PhD, University of Cincinnati, 2021, Engineering and Applied Science: Aerospace Engineering

The overarching objective of this research is to investigate novel aircraft engine exhaust nozzle concepts in search of acoustically quieter designs that offer benefits in multiple areas of operations. They include transonic to supersonic regime operations in both civil and military aviation sectors. Exhaust systems are a crucial component of aerial vehicles that tremendously impact their aero-propulsive as well as acoustic and stealth characteristics. Thus, in this endeavor to investigate a nozzle design that has the potential to meet current generations' highly demanding performance requirements, this research focuses on a type of rectangular nozzle called Single Expansion Ramp Nozzle (SERN), with moderate to high aspect ratio configurations. The overarching objective of this research is to investigate novel aircraft engine exhaust nozzle concepts in search of acoustically quieter designs that offer benefits in multiple areas of operations. They include transonic to supersonic regime operations in both civil and military aviation sectors. Exhaust systems are a crucial component of aerial vehicles that tremendously impact their aero-propulsive as well as acoustic and stealth characteristics. Thus, in this endeavor to investigate a nozzle design that has the potential to meet current generations' highly demanding performance requirements, this research focuses on a type of rectangular nozzle called Single Expansion Ramp Nozzle (SERN), with moderate to high aspect ratio configurations.

Committee: Ephraim Gutmark Ph.D (Committee Chair); Kazhikathra| Kailasanath Ph.D (Committee Member); Daniel Cuppoletti Ph.D (Committee Member); Mark Turner Ph.D (Committee Member) Subjects: Aerospace Engineering; Aerospace Materials

Doctor of Philosophy, The Ohio State University, 2022, Aerospace Engineering

The propulsion/airframe integration benefits of non-axisymmetric nozzles have led to renewed interest in their integration into future generations of aircraft design. Rectangular nozzles can offer significant benefits in terms of drag reduction, improved mixing for heat signature reduction and ease of implementing thrust vectoring. Aircraft with high- and low-aspect-ratio rectangular nozzles have already been operational for years and the recent interest in developing manned and unmanned platforms with such nozzles integrated with the airframe underscores the need for further development and understanding the physics of flow in such geometries. Jet noise emitted from the hot, high-speed jet plumes of high-performance tactical fighters severely affects the crew and communities exposed to it. Furthermore, interaction and coupling of jet plumes in twin-engine tactical aircraft has the potential to cause structural fatigue and failure due to elevated near-field pressure fluctuations. This work seeks to address these issues by studying the physics of flow and acoustics in low aspect ratio rectangular twin jets (RTJs) and implementing active flow control to alleviate near-field pressure fluctuations and far-field noise. One of the major contributions of the present work is the extensive characterization of baseline RTJs in a wide range of operation conditions (jet Mach number, Mj, or nozzle pressure ration, NPR) to better understand the underlying processes that drive flow and acoustic behavior of these jets. The second contribution of this work is to implement active flow control with localized arc filament plasma actuators (LAFPAs), the control authority of which has been demonstrated in a wide range of high-speed flows. In the present work, LAFPAs have been used to manipulate the growth and development of large-scale structures (LSS) in jet shear layers and thus affect the flow-field and acoustics of RTJs. The twin jet setup studied in this work consists of two milit (open full item for complete abstract)

Committee: Mo Samimy (Advisor); Lian Duan (Committee Member); Datta Gaitonde (Committee Member); Nathan Webb (Committee Member) Subjects: Aerospace Engineering

PhD, University of Cincinnati, 2021, Engineering and Applied Science: Aerospace Engineering

In streamlined airplane configurations, additional noise sources can be created from interactions between the jet flow and surfaces on an aircraft's body. During takeoff and landing procedures the ground itself is close enough to already cause jet-surface interference. During these procedures, workers on an aircraft carrier are exposed to such noise radiation and can suffer from varying degrees of hearing loss, providing an impetus from the Navy to study these conditions. To assess this interaction, the presence of a flat plate impinging on a supersonic jet of a low aspect ratio (2:1) rectangular nozzle of equivalent exit diameter of De=20.65mm, is studied from the minor and major axis orientation. The impact of the plate, large enough in length to resemble an aircraft carrier deck relative to the nozzle in scale, is studied at supersonic nozzle pressure ratios (NPRs) of 2.5–4.5. These surface interactions have been studied in large part on cold jets to a certain extent, as these are more easily replicated in a laboratory setting. However more realistic applications involve a heated jet flow, particularly in Navy jet applications. This leads to tested operating conditions produced here of the cold flow temperature ratio (TR=1.0) to a heated jet up to TR=3.0. To investigate this application, acoustic data from near-field and far-field arrays is captured, and combined with visual flow measurements from streamwise particle image velocimetry (PIV) and shadowgraph/Schlieren photography. From PIV data, average velocity and turbulence kinetic energy (TKE) of the flow can be extracted, while shadowgraph provides various density gradients across the flow field. Further processing in the form of proper orthogonal decomposition (POD) can be applied here to extract dominant modes of the jet's movement. Plate offset (h) distances of h/De=?0, 1, 2, and 3 from the nozzle lip are studied to assess trends related to shock cell spacing, potential core length, and shear layer devel (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D. (Committee Chair); Shaaban Abdallah Ph.D. (Committee Member); Jeffrey Kastner Ph.D. (Committee Member); Mark Turner Sc.D. (Committee Member) Subjects: Aerospace Materials

PhD, University of Cincinnati, 2015, Engineering and Applied Science: Aerospace Engineering

An experimental investigation into the effect of azimuthal variance of chevrons and fluidically enhanced chevrons applied to supersonic jets is presented. Flow field measurements of streamwise and cross-stream particle imaging velocimetry were employed to determine the causes of noise reduction, which was demonstrated through acoustic measurements. Results were obtained in the over- and under- expanded regimes, and at the design condition, though emphasis was placed on the overexpanded regime due to practical application. Surveys of chevron geometry, number, and arrangement were undertaken in an effort to reduce noise and/or incurred performance penalties. Penetration was found to be positively correlated with noise reduction in the overexpanded regime, and negatively correlated in underexpanded operation due to increased effective penetration and high frequency penalty, respectively. The effect of arrangement indicated the beveled configuration achieved optimal abatement in the ideally and underexpanded regimes due to superior BSAN reduction. The symmetric configuration achieved optimal overexpanded noise reduction due to LSS suppression from improved vortex persistence. Increases in chevron number generally improved reduction of all noise components for lower penetration configurations. Higher penetration configurations reached levels of saturation in the four chevron range, with the potential to introduce secondary shock structures and generate additional noise with higher number. Alternation of penetration generated limited benefit, with slight reduction of the high frequency penalty caused by increased shock spacing. The combination of alternating penetration with beveled and clustered configurations achieved comparable noise reduction to the standard counterparts. Analysis of the entire data set indicated initial improvements with projected area that saturated after a given level and either plateaued or degraded with additional increases. Optimal reductio (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D. D.Sc. (Committee Chair); James Bridges Ph.D. (Committee Member); Kailas Kailasanmath Ph.D. (Committee Member); Steve Martens Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member); Paul Orkwis Ph.D. (Committee Member) Subjects: Aerospace Materials

PHD, Kent State University, 2013, College of Arts and Sciences / Department of Physics

Historically, the National Aeronautics and Space Administration (NASA) has used rocket-powered vehicles as launch vehicles for access to space. A familiar example is the Space Shuttle launch system. These vehicles carry both fuel and oxidizer onboard. If an external oxidizer (such as the Earth's atmosphere) is utilized, the need to carry an onboard oxidizer is eliminated, and future launch vehicles could carry a larger payload into orbit at a fraction of the total fuel expenditure. For this reason, NASA is currently researching the use of air-breathing engines to power the first stage of two-stage-to-orbit hypersonic launch systems. Removing the need to carry an onboard oxidizer leads also to reductions in total vehicle weight at liftoff. This in turn reduces the total mass of propellant required, and thus decreases the cost of carrying a specific payload into orbit or beyond. However, achieving hypersonic flight with air-breathing jet engines has several technical challenges. These challenges, such as the mode transition from supersonic to hypersonic engine operation, are under study in NASA's Fundamental Aeronautics Program. One propulsion concept that is being explored is a magnetohydrodynamic (MHD) energy- bypass generator coupled with an off-the-shelf turbojet/turbofan. It is anticipated that this engine will be capable of operation from takeoff to Mach 7 in a single flowpath without mode transition. The MHD energy bypass consists of an MHD generator placed directly upstream of the engine, and converts a portion of the enthalpy of the inlet flow through the engine into electrical current. This reduction in flow enthalpy corresponds to a reduced Mach number at the turbojet inlet so that the engine stays within its design constraints. Furthermore, the generated electrical current may then be used to power aircraft systems or an MHD accelerator positioned downstream of the turbojet. The MHD accelerator operates in reverse of the MHD generator, re-accelerating t (open full item for complete abstract)

Committee: David Allendar PhD (Committee Co-Chair); Isaiah Blankson PhD (Committee Co-Chair); John Portman PhD (Committee Member); Mark Manley PhD (Committee Member); John West PhD (Committee Member); Jonathan Maletic PhD (Committee Member) Subjects: Aerospace Engineering; Electromagnetics; Theoretical Physics

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

An experimental investigation into the wave sources responsible for the noise generation mechanisms of supersonic jet noise and their mitigation using MVG nozzles is presented. Flow field measurements using PIV revealed that MVGs mitigate noise through two mechanisms; they generate internal oblique shock waves that weakens the shock cell, and they substantially increase the shear layer mixing and its entrainment of ambient fluid which reduces the length scales and velocities of the convecting coherent structures. In addition, time-resolved Schlieren visualizations were spectrally analyzed to decompose and reconstruct the hydrodynamic waves in the flow field and the generation process of the acoustic wave emission directly from the jet providing insights into the noise generation mechanism and their suppression by inducing the peak wave instabilities to shift to larger wavenumber values to reduce their acoustic emission efficiencies, which were confirmed by acoustic measurements. The findings from this investigation show direct visualization of the acoustic wave emission from the sources in the flow field. These waves have downstream components that are emitted from the modulation of the shear layer inducing spatial coherence of the turbulent vortical structures. This modulation is induced by the passing of the upstream acoustic waves along the shear layer from all shock cells synchronized as a phased array with regions of constructive and destructive interference patterns which steers the emitted acoustic radiation beams. The intense acoustic beam perturbs the shear layer and induces the formation of the internal trapped waves with phase velocities propagating upstream of the supersonic jet flow. Based on these findings, new noise generation mechanisms are proposed that cover various aspects of the physical mechanisms of jet noise components for the turbulent mixing noise, the broadband shock associated noise and the screech resonance tone, along with reduction and (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D. (Committee Chair); Junhui Liu Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member); Paul Orkwis Ph.D. (Committee Member) Subjects: Aerospace Engineering

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

Supersonic flows have been a topic of active research due to their application in various propulsion systems such as aircraft jet engines, rocket engines and more recently rotating detonation engines. Due to the inherent complexity of supersonic jets, it is critical to develop a comprehensive understanding of various flow aspects such as acoustics and flow field development to fully define them. Supersonic jet screech is a well-known flow behavior that leads to generation of sustained flow instabilities. While a vast body of research on standard circular nozzles has been conducted exploring screech, there are several aspects of screech that are not fully understood. This research focuses on the origin, growth and propagation of screech instabilities in two types of nozzle cross-sections: rectangular and square nozzle exits. Experimental studies including acoustic tests and flow visualization studies were conducted to reveal intricate mode variations in convergent-divergent rectangular jets designed Mach 1.5. A multi-mode instability mode was discovered in the rectangular jet that consisted of simultaneous symmetric and antisymmetric oscillations propagating across all shear layers. Spatial decomposition led to the identification of the interaction between the Kelvin-Helmholtz (KH) instabilities and the flow shock cell as source of symmetric instability. Additionally, the energy decomposition into the Guided Jet Mode (G-JM), shock leakage and acoustic component was demonstrated. It was deduced that sub-optimal interactions trigger multi-mode and symmetric instabilities while antisymmetric modes while optimal interactions trigger antisymmetric modes. Twin square nozzles were studied through experimental and numerical techniques. Two different types of coupled instability modes were identified. The first was an antisymmetric mode propagating along each jet with a half period phase offset. The second was a symmetric oscillation mode that led to phase locked instabi (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D. D.Sc. (Committee Chair); Shaaban Abdallah Ph.D. (Committee Member); Paul Orkwis Ph.D. (Committee Member) Subjects: Aerospace Materials

PhD, University of Cincinnati, 2023, Engineering and Applied Science: Aerospace Engineering

This thesis delves into the topic of supersonic jet noise, aiming to advance the development of practical noise mitigation technologies suitable for full-scale applications. While the primary focus remains on supersonic jet noise, this research covers diverse dimensions, encompassing optimization, performance tradeoffs, and ease of implementation. The centerpiece of this investigation is a bi-conic Converging-Diverging scaled nozzle, representative of the F404-GE- 400 variable exhaust nozzle, which has evolved into a benchmark for validating and implementing technologies in practical full-scale applications. Three noise mitigation devices, namely Micro Vortex Generators, Chevrons, and Sweeping Jets, take the center stage in the pursuit of supersonic jet noise reduction. These devices are at the heart of three main sections spanning chapters 2 through 4. Chapter 2 provides a comprehensive review of supersonic jets. Theory on supersonic jets flow characteristics and the ensuing acoustic signature, supporting the study's foundation is provided. Within this chapter, a review on micro vortex generators and their current applications in different aerodynamic and flow control applications is provided highlighting the key features that make them ideal for supersonic jet noise mitigation applications. A review of chevrons encompasses pivotal breakthroughs in supersonic jet noise, while simultaneously acknowledging the existing limitations. Additionally, a glance at fluidic injections sets the stage for Sweeping Jets, a device designed to be embedded into the baseline nozzle's wall to provide modulated fluidic injection. Notably, the advantage of Sweeping Jets lies in their ability to produce oscillatory jets without the need for moving parts or mechanical actuators. Instead, a self-sustained jet emerges from a cavity with feedback channels, ushering in a novel approach to noise mitigation. Chapter 3 covers the details of the experimental and numerical te (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D. (Committee Chair); Paul Orkwis Ph.D. (Committee Member); Junhui Liu Ph.D. (Committee Member); Shaaban Abdallah Ph.D. (Committee Member) Subjects: Aerospace Engineering

Doctor of Philosophy, The Ohio State University, 2019, Aero/Astro Engineering

Over the last 80 years, high-performance military aircraft have relied on increasingly sophisticated jet engine technologies to enable performance and fulfill mission requirements. To satisfy these engineering demands, several reliable technologies are often combined into one composite engine configuration. Although the behavior of these composite configurations is fairly well known at the engineering level, their rapid advancement is constrained by a lack of knowledge of the fundamental fluid dynamics, particularly the dominant unsteady turbulent mechanisms. This has in turn limited the effectiveness of design tools. Nozzle designs are also becoming more exotic as they conform to increasingly complicated emerging engine architectures, such as variable-cycle engines, whose flowfields are inherently complex due to the multitude of compressible shear layers that evolve in the presence of pressure gradients. The flowfield complexity is further exacerbated for supersonic flight where the engine is integrated into the airframe, and the turbulent, high-speed, shock-containing exhaust interacts with proximal surfaces of the aircraft. In this work, high-fidelity Large-Eddy Simulations are employed to examine the fluid dynamics of a nozzle configuration relevant to emerging airframe-integrated variable-cycle engine architectures. The nozzle comprises two rectangular streams; the upper (core) Mach 1.6 single-sided expansion ramp stream is separated by a splitter plate from a sonic, lower (deck) stream issuing in a wall-jet arrangement over an aft-deck. Simulation results are validated with available experimental measurements performed at Syracuse University, and are probed using a variety of techniques to characterize the composite flowfield. Even at design conditions, the asymmetry induced by the single-sided expansion and the aft-deck results in a highly three-dimensional shock train, whose interactions with the bounding shear layers influence numerous aspects of the flo (open full item for complete abstract)

Committee: Datta Gaitonde PhD (Advisor); Jen-Ping Chen PhD (Committee Member); Mei Zhuang PhD (Committee Member) Subjects: Aerospace Engineering

Doctor of Philosophy, The Ohio State University, 2017, Aero/Astro Engineering

Jet noise has been a major source of concern for commercial and military aviation sectors alike. The need to assuage the adverse impact of jet noise on human health has led to increased interest in jet noise source identification and noise level minimization/mitigation. Most previous works on common round supersonic jets have primarily explored the ideal case of simple, perfectly expanded jet configurations. In real world scenarios however, many of these simplifications do not hold. Two such considerations are examined in this work. The first is a simple configuration operating at complex operating conditions, specifically an imperfect expansion i.e. where the jets are operating at off-design conditions. The second concerns a complex configuration at simple conditions: specifically two jets (twin-jets such as those on fighter aircraft) operating in close proximity to each other. In this work, Large Eddy Simulation (LES) based high-fidelity computations are used to understand the dynamics of imperfectly expanded and twin-jets respectively, with the following objectives: 1) Identify the impact of active flow control techniques on the plume dynamics and acoustic characteristics of underexpanded jets, and 2) Investigate the interaction dynamics of the twin-jet plumes and study its associated sound field which exhibits complex radiation characteristics. To meet the first objective, a single-jet with a fully expanded Mach 1.3 jet, issuing from a converging nozzle operating at underexpanded conditions is considered. After selecting the appropriate grid based on mesh resolution studies, flow validation is conducted which indicates an excellent qualitative and quantitative agreement of the computed jet plume characteristics with the experimental observations. The analysis of underexpanded flow-field at two different Reynolds numbers indicates a relative independence of the jet flow characteristics and downstream plume evolution to the variation in Reynolds number. A det (open full item for complete abstract)

Committee: Datta Gaitonde Professor (Advisor); Mo Samimy Professor (Committee Member); Mei Zhuang Professor (Committee Member); Jen-Ping Chen Associate Professor (Committee Member) Subjects: Acoustics; Aerospace Engineering; Fluid Dynamics

PhD, University of Cincinnati, 2016, Engineering and Applied Science: Aerospace Engineering

This work focuses in the investigation of crackle and Mach wave radiation in heated supersonic jets. The skewness and kurtosis of the acoustic pressure signal and its time derivative were adopted as metrics for identifying crackling jets and quantifying levels of crackle. Cold and heated jets from supersonic nozzles with different geometric parameters and scales are analyzed to draw conclusions on noise sources and propagation. In order to complement the investigation, results are also presented for the mixing noise, broadband shock-associated noise and screech. Chapter 4 focuses on the impact of jet operating condition on the skewness and kurtosis levels of a jet issuing from a converging-diverging conical nozzle, with a 1.5 design Mach number. An increase in convective Mach number, achieved by increasing jet temperature, proved to be related to elevated values of OASPL, skewness, and kurtosis, in both the near and far fields. Intense levels of the dP/dt high-order statistics appear to be generated at different locations in the shear layer of the jet and strengthen away from the jet by non-linear propagation effects. Chapter 5 studies how adding chevrons to a converging-diverging nozzle impacts Mach wave radiation and crackle. The chevrons decreased OASPL in the downstream angles but increased the broadband shock-associated noise. Pressure skewness, dP/dt skewness and kurtosis were all reduced by the chevrons in the near field and far field, and thus they effectively mitigated crackle and Mach wave radiation; however, chevrons showed no evidence of changing the convective Mach number. The evolution of noise signals was analyzed in the near-field to the far-field to identify the strengthening of skewness through nonlinear propagation effects. Chapter 6 investigates a jet exhausting over a plate at different stand-off distances, to simulate jets exhausting over airframe surfaces and jet-ground interaction during take-off and landing operations. Far-field acoustics (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D. D.Sc. (Committee Chair); Kailas Kailasanath Ph.D. (Committee Member); Jeffrey Kastner Ph.D. (Committee Member); Mark Turner Sc.D. (Committee Member) Subjects: Aerospace Materials

PhD, University of Cincinnati, 2013, Engineering and Applied Science: Aerospace Engineering

Supersonic jets provide unique challenges in the aeroacoustic field due to very high jet velocities, shock associated noise components, flow dependence on jet expansion, and stringent performance requirements. Current noise suppression technology for commercial and military jet engines revolves around using chevrons or mechanical vortex generators to increase mixing near the nozzle exit, subsequently reducing peak turbulence levels in the mixing region. Passive noise control methods such as mechanical chevrons cause thrust loss throughout the flight envelope and performance can vary with the engine operating condition. Development of active noise control methods have the potential of improved performance throughout the flight envelope and the benefit of being deactivated when noise control is unnecessary. Fluidic injection of air into a supersonic jet is studied as an active control method with an emphasis on understanding the physics of the problem and identifying the controlling parameters. An experimental investigation with computational collaboration was conducted to understand the effect of nozzle design on supersonic jet noise and to develop various fluidic injection techniques to control noise from a supersonic jet with a design Mach number of 1.56. The jet was studied at overexpanded, ideally expanded, and underexpanded conditions to evaluate the effects throughout the operational envelope. As a passive noise control method, the internal contour of a realistic nozzle was modified to investigate the effect on acoustics and performance. Thrust was improved up to 10% with no acoustic penalties through nozzle design, however it was found that the shock noise components were highly sensitive to the shock structure in the jet. Steady fluidic injection was used to generate vorticity at the trailing edge of the nozzle showing that noise reduction is achieved through vorticity generation, modification of the shock structure, and interference with the screech feedb (open full item for complete abstract)

Committee: Ephraim Gutmark Ph.D., D.Sc. (Committee Chair); Steve Martens Ph.D. (Committee Member); Awatef Hamed Ph.D. (Committee Member); Jeffrey Kastner Ph.D. (Committee Member); David Munday Ph.D. (Committee Member); Mark Turner Sc.D. (Committee Member) Subjects: Aerospace Materials

MS, University of Cincinnati, 2012, Engineering and Applied Science: Aerospace Engineering

Proper Orthogonal Decomposition (POD) is used in order to better understand the coherent structure of a non-ideally expanded supersonic single flow jet. The method has been performed using numerical results obtained from Large Eddy Simulation (LES) obtained by Junhui Liu and Dr. K. Kailasanath from Naval Research Laboratory and from Particle Image Velocimetry (PIV) performed by Nick Heeb and Dr. D. Munday from the Propulsion and Gas Dynamics Laboratory at the University of Cincinnati. Experimental techniques, such as Particle Image Velocimetry (PIV) are available to visualize the structure of supersonic flow. The flow domain which can be observed by these experimental techniques is a plane embedded in the entire flow. The LES flow domain is a 3D domain. A numerical grid is created to match the PIV domain size. Comparison between PIV and LES showed that the flow domain size acts as a spatial filter. The largest scale structure that can be observed is limited by the domain size, which is dictated by the PIV technologies features, such as laser energy, Charge-Coupled Device (CCD) camera resolution. The numerical results for their part are very sensitive to the grid employed. Proper Orthogonal Decomposition was performed on two different grids for the baseline nozzle. It is seen that the grid size is a very sensitive parameter when educing the coherent structure of a supersonic non-ideally expanded jet. It acts as a spatial filter of turbulent scales. iii The POD analysis is used to compare baseline and chevrons nozzles flows and how chevrons impact the baseline coherent structure. It is observed that the chevrons increase the Turbulent Kinetic Energy near the nozzle exit and reduce it downstream. By studying the POD modes, energy, and Modal Amplifications Coefficients, it can be concluded that the chevrons break down large stream wise oriented energy containing structures near the nozzle exit and transfer this energy to smaller radial oriented structures and therefore (open full item for complete abstract)

Committee: Ephraim Gutmark PhD DSc (Committee Chair); Mihai Mihaescu PhD (Committee Member); Shaaban Abdallah PhD (Committee Member); Jeffrey Kastner PhD (Committee Member) Subjects: Aerospace Materials

PhD, University of Cincinnati, 2010, Engineering and Applied Science: Aerospace Engineering

This research project examines supersonic jets from nozzles representative of the practical variable-geometry convergent-divergent nozzles used on high-performance military aircraft. The nozzles employed have conical convergent sections, sharp throats and conical divergent sections. Nozzles with design Mach numbers of 1.3, 1.5, 1.56 and 1.65 are tested and the flow and acoustics examined. Such nozzles are found to produce a double-diamond shock structure consisting of two overlapping sets of shock cells, one cast from the nozzle lip and one cast from the nozzle throat. These nozzles are found to produce no shock-free condition at or near the design condition. As a result they produce shock-associated noise at all supersonic conditions. The shock cell spacing, broad-band shock-associated noise peak frequency and screech frequency all match those of more traditional nearly isentropic convergent-divergent nozzles. A correlation is proposed which improves upon the Prandtl-Pack relation for shock cell spacing in that it accounts for differences in nozzle design Mach number which the Prandtl-Pack relation does not. This proposed relation reverts to the Prandtl-Pack equation for the case of a design Mach number of 1.0. Chevrons are applied to the nozzles with design Mach numbers of 1.5 and 1.56. The effective penetration of the chevrons is found to be a function of the jet Mach number. Increasing jet Mach number increases effective penetration of the chevrons and increases the magnitude of all chevron effects. Chevrons on supersonic jets are found to reduce shock cell length, increase mixing and spreading, decrease turbulent kinetic energy at the end of the potential core and increase it near the nozzle. Chevrons corrugate the shear layer but not the shock structures inside the jet which remain axisymmetric. Chevrons thicken the shear layer, reducing the sonic diameter and reducing the diameter of the shock cells. By reducing their diameter they also reduce the shock cell (open full item for complete abstract)

Committee: Ephraim Gutmark PhD, DSc (Committee Chair); Shaaban Abdallah PhD (Committee Member); Paul Orkwis PhD (Committee Member); James Bridges PhD (Committee Member); Kailas Kailasanmath PhD (Committee Member) Subjects: Aerospace Materials

Doctor of Philosophy (Ph.D.), Bowling Green State University, 2008, Photochemical Sciences

The spectroscopy and exciton coupling in a series of conformationally flexible bichromophoric molecules have been investigated in a supersonic jet. The investigated molecules are: 1,2-diphenylethane (DPE), 1,2-bis(4-methylphenyl)ethane (M2DPE), 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (THDC), cis and trans isomers of 1,2-diphenylcyclopropane (DPCP), 2-phenylindane(2PI), and 2-(4-fluorophenyl)indane (2FPI). Resonant enhanced two-photon ionization (R2PI) spectra of these compounds and many of their deuterated isotopomers have been recorded for the first time. The observed spectral features have been assigned. The experimental results are compared with the predictions of the dipole-dipole based Forster theory, and the supramolecular model of bichromophoric molecules with identical chromophores. Analysis of the experimental data is facilitated by the spectral analysis of the single chromophore analogues of the investigated bichromophoric molecules. These include ethylbenzene-d0, α-ethylbenzene-d1, cis and trans isomers of 1-methyl-2-phenylcyclopropane, and 2-methylindane. The molecular structures and other properties of the investigated molecules in their ground and exited singlet states have been computed at various levels of theory. The calculations predict energetic preference of localized electronic excitation of most of the investigated molecules, suggesting weak inter-chromophore interactions in the excited states. Transition density surfaces indicate that the lowest excited singlet states of most of the investigated molecules have Lb ππ* character. Two conformers of DPE, M2DPE, THDC, and 2-methylindan have been observed. The rest of the investigated molecules are found to exist in a single conformation. The S2←S0 transitions of both anti and gauche conformers of DPE, the anti conformer of M2DPE, the chair conformer of THDC, and the trans isomer of DPCP are forbidden. However, the S2←S0 transitions of the asymmetrically deuterated isotopomers are observed, (open full item for complete abstract)

Committee: John R. Cable (Advisor); Douglas C. Neckers (Committee Member); Sheila J. Roberts (Committee Member); Deanne L. Snavely (Committee Member) Subjects: Chemistry

Doctor of Philosophy, University of Akron, 2005, Chemistry

Excitation-energy dependence of fluorescence intensity and lifetime has been measured for 4-dimethylaminobenzonitrile (DMABN), 4-aminobenzonitrile (ABN), 4-diisopropylaminobenzonitrile (DIABN) and 1-cyanonaphthalene (1CNN) in a supersonic free jet. In all cases, the fluorescence yield decreases rather dramatically, whereas the fluorescence lifetime decreases only moderately for ( ) excess vibrational energy exceeding about 1000cm-1. This is confirmed by the normalized fluorescence excitation spectrum with the absorption spectrum of the compound in the vapor phase. The result indicates that the strong decrease in the relative fluorescence yield at higher energies is due mostly to a decrease in the radiative decay rate of the emitting state. Comparison of the experimental results with the Time Dependent Density Functional Theory (DDFT) potential energy curves for excited states strongly suggests that the decrease in the radiative decay rate of the amino-benzonitriles at higher energies is due to the crossing of the singlet state by the lower-lying singlet state of very small radiative decay rate. The threshold energy for the fluorescence “break-off” is in good agreement with the computed energy barrier for the crossing. For 1CNN, on the other hand, the observed fluorescence break-off can be best attributed to the crossing of the singlet state by the triplet state. A concerted experimental (mass-selective spectroscopy) and theoretical (correlated quantum chemistry calculation) study of hydrogen-bonded clusters of 1-cyanonaphthalene (1CNN) with water has been carried out to probe geometries of the conformational isomers. The structures of the two low-energy conformers of 1CNN(H2O) and 1CNN(H2O)2 predicted by MP2/cc-pVDZ calculation, are consistent with ionization-loss (ion-dip) infrared spectra of C-H and O-H stretches of the two conformers, identified by ionization-detected hole-burning spectroscopy. The facile loss of a neutral water molecule from the cluster ion of 1 (open full item for complete abstract)

Committee: Edward Lim (Advisor) Subjects: Chemistry, Physical