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Daniel Guggenheim School of Aerospace Engineering

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End date: 2022 × Start date: 2020 × search.filters.applied.f.isOrgUnitOfPublicationTitle: College of Engineering × search.filters.applied.f.resourcesubtype: Thesis ×

Publication Search Results

Now showing 1 - 10 of 22
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Verification Of Adversarially Robust Reinforcement Learning Mechanisms In Autonomous Systems

2022-12-07 , Seo, Taehwan

Artificial Intelligence (AI) is an effective algorithm for satisfying both optimality and adaptability in autonomous control systems. However, the policy generated from the AI is black-box, and since the algorithm cannot be analyzed in advance, this motivates the performance measurement of the AI model with verification. The performance and safety of the Cyber-Physical System(CPS) are subject to cyberattacks that intend to fail the system in operation or to interrupt the system from learning by modulation of learning data. For the safety and reliability scheme, verifying the impact of attacks on the CPS with the learning system is critical. This thesis proposal focuses on proposing one verification framework of adversarially robust Reinforcement Learning (RL) policy using the software toolkit ‘VERIFAI’, providing robustness measures over adversarial attack perturbations. This allows an algorithm engineer would be equipped with an RL control model verification toolbox that may be used to evaluate the reliability of any given attack mitigation algorithm and the performance of nonlinear control algorithms over their objectives. For this specified work, we developed the attack mitigating RL on nonlinear dynamics by the interconnection of off-policy RL and on-off adversarially robust mechanisms. After that, we connected with the simulation and verification toolkit for testing both the verification framework and integrated algorithm. The simulation experiment of the whole verification process was performed with two different control problems, one is a cart-pole problem from OpenAI gym, and the other problem is the attitude control of Cessna 172 in X-plane 11. From the experiment, we analyzed how the attack-mitigating RL algorithm performed with gain varying specific adversary attacks, and evaluated the generated model performance over the changing environmental parameters.

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Characterization of Non-volatile Particulate Matter in Pressurized Premixed Laminar Jet-A Flames Via Thermophoretic Sampling

2022-08-16 , Manikandan, Sundar Ram

Production and subsequent emissions of non-volatile particulate matter (nvPM) pose a challenge for both optical diagnostics and physical probing, especially at conditions relevant to practical combustors. Key to enabling nvPM mitigation is in-situ optical measurements, particularly laser induced incandescence (LII). However, interpreting the LII signals is challenging. To quantitatively use LII in gas turbines, their measurements must be calibrated and validated against physical nvPM samples. The preferred approach for extracting these physical samples is in-situ thermophoretic soot sampling followed by transmission electron microscope (TEM) imaging. This thesis work deals with the design of a multi-probe thermophoretic soot sampling system capable of extracting nvPM samples in laminar, rich flames of prevaporized jet-A/air premixtures at elevated pressures. The flames under investigation were observed to exhibit thermal-diffusive instabilities, that are responsible for the flame to form corrugated structures. Moreover, these instabilities cause the corrugated flame to exhibit spatio-temporal variations, which exacerbate the challenges in implementing diagnostics. For the soot sampler, a significantly larger sampling time of 125 ms was required to obtain sufficient soot deposition on the TEM grids, which can enhance the extent of restructuring in the deposited soot particles. Visualization of the data through the TEM revealed (i) a wide range of soot particle size varying between 10 – 250 nm; (ii) presence of non-soot organic matters that include (1) fibers, (2) sharp contrasted mineral-like structures, and (3) uniform and porous spherical structures with varying contrast; and (iii) the dominant morphological characteristics of the flame generated soot particles that are indicative of its chemically reactive nature and restructuring. Furthermore, the quantitative results show (i) increasing soot particle size with pressure, and (ii) an increasing-decreasing trend for the mean soot particle size with height above the burner. While the effect of pressure is explained by the enhanced extent of graphitization and maturity in the nanostructures of soot particles at elevated pressures; the dependence on height can be explained through particle agglomeration for the initial increase in size with height, followed by oxidation of the particles respectively. However, considering the range of tested HAB when compared to the flame length, the possibility for inconclusive variation stems from preheat temperature variations and restructuring effects

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Evaluation of Boundary Condition Treatments and Simulation Environments for Improved Near-Body Solutions in Lattice-Boltzmann Flow Simulations

2022-05-23 , Fernandez, Isabel Faith

In this study, different wall boundary conditions and methodologies for improving near-body flow solutions for more complex geometric shapes in a GPU accelerated Lattice-Boltzmann method (LBM) framework were implemented and assessed by comparison to experimental data. Boundary conditions that account for curved geometry, an interpolated bounce-back method, an extrapolation based ghost method, and a unified boundary treatment, were implemented in the current Lattice-Boltzmann framework and the flow around a ROBIN fuselage body was evaluated based on the surface pressure distribution. The boundary conditions were implemented using both no-slip assumptions and slip/moving-wall assumptions. It was found that different types of boundary treatments had little effect on the near-body flow solution, but the slip vs. no-slip assumption had a significant impact on the near-body results. Applying a boundary treatment with a slip assumption, the flow separation expected around the fuselage was captured and the predicted pressures correlated well with experimental data, whereas the no-slip boundary treatment caused the flow separation region around the object to be over-estimated. For both the no-slip and slip boundary treatments, resolution and domain size were found to have little effect on the near-body flow solution in terms of surface pressure distribution. The no-slip boundary conditions, in addition to giving a less accurate near-body flow solution, also showed greater velocity fluctuations and more turbulent energy downstream, indicating that the wall treatments at the fuselage also have an effect on the flow field further downstream. The GPU accelerated LBM was found to have a significantly lower computational expense than the higher-fidelity Helios solvers being compared against.

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Investigating Lean Blowout of an Alternative Jet Fuel in a Gas Turbine Combustor

2022-01-05 , Narayanan, Vijay

In the global effort to reduce the climate impact of combustion emissions, sustainable aviation fuels offer the ease and reliability of conventional petroleum-derived jet fuels without the significant pollutant effects. Ongoing research efforts include experimental testing of alternative jet fuels to identify fuel candidates that produce less pollutant combustion products and are cheaper and environmentally cleaner to source than conventional jet fuels. Fuel lean combustion already reduces the emissions of jet engines and increases fuel efficiency, but lean blowout (LBO) can occur at reduced throttle and minimum power scenarios such as descent. Lean blowout (LBO) has been identified as a critical figure of merit to ensure the stability of alternative jet fuels in the place of conventional fuels. This work aimed to further understand the LBO phenomenon, leveraging computational studies of the alternative fuel designated C-5 by the National Jet Fuel Combustion Program (NJFCP). The fuel sensitivity of LBO has been established by the NJFCP’s participants recently. In this thesis, the chemical kinetics for C-5 is first verified using zerodimensional (0-D) and one-dimensional (1-D) studies and then this is followed by three dimensional (3D) large-eddy simulations (LES). In LES to observe LBO, a direct-step and gradual equivalence ratio reduction were separately employed to assess fuel sensitivity of LBO against available experimental data. The time histories of pressure, temperature, and composition were analyzed for precursor signatures of LBO both inside and outside the flame. Localized extinction, a reduction in the vortex breakdown bubble size and magnitude, and a reduction in the exhaust velocity were all observed to occur during the LBO event.

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An analysis on the application of algebraic geometry in Initial Orbit Determination problems

2022-12-01 , Mancini, Michela

Initial Orbit Determination (IOD) is a classical problem in astrodynamics. The space around Earth is crowded by a great many objects whose orbits are unknown, and the number of space debris is constantly increasing because of break-up events and collisions. Reconstructing the orbit of a body from observations allows us to create catalogs that are used to avoid collisions and program missions for debris removal. Also, comparing the observations of celestial bodies with predictions of their positions made based on our knowledge of the universe has been in the past, and is still today, one of the most effective means to make improvements in our cosmological model. In this work, a purely geometric solution to the angles-only IOD problem is analyzed, and its performance under various scenarios of observations is tested. The problem formulation is based on a re-parameterization of the orbit as a disk quadric, and relating the observations to the unknowns leads to a polynomial system that can be solved using tools from numerical algebraic geometry. This method is time-free and does not require any type of initialization. This makes it unaffected by the problems related to the estimate of the time-of-flight, that usually affects the accuracy of the solution. A similar approach may be used to analyze the performance of the solver when streaks are used, together with lines of sight, as inputs to the problem. Streaks on digital images form, together with the camera location, planes that are tangent to the orbit. This produces two different types of constraints, that can be written as polynomial equations. The accuracy and the robustness of the solver are decreased by the presence of streaks, but they remain a valid input when diversity in the observed directions guarantees the departure from the singular configuration of almost coplanar observations.

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Reshock Gas Curtain Mixing Study

2022-06-25 , Risley, Karl Robert

The current work investigates the behavior of gas curtain instabilities. A gas curtain can be visualized as an A − B − A domain, where A and B are light and heavy fluids respectively, creating a ”curtain” of heavy fluid B that is surrounded by a light fluid A. Specifically, the behavior of gas curtains following an initial shock passage and the passage of a reflected shock (reshock) through the entirety of the curtain are investigated. A gas curtain instability commonly occurs physically in a wide range of applications such as during afterburning of an explosion, inertial confinement fusion, and even supernovae explosions. Previous studies have emphasized that the physics occurring during the reshock of a gas curtain are far more complex than the behavior of a re-shock Richtmyer-Meshkov Instability, due to the interactions between the two interfaces and wave reverberations occurring. The current work attempts to understand the relationship between a gas curtain’s initial conditions and its behavior to reshock through two-dimensional numerical simulations that utilize the viscous Navier-Stokes equations. More specifically, the current work isolates the effects of the curtain’s initial thickness and shape on the post reshock mixing layer growth rate and molecular mixing of the curtain. The results for all cases indicate that the post-reshock growth rate of the curtain’s width is a function of initial thickness. The sensitivity of the curtain’s post-reshock growth rate to the initial thickness, however, depends on the curtain’s initial perturbation shape. As the initial thickness of the curtain is decreased, the interactions between the curtain’s interfaces grow in strength and impede perturbation growth, thus reducing the post reshock growth rate of the curtain’s structure width. Similarly, the results strongly suggest that a reduction in initial curtain thickness increases the late-time asymptotic molecular mixing fraction value. This result is significant, especially for reacting flows, because it indicates that faster combustion (or afterburning in an explosion) could be reached with the thinning of the gas curtain in flow systems.

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UNCERTAINTY QUANTIFICATION OF DIMP PYROLYSIS KINETICS

2022-05-13 , Patel, Pavan

To develop effective explosives and strategies for the rapid destruction of sarin stockpiles, a reliable understanding of sarin’s chemical kinetics is needed. Kinetic mechanisms of sarin simulants such as di-isopropyl methyl phosphonate (DIMP) are developed instead because they have a similar chemical structure as sarin and are less toxic. A detailed DIMP kinetics mechanism has been developed in the past; however, there is a considerable amount of uncertainty surrounding it. This uncertainty manifests through the choice of pathways, and their respective reaction rates, leading to large variations in outcomes predicted through simulations. Out of the many reaction pathways involved in the decomposition of DIMP, the initiating steps are the most crucial. Out of the two possible initiating pathways in the destruction of DIMP, the lower activation energy pathway is dominant for all temperatures. The purpose of this study is to investigate the uncertainties associated with the dominant initiating pathways of the DIMP kinetics mechanism. Propagating rate parameter uncertainties of the dominant pathway through computational models yields large uncertainties in predicting DIMP survivability at different temperatures. The prediction uncertainties are larger at lower temperatures than at high temperatures. This can significantly impact the ability to precisely predict collateral damage caused by partially destroyed DIMP in the far-field of an explosion. After reducing these rate parameter uncertainties, using Bayesian inference, the prediction uncertainties were within reasonable limits. The results here provide a reduced subspace for uncertainties associated with the first and most important step in the breakdown of DIMP, which shall enable more reliable predictions.

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Numerical Simulations of the HVAB Rotor in Hover

2022-11-29 , Mali, Hajar

Numerical simulations of compressible viscous flow over the Hover Validation and Acoustic Baseline (HVAB) rotor in hover are presented. The commercial flow solver, ANSYS Fluent, has been employed. The effects of transition are modeled using the Langtry-Menter k-ω SST-γ-Reθ model at three different pitch settings. Comparisons with HVAB test data and other publications are discussed. These include integrated thrust and power coefficients, figure of merit, velocity inflow, surface pressure distribution, lift distribution, transition locations, and vortex structures.

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High-Speed, Low-Power, Low-Profile Design Fiber-Optic Communication System for CubeSat

2022-06-08 , Kotani, Kohei

Today, the demand for big data, such as high-resolution images, has been rapidly increasing in space missions. However, the means to achieve multi-Gbps transmission is limited to ethernet, coax, or FFC in CubeSat design. This research describes the development of a lightweight and low-power consumption high-speed communication system suitable for small satellites. A high volume of data from two high-resolution cameras is transmitted to a Raspberry Pi Compute Module 4 running Linux using a fiber-optic link as an interconnect, and the dual images are displayed on a monitor. The FPGA with a high-speed transceiver is extensively used to achieve high-speed communication. It is also verified that the fiber-optic module operates at up to 6.25 Gbps with a power consumption of 90 mW. This research includes the hardware and software development details. All the materials, including the schematics, PCB design, and programming codes, can be found in the Github repository. Furthermore, this thesis includes the discussion of fiber-optic module usage in the space environment and comparing fiber-optic with ethernet, coax, and FFC, along with the selection guides CubeSat developers can refer to. The final deliverable of this research is the high-speed fiber-optic interconnection designed to fit into a CubeSat platform, demonstrating the dual-image display from two HD cameras. The prototype can be extended to implement high-volume data applications such as stereo imaging for proximity operations, free-space inter-satellite links, and high-speed intra-satellite communications for CubeSat platforms.

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Conceptual Effectiveness-Based Hypersonic Evaluation (CEBREN)

2022-05-03 , Van Der Linden, James C. A.

For decades, the United States has largely been uncontested in its quest to advance its national interests in every domain – to protect the American people, promote prosperity, preserve peace, and advance American influence. To maintain technological superiority, the National Security Strategy calls upon the military to field new capabilities that clearly overmatch US adversaries in lethality. Furthermore, the US military has identified hypersonics as an area of interest to stay competitive on the global stage. Hypersonics have been around for over 70 years ranging from the X-20 to the Space Shuttle; however, these projects were products of the traditional design-build-test methodology which often never saw flight. This design-build-test methodology is unable to meet the demands of technological growth and complexity and often drives up costs and overruns. Thus, there is a need to develop a new methodology for assessing hypersonic weapon capability rapidly to support interactive decision making for conceptual development. Hypersonic conceptual design distinguishes itself from traditional aircraft design because the disciplines that must be considered are highly coupled and tightly integrated which drastically increases design risk due to sources of uncertainty. Additionally, it is difficult for engineers to evaluate revolutionary designs because the historical data necessary to perform initial analysis likely is unavailable. Due to this uncertainty, conceptual design is critical because the decisions made have profound ramifications throughout the entire process. To address this uncertainty, physical experiments are required to provide the highest quality of data; however, they are extremely limited in scope and expensive. Hence, there is a need to make well informed decisions at the conceptual design level when designing novel hypersonic vehicles. Due to the coupling of disciplines within hypersonic conceptual design, a Multidisciplinary Design Analysis and Optimization (MDAO) environment was used to design novel hypersonic vehicles. To aid in evaluating these alternatives, agent-based modelling was used to study the effectiveness of the vehicles through operational analysis (OA). By integrating an MDAO environment with an OA framework, novel hypersonic vehicles were constructed, and their capabilities assessed through a process known as effectiveness-based design (EDB). Within EBD, the design objective is shifted from performance metrics (e.g., weight, range, etc.) to effectiveness metrics (e.g. targets killed, survival, etc.) which allows decision makers to consider and understand the implications of design-space-limiting decisions earlier in the process. This shifts away from over-defining requirements before exploring potential best solutions to the problem. Thus, the purpose of this thesis presents a new methodology to address the need of designing and rapidly assessing hypersonic capability to better inform the decision maker through the integration of OA within an MDAO environment thereby closing the loop by coupling the effectiveness to vehicle design parameters.

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