DSpace@RPI
DSpace@RPI is a repository of Rensselaer Polytechnic Institute's theses and dissertations which are available in digital format, largely from 2006 to present, along with other selected resources.
Recent Submissions
Item Tour length estimation guided vehicle routing(Rensselaer Polytechnic Institute, Troy, NY, 2025-12)The Vehicle Routing Problem (VRP) is a fundamental challenge in logistics, with most state-of-the-art heuristics relying on operators that directly manipulate explicit route sequences. This work proposes a heuristic method based on the 2-Stage Assignment-Routing formulation, which reframes the VRP as the problem of first partitioning customers among vehicles and then solving for the routes. The primary challenge of this formulation is the intractability of evaluating an assignment's cost, which requires solving a Traveling Salesman Problem (TSP). We address this challenge by replacing the exact TSP cost with a fast tour length estimator, creating a tractable objective function to guide a metaheuristic search. We evaluate a variety of existing estimators and introduce a novel Geometrically Assisted Regression Tree (GART), which employs machine learning to predict the TSP-to-MST cost ratio using a rich set of spatial and topological features. A comprehensive benchmark demonstrates that GART is significantly more precise than existing estimator models across a wide range of TSP instances. When integrated into a powerful VRP metaheuristic, the estimator-driven search improves upon initial solutions generated by constructive heuristics. However, a local optimizer of the estimator is not always consistent with the true VRP objective function. Our numerical experiments demonstrate that high-quality VRP solutions can be obtained by guiding the second stage with TSP tour length estimations rather than solving the full TSP directly. The effectiveness of this approach, however, depends critically on the precision of the estimation method: while reliable estimators enable strong performance, medium- to large-scale errors can cause the search to diverge from the true objective.Item Stochastic functional time-series modeling for aerospace systems: regularization and bayesian methods towards robust state estimation(Rensselaer Polytechnic Institute, Troy, NY, 2025-12)Future intelligent aerospace structures and aerial vehicles will possess the ability to "feel," "think," and "react" instantaneously through advanced state estimation, awareness, and self-diagnostic capabilities. To achieve this, a structural health monitoring (SHM) system is crucial, providing the aircraft with vital structural insights into both damage states and environmental and operational conditions. Current developments in vibration-based stochastic SHM methods have demonstrated robust and accurate structural state estimation results on lab-scale coupons and elements. However, the extension of vibration based methods to complex structural components remains an open field of study. As such, a thorough study is needed to address the challenges of upscaling current vibration based SHM methods to complex structures: 1) modeling structural dynamics under uncertainty, 2) formulating probabilistic state estimations, 3) alleviating ill-posedness in inverse estimations, and 4) formulating statistical assessments for the performance of the methodology. Serving this purpose, this study proposes a novel vibration-based stochastic SHM method based on the Vector-dependent Functionally Pooled Stochastic Time Series (VFP-STS) model, addressing the need for a data-driven stochastic model of the structure under varying conditions. A novel regularized stochastic system identification framework is developed for the VFP-STS model, introducing sparse regularization through the Least Absolute Shrinkage and Selection Operator (LASSO) and Adaptive LASSO (ALASSO) penalties to the Weighted Least Squares (WLS) model estimation process. The Extended Regularized Information Criterion (ERIC) is introduced as a robust criterion for model structure selection. The proposed framework enables optimized functional basis selection and consistent estimation of model parameters while accounting for heteroskedasticity. Model parsimony and robustness are improved under uncertainty. As a result, ill-posedness in the inverse damage state estimation is reduced, leading to robust state estimation. Subsequently, the Cram\'er-Rao Lower Bound (CRLB) is developed and used as a statistical criterion to assess the best achievable estimator performance. The inverse estimation of structural states is formulated as an uncertainty quantification (UQ) problem, where the comprehensive distributions of structural state parameters are inferred via the Monte-Carlo Markov Chain (MCMC) algorithm with the Adaptive Metropolis (AM) method. This approach allows for the quantification of out-of-sample uncertainty, increasing the robustness of the estimator when performing online diagnosis. Additionally, multiple prior selection strategies are proposed to circumvent the effect of globally ill-posed inverse estimation by leveraging locally well-posed convex regions within the inverse problem setup. The proposed framework is evaluated thoroughly via simulated Monte-Carlo studies, which demonstrate the model selection consistency, asymptotic statistical characteristics, and modal reconstruction accuracy. Additionally, the framework is evaluated in modal vibration experiments and wind-tunnel experiments. In general, the obtained results demonstrate the improvement in accuracy and robustness by applying ALASSO regularization and Bayesian inversion to the SHM framework centered on the VFP-STS model, constituting the first step towards up-scaling the vibration-based SHM to complex structural components.Item Characterization of local anisotropic swelling and fragmentation at the top of metallic nuclear fuel slugs and its significance on fuel performance(Rensselaer Polytechnic Institute, Troy, NY, 2025-12)U-Zr and U-Pu-Zr metallic fuel alloys have been widely studied as a possible fuel type for sodium fast reactors and other liquid metal cooled reactor concepts. Major experiments such as the Experimental Breeder Reactor II (EBR-II) and Fast Flux Test Facility (FFTF) subjected metallic fuels to extensive irradiation campaigns to optimize metallic fuel designs and compositions. During the operation of these experiments radiography of metallic fuel pins showed the formation of a highly porous, irregularly shaped structure at the top of the fuel pins. This structure has been given the name metallic fuel free-surface swelling and will be referred to as “MFFS” for the remainder of this dissertation. Destructive post irradiation examinations (PIE) of the top of an irradiated EBR-II pin showed that MFFS is predominantly comprised of fuel elements in similar proportions to as cast fuel. The presence of fissile atoms in the MFFS structure could lead to unexpected neutronics behavior or even safety concerns if the MFFS structure becomes detached under a reactor transient. The focus of this dissertation has been to further investigate the MFFS structure and gain insights into its formation mechanism through a combination of reviewing legacy data from EBR-II, separate effects studies performed at RPI, and new PIE performed at Idaho National Laboratory (INL). Chapter 1 of this report gives a thorough background on metallic fuels, their swelling behavior under irradiation, and previous work that has been done to investigate the MFFS structure. Chapter 2 reviews legacy radiographs and operating data from EBR-II to correlate MFFS content with key reactor operating conditions. Chapters 3 and 4 compare 3D characterizations performed on cubes taken from the MFFS and bulk fuel regions. Chapter 5 discusses macro-scale characterizations and optical microscopy performed on newly sectioned MFFS cross section. Chapter 6 discusses microscale characterizations of low burnup U-10Zr fuel samples under low irradiation temperatures. Finally, Chapter 7 discusses separate effect creep testing performed on porous uranium and U-10Zr samples at elevated temperature. The completed work currently suggests the MFFS forms due to there being high a high proportion α-phase uranium at the top of metallic fuel. α-phase uranium undergoes highly anisotropic swelling which causes it to form jagged highly porous structures in the absence of mechanical constraint at the top of the fuel slug. Fuel creep behavior at the top of the fuel slug also likely contributes to MFFS formation.Item Influence of dynamic motions on the flow physics of a tailless chine-forebody slender delta wing aircraft(Rensselaer Polytechnic Institute, Troy, NY, 2025-12)At moderate to high angles of attack, delta wing aircraft develop vortices over their wingsthat form at the sharp leading edge. The vortex formation over a chined-forebody delta wing aircraft is affected at different pitch, roll, or yaw angles. Differences between static and dynamic motions on vortex development were also observed. Experimental wind tunnel tests conducted at a Reynolds number of Rec = 2.53 × 10^5 using strain gauge load balance showed how the aerodynamic loads were impacted by different maneuvers and aircraft attitudes. Changes observed in loads were then correlate the changes in the flow field using stereo particle image velocimetry data. The results show that at angles of attack α > 28◦, the model experienced an asymmetric vortex breakdown, causing a decrease in the lift and drag, as well as a pitch moment and roll moment. Increasing a roll or yaw angle can further decrease the overall lift and drag at these higher angles (α > 25◦), while further compounding the asymmetries led to an increase in side force, roll moment, and yaw moment. Performing dynamic pitch, roll, and yaw motions at low angles of attack (α < 25◦) showed negligible hysteresis in the forces or moments, while similar dynamic motions performed at α > 25◦ showed larger hysteresis loops. From the SPIV data, it was concluded the hysteresis was caused by a delay in breakdown during dynamic pitching up or increasing the roll angle, while pitching down or decreasing the roll angle promoted breakdown. The changes in the axial velocity and the circulation were evaluated to quantify the effects of the dynamic motions on the flow field. A decrease in the axial velocity within the vortex, as well as a decrease in the circulation was calculated in most of the dynamic cases compared to the corresponding static case. Additional SPIV work was conducted to observe how a synthetic jet interacted with a steady vortex for possible flow control applications. A rectangular orifice synthetic jet was placed downstream of a vortex generator. The jet orifice was pitched downstream, and skewed by 45◦ with respect to the freestream to generate a vortex that would augment the steady vortex. The synthetic jet was operated at two blowing ratios of C_b = 1.0 and C_b = 0.5. The location (relative to the synthetic jet) and height of the vortex generator was also varied from 1δ, 2δ, or 3δ with respect to the local boundary layer height at the vortex generator location, while the spanwise location was adjusted from z/δ = 0 − ±1.3 with respect to the centerline of the synthetic jet. Results showed that the synthetic jet was able to forma streamwise vortex downstream even in the absence of the vortex generator. When the vortex generator was placed at the center or closer to the side the synthetic jet was facing, the steady vortex and the vortex from the synthetic jet compounded and formed a larger vortex with higher vortex core velocity. However, when the vortex generator was placed at the side the synthetic jet was facing away from, separate vortices remained downstream, that were comparably weaker. A stronger synthetic jet, with C_b = 1.0, had greater influence over the interactions. However, for the smallest 1δ vortex generator, that formed a weaker and smaller streamwise vortex, the synthetic jet destroyed it and only the structures from the synthetic jet propagated downstream.Item On the study of impact induced hydrothermal systems as potential environments for the origins of life on earth(Rensselaer Polytechnic Institute, Troy, NY, 2025-12)Understanding how life emerged on Earth requires exploring environments that couldpotentially have supported prebiotic chemistry reactions. Impact-induced hydrothermal systems offer a unique combination of energy, mineral composition, and organic material that may have fostered chemical evolution on the early Earth. This dissertation investigates the fate of simple organic compounds delivered by meteorites within these dynamic hydrothermal environments. First, the implementation of two analytical techniques will be presented for their ability to detect small organic molecules in complex, saline fluids representative of the early Earth oceans. These techniques, Direct Analysis in Real Time mass spectrometry (DART-MS) and nuclear magnetic resonance (NMR), will be discussed in detail for how effectively they can characterize organic mixtures with minimal sample processing. Building on this analytical foundation, a series of high-pressure, high-temperature experiments will be presented. These experiments simulate the chemical alteration of meteoritic soluble organic compounds within subaerial and submarine impact-induced hydrothermal systems. It will be shown how temperature, mineral composition, redox state, and salinity all play crucial roles in determining the direction and complexity of organic transformations. Finally, the design and implementation of a novel flow-through reactor system capable of simulating the dynamic, flowing conditions within impact craters will be shown. Ultimately, the research presented will offer new insights and fundamental aspects about how postimpact environments may have contributed to the emergence of life on the early Earth.
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