Mechanical and Aerospace Engineering electronic theses and dissertations (MU)
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The items in this collection are the theses and dissertations written by students of the Department of Mechanical and Aerospace Engineering. Some items may be viewed only by members of the University of Missouri System and/or University of Missouri-Columbia. Click on one of the browse buttons above for a complete listing of the works.
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Item Acoustic metamaterials : microperforated shell and Helmholtz resonator(University of Missouri--Columbia, 2025) Yu, Yukai; Huang, Guoliang[EMBARGOED UNTIL 12/01/2026] Metamaterials have been extensively developed in many areas over the past two decades, a great deal of research has been conducted on acoustic metamaterials exhibiting unusual dynamic effective material properties produced by artificially engineered microstructures. While most of the studies are still scattered, superficial delineations and lack of effective practical application. To promote the application of metamaterials, we designed the structures to explore the metamaterials application in acoustic wave modulation where these techniques can be used to reduce to noise inside the structure and ensure the safety of the target. Acoustic metamaterials have been studied intensively recently since they can expose unnatural-born properties, potentially breaking the capacity limits of conventional acoustic materials. Since these interesting properties are mostly observed around metamaterials' local resonances/anti-resonance, resonance-based acoustic metamaterials are most popular in developing metamaterials. Employing resonance-based unnatural born properties such as effective negative mass density, effective negative bulk modulus, and acoustic hyper-damping on designing noise control solutions can give excellent devices showing such high performance that conventional acoustic material cannot achieve. This dissertation is an effort to employ acoustic metamaterials in designing efficient noise control. Helmholtz has been used to design a high-performance and broadband acoustic silencer. Specifically, five slit-type Helmholtz resonators, which possess a massive viscous area, are packed together to create a single-layer silencer. In turn, two single-layer silencers are combined to form a double-layer silencer, which in theory double performance on noise blocking of the single-layer silencer. Theoretical models of slit-type Helmholtz resonators and silencers are developed completely and well validated with simulation and experimental results.Item A study of wave propagation in time-varying and non-Hermitian elastic media(University of Missouri--Columbia, 2025) Wang, Shaoyun; Yu, Qingsong[EMBARGOED UNTIL 12/01/2026] Elastic wave control in solids traditionally focuses on time-invariant and Hermitian media, where wave frequency, momentum, and dispersion are fixed by constant and symmetric material parameters. Emerging theoretical advances show that relaxing these constraints opens fundamentally richer regimes of wave physics. Space–time duality provides a unifying viewpoint: mapping spatial processes into the time domain enables temporal refraction, frequency shifting, and dynamical waveform manipulation, while mapping temporal evolution into effective spatial dimensions reveals synthetic topological phenomena and higher-dimensional band structure. In parallel, generalized constitutive and symmetry-breaking mechanisms further expand elastic dynamics. Non-Hermitian elasticity introduces asymmetric coupling, and Willis media add momentum–strain and stress–velocity interactions that generate intrinsic nonlocality and break reciprocity. Although each framework enriches wave behavior in distinct ways, their combined effects remain largely unexplored, and a unified understanding of how non-Hermitian and Willis mechanisms interact is still missing. These perspectives identify a broad frontier in which temporal modulation, synthetic dimensions, and generalized constitutive behavior collectively enrich the landscape of mechanical wave phenomena. Yet despite their promise, these concepts remain insufficiently unified and lack comprehensive experimental validation in elastic systems. First, this dissertation establishes the experimental foundations of space–time duality in elastic media by realizing temporal refraction and reflection of flexural waves in a time-modulated metabeam. Through sub-microsecond control of bending stiffness, sharp temporal interfaces implement the timedomain analogs of Snell's and Fresnel's laws, enabling direct observation of momentum conservation, frequency conversion, and waveform redistribution across temporal boundaries. Additional smoothly varying modulations achieve temporal impedance matching, waveform morphing, and programmable frequency shifts, demonstrating time modulation as a practical and powerful degree of freedom for elastic wave control. Second, the dissertation demonstrates topological pumping of Rayleigh surface waves by mapping temporal evolution onto a synthetic spatial dimension encoded by the phason degree of freedom in a quasiperiodically patterned metasurface. Adiabatic sweeping of the phason traces a closed loop on a synthetic torus and produces quantized edge-to-edge transport protected by a non-zero Chern number. This experiment provides a direct mechanical realization of higher-dimensional topological physics in a two-dimensional elastic platform, confirming that synthetic dimensions allow temporal concepts--such as quasienergy winding and adiabatic pumping--to emerge through purely spatial patterning. Third, a dynamic homogenization framework is developed for metabeams containing active or self-sensing scatterers, yielding a non-Hermitian Willis medium that captures frequency- and wavenumber-dependent inertia, stiffness, and cross-coupling. This effective model reproduces the full complex dispersion across the Brillouin zone and reveals a range of non-Hermitian and nonlocal behaviors, including directional amplification, shear-enhanced flexural modes, and non-Hermitian skin localization under open boundaries. By formulating a non-Bloch band theory and deriving a closedform bulk–boundary correspondence within the Willis continuum, the framework analytically connects spectral winding under periodic boundaries to spatial mode localization under open boundaries, establishing a general route to non-Hermitian topological mechanics. Together, these studies show that elastic structures provide a powerful and versatile platform for integrating time-varying modulation, synthetic-dimensional mappings, and non-Hermitian–Willis coupling within a unified physical framework. The insights developed in this dissertation broaden the conceptual and practical design space of mechanical metamaterials, demonstrating that temporal structure, synthetic dimensions, and generalized constitutive behavior form complementary degrees of freedom for shaping wave dynamics. This unified perspective points toward programmable and dynamically reconfigurable architectures that leverage time as a central design dimension, offering promising routes for future advances in mechanical wave control, topological transport, and active metamaterial technologies.Item Solid state laser synthesis of high entropy nanomaterials as elecrocatalysts for water electrolysis and biosensing(University of Missouri--Columbia, 2025) Tan, Aik Jong; Lin, Jian[EMBARGOED UNTIL 12/01/2026] This dissertation is about synthesis and applications of high-entropy nanomaterials produced by a simple solid-state CO₂ laser process to solve two practical problems: producing green hydrogen from natural seawater and building more stable, enzyme-free glucose sensors. First, it addresses the challenge that most water electrolysis systems need either precious-metal catalysts or corrosive acidic/alkaline electrolytes, which are costly and less practical for large-scale use with seawater. To address this, the work develops FeNiCoRu medium-entropy alloy nanoparticles directly on carbon paper using rapid CO₂ laser induction under ambient conditions. The resulting porous, single-phase nanostructures act as durable, efficient bifunctional electrocatalysts for hydrogen and oxygen evolution in neutral seawater, offering a pathway toward more affordable and scalable seawater electrolysis. Second, the dissertation addresses the limitations of conventional enzymatic glucose sensors, which suffer from enzyme instability, short lifetimes, and dependence on expensive noble metals. It introduces a FeNiCoCuZn high-entropy alloy nanomaterial, also synthesized in situ by CO₂ laser on carbon paper, as a non-enzymatic glucose sensing electrode. The multimetallic surface forms active oxyhydroxide species in alkaline media, enabling direct electrocatalytic oxidation of glucose with good sensitivity, stability, and selectivity, including in artificial sweat samples. Overall, the dissertation shows that direct CO₂ laser writing is a versatile and scalable method for creating high-entropy nanomaterials and demonstrates how these materials can be engineered to solve real-world problems in clean energy conversion and biomedical sensing.Item Theoretical analysis and experimental investigation of ejector heat pumps and its water heating applications(University of Missouri--Columbia, 2025) Spitzenberger, Jeremy Shields; Ma, Hongbin[EMBARGOED UNTIL 12/01/2026] This dissertation presents both theoretical and experimental work on ejector heat pump (EHP) systems and its water heating applications. The main goal is to improve performance, identify environmentally friendly working fluids, and determine how EHP technology can be applied to residential and commercial heating systems. The work combines analytical modeling, refrigerant screening, ejector geometry studies, and full-scale lab testing to create a design and evaluation framework for different EHP setups. Three theoretical models are presented. (1) A steam-based gas-fired EHP model was developed utilizing isentropic flow relations and a constant-pressure mixing assumption to study the effects of high- and low-temperature evaporator (HTE and LTE) conditions and ejector back pressure on COP. (2) A thermodynamic screening model used to evaluate ultra-low GWP refrigerants, identifying R1233zd(E) and several others capable of meeting the condensing temperature required for domestic water heating. (3) A simplified version of the steam EHP model was then applied to these top-performing refrigerants, focusing on the ejector's geometry and mapped out how area ratio, PF and SF temperatures, and critical condensing temperature affect the entrainment ratio and heating COP. Four experimental systems were built and tested based on these models. The standalone steam EHP was used to study the effect of nozzle exit position, primary nozzle size, back pressure, and COP under subcritical operation. The standalone R1233zd(E) EHP reached higher condensing temperatures and was able to operate at critical conditions for longer, outperforming steam in sustained efficiency. Updating the theoretical model with real-fluid properties allowed it to match experimental results within 5%. A gas-fired EHP (GFEHP) prototype with a combustion-based HTE showed performance improvements over the standalone R1233zd(E) system, while also identifying key areas to improve, such as heat losses, PF flow stability, and SF throttling. Finally, a vapor-compression ejector heat pump (VCEHP) water heater using R134a provided a baseline for integrating ejectors into high-efficiency heat pump systems and showed where nozzle and ejector geometry changes could improve flow rates and evaporator performance. Overall, the results show that EHP systems, especially with R1233zd(E), has promising potential to meet water heating performance targets while reducing environmental impact. The models, tools, and experimental results from this work give a strong foundation for optimizing EHP performance and moving the technology toward real-world use in sustainable heating applications.Item Dynamic behavior and fatigue performance of closed cell aluminum foams reinforced with graphene nanoplatelets and carbon nanotubes(University of Missouri--Columbia, 2025) Devapati, Tri Sai Veera Rupesh; Khanna, Sanjeev[EMBARGOED UNTIL 12/01/2026] "Closed-cell aluminum foams represent a class of cellular metals whose unique combination of low density, high specific stiffness, and energy-absorption capacity makes them attractive for transportation, defense, and multifunctional engineering systems [1][2]. Their compressive stress–strain response is typically divided into three regimes: an initial linear elastic region governed by bending of intact cell walls, a long plateau region associated with progressive cell-wall buckling and collapse, and a densification regime where the porous structure collapses and the material transitions toward the response of the fully dense parent alloy [2][3]. This characteristic behavior, combined with manufacturability and tunable geometry (more recently) has positioned aluminum foams as promising candidates for impact mitigation, vibration damping, thermal management, and lightweight structural design [4][5]". -- first page