Doctor of Philosophy with a Major in Aerospace Engineering

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Publication Search Results

Now showing 1 - 10 of 1161
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    An Experimental Characterization of a Multi-Element Lean Premixed Pre-Vaporized Combustor for Supersonic Transport Applications
    (Georgia Institute of Technology, 2023-12-12) Passarelli, Mitchell Louis
    Recent trends have driven a re-emergence of research and development in aircraft engines for commercial supersonic transport (CST). Despite the vast body of literature that exists for gas turbine combustors operating at conditions relevant to conventional subsonic flight, there is little to validate extensions of this knowledge to the conditions encountered by CST engines. Further complications arise from advanced combustor designs that involve multiple different flames or flow devices. The interactions of such combustor elements can lead to individual behaviours that differ from that of single elements. The existing literature on flame and flow interactions is focused on conditions relevant to the operation of conventional, subsonic aircraft engines. While such works provide a baseline understanding of the physical phenomena involved in such interactions, they do not necessarily predict the behaviours exhibited by different combustor configurations and/or at different conditions. Some recent studies have employed numerical simulations to determine the characteristics of various combustor schemes, including lean direct injection and lean premixed pre-vaporized (LPP) designs. These studies are limited by the lack of empirical data for validation and model development. The work presented herein aims to characterize experimentally the flow field, flame dynamics and operating limits of a multi-element LPP combustor operating at CST-relevant conditions. Simultaneous laser and probe-based diagnostics were employed to obtain measurements of pollutant emissions, flow velocities, heat release rate, fuel-air mixing and thermoacoustic dynamics. The effects of combustor inlet pressure, temperature and fuel-air ratio are studied via corresponding parameter sweeps. Numerical chemistry simulations provide estimates of relevant flame properties, complementary to the experimental results. A second set of experiments investigated the forced response of the combustor. Overall, the results presented in this thesis demonstrate the importance of flame and flow interactions. In particular, the interactions of the pilot flame with neighbouring main flames are found to be critical in determining the stable operating range of the combustor. Furthermore, the pilot is found to dominate the dynamics of the combustor at forced and unforced conditions. Empirically-computed flame transfer functions at different forcing frequencies show that the pilot is most sensitive to acoustic perturbations and that this sensitivity is enhanced by interactions of the pilot with the main flames. This work also demonstrates the viability of LPP combustors for CST applications in three aspects. First, the pollutant emissions characteristics of the combustor studied are in line with future emissions targets. Second, the mean flow field, flame and dynamical characteristics do not vary strongly with operating conditions or undergo sudden or unexpected bifurcations, except when exceeding blowoff limits. A Damköhler number (Da)-based blowoff analysis shows that this combustor design exhibits enhanced stability compared with previously reported bluff-body stabilized flames. The analysis itself also demonstrates the robustness of a simple Da correlation for blowoff prediction, which works for a complex geometry such as the one studied in this work.
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    Learning, sampling and inference with stochastic differential equations
    (Georgia Institute of Technology, 2023-12-06) Zhang, Qinsheng
    Stochastic differential equations (SDEs) constitute a formidable tool for modeling the dynamics of continuous-time stochastic processes, and offer a natural framework for the probabilistic modeling of high-dimensional data. Consequently, they have garnered increasing attention in generative machine learning. Despite their promise, the applications of SDEs in machine learning have been limited due to the lack of scalable learning approaches that can train flexible neural networks to approximate stochastic processes, and the difficulty of conducting tractable inference and sampling caused by inefficient SDE solvers. In this dissertation, I outline my efforts to develop novel computational models capable of efficient and scalable learning, sampling, and inference from SDEs. Specifically, I introduce several approaches to learning SDEs for probabilistic modeling, including fitting non-linear forward and backward SDEs with neural networks and learning with limited data. Next, I present a novel deep model designed to learn SDE dynamics while satisfying given constraints on the marginal probability of the SDE. Furthermore, I develop an efficient algorithm for drawing samples from high-dimensional SDEs, which proves effective in generating diverse and high-fidelity data, such as realistic images and videos.
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    Investigation of Explicit Residual Filtering for LES of the Compressible Navier-Stokes Equations
    (Georgia Institute of Technology, 2023-12-06) Theuerkauf, Scott William
    Large Eddy Simulation (LES) is a useful tool for modeling turbulent, non-statistically-stationary flow without resolving the entire range of turbulent scales. While it is frequently calculated using the implicit filter applied by the numerical discretization used to solve the equations of flow, another form of LES uses an explicitly-defined filter to control the range of scales present in the flow. This method seeks to decouple the LES solution from the grid and numerical method in order to effectively eliminate competition between the required subfilter-scale models and the numerical discretization errors. Several methods for Explicitly Filtered LES (EFLES) have been attempted in the past with this goal and others in mind. This work uses a familiar method, grounded in novel derivation from the Navier-Stokes equations governing compressible, reacting flows. By applying a discrete, sufficiently commuting, low-pass filter that is sufficiently sharp and wide relative to the filtering effects of the underlying numerical scheme to the numerical residual of each equation, the effects of the implicit filter can be minimized, overriding them with those of the explicitly-defined filter. Additionally, by applying the filter once to each equation, the cost of successive filter operations is minimized, reducing the cost of EFLES. Finally, the structure of this method allows existing Implicitly Filtered LES (IFLES) numerical methods to be adapted to EFLES in a straightforward and computationally efficient manner. The following work validates this EFLES method on a Taylor-Green Vortex for both incompressible and compressible cases. A stability analysis examines the requirement to apply a residual filter as part of the EFLES method to each conservation equation and explores the stabilizing effect of residual filtering. Identifying the computational costs of this approach in context with its advantages provides the advanced knowledge necessary to achieve grid- and scheme-independent results using EFLES for more advanced flows.
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    Large Deflection Effects on the ERR and Mode Partitioning of the Single and Double Cantilever Beam Sandwich Debond Configurations
    (Georgia Institute of Technology, 2023-12-05) Okegbu, Daniel O.
    The goal of this study is to investigate the effects of large deflections in the energy release rate and mode partitioning of face/core debonds for the Single and Double Cantilever Beam Sandwich Composite testing configurations, which are loaded with an applied shear force and/or bending moment. Studies on this topic have been done by employing geometrically linear theories (either Euler-Bernoulli or Timoshenko beam theory). This assumes that the deflection at the tip of the loaded debonded part is small, which is not always the case. To address this effect, we employ the elastica theory, which is a non-linear theory, for the debonded part. An elastic foundation analysis and the linear Euler-Bernoulli theory are employed for the "joined" section where a series of springs is employed to represent the interfacial bond between the face and the substrate (core and bottom face). The derivation/solution is done for a general asymmetric sandwich construction. A $J$-integral approach is subsequently used to derive a closed-form expression for the energy release rate. Furthermore, in the context of this Elastic Foundation model, a mode partitioning measure is defined based on the transverse and axial displacements at the beginning of the elastic foundation. The results are compared with finite element results for a range of core materials and show very good agreement. Specifically, the results show that large deflection effects reduce the energy release rate but do not have a noteworthy effect on the mode partitioning. Conversely, a small deflection assumption can significantly overestimate the energy release rate.
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    A Method for the Conceptual Design of Integrated Variable Cycle Engines and Aircraft Thermal Management Systems
    (Georgia Institute of Technology, 2023-11-28) Clark, Robert Arthur
    Development efforts for current and future military fighter aircraft are tasked with fulfilling strenuous requirements, many of which are at odds with each other. An increased demand for high power electronics and weapons systems has put the need for auxiliary power generation and heat dissipation on par with the more traditional military aircraft requirements of extended range and speed, stealthiness, and enhanced maneuverability. All of these requirements can be traced back in some way to the propulsion system, which is arguably the single most important subsystem on any aircraft, military or commercial. For decades, the low bypass ratio mixed flow turbofan (MFTF) has been the architecture of choice for the propulsion systems that power military fighter aircraft. However, the competing nature of modern aircraft requirements has begun to highlight the drawbacks of the fixed cycle MFTF, and has led to the development of variable cycle engines (VCEs). Variable cycle engines show promise in increasing thrust, reducing fuel consumption, and improving heat dissipation capability, all of which are critical requirements for military aircraft. There has been a further recognition that thermal management requirements need to be assessed earlier in the conceptual design phase in concert with the propulsion system, given that current aircraft such as the F-35 struggle to meet heat dissipation requirements. Unfortunately, the existing conceptual cycle design methods used to select engine cycles were not developed with variable cycle engines in mind. The objective of this research is to enhance conceptual design-level modeling methods for integrated design of variable cycle engines and thermal management systems in order to better achieve aircraft-level mission requirements. The key difference between a variable cycle engine and a traditional fixed cycle engine is the presence of variable geometry features whose positions are modulated specifically to move air between the different streams in the engine. A method of variable cycle engine design is presented that accounts for these variable geometry as a means of aiding the propulsion system designer during the conceptual design phase of the propulsion system. A series of research questions, hypotheses, and experiments that build on each other are posed in order to address the need for conceptual cycle designers to better understand how variable cycle engines impact the cycle design process, especially in the context of integrated propulsion and thermal management systems. The first research question and experiment address the need to determine optimum variable geometry positions for off-design analysis of a variable cycle engine throughout the complete flight envelope of a fighter aircraft. Existing design methods require an optimizer to determine variable geometry position targets at every off-design operating condition used during aircraft mission analysis, which, for refined mission analysis methods can be hundreds or thousands of off-design points. This results in significant cost due to the repeated use of the optimizer. This thesis develops a method for determining variable geometry schedules, which can be generated cheaply with only a small number of optimizer calls, and then used in place of the optimizer during off-design evaluation of the variable cycle engine. The use of variable geometry schedules during the off-design process is shown to significantly reduce the computational cost of off-design analysis of variable cycle engines. The second research question and experiment examine the design process for variable cycle engines and incorporate the use of the variable geometry schedules directly into the engine design process. Current design methods in the literature utilize nested optimization techniques in order to determine the optimum positions of variable geometry features during the design process. The method in this thesis takes the variable geometry schedules, shown in the first experiment to be useful for off-design analysis, and incorporates them directly into an engine design loop. The use of variable geometry schedules during the design process is shown to reduce the overall number of required engine design iterations by two orders of magnitude, relative to current design methods in the literature. The third research question and experiment address the need to assess the impact of integrating a thermal management system into the design of the variable cycle engine. The literature is sparse on how incorporating the design of a thermal management system directly into the engine design process impacts the selection of the design cycle for a variable cycle engine. This thesis demonstrates how design integration of the engine and thermal management system shifts the location of the optimal cycle within the cycle design space of a variable cycle engine. Furthermore, the utility of variable geometry schedules is demonstrated through a cycle design scenario, where schedules that minimize fuel burn or maximize heat dissipation capability for the aircraft are shown to lead the cycle designer to different locations in the optimized cycle design space. The design methods utilized for each of these experiments are synthesized into an overall conceptual design method called PREHEAT-V (Preliminary/Conceptual Design Method for Handling Heat and Aircraft Thermal Management in Variable Cycle Engines), which incorporates variable geometry optimization techniques directly into a multiple design point cycle design process. The PREHEAT-V design method allows cycle designers to evaluate large candidate variable cycle engine design spaces in a computationally efficient manner, and assess the impact of heat dissipation requirements on the optimum design cycle. The PREHEAT-V method emphasizes evaluating aircraft-level mission requirements, rather than engine-level requirements, since the ultimate barometer of success for military aircraft is mission capability, not engine capability.
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    A Kinetic Energy Preserving and Entropy Conserving Scheme for Stable Simulation of Fluid Flow
    (Georgia Institute of Technology, 2023-10-31) Schau, Kyle Arthur
    This work contributes to the non-linear stability of fluid flow simulation by indirectly enforcing entropy evolution through the numerical internal energy flux. Existing numerical methods are improved and extended to a fully discrete numerical method for solving the Euler equations. The presented scheme preserves the accurate evolution of kinetic energy and is entropy conservative. The developed scheme demonstrates improved stability and accuracy over existing methods by formulating the numerical internal energy flux as a temperature weighted average. The presented scheme is extended to multicomponent simulations and demonstrates improved entropy properties over existing methods; however, numerical mixing rules prevent desired entropy conservation. Finally, a shock capturing extension of the presented scheme is designed from kinetic energy preserving and entropy stability principles. The scheme demonstrates the ability to resolve shocks and provides a novel method for prescribing entropy dissipation through the internal energy dissipation.
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    An Integrated Framework for Evaluating Commercial Supersonic Aircraft Design Trade-offs and Operational Constraints
    (Georgia Institute of Technology, 2023-08-25) Wen, Jiajie
    Ever since the Concorde performed its final flight in 2003, the world might finally see a new commercial supersonic transport (SST) by the end of the decade. Although the COVID-19 pandemic has significantly impacted the commercial aviation industry, an SST could provide operators with the opportunity to offer unique services and differentiate themselves from competitors when the industry recovers. A civil supersonic aircraft can greatly boost the productivity of onboard passengers by significantly reducing trip time. However, this benefit comes at the expense of additional fuel consumption and en-route noise. Most countries prohibit civil supersonic overland flight due to the disturbance of sonic boom, and such restriction is not likely to be lifted for a large commercial supersonic aircraft to cruise over land at full supersonic speeds. By analyzing the performance characteristics of SSTs, as well as the commercial aviation flight network and market demand, it becomes obvious that SSTs should be regarded as specialty products. Traditional aircraft design is driven by a fixed set of design requirements. These requirements are imposed during aircraft sizing in the conceptual design stage and followed by appropriate network and operations analyses. Due to the relatively limited use cases of an SST, conducting network-level operational analysis can greatly inform the definition of design requirements (such as supersonic cruise Mach number and design range). Furthermore, operational considerations such as limitations on overland cruise Mach number and en-route sonic boom propagation can both have direct impact on the success of future commercial supersonic operations. This research does not take into account low-boom designs, as they are improbable choices for larger commercial supersonic jets. Instead, this thesis attempts to address the lack of feedback between conventional SST design requirement definition and its network as well as operations. The research consists of three main steps: • Improving the current supersonic flight routing capability (based on rasterized search algorithm) by including aircraft mission analysis and sonic boom carpet estimation. • Creating a network simplification technique that simplifies a forecasted supersonic flight network in 2050 while retaining its underlying structure. • Using the developed flight routing capability and network simplification technique to evaluate the impact of different mission performance requirements and operational constraints.
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    Mechanical Metamaterial Lattices via Direct Methods
    (Georgia Institute of Technology, 2023-08-23) Gloyd, James Todd
    Discrete lattices use individually manufactured unit cell building blocks, which are then assembled to form large lattice structures, the quality of which is not significantly influenced by the scale of the structure. Furthermore, the size of the final structure is not bounded by the footprint of the manufacturing equipment. Tuning the elastic behavior of materials and structures provides the possibility for significant improvements to overall performance, as demonstrated by structural and topology optimization studies. Similar improvements can be made in discrete lattice applications, as shown by the Coded Structures Laboratory at NASA. Improvements made to performance of discrete lattice structures are, so far, limited by the lack of a systematic, direct method to dictate the behavior---that is, prescribe the deformation---of the final structure. Here we present a direct method of prescribed structural behavior integrating structural and topology optimization, for both discrete lattice structures and general structures. Also presented are formulas and methods for calculating the determinant and inverse of a linear combination of matrices, which originally stemmed from the development of prescribed behavior methods however, while applicable to prescribed deformation problems, are much more useful in other situations. The direct methods of prescribed deformation presented here automatically produce dictated behavior from the candidate structure when possible and produce an approximation when the desired behavior is impossible. These methods are shown to move towards a minimizer with quadratic convergence, with improved results in situations with fewer limits on the prescribed behavior. Additionally, the presented formula for calculation of the determinant of a linear combination of matrices provides exact results in as little as one tenth of the time of traditional approximation methods, and the exact inverse of the linear combination is calculated in as little as one quarter of the time of traditional exact methods. We show these formulas provide significant computational and conceptual improvement to current methods and provide unmatched performance in parallel computing settings.
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    Integrated Framework for Aircraft Design and Assembly Tradeoffs
    (Georgia Institute of Technology, 2023-08-15) Huynh, Dat
    Aircraft passenger traffic is expected to increase and lead to demand for 40,000 new aircraft by 2040. Aircraft production rates have been rising to meet this demand, but delivery backlogs are growing at even faster rates. Large backlogs can lead to missed deliveries, canceled orders, and traffic congestion due to too few planes for too many passengers. This comes at a time when the two primary aircraft manufacturers, Boeing and Airbus, are competing for dominance in a market that a third competitor, Comac, is poised to enter as well. Increased production rates to better meet customer demand would thus also allow one of them to gain the edge over the others. Aircraft production rates must be increased and done so without inducing enormous costs to meet passenger demand and to stay competitive. Changes to aircraft assembly, which constitutes up to 50% of total production time and up to 30% of total production cost, during the design process can address this. Current Design for Assembly methods addressing assembly changes during the design process range from Product Lifecycle Management techniques to various methods in Systems Engineering and have been used to great success. However, few such methods consider aircraft design in their analysis, which would enable further tradeoff capabilities and greater production rate and cost improvements. Those methods that do explicitly incorporate aircraft design analysis alongside the assembly analysis insufficiently consider several key assembly aspects such as assembly sequence planning (ASP) and more detailed assembly line balancing (ALB), which can be used to optimize the assembly line. This work establishes a better connection between the aircraft design and assembly disciplines and joins them by accounting for geometry and material factors common to both using ASP and ALB. First, the correct analysis fidelity for the aircraft design process and ASP's geometric reasoning process is determined to allow geometry data to easily flow between the two, linking them. Then, the ASP and ALB analyses are combined and augmented to account for the novel materials traded during aircraft design and the manufacturing processes used to make them. Afterwards, the most promising assembly sequences are optimized for using metrics representative of both ASP and ALB so that sub-optimal assembly sequences are not line balanced, reducing the overall problem size to explore the large design space more efficiently. From all this an integrated aircraft design and assembly framework is made that strives to obtain higher production rates, lower costs, and better tradeoffs by leveraging the additional feedback loops produced via consideration of variables common to both disciplines. Finally, this framework is tested and compared with a state-of-the-art framework on a representative aircraft's wingbox and its production system. The developed framework demonstrates it is able to obtain significantly higher throughput and lower cost values by simultaneously: sizing the aircraft to meet its performance requirements; accounting for the aircraft's geometry via ASP determining assemblability and ALB determining the consequent task time, cost, and space requirements; incorporating the aircraft's material system via usage of specialized sequences in ASP and identification of optimal line balances in ALB given the material's manufacturing process and its subsequent resource requirements; and flowing all this manufacturing information back upstream to maximize the aircraft's manufacturability during its sizing. The developed framework is thus able to make tradeoffs such as what size the aircraft should be for a given performance, throughput, and cost requirement, what the maximum production rate is given a design, material, manufacturing process, and spatial constraint, and what costs are incurred given a desired production rate. This provides the designer with a greater understanding of the problem and its constraints and allows them to see what factors can help them increase production rate as well as what the associated costs are.
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    Assessment and Analysis of Turbulent Flame Speed Measurements of Hydrogen-Containing Fuels
    (Georgia Institute of Technology, 2023-08-14) Johnson, Henderson
    Global efforts to reduce greenhouse gas emissions and achieve a carbon-neutral economy have spurred the exploration of integrating hydrogen into various aspects of the global energy infrastructure. This can involve incorporating hydrogen into existing power generation applications or utilizing fuels with significant hydrogen content, such as syngas. However, the introduction of hydrogen poses significant challenges due to its potential to greatly impact the combustion process, with many aspects of its behavior not yet fully understood under practical gas turbine operating conditions. This thesis aims to investigate the influence of thermodynamic, fluid mechanic, and fuel factors on the turbulent global consumption speed, ST,GC, across different fuel types containing up to 90% hydrogen. This parameter represents the average rate of conversion of reactants to products relative to a specific iso-surface. The presented database encompasses three distinct fuel types: H2/CO, H2/CO/CH4/N2, and H2/CH4¬, which represent fuels that are either commonly encountered in practical applications or are of interest for future applications. The latter two fuels are new to the overall Georgia Tech database of turbulent flame speed measurements which increase the amount of high pressure data (up to 20 atm), and add data at preheat temperatures up to 500 K. The addition of this data is of great importance as it allows for further exploration of thermodynamic and fuel effects on ST,GC¬. The analysis of this database reveals several key findings. Firstly, regardless of whether the unstretched laminar flame speed, SL,0, is held constant, higher pressures lead to an increase in ST,GC across all fuel types. The preheat temperature is also shown to increase ST,GC, but when normalized by the laminar flame speed, it demonstrates a decrease. Moreover, the effects of hydrogen addition in H2/CO and H2/CO/CH4/N2 fuel blends are more pronounced compared to those in H2/CH4 fuels. Building upon prior studies that link these observations to mixture stretch sensitivity, the database is analyzed within the framework of a quasi-steady leading points concept model. In this framework, the maximum stretched laminar flame speed, SL,max, serves as the normalizing parameter. This approach proves effective for the H2/CO fuels discussed in this work, as it captures fuel effects at a fixed pressure and preheat temperature. However, a notable limitation arises in its inability to account for systematic differences in pressure and preheat temperature, indicating the need for a second correlating parameter. To identify this second parameter, a systematic investigation of three additional dimensionless numbers, namely the turbulent Reynolds number, Ret, time scale ratio, and acceleration ratio, is presented. Each of these numbers represents a different physical phenomenon that could potentially account for the observed variation in the data reported. The addition of Ret was considered in prior work; however, we identify that is insufficient as an appropriate scaling number due to its inconsistent correlation with preheat temperature. The acceleration ratio was introduced as a novel means of attempting to capture the ability of a flame to accelerate relative to the flow field. Similar to the Reynolds number, this approach showed limited ability to capture both pressure and preheat temperature effects; nevertheless, it does offer a new way to think about turbulence-flame interactions. Ultimately, the time scale ratio emerges as the optimal second correlating parameter due to its lesser degree of scatter compared to the acceleration ratio. This finding is significant, as it aligns with prior analyses that incorporated the time scale ratio to quantify non-quasi-steady chemistry effects at the leading point and demonstrates its promise as an appropriate scaling approach across a wide variety of conditions.