanism. The results provide the first known experimental evidence that baroclinic vorticity could play a role in triggering flame-acoustic interactions associated with LRE shear coaxial injectors. Parametric studies investigating the sensitivity of flame-acoustic interaction on key physical parameters that govern shear coaxial injector operations (including density ratio, velocity ratio, momentum ratio and chemical composition of the fuel) were conducted by varying the parameter of interest independently while holding the other parameters relatively constant. Density ratios ranging from 1 to 16, velocity ratios ranging from 3.02 to 5.27, momentum ratios ranging from 0.67 to 2.12 and fuel mixtures ranging from pure hydrogen to 10%-90% GH2-GCH4 combination were tested. It is shown that in the ranges considered, flame-acoustic interaction is most sensitively affected by density ratio changes. Spectral measurements of flame front oscillations using local chemiluminescence measurements further revealed the non-linear nature of the interaction process : a flame system forced at 1150 Hz gave rise not only to 1150 Hz oscillations but also triggered flame oscillations occurring at substantially lower frequencies. Analytical models were developed to interpret and predict acoustic modes of a combustion chamber containing a density stratified flowfield subjected to transverse acoustic disturbances. Incorporating both the known phenomenon of jet mixing length and the new experimental result of preferential excitation, the models allow different resonant behaviors to occur for separate regions of the combustor bounded by sudden changes in density. For isothermal experiments where the flow temperature was known, calculated Eigen frequencies were in good agreement with measured frequencies. Overall, the identification of fuel-oxidizer density ratio as a critical parameter and the identification of baroclinic vorticity as a potential mechanism in flame acoustic coupling are significant because a reduction in the density gradient between fuel and oxidizer can be used as a control mechanism to improve flame stability in liquid rocket engines.