Department: Engineering : Aerospace Engineering ![[clear]](close-x.png "Remove this limiter")
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1.
Abd El-Nabi, Bassam.
Single Annular Combustor: Experimental investigations of Aerodynamics, Dynamics and Emissions.
Degree: PhD, Engineering : Aerospace Engineering, 2010, University of Cincinnati
► The present work investigates the aerodynamics, dynamics and emissions of a Single…
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▼ The present work investigates the aerodynamics, dynamics and emissions of a Single Cup Combustor Sector. The Combustor resembles a real Gas Turbine Combustor with primary, secondary and dilution zones (also known as fuel rich dome combustor). The primary jets considerably contribute to the heat release process at high power conditions. Also, the primary jets drastically impact the flow field structure. Therefore, the parameters influencing the primary jets are studied using PIV (pressure drop, jets size, off-centering, interaction with convective cooling air, jet blockage and fuel injection). This study is referred to as a jet sensitivity study. The results indicate that the primary jets can be used effectively in controlling the flow field structure. A pressure drop of 4.3% and 7.6% result in similar flows with no noticeable effect on the size of the CRZ and the four jets wake regions. On the other hand, the results show that the primary jets are very sensitive to perturbations. The cooling air interacts with the primary jet and influences the flow field although the momentum ratio has an order of magnitude of 100:1. The results also show that the big primary jets dictate the flow field in the primary zone as well as the secondary zone. However, relatively smaller jets mainly impact the primary zone. Also, the results point to the presence of a critical jet diameter beyond which the dilution jets have minimum impact on the secondary region. The jet off-centering shows significant effect on the flow field though it is on the order of 1.0 mm. The jet sensitivity study provides the combustion engineers with useful methods to control the flow field structure, an explanation for observed flow structure under different conditions and predictable flow field behavior with engine aging. All results obtained from the jet sensitivity study could be explained in terms of jet opposition. Hence, similar results are expected under reacting conditions. The combustion instabilities are studied using a microphone, high speed camera and regular cameras. The frequency spectrum for the sector is established at different pressure drops (2, 4 and 6%) as well as different pre-heat temperatures (200, 400 and 600F). The acoustic spectrum suggests that there are three frequencies of concern (280, 400 and 600 HZ). The high frequency appears to be related to the combustor ¼ longitudinal wave. The 280 Hz is due to a rotating instability while the 400 Hz is related to the primary jets. The emissions emanating from the combustor are studied using FTIR at pressure drop of 4% and different power conditions. The sector emissions characteristics are determined. Water injection is also used to control the pollutant emissions. Water fuel ratio of 100% and 50% results in a corresponding reduction in the NOx concentration with 50% and 22%. No noticeable effects are observed on the NOx and CO at low power conditions. A high degree of homogeneity in the emissions contours is observed at the combustor exit at low power conditions. However, this homogeneity is noticeably reduced at high power conditions.
Advisors/Committee Members: Jeng, San-Mou.
Subjects: Aerospace materials
Keywords: Primary jets senstivity; Swirling flow; Combustion Aerodynamics; Combustion Dynamics; Emissions control; Engine Aging effects
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2.
ABU-NABAH, BASSAM ABDEL JABER.
EDDY CURRENT SPECTROSCOPY FOR NEAR-SURFACE RESIDUAL STRESS PROFILING IN SURFACE TREATED NONMAGNETIC ENGINE ALLOYS.
Degree: PhD, Engineering : Aerospace Engineering, 2007, University of Cincinnati
► Recent research results indicated that eddy current conductivity measurements can be exploited…
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▼ Recent research results indicated that eddy current conductivity measurements can be exploited for nondestructive evaluation of near-surface residual stresses in surface-treated nickel-base superalloy components. Most of the previous experimental studies were conducted on highly peened (Almen 10-16A) specimens that exhibit harmful cold work in excess of 30% plastic strain. Such high level of cold work causes thermo-mechanical relaxation at relatively modest operational temperatures; therefore the obtained results were not directly relevant to engine manufacturers and end users. The main reason for choosing peening intensities in excess of recommended normal levels was that in low-conductivity engine alloys the eddy current penetration depth could not be forced below 0.2 mm without expanding the measurements above 10 MHz which is beyond the operational range of most commercial eddy current instruments. As for shot-peened components, it was initially felt that the residual stress effect was more difficult to separate from cold work, texture, and inhomogeneity effects in titanium alloys than in nickel-base superalloys. In addition, titanium alloys have almost 50% lower electric conductivity than nickel-base superalloys; therefore require proportionally higher inspection frequencies, which was not feasible until our recent breakthrough in instrument development. Our work has been focused on six main aspects of this continuing research, namely, (i) the development of an iterative inversion technique to better retrieve the depth-dependent conductivity profile from the measured frequency-dependent apparent eddy current conductivity (AECC), (ii) the extension of the frequency range up to 80 MHz to better capture the peak compressive residual stress in nickel-base superalloys using a new eddy current conductivity measuring system, which offers better reproducibility, accuracy and measurement speed than the previously used conventional systems, (iii) the lift-off effect on high frequency eddy current spectroscopy, (iv) the development of custom-made spiral coils to allow eddy current conductivity characterization over the whole frequency range of interest with reduced coil sensitivity to lift off, (v) the benefits of implementing a semi-quadratic system calibration in reducing the coil sensitivity to lift-off, and (vi) the feasibility of adapting high-frequency eddy current residual stress characterization for shot-peened titanium alloys.
Advisors/Committee Members: Nagy, Dr. Peter B.
Keywords: Eddy current, Conductivity, Spectroscopy, Residual stress, Shot peening
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3.
ADIBHATLA, GAGAN.
DESIGN AND IMPLEMENTATION OF A COMPLIANCE CONTROLLER FOR THE PA10-7CE SEVEN DEGREE OF FREEDOM DEXTEROUS ROBOT.
Degree: MS, Engineering : Aerospace Engineering, 2008, University of Cincinnati
► In many applications, such as spacecraft servicing or automated drilling by a…
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▼ In many applications, such as spacecraft servicing or automated drilling by a surgical robot, misalignments due to contact can result in excessive stresses at the interface or unwanted motion of the target. To mitigate these effects it is essential that the robotic manipulator incorporate both compliance and energy dissipation. Compliance enables the robotic manipulator to flex under high stresses, thus minimizing the possibility of damage, and the bounce resulting from impulsive loading aids in centring the tool in the desired location on the target. Energy dissipation ensures that the transient motion resulting from contact eventually dies out, allowing finer dexterous manipulation to occur. The objective of this work is to develop and demonstrate an active compliance controller for a dexterous robot, namely, the Mitsubishi PA10-7CE. The compliance controller, which is a type of impedance controller, introduces flexing, bounce, and energy dissipation on contact by controlling the tip velocities by a set of dynamic equations that emulate a spring-mass-damper system in all six degrees of freedom. Experimental results showed that the robot with active compliance control achieved the desired performance.
Advisors/Committee Members: Bosse, Dr. Albert B.
Keywords: Compliance; controller; seven degree freedom; Spring mass damper; servo; robot; Mitsubishi; pa10; impedance
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4.
ALLGOOD, DANIEL CLAY.
AN EXPERIMENTAL AND COMPUTATIONAL STUDY OF PULSE DETONATION ENGINES.
Degree: PhD, Engineering : Aerospace Engineering, 2004, University of Cincinnati
► Research studies investigating the performance optimization and fundamental physics of pulse detonation…
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▼ Research studies investigating the performance optimization and fundamental physics of pulse detonation engines (PDE) were performed. Experimental and computational methods were developed and used in these studies. Four primary research tasks were established. The first research task was to obtain detailed measurements of a PDE exhaust plume for a variety of operating conditions and engine geometries. Shadowgraph visualizations in conjunction with OH* and CH* chemiluminescence imaging were performed. The PDE plume visualizations provided a means of studying the flowfield behavior associated with PDE ejectors and exhaust nozzles as well as providing explanations for the observed acoustic behavior of the PDE. The second research task was to quantify the thrust augmentation of PDE-ejectors. Significant losses in the ejector entrainment were observed when the ejector inlet was not of an aerodynamic shape. Performance measurements of axisymmetric PDE-ejector systems showed the thrust augmentation to be a strong function of the ejector length-to-diameter ratio, ejector axial placement and PDE fill-fraction. Peak thrust augmentation levels were recorded to be approximately 20% for a straight-ejector and 65% for a diverging-ejector. An increase in thrust augmentation was obtained with a reduction in fill-fraction. Performance measurements of PDE converging and diverging exhaust nozzles were also obtained at various operating conditions of the engine. At low fill-fractions, both converging and diverging exhaust nozzles were observed to adversely affect the PDE performance. At fill-fractions close to and greater than 1, the converging nozzles showed the best performance due to increased PDE blow-down time (maintaining PDE chamber pressure) and acceleration of the primarily subsonic exhaust flow. The fourth research task was to perform a detailed far-field study of PDE acoustics. The acoustic energy of the PDE blast-wave was observed to be highly directional. Very good agreement was obtained between the experimental data and model predictions for the radial decay in peak pressure as well as the characteristic times of the blast-wave pulses. Converging exhaust nozzles were observed to produce a global reduction in PDE noise, while diverging nozzles affected only the downstream noise.
Advisors/Committee Members: Gutmark, Dr. Ephraim.
Subjects: Engineering, Aerospace
Keywords: pulse detonation engine; detonation; nozzles; ejectors; acoustics; shadowgraph; blast waves; computational fluid dynamics; chemiluminescence
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5.
ALONSO, ELENA.
CONTROL DESIGN AND IMPLEMENTATION FOR THE SELF-SEPARATION OF IN-TRAIL AIRCRAFT.
Degree: MS, Engineering : Aerospace Engineering, 2005, University of Cincinnati
► One of the responsibilities of air traffic controllers is to maintain aircraft…
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▼ One of the responsibilities of air traffic controllers is to maintain aircraft separation in flight. However, with growing air traffic demands, current research is investigating the possibility of delegating some of that responsibility to the cockpit. One suggested implementation of this concept is the design of a control law that will assist pilots in a stream of in-trail aircraft to maintain separation. The goal of this thesis is to develop and apply control laws that can aid in the maintenance of separation of long strings of aircraft. Automatic Dependence Surveillance – Broadcast (ADS-B) is used to communicate state vectors. The dynamics and controls of the aircraft are numerically simulated with the design of a MATLAB program. The results for the tests run show promising results. Further tests can be run to potentially optimize the control schemes chosen as well as analyze different flight trajectories.
Advisors/Committee Members: Slater, Dr. Gary.
Subjects: Engineering, Aerospace
Keywords: In-Trail Aircraft; Self-Separation; Dynamics and Controls
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6.
AMINJIKARAI, SRINIVASA BABU.
A STRAIN RATE DEPENDENT 3D MICROMECHANICAL MODEL FOR FINITE ELEMENT SIMULATIONS OF PLAIN WEAVE COMPOSITE STRUCTURES.
Degree: MS, Engineering : Aerospace Engineering, 2003, University of Cincinnati
► A 3D micromechanical model has been developed for Plain Weave Composites (PWC)…
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▼ A 3D micromechanical model has been developed for Plain Weave Composites (PWC) and implemented in the explicit finite element software DYNA3D. The model accounts for the strain rate dependency, inherent material nonlinearity, and progressive failure of constituents of PWC. The micromechanical equations have been obtained for a Representative Volume Cell (RVC) which is assumed to represent the behavior of a PWC lamina. The model implemented in DYNA3D can be used for the simulation of the mechanical behavior of PWC structures under various loads such as multi-axial and impact. The yarns were assumed to be transversely isotropic till initial failure. A viscoplastic constitutive model was used for the resin constituent as it was the primary reason for the rate dependency of PWC. The nonlinear behavior in shear was modeled by updating the shear moduli of the constituents based on their current stress state. Progressive failure was modeled by defining a set of maximum strain criteria for detecting failure in constituents and degrading the properties depending on the failure mode. The implemented model was validated in different loading conditions by comparing its prediction with experimental results available in the literature. Good correlation was observed between the predicted and the experimental results.
Advisors/Committee Members: Tabiei, Dr. Ala.
Keywords: plain weave composites; material model; micromechanical; strain rate dependent; constitutive model
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7.
Aminjikarai Vedagiri, Srinivasa Babu.
An Automated Dynamic Fracture Procedure and a Continuum Damage Mechanics Based Model for Finite Element Simulations of Delamination Failure in Laminated Composites.
Degree: PhD, Engineering : Aerospace Engineering, 2009, University of Cincinnati
► An active field of research that has developed due to the increasing…
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▼ An active field of research that has developed due to the increasing use of computational techniques like finite element simulations for analysis of highly complex structural mechanics problems and the increasing use of composite laminates in varied industries such as aerospace, automotive, bio-medical, etc. is the development of numerical models to capture the behavior of composite materials. One of the big challenges not yet overcome convincingly in this field is the modeling of delamination failure which is one of the primary modes of damage in composite laminates. Hence, the primary aim of this work is to develop two numerical models for finite element simulations of delamination failure in composite laminates and implement them in the explicit finite element software DYNA3D/LS-DYNA.Dynamic fracture mechanics is an example of a complex structural analysis problem for which finite element simulations seem to be the only possible way to extract detailed information on sophisticated physical quantities of the crack-tip at any instant of time along a highly transient history of fracture. However, general purpose, commercial finite element software which have capabilities to do fracture analyses are still limited in their use to stationary cracks and crack propagation along trajectories known a priori. Therefore, an automated dynamic fracture procedure capable of simulating dynamic propagation of through-thickness cracks in arbitrary directions in linear, isotropic materials without user-intervention is first developed and implemented in DYNA3D for its default 8-node solid (brick) element. Dynamic energy release rate and stress intensity factors are computed in the model using integral expressions particularly well-suited for the finite element method. Energy approach is used to check for crack propagation and the maximum circumferential stress criterion is used to determine the direction of crack growth. Since the re-meshing strategy used to model crack growth explicitly in the model induces spurious high-frequency oscillations in the finite element results after crack initiation, a “gradual nodal release” procedure is implemented as part of the model to overcome this problem. Also, an in-built contact algorithm of DYNA3D is modified to adapt it to the re-meshing strategy to maintain proper contact conditions at newly added elements. Finally, the model is suitably modified for simulating delamination failure in laminated composites and used to predict delamination resistance characteristics which are important considerations for effective use of composite structures. Continuum damage mechanics is a popular approach for modeling the in-plane failure modes in composites. However, its applicability to modeling delamination has not been sufficiently analyzed yet. Hence, as the second part of this dissertation work, a new material model is developed for unidirectional polymer matrix composites in which this approach is used to predict delamination failure and used to perform a qualitative study of the damage mechanics approach to modeling delamination. The new material model is developed using micro-mechanics and accounts for the strain-rate dependent behavior of polymer matrix composites. It is implemented for three different element formulations with different transverse shear strain assumptions and the effect of these assumptions on the delamination prediction using this approach is analyzed.
Advisors/Committee Members: Tabiei, Dr. Ala.
Subjects: Aerospace materials; Automotive materials; Mechanical engineering; Mechanics
Keywords: automated fracture procedure; dynamic fracture mechanics; energy release rate; stress intensity factors; gradual nodal release technique; delamination; delamination resistance characteristics; continuum damage mechanics model; micro-mechanical model
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8.
Anning, Grant Hugh Gary.
The Effect of Fuel Injector Geometry on the Flow Structure of a Swirl Stabilized Gas Turbine Burner.
Degree: MS, Engineering : Aerospace Engineering, 2002, University of Cincinnati
► This thesis presents an experimental study on the effect of geometry on…
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▼ This thesis presents an experimental study on the effect of geometry on the flow structure of experimental swirl-stabilized gas turbine burners. The purpose of this project was to test a number of geometric changes to a burner and determine how these changes affected the flow field in order to evaluate how they might affect a combustion process. Tests were initially conducted in a water channel on several full-scale experimental burner models, which included the use of elliptic outlet geometries and a vortex breakdown stabilizer. Velocity and phase-averaged PLIF measurements are presented for the different burner configurations. More extensive measurements were then conducted in a cold flow air channel on a series of one-quarter scale experimental swirlers. The scale models were designed with different geometries in order to test the effect of different blade angles and spacings. The models were also tested with a vortex breakdown stabilizer and a mixing section located between the swirler outlet and the sudden expansion. A scale model burner with an elliptic outlet was also tested. Stereo PIV measurements are presented for the different configurations. Water channel measurements identified axial oscillation of the vortex breakdown bubble as the primary driving mechanism behind combustion instabilities. Tests showed that the elliptic geometry damped oscillation of the breakdown bubble and the vortex breakdown stabilizer physically anchored the breakdown bubble inside the burner cone. Tests in the air channel showed that wider blade spacing resulted in a reduced swirl number and a weaker vortex breakdown. A wider blade angle was observed to increase the swirl and the strength of the vortex breakdown. Certain configurations of the breakdown stabilizer were seen to increase the strength of the vortex breakdown while other configurations forced it to form in an unstable configuration. The use of a mixing section at the swirler outlet caused the vortex breakdown bubble to propagate upstream inside of the mixing tube. The elliptic swirler exhibited a very complex flow structure, indicative of axis-switching.
Advisors/Committee Members: Gutmark, Ephraim J.
Keywords: combustion; passive control; gas turbine; swirler
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9.
BAI, XINWEN.
EFFECT OF L/D AND YAW ANGLE ON FLOW OSCILLATIONS IN SUBSONIC RECTANGULAR CAVITIES.
Degree: MS, Engineering : Aerospace Engineering, 2003, University of Cincinnati
► An experimental investigation was undertaken to examine the effect of cavity depth…
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▼ An experimental investigation was undertaken to examine the effect of cavity depth and yaw angle on flow oscillations that occur in an open cavity placed within a subsonic boundary layer. A rectangular cavity was placed within a thick turbulent boundary layer with a corresponding Reynolds number based on the cavity length of 61,935. Pressure time histories were acquired at five separate cavity depths (or L/D values) at each yaw angle from 5 degree to 90 degree using microphone-type pressure transducers. The spectral character of these signals was analyzed, and pressure level and dominant frequencies were determined. This study indicates that large changes in the pressure level occur as L/D varies from 0.5 to 2.1 and as the yaw angle varies from 5 to 90 deg. Relative sound pressure level calculations indicate that the energy within the cavity compared with that of the boundary layer was observed to increase by approximately 22 percent at L/D=2.1 and 2.2 times at yaw angle=60 deg.
Advisors/Committee Members: Disimile, Dr. Peter J.
Subjects: Engineering, Aerospace
Keywords: cavity; flow oscillation; boundary; pressure oscillation
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10.
Balawi, Shadi Omar.
Effective Mechanical Behavior of Honeycombs: Theoretical and Experimental Studies.
Degree: PhD, Engineering : Aerospace Engineering, 2007, University of Cincinnati
► Honeycombs are discrete materials at the macro-scale that can be used as…
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▼ Honeycombs are discrete materials at the macro-scale that can be used as standalone materials or placed as cores between composite facesheets to form sandwich structures. The prediction of their effective mechanical properties as a continuum material is essential to the analysis and design of honeycomb sandwich structures and other honeycomb structures. In this research work, the effective mechanical behavior of honeycombs was studied by analytical and numerical means and correlated with experimental results for aluminum hexagonal honeycombs. The analytical methods included continuum formulations and models based on strength of materials including a variety of beam theories. The numerical analyses included finite element analyses and the experimental program consisted of the mechanical characterization of the honeycombs under both in-plane and out-of-plane loading. The effective in-plane properties including elastic moduli and Poisson’s ratios of the honeycombs were studied as a function of their relative density with existing beam models. It was shown experimentally that the beam models describe well the material response in the direction of the honeycomb double wall. However, it was concluded that the effective elastic moduli for honeycombs with low relative densities are not similar in the two in-plane directions as predicted by previous studies. A refined model that predicts the effective honeycomb properties was developed to take into consideration the curvature that is present in the intersection points of hexagonal honeycombs due to corrugation or expansion during manufacturing. The developed refined model is not only capable of explaining the experimentally observed difference between the effective in-plane elastic moduli but can also be expanded to predict the effective moduli of honeycombs of any relative density.
Advisors/Committee Members: Abot, Dr. Jandro L.
Keywords: Honeycomb Structures; Mechanical Properties; Cellular Structures
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11.
Barone, Dominic L.
Investigation of TDLAS Measurements in a Scramjet Engine.
Degree: MS, Engineering : Aerospace Engineering, 2010, University of Cincinnati
► An investigation of the viability of tunable diode laser absorption spectroscopy for…
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▼ An investigation of the viability of tunable diode laser absorption spectroscopy for use as a flow measurement device in a scramjet engine was completed. First, the effects on TDLAS measurements across a temperature jump that is common in scramjet combustor flow-paths was studied using a flat flame burner designed with four independently fueled quadrants. Rigorous thermocouple mapping of the burner was performed and a discussion of multi-thermocouple radiation correction techniques is presented. The fundamental mass capture measurements (temperature, water number density, pressure, and velocity) were then made in the isolator section of a direct-connect scramjet engine and compared to a scramjet performance analysis code. Post-combustion measurements (temperature and water number density) were measured in the exhaust section of the model engine. The results of the measurements and an in-depth discussion of analysis routines used in the processing of raw absorption measurements is presented.
Advisors/Committee Members: Jeng, San-Mou.
Subjects: Aerospace materials
Keywords: Tunable Diode Laser; Absorption; Spectroscopy; TDLAS; Scramjet
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12.
BASCIANO JR., THOMAS E.
THE UNIVERSITY OF CINCINNATI H.A.V.O.C. SOUNDING ROCKET PROJECT DESIGN STUDY AND FINAL RESULTS.
Degree: MS, Engineering : Aerospace Engineering, 2001, University of Cincinnati
► The thesis presented here is meant to give an overview of the…
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▼ The thesis presented here is meant to give an overview of the objectives and technical aspects of designing, testing, and launching a sounding rocket. It is a compilation of the many lessons learned by the University of Cincinnati Student Sounding Rocket Team. The information contained in this thesis will help new student groups design launch vehicles and understand the philosophy behind NASA support, testing and launching of sounding rockets.
Advisors/Committee Members: Richardson, Dr. David L.
Subjects: Engineering, Aerospace
Keywords: sounding rocket; rocket design; student aerospace projects; student rockets; college aerospace programs
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13.
Basu, Debashis.
Hybrid Methodologies for Multiscale Separated Turbulent Flow Simulations.
Degree: PhD, Engineering : Aerospace Engineering, 2006, University of Cincinnati
► The goal of the present research is to develop and assess multiscale…
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▼ The goal of the present research is to develop and assess multiscale and hybrid turbulence models in simulation of separated turbulent flows at high Reynolds numbers in terms of grid refinement and CPU resources required for a certain level of spectral resolution of the separated flow. These investigated multiscale turbulence models include the DES (Detached Eddy Simulation), hybrid RANS (Reynolds-Averaged Navier Stokes) /LES (Large Eddy Simulation) and PANS (Partially Averaged Navier-Stokes) closure models. These techniques adapt a turbulence model, that function as a RANS model in regions where the grids are highly stretched and the high Reynolds number boundary layer is attached, to function as a sub-grid scale LES type model where the grid is nearly isotropic in the separated flow regions. This accomplished by reducing the turbulence eddy viscosity to promote resolution of more turbulence scales in these regions while still reverting to the original RANS behavior in attached flow and near wall regions. The sensitivity of the computed results to multiscale closure model parameters are compared for three developed formulations of the DES model, one variant of hybrid RANS/LES model, a proposed adaptation of the PANS model and the original multiscale SST-DES model for a number of problems involving unsteady separated high Reynolds number flow. The flow configurations include transonic flow over open cavity, subsonic flow over a back facing step and flow over wall-mounted hump. Simulation predictions are compared with experimental data and also equivalent LES simulations. Simulated results show that these models perform better when there is a distinct demarcation between the attached and separated regions and unsteady shear layers dominate the flow. Computed results show that multiscale methods based on the modification of the turbulent kinetic energy dissipation rate provide the most accurate results and the computed results for the unsteady spectra amplitude and frequency are significantly influenced by the model parameters and the grids. These models provide a useful tool for predicting complex 3-D separated unsteady flows over an expansive dynamic range at high Reynolds number and are comparable to LES predictions at (1/6)th - (1/10)th the corresponding LES CPU resources.
Advisors/Committee Members: Haned, Dr. Awatef.
Subjects: Engineering, Aerospace
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14.
Bolender, Michael Alan.
Scheduling and Control Strategies for the Departure Problem in Air Traffic Control.
Degree: PhD, Engineering : Aerospace Engineering, 2000, University of Cincinnati
► Two problems relating to the departure problem in air traffic control automation…
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▼ Two problems relating to the departure problem in air traffic control automation are examined. The first problem that is addressed is the scheduling of aircraft for departure. The departure operations at a major US hub airport are analyzed, and a discrete event simulation of the departure operations is constructed. Specifically, the case where there is a single departure runway is considered. The runway is fed by two queues of aircraft. Each queue, in turn, is fed by a single taxiway. Two salient areas regarding scheduling are addressed. The first is the construction of optimal departure sequences for the aircraft that are queued. Several greedy search algorithms are designed to minimize the total time to depart a set of queued aircraft. Each algorithm has a different set of heuristic rules to resolve situations within the search space whenever two branches of the search tree with equal edge costs are encountered. These algorithms are then compared and contrasted with a genetic search algorithm in order to assess the performance of the heuristics. This is done in the context of a static departure problem where the length of the departure queue is fixed. A greedy algorithm which deepens the search whenever two branches of the search tree with non-unique costs are encountered is shown to outperform the other heuristic algorithms. This search strategy is then implemented in the discrete event simulation. A baseline performance level is established, and a sensitivity analysis is performed by implementing changes in traffic mix, routing, and miles-in-trail restrictions for comparison. It is concluded that to minimize the average time spent in the queue for different traffic conditions, a queue assignment algorithm is needed to maintain an even balance of aircraft in the queues. A necessary consideration is to base queue assignment upon traffic management restrictions such as miles-in-trail constraints. The second problem addresses the technical challenges associated with merging departure aircraft onto their filed routes in a congested airspace environment. Conflicts between departures and en route aircraft within the Center airspace are analyzed. Speed control, holding the aircraft at an intermediate altitude, re-routing, and vectoring are posed as possible de-confliction maneuvers. A cost assessment of these merge strategies, which are based upon 4D flight management and conflict detection and resolution principles, is given. Several merge conflicts are studied and a cost for each resolution is computed. It is shown that vectoring tends to be the most expensive resolution technique. Altitude hold is simple, costs less than vectoring, but may require a long time for the aircraft to achieve separation. Re-routing is the simplest, and provides the most cost benefit since the aircraft flies a shorter distance than if it had followed its filed route. Speed control is shown to be ineffective as a means of increasing separation, but is effective for maintaining separation between aircraft. In addition, the effects of uncertainties on the cost are assessed. The analysis shows that cost is invariant with the decision time.
Advisors/Committee Members: Slater, Gary.
Subjects: Engineering, Aerospace
Keywords: air traffic control; departure scheduling; 4D tragectories
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15.
BUSH, SCOTT M.
CHARACTERIZATION OF FLAME STABILIZATION TECHNOLOGIES.
Degree: PhD, Engineering : Aerospace Engineering, 2006, University of Cincinnati
► To experimentally explore and characterize a V-gutter stabilized flame, this research study…
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▼ To experimentally explore and characterize a V-gutter stabilized flame, this research study developed a Combustion Wind Tunnel Test Facility capable of effectively simulating the freestream Mach #'s and temperatures achieved within the back end of a gas turbine jet engine. After validating this facility, it was then used to gain a better understanding of the flow dynamics and combustion dynamics associated with the V-gutter configuration. The motivation for studying the V-gutter stabilized flame is due to the concern in industry today with combustion instabilities that are encountered in military aircraft. To gain a better understanding of the complex flow field associated with the V-gutter stabilized flame, this research study utilized Particle Image Velocimetry to capture both non-reacting and reacting instantaneous and mean flow structures formed in the wake region of the three dimensional V-gutter bluff body. The results of this study showed significant differences between the non-reacting and reacting flow fields. The non-reacting case resulted in asymmetric shedding of large scale vortices from the V-gutter edges while the reacting case resulted in a combination of both symmetric and asymmetric shedding of smaller scale vortical structures. A comparison of the mean velocity components shows that the reacting case results in a larger region of reversed flow, experiences an acceleration of the freestream flow due to combustion, and results in a slower dissipation of the wake region. Simultaneous dynamic pressure and CH* chemiluminescence measurements were also recorded to determine the coupling between the flow dynamics and combustion dynamics. The results of this study showed that only low frequency combustion instabilities were encountered at various conditions within the envelope of stable operation because of the interaction between longitudinal acoustic waves and unsteady heat release. When approaching rich blow out, rms pressure amplitudes were as high as 2 psi, and approaching lean blow out lead to rms pressure amplitudes around 0.2 psi. These studies also showed the instability frequency increasing with increases in either inlet temperature or inlet Mach #. Additionally, increasing the inlet velocity or the DeZubay parameter reduced the stability limits of operation for the V-gutter stabilized flame.
Advisors/Committee Members: Gutmark, Dr. Ephraim J.
Subjects: Engineering, Aerospace
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16.
CAI, JUN.
AERODYNAMICS OF LEAN DIRECT INJECTION COMBUSTOR WITH MULTI-SWIRLER ARRAYS.
Degree: PhD, Engineering : Aerospace Engineering, 2006, University of Cincinnati
► Multi-point Lean Direct Injection (LDI) combustion focuses on low NOx emissions under…
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▼ Multi-point Lean Direct Injection (LDI) combustion focuses on low NOx emissions under extremely high temperature and pressure environment as might be used in high performance aircraft engines in the near future. The performance of the multi-point LDI combustor directly depends on the aerodynamics of the combustor swirlers and their different combinations. Therefore, further understanding of the underlying physical performance of swirler arrays is one of the primary requirements for advanced LDI combustor design. The focus of this dissertation is to investigate the aerodynamics of co-rotating and counter-rotating multi-swirler arrays. This study covers two aspects of investigation, experimental and computational. Experimental measurements were conducted on the flow fields of a macro-laminated radial swirler, a discrete-jet swirler with different size confinements, and co-rotating and counter-rotating swirler arrays using the discrete-jet swirler. Computational studies of the flow in the swirler and the downstream tube were carried out using k-ε and Reynolds stress turbulence models and the results were compared with experimental data. The experimental measurements show that the swirling flow generated by the macro-laminated swirler is not uniform. Discrete air jets can be found near the exit. The square chamber strongly affects the velocity distribution. The study on the flow structures of a discrete-jet swirler in three different size confinements shows that the confinement size strongly affects flow structure such as the recirculation zone size, velocity distribution and turbulence levels. The experimental results of co-rotating and counter-rotating swirler array indicate that the considered flow fields have very complicated structures. The existence of recirculation zones and strong turbulence levels indicate the potential for good combustion performance and Low NOx emissions. For the single swirler case, both the k-ε and the Reynolds stress models were able to predict the flow structure very well. However for the co-rotating swirler array, the Reynolds stress model proved superior to the k-ε model, and was able to predict the extremely complicated flow structure well.
Advisors/Committee Members: Jeng, Dr. San-Mou.
Subjects: Engineering, Aerospace
Keywords: Swirling Flow, LDI, NOx, Swirler array
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17.
CALLENDER, WILLIAM BRYAN.
AN INVESTIGATION OF INNOVATIVE TECHNOLOGIES FOR REDUCTION OF JET NOISE IN MEDIUM AND HIGH BYPASS RATIO TURBOFAN ENGINES.
Degree: PhD, Engineering : Aerospace Engineering, 2004, University of Cincinnati
► This research project has developed a new, large-scale, nozzle acoustic test rig…
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▼ This research project has developed a new, large-scale, nozzle acoustic test rig capable of simulating the exhaust flows of separate flow exhaust systems in medium and high bypass turbofan engines. This rig has subsequently been used to advance the understanding of two state-of-the-art jet noise reduction technologies. The first technology investigated is an emerging jet noise reduction technology known as chevron nozzles. The fundamental goal of this investigation was to advance the understanding of the fundamental physical mechanisms responsible for the acoustic benefits provided by these nozzles. Additionally, this study sought to establish the relationship between these physical mechanisms and the chevron geometric parameters. A comprehensive set of data was collected, including far-field and near-field acoustic data as well as flow field measurements. In addition to illustrating the ability of the chevron nozzles to provide acoustic benefits in important aircraft certification metrics such as effective perceived noise level (EPNL), this investigation successfully identified two of the fundamental physical mechanisms responsible for this reduction. The flow field measurements showed the chevron to redistribute energy between the core and fan streams to effectively reduce low frequency noise by reducing the length of the jet potential core. However, this redistribution of energy produced increases in turbulent kinetic energy of up to 45% leading to a degradation of the chevron benefit at higher frequencies. Trends observed with respect to the chevron geometry showed that the chevron penetration could be matched to the exhaust flow conditions to optimally balance the trade between low frequency reduction and high frequency increase to maximize reductions in EPNL. Secondly, a completely new technology, known as fluidic injection, was investigated. This technology consists of applying continuous air injection, from a number of small injection jets, at the nozzle exit plane to reduce jet noise. The principal advantage of such an approach is that it is an active technology that can be activated as needed and, as such, may be more acceptable in aircraft engines from a performance standpoint than passive technologies. This study successfully demonstrated the feasibility of this technology by showing that effective jet noise reduction can be provided in a broad range of flow conditions using less than 1% of the mean jet mass flow. An investigation of injection geometric parameters identified the injection pitch angle as the most influential parameter with respect to jet noise reduction. Furthermore, an investigation of scaling effects showed a momentum ratio of approximately 1.5% to provide reductions in sound pressure level between 1 and 2 dB across a wide range of frequencies for a wide range of flow conditions and scales including both single stream and dual stream flows. PIV flow field measurements identified the fundamental physical mechanism of the noise reduction to be a near uniform reduction in shear layer turbulence.
Advisors/Committee Members: Gutmark, Dr. Ephraim J.
Keywords: Aeroacoustics; Jet Noise; Separate Flow Exhaust Systems; Separate Flow Exhaust Nozzles; Chevron Nozzles; Chevrons; Fluidic Injection; Fluidics
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18.
Das, Kaushik.
Numerical Simulations of Icing in Turbomachinery.
Degree: PhD, Engineering : Aerospace Engineering, 2006, University of Cincinnati
► Safety concerns over aircraft icing and the high experimental cost of testing…
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▼ Safety concerns over aircraft icing and the high experimental cost of testing have spurred global interest in numerical simulations of the ice accretion process. Extensive experimental and computational studies have been carried out to understand the icing on external surfaces. No parallel initiatives were reported for icing on engine components. However, the supercooled water droplets in moist atmosphere that are ingested into the engine can impinge on the component surfaces and freeze to form ice deposits. Ice accretion could block the engine passage causing reduced airflow. It raises safety and performance concerns such as mechanical damage from ice shedding as well as slow acceleration leading to compressor stall. The current research aims at developing a computational methodology for prediction of icing phenomena on turbofan compression system.Numerical simulation of ice accretion in aircraft engines is highly challenging because of the complex 3-D unsteady turbomachinery flow and the effects of rotation on droplet trajectories. The aim of the present research focuses on (i) Developing a computational methodology for ice accretion in rotating turbomachinery components (ii) Investigate the effect of inter-phase heat exchange (iii) Characterize droplet impingement pattern and ice accretion at different operating conditions. The simulations of droplet trajectories are based on a Eulerian-Lagrangian approach for the continuous and discrete phases. The governing equations are solved in the rotating blade frame of reference. The flow field is computed by solving the 3-D solution of the compressible Reynolds Averaged Navier Stokes (RANS) equations. One-way interaction models simulate the effects of aerodynamic forces and the energy exchange between the flow and the droplets. The methodology is implemented in the code TURBODROP and applied to the flow field and droplet trajectories in NASA Roto-67r and NASA-GE E3 booster rotor.The results highlight the variation of impingement location and temperature with droplet size. It also illustrates the effect of rotor speed on droplet temperature rise. The computed droplet impingement statistics and flow properties are used to calculate ice shapes. It was found that the mass of accreted ice and maximum thickness is highly sensitive to rotor speed and radial location.
Advisors/Committee Members: Hamed, Dr. Awatef.
Subjects: Engineering, Aerospace
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19.
DAVIDZ, HEIDI L.
USE OF NEAR-FROZEN ORBITS FOR SATELLITE FORMATION FLYING.
Degree: MS, Engineering : Aerospace Engineering, 2001, University of Cincinnati
► There is growing interest in flying coordinated clusters of small spacecraft to…
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▼ There is growing interest in flying coordinated clusters of small spacecraft to perform missions once accomplished by single, larger spacecraft. Using these satellite clusters reduces cost, improves survivability, and increases the flexibility of the mission. One challenge in implementing these satellite clusters is maintaining the formation as it experiences orbital perturbations, most notably due to the non-spherical Earth. Certain aspects of the orbital geometry can remain virtually fixed over extended periods of time due to a natural phenomenon called a frozen orbit. Specifically, the elements of the orbital geometry that can remain fixed are the argument of perigee (a measure of where the orbit is closest to the Earth) and the eccentricity (a measure of how circular or elliptical the orbit is). For satellite formations, using this frozen orbit phenomenon results in considerable propellant savings. In this study, a discussion of current literature on this topic is given. Some examples of formations at near-frozen conditions are shown. There is also a discussion of the propellant impact of near-frozen conditions. If two orbits meet a certain set of initial conditions, the orbits will naturally stay in the vicinity of each other. These orbits are sometimes called Hill’s orbits. An algorithm is developed here that determines if the Hill’s orbit conditions will be met given the initial differences in eccentricity and argument of perigee for two satellites.
Advisors/Committee Members: Williams, Dr. Trevor.
Subjects: Engineering, Aerospace
Keywords: FROZEN ORBIT; SATELLITE FORMATION FLYING
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20.
Davis, John Matthew.
An investigation into the effect of surface-mounted circular obstructions on flow driven diffusion flames.
Degree: MS, Engineering : Aerospace Engineering, 2009, University of Cincinnati
► With little data currently available, designers of aircraft engine nacelle fire-test simulators…
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▼ With little data currently available, designers of aircraft engine nacelle fire-test simulators have little basis to determine the size of geometric detail required to provide a sufficient representation of the actual specimen. The current study presents an experimental investigation into the minimum geometry size required for accurate aircraft engine simulator design by examining the turbulent shear flow downstream of surface mounted circular cylinders of diameters of 5, 10, 20, and 40 mm, and, in-turn, the cylinder's effect on the flame stability of a downstream pool fire. To facilitate this investigation, each cylinder was individually flush-mounted on the test section floor of a wind tunnel specifically designed to simulate an aircraft engine nacelle, which operated at a freestream velocity and turbulence intensity of 8.4 m/s and 1%, respectively. Results show a changed in the flow phenomenon between the 10 and 20 mm cylinders, and indicate a possible minimum geometry size.
Advisors/Committee Members: Disimile, Peter.
Subjects: Fluid dynamics
Keywords: Shear Flow; Surface Mounted Obstructions; Boundary Layer; Flame Stability
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21.
DRENSKY, GEORGE K.
EXPERIMENTAL INVESTIGATION OF COMPOSITE MATERIAL EROSION CHARACTERISTICS UNDER CONDITIONS ENCOUNTERED IN TURBOFAN ENGINES.
Degree: PhD, Engineering : Aerospace Engineering, 2007, University of Cincinnati
► The design and development of high performance turbomachinery operating in both ambient…
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▼ The design and development of high performance turbomachinery operating in both ambient and high temperature environment in the presence of solid particles requires a thorough knowledge of the fundamental phenomena associated with particulate flow. Because of the serious consequences of turbomachinery erosion on their performance and life expectancy, it is important to have reliable methods for predicting their erosion when solid particles are ingested with the incoming flow. The ingestion of these solid particles over a period of time will reduce the efficiency of the propulsion system, causing increased fuel consumption and reduction in performance and thrust. Many studies, essential to predicting blade surface erosion intensity and pattern, have been conducted at the University of Cincinnati’s Propulsion Laboratory in the past. The studies and experiments at the (UC) laboratory were performed in order to obtain a better understanding and a more realistic prediction of erosion rates of various conventional materials and coatings, while varying impingement angle, particle velocity, particle concentration, particle size, temperature and other important erosion parameters. Solid particle erosion is a complicated process which becomes even more complicated when it comes to composite material structures. In composite materials the mechanisms of erosion are complex, difficult to determine and even more difficult to predict due to the non-homogeneity of the material. Attempts were made to understand some of the basic mechanisms of erosion as early as the beginning of the (20th) century and continue even today. Over the years most of the attention of scientists was concentrated toward understanding the mechanisms occurring in conventional materials. However, due to the growing potential of composite materials and their desirable properties, they became a more focal point of interest.
Advisors/Committee Members: Tabakoff, Dr. Widen.
Subjects: Engineering, Aerospace
Keywords: Composite materials, Erosion Tunnel, Erosion, Models and Correlations, Jet Engine Performance
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22.
DRENSKY, GEORGE KERILOV.
AMBIENT AND HIGH TEMPERATURE EROSION INVESTIGATION OF MATERIALS AND COATINGS USED IN TURBOMACHINERY.
Degree: MS, Engineering : Aerospace Engineering, 2002, University of Cincinnati
► The design and development of high performance turbomachinery operating in an ambient…
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▼ The design and development of high performance turbomachinery operating in an ambient environment with solid particles require a thorough knowledge of the fundamental phenomenon associated with particulate flows. Because of the serious consequences of turbomachinery erosion on their performance and life, it is important to have reliable methods for predicting their erosion when solid particles are ingested with the incoming flow. The ingestion of these solid particles over a period of time will reduce the efficiency of the propulsion system, causing increased fuel consumption and decrease in thrust. Several studies, which are essential to predicting blade surface erosion intensity and pattern, have been conducted at the University of Cincinnati's Propulsion Laboratory over the past (30ty) years. This particular work describes only some investigation done on erosion behavior of materials and coatings exposed to different types of solid particles, velocities, temperatures, and impingement angles. For the present work the following materials and coatings were evaluated: (AA-22) Ceramic Material, Tungsten Carbide Cobalt (WC-Co) coating, Chromium Carbide Nickel (Cr 3C 2Ni) coating, and Titanium Nitride (TiN) coating. The erosive wear of the samples was studied experimentally by exposing them to particle-laden flow at velocities from (100 to 1000 ft/sec). The studied temperatures were between ambient (70°) and (1400°F) and impingement angles from (15° to 90°) degrees. The erosive particles used for the ambient and high temperature testing were: Runway Sand (100 and 1000 microns), Aluminum Oxide (Al 2 0 3 -27.5 microns), Crushed Quartz (60 microns), Arizona Test Dust (20 and 100 microns), and Silica Carbide (30 microns).
Advisors/Committee Members: Tabakoff, Dr. Widen.
Keywords: erosion; coatings; turbomachinery; erosion tunnel facility; Titanium Nitride protective coats
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23.
ELKADY, AHMED MOSTAFA.
EXPERIMENTAL INVESTIGATION OF AERODYNAMICS, COMBUSTION, AND EMISSIONS CHARACTERISTICS WITHIN THE PRIMARY ZONE OF A GAS TURBINE COMBUSTOR.
Degree: PhD, Engineering : Aerospace Engineering, 2006, University of Cincinnati
► The present work investigates pollutant emissions production, mainly nitric oxides and carbon…
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▼ The present work investigates pollutant emissions production, mainly nitric oxides and carbon monoxide, within the primary zone of a highly swirling combustion and methods with which to reduce their formation. A baseline study was executed at different equivalence ratios and different inlet air temperatures. The study was then extended to investigate the effects of utilizing transverse air jets on pollutant emission characteristics at different jet locations, jet mass ratio, and overall equivalence ratio as well as to investigate the jets’ overall interactions with the recirculation zone. A Fourier Transform Infrared (FTIR) spectrometer was employed to measure emissions concentrations generated during combustion of Jet-A fuel in a swirl-cup assembly. Laser Doppler Velocimetry (LDV) was employed to investigate the mean flow aerodynamics within the combustor. Particle Image Velocimetry (PIV) was utilized to capture the instantaneous aerodynamic behavior of the non-reacting primary zone. Results illustrate that NOx production is a function of both the recirculation zone and the flame length. At low overall equivalence ratios, the recirculation zone is found to be the main producer of NOx. At near stoichiometric conditions, the post recirculation zone appears to be responsible for the majority of NOx produced. Results reveal the possibility of injecting air into the recirculation zone without altering flame stability to improve emission characteristics. Depending on the jet location and strength, nitric oxides as well as carbon monoxide can be reduced simultaneously. Placing the primary air jet just downstream of the fuel rich recirculation zone can lead to a significant reduction in both nitric oxides and carbon monoxide. In the case of fuel lean recirculation zone, reduction of nitric oxides can occur by placing the jets below the location of maximum radius of the recirculation zone.
Advisors/Committee Members: Jeng, Dr. San-Mou.
Keywords: Swirling flow, recirculation zone, emissions, NOx, primary jets, dilution
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24.
ELSHAMY, OMAR M.
EXPERIMENTAL INVESTIGATIONS OF STEADY AND DYNAMIC BEHAVIOR OF TRANSVERSE LIQUID JETS.
Degree: PhD, Engineering : Aerospace Engineering, 2007, University of Cincinnati
► The injection of a liquid jet into a crossflow of air provides…
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▼ The injection of a liquid jet into a crossflow of air provides a means of higher penetration and rapidly mixing liquid fuel and air for combustion applications. The structure of the spray, formed is investigated. To attain this goal, the problem is divided into the following tasks which involve: (1) characterize the penetration, breakup, atomization, mixing, and breakup of liquid jet injected into crossflow at conditions relevant to real engine conditions, (2) establish an understanding of the structure of that transverse jet near the injection point, and (3) study the dynamics behavior of the transverse jet and propose new method to control the transverse liquid jet in crossflow. Two breakup modes have been observed, column and surface breakup. The agreement between the breakup map developed in the present study with the existing ones is quite good. The agreement between the PIV and LDV measurements was good and within 10% accuracy. PIV probe has been proven as a good tool to capture the aero-structure of spray generated by liquid jet in cross flows by comparing its results with the corresponding LDV results. Droplet velocity exhibits a minimum in the spray core. As the momentum ratio increases, the transverse location as well as the droplet velocity of the spray core increase, while the droplet velocity at the outer periphery decreases. Elevating the ambient pressure slightly decreases the penetration and decreases the spray spread. At higher ambient pressure, shorter axial distance is required for the droplet to follow the air flow. Mechanically exciting the transverse liquid jet can have a significant effect on the mixing, spreading, and penetration of the liquid jet in crossflow. The penetration of the jet may increase by more than 40 % while the spread of the jet by 100 % at an axial location of about ten diameters downstream of the injection point. The optimum excitation Strouhal number is about 0.0047, at which homogenous droplet average velocity distribution and maximum interaction between the liquid jet and the crossflow are observed. Novel correlations that describe the outer and inner boundaries of the dynamic jet are developed.
Advisors/Committee Members: Jeng, Dr. San-Mou.
Subjects: Engineering, Aerospace
Keywords: Dynamic Jet, PIV, LDV, Crossflow, Atomozation, LPP, Excitation, Modulation
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25.
FAIDI, WASEEM IBRAHIM.
THEORETICAL INVESTIGATIONS OF THE THERMOELECTRICALLY INDUCED MAGNETIC FIELD IN THERMOELECTRIC METAL MATERIALS CHARACTERIZATION.
Degree: PhD, Engineering : Aerospace Engineering, 2002, University of Cincinnati
► The objective of this study is to demonstrate that the thermoelectric coupling…
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▼ The objective of this study is to demonstrate that the thermoelectric coupling inherent in metal materials can be exploited as a viable mean in characterizing material imperfections in a noncontacting way. These include, but not limited to, inclusions, inhomogeneity, and anisotropy. Essentially all existing thermoelectric techniques are based on the well-know Seebeck effect that is used in thermocouples to measure temperature at the junction of two different conductors. These conventional techniques are solely sensitive to intrinsic material variations regardless of the shape and surface quality of the specimen to be inspected. However, they require a very good metallic contact between the specimen and the reference electrode; therefore the detectability of small material variations and imperfections is limited by the presence of the imperfect contact. On the other hand, the new noncontacting thermoelectric method uses the surrounding intact material as the reference electrode; thus provides perfect interface between the region to be tested and the surrounding material. The first part of this work deals with infinitely extended media and develops theoretical models to predict the thermoelectric magnetic field around the material imperfections. The already available results for conical inclusion geometries are extended to present more complicated-geometry inclusions. The effects of the shape and orientation of the inclusion on the signal magnitude were investigated. Furthermore, the presence of the material anisotropy is introduced and modeled. Results are obtained for the case of holes and inclusions in infinitely extended media. Like most other methods used in nondestructive evaluation, the detection sensitivity of the noncontacting thermoelectric method is limited by unwanted background signal that interferes with, and often, conceals the flaw signal to be detected. The second part of this work is devoted to model such background signal in finite-size specimens. Two sources of the thermoelectric background signal are considered, namely, material inhomogeneity and anisotropy. The predictions of the analytical models resemble the experimental results recorded in the literature.
Advisors/Committee Members: Nayfeh, Dr. Adnan H.
Subjects: Engineering, Aerospace
Keywords: nandestructive evaluation; thermoelectric coupling; inclusions; metal characterization; magnetic detections
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26.
Fang, Kuan-Chieh.
Unsteady Incompressible Flow Analysis Using C-Type Grid with a Curved Branch Cut.
Degree: PhD, Engineering : Aerospace Engineering, 2000, University of Cincinnati
► For an unsteady viscous flow simulation on a two-dimensional body at high…
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▼ For an unsteady viscous flow simulation on a two-dimensional body at high angle of attack, the calculation of unsteady aerodynamic forces acting on the body is influenced not only by the unsteady separated flow near the body but also by the unsteady wake behind the body. To resolve the wake flow behind the trailing edge, an orthogonal C-grid topology with a curved branch cut aligned with the inviscid stagnation streamline is generated using a conformal mapping technique. This permits the desired grid clustering in the wake region and leads to better flow results in that region. The conformal mapping technique also provides analytical Jacobian metrics for the coordinate transformation and an inviscid solution which is useful in initiating the viscous flow of the impulsively started motion. The use of analytical metric coefficients facilitates the direct determination of part of the coefficients in the governing equations without introducing numerical errors. The unsteady two-dimensional incompressible Navier-Stokes equations in generalized orthogonal coordinates are solved using a vorticity-stream function formulation. The analysis also requires coupling of flow circulation in the far field. As a result, the vorticity-stream function formulation introduced in the present study contains the spatially elliptic equation for the disturbance stream function coupled with the temporally parabolic vorticity transport equation. An efficient direct Block-Gaussian Elimination (BGE) technique is used to solve the stream function Poisson problem subject to Neumann and Dirichlet boundary conditions. The vorticity transport equation is solved using the Alternating Direct Implicit (ADI) method. In addition, the Jacobian at the grid points along the curved branch cut is multi-valued and the metric coefficients are found to be discontinuous across the branch cut. Hence, a special finite element interpolation is implemented in the governing equations at those points in order to overcome this discontinuity. To achieve the objective stated above, the unsteady flow over a stationary NACA 0015 airfoil at various angles of attack is selected in the present study.
Advisors/Committee Members: Ghia, Kirti.
Keywords: Computational Fluid Dynamics; Grid Generation
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27.
FU, YONGQIANG.
Aerodynamics and Combustion of Axial Swirlers.
Degree: PhD, Engineering : Aerospace Engineering, 2008, University of Cincinnati
► A multipoint lean direct injection (LDI) concept was introduced recently in non-premixed…
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▼ A multipoint lean direct injection (LDI) concept was introduced recently in non-premixed combustion to obtain both low NOx emissions and good combustion stability. In this concept, a key feature is the injection of finely atomized fuel into the high-swirling airflow at the combustor dome that provides a homogenous, lean fuel-air mixture. In order to achieve the fine atomization and mixing of the fuel and air quickly and uniformly, a good swirler design should be studied. The focus of this dissertation is to investigate the aerodynamics and combustion of the swirling flow field in a multipoint lean direct injector combustor. A helical axial-vaned swirler with a short internal convergent-divergent venturi was used. Swirlers with various vane angles and fuel nozzle insertion lengths have been designed. Three non-dimensional parameter effects on non-reacting, swirling flow field were studied: swirler number, confinement ratio and Reynolds number. Spray and combustion characteristics on the single swirler were studied to understand the mechanism of fuel-air mixing in this special configuration. Multi-swirler interactions were studied by measuring the confined flow field of a multipoint swirler array with different configurations. Two different swirler arrangements were investigated experimentally, which include a co-swirling array and a counter-swirling array. In order to increase the range of stability of multipoint LDI combustors, an improved design were also conducted. The results show that the degree of swirl and the level of confinement have a clear impact on the mean and turbulent flow fields. The swirling flow fields may also change significantly with the addition of a variety of simulated fuel nozzle insertion lengths. The swirler with short insertion has the stronger swirling flow as compared with the long insertion swirler. Reynolds numbers, with range of current study, will not alter mean and turbulent properties of generated flows. The reaction of the spray dramatically changes the gas phase velocity distribution, while the convergent-divergent nozzle strongly affects the spray velocity profiles. The multipoint flow field has a very complicated structure, especially for the flow structure near the swirler exit, where very strong interactions exit among the adjacent swirlers. Multipoint swirler arrays with the recessed center swirler will alter flow structure significantly. There is a short strong central recirculation zone in both co-swirler and counter-swirler recessed arrays, which may increase the operability range of the multipoint swirl-venturi LDI combustor.
Advisors/Committee Members: Jeng, Dr. San-Mou.
Subjects: Engineering, Aerospace
Keywords: Swirling Flow, Axial swirler, Multipoint, LDI, Aerodynamics, NOx
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28.
Galbraith, Marshall Chistopher.
Implicit Large Eddy Simulation of Low-Reynolds-Number Transitional Flow Past the SD7003 Airfoil.
Degree: MS, Engineering : Aerospace Engineering, 2009, University of Cincinnati
► A laminar separation bubble is known to be detrimental to the performance…
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▼ A laminar separation bubble is known to be detrimental to the performance of airfoils operating at low Reynolds numbers (Re < 105). With increasing interest in Micro Air Vehicles (MAV), a clear understanding of the formation and subsequent turbulent breakdown of laminar separation bubbles is required for improved handling, stability, and endurance of MAV’s. A computational investigation of flow past the SD7003 airfoil over the Reynolds number range 104 < Re < 9x104 is presented. This airfoil was selected due to its robust laminar separating bubble and the availability of high-resolution experimental data. A high-order implicit large-eddy simulation (ILES) approach capable of capturing the laminar separation and subsequent three-dimensional breakdown is shown. The ILES methodology also predicts, without change in parameters, the passage into full airfoil stall at high incidence. In addition, computed separation, reattachment, and transition locations, as well as aerodynamic loads generally agree well with experimental data. Finally, a blowing/suction slot positioned near the leading edge was shown to energize the two-dimensional mode and reduced spanwise instabilities of the shear layer. This caused transition to occur further downstream and effectively eliminated the time mean laminar separation bubble.
Advisors/Committee Members: Orkwis, Paul.
Subjects: Aerospace materials; Engineering
Keywords: Large Eddy Simulation; Laminar Separation Bubble; Transitional Flow; SD7003
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29.
GANGWAR, ASHUTOSH.
Source Term Modeling of Rectangular Flow Cavities.
Degree: MS, Engineering : Aerospace Engineering, 2001, University of Cincinnati
► The presence of small cavities has an effect on the primary fluid…
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▼ The presence of small cavities has an effect on the primary fluid flow and should be modeled properly. The goal of this research is to develop a source term module that models the effect of these cavities for gas-path only simulations. In this approach the unsteady effect of the cavity in the solution is modeled by adding source terms to the right hand side of the Navier-Stokes equation without actually having to resolve the complex cavity geometry. This idea increases considerably the numerical efficiency of the scheme by avoiding the computation of small-scale fluid dynamic structures and complex geometric details of the cavity. Unsteady flow phenomena governing both subsonic and supersonic cavity oscillations were studied. The exact cycle for both cases has been presented and compared with those presented by other researchers. These unsteady solutions have been used to determine deterministic source terms. Source terms were then inserted in steady solutions and used to demonstrate that they can be used to model accurately cavity unsteadiness. Also two methods to model these source terms were briefly reviewed.
Advisors/Committee Members: Orkwis, Dr.Paul.
Subjects: Engineering, Aerospace
Keywords: CFD/CAVITY; SUPERSONIC; AEROSPACE
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30.
Garmann, Daniel J.
High-Fidelity Simulations of Transitional Flow Over Pitching Airfoils.
Degree: MS, Engineering : Aerospace Engineering, 2010, University of Cincinnati
► Presented is a high-fidelity, computational study of transitional flow over an airfoil…
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▼ Presented is a high-fidelity, computational study of transitional flow over an airfoil as it is pitched up from an initial zero incidence to 40 degrees at a nominally constant pitch rate, held, and then returned in a similar manner. The Reynolds numbers were chosen to bracket the regions of laminar and transitional flows applicable to prototypical micro air vehicle conditions, 5000 < Re < 40,000. A high-order, implicit large eddy simulation technique was employed to show the degree of fidelity required to capture these highly transitional flows. Two-dimensional analyses examining the effects of Reynolds number and pitch rate were conducted and a discussion is provided. Additionally, the impact of transition and spanwise extent on the flowfield and force histories were explored through three-dimensional, spanwise periodic simulations. These simulations were shown qualitatively to compare extremely well with available experimental observations.
Advisors/Committee Members: Orkwis, Paul.
Subjects: Aerospace materials
Keywords: ILES; compact finite difference; implicit large eddy simulation; perching; pitching airfoils; micro air vehicle
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