PhD, University of Cincinnati, 2010, Engineering : Mechanical Engineering

Simulation-based study of human auditory response characteristics and development of a prototype for advanced noise guideline are two major focuses of this dissertation research. This research was conducted as a part of the long-term effort to develop an improved noise guideline for better protection of the workers exposed to various noise environments. The human auditory responses were studied with simulation models. A human full-ear model derived from an existing model, Auditory Hazard Assessment Algorithm for Human (AHAAH), was utilized as a baseline for the study. Frequency- and time-domain responses of well-known human middle ear network models were cross-compared to estimate expected accuracy of the models and understand their proper use. Responses of the stapes to impulsive noises were investigated by using the middle ear models to understand the effects of the temporal characteristics of impulsive noises on the responses. Available measured transfer functions between the free-field pressure and the stapes response for human and chinchilla were also used to study the auditory response characteristics. The measured transfer functions were refined and reconditioned to make them have equivalent formats. Using the reconstructed transfer functions, time-domain stapes responses of human and chinchilla to impulsive and complex type noises were calculated and compared. Applicability of the noise metrics defined in terms of the stapes response to assess the risk of the noise induced hearing loss was studied. A prototype of an improved noise guideline was developed from existing chinchilla noise exposure data. Applying a new signal processing technique to the time histories of the exposed noises and studying the relationship between the noise metric and the permanent threshold shift (PTS), the dose-response relationship (DRR) was established in a compatible way with the definition used in current human noise guidelines. From the DDR, noise induced hearing loss (NIHL (open full item for complete abstract)

Committee: J. Kim PhD (Committee Chair); William Murphy PhD (Committee Member); Mark Schulz PhD (Committee Member); Teik Lim PhD (Committee Member) Subjects: Mechanical Engineering

PhD, University of Cincinnati, 2024, Engineering and Applied Science: Aerospace Engineering

A rigorous literature survey on combustion instability, acoustic modeling, dynamic flame modeling, and reduced-order modeling (ROM) was presented to acknowledge the difficulty of accurately predicting combustion instability for a lean, fully premixed combustor operating at speed beyond 5% of the speed of sound while producing significantly long flame. The objective is to validate the feasibility of predicting thermoacoustic instability using ROM for changes in boundary condition (exit blockage ratio) and flow rate on a combustor that produces a non-compact flame. A revised ROM was derived with convective flow rather than assuming stationary flow to predict combustion instability in the combustor that operates at a speed beyond 5% of the speed of sound. The structure of the ROM was revised to account for spatially distributed heat release rather than assuming a singular compact flame. A proposed end boundary condition model was used to improve the solution of the ROM. A bluff body combustor was tested to establish a combustor that can operate at speeds greater than 5% of the speed of sound. The bluff body combustor produced stable and unstable non-compact flame under the same flow condition (inlet mach number and equivalence ratio) as the combustor configuration changes (combustor length and blockage ratio). The stable flame combustor configuration is used for flame transfer function measurement. In contrast, the unstable flame combustor configuration is used for ROM prediction validation. The exit pressure reflection coefficients were measured for three different blockage ratios (69%, 56%, 0%) without combustion as the temperature, flow rate, and frequency change to validate the end boundary model proposed in the literature. The model used to characterize the acoustic exit boundary condition for the revised ROM was successfully validated using the multiple microphone method downstream of the bluff body flame holder. An inlet Mach number of 0.06-0.13 at an equ (open full item for complete abstract)

Committee: Jongguen Lee Ph.D. (Committee Chair); Kwanwoo Kim Ph.D. (Committee Member); Paul Orkwis Ph.D. (Committee Member); Prashant Khare Ph.D. (Committee Member) Subjects: Aerospace Materials

Doctor of Philosophy (Ph.D.), University of Dayton, 2016, Engineering

Atmospheric turbulence as an agency affecting the propagation of electromagnetic (EM) waves in different regions of the earth relative to the ground plane has been studied extensively over the past several decades. Mathematical models describing turbulence itself relative to EM waves have been developed by a variety of investigators in the last 50 or more years. It turns out that the majority of these models are essentially in the spatial domain, involving transverse spatial coordinates and their spatial frequency counterparts in the spectral domain. Most turbulence models start out by assuming a random dependence of the medium permittivity on the turbulence. This leads to a random model describing what is commonly referred to as the refractive index power density spectrum. It is well known that propagation through standard atmospheric turbulence creates ripples, random distortions, phase variations and also for monochromatic cases scintillations in the recovered signals. One idea that was proposed to the investigators of this research was that perhaps pre-packaging the EM signal inside a trackable chaos waveform might offer some measure of shielding for the signal even as the overall EM wave passes through turbulence. With this objective in mind, this work began by first establishing standard numerical simulations of EM propagation through homogeneous regions upon passage through a variety of apertures. This standard application involved the use of the Fresnel-Kirchhoff diffraction integral implemented in two ways: (a) as a direct propagation from an object to an image plane, and (b) segmented propagation over uniform incremental layers of the medium in the longitudinal direction. The latter approach was put into place in anticipation of the later introduction of a turbulent layer in the system. Following successful implementation of this technique, turbulence was inserted once again in two different ways: (a) assuming a relatively narrow region of turbulenc (open full item for complete abstract)

Committee: Monish Chatterjee Ph.D. (Advisor); Partha Banerjee Ph.D. (Committee Member); Eric Balster Ph.D. (Committee Member); Muhammad Islam Ph.D. (Committee Member) Subjects: Electrical Engineering; Engineering; Optics

Doctor of Philosophy (PhD), Wright State University, 2013, Environmental Sciences PhD

Time series that exhibit multiple scaling properties in the frequency domain are common in natural systems (e.g., temperature through geologic time). NOAA verified hourly water level data ranging from 20 to 30 years in duration for nine stations in the North American Great Lakes is converted to the frequency domain using a complex discrete fast Fourier transform (FFT) and then expressed as a power spectrum in terms of frequency versus power. To quantify power law scaling behavior, a scaling exponent (β) is determined by fitting a power function to a log-log plot of frequency (f ) or period (T) versus power in the frequency domain. For water level fluctuations in the Great Lakes, the frequency domain exhibits four distinct regions of power law scaling. The mathematical relationship of the scaling exponent (β) to 1/f time series behavior is examined employing Bode analysis. Variations in scaling behavior of water level data, indicated by the patterns of change in amplitude and phase across frequencies, can be expressed through transfer functions. The transfer functions are created using Laplace transforms. Each Laplace term (s) has a fractional exponent based on the scaling exponent (β) derived from the Bode magnitude plot. Convolution of the transfer function in the time domain is equivalent to multiplication in the frequency domain (Laplace space). Combining the transfer functions for all frequencies yields a Frequency Response Model and provides a basis to determine how the system that created the time series will respond to any given input over all frequencies. For water level fluctuations in the Great Lakes, the scaling behavior pattern is well approximated by a combination of four linear differential equations or transfer functions, one primary equation for each distinct scaling region. The collective interactions of all equations over all frequencies create the Great Lakes Frequency Response Model and represent the underlying physical dynamics of the Great La (open full item for complete abstract)

Committee: Sarah Tebbens Ph.D. (Advisor); Christopher Barton Ph.D. (Committee Member); John Flach Ph.D. (Committee Member); Paul Seybold Ph.D. (Committee Member); Brian Tsou Ph.D. (Committee Member) Subjects: Applied Mathematics; Environmental Science; Geophysical; Geophysics; Hydrologic Sciences; Mathematics; Systems Design; Systems Science; Water Resource Management