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- Title
- Atomic simulation on chemical-mechanical coupled deformations in complex nano structures
- Creator
- Liu, Jialin (Graduate of Michigan State University)
- Date
- 2019
- Collection
- Electronic Theses & Dissertations
- Description
-
"Nano-structured materials often exhibit very different mechanical properties comparing with their bulk counterpart and are more sensitive and active to chemical interactions with the environments due to the large surface-to-volume ratio. In this thesis, predictive modeling techniques including density functional theory (DFT) and reactive molecular dynamics method (MD) are designed and applied to understand the deformation mechanisms of complex nano-structured material and describe chemical...
Show more"Nano-structured materials often exhibit very different mechanical properties comparing with their bulk counterpart and are more sensitive and active to chemical interactions with the environments due to the large surface-to-volume ratio. In this thesis, predictive modeling techniques including density functional theory (DFT) and reactive molecular dynamics method (MD) are designed and applied to understand the deformation mechanisms of complex nano-structured material and describe chemical-mechanical coupled interactions. Three technologically important materials are investigated, to understanding the high strain rate toughening mechanism in nacre, predicting the formation and fracture of aluminum oxide bifilms in aluminum castings, and revealing the lithium growth morphology as a function of oxygen partial pressure. For nacre, its hierarchical structure and toughening mechanisms have inspired many materials developments. Recently, a new toughening mechanism, deformation twins was observed in nacre after dynamic loading (103 s--1). The deformation twinning tendency and the competition between fracture and deformation twinning were revealed by DFT calculations. We discovered that the ratio of the unstable and the stable stacking fault energy in aragonite is hitherto the highest in a broad range of metallic and oxide materials and the bonding nature for this high ratio is explained. Both aluminum and lithium have high oxygen affinity. Their interaction with the oxygen environment affects the mechanical properties and vice versa. During casting of aluminum, it has long been proposed that the entrapped alumina "bifilms" are detrimental to the fatigue properties of the cast product. However, its properties have never been measured due to experimental limitations. Therefore, a ReaxFF based MD protocol was designed to simulate aging, folding, and fracture of oxide bifilms. The predicted fracture energy, fracture location, and differences between old and young oxides are explained a series of experimental observations. To illustrate the Li-growth mechanism in a solid-state-battery testing platform, we modeled the morphology of Li nano-structure growth in oxygen environment via ReaxFF-based MD. The simulation revealed that the competition of the Li growth rate and oxidation rate leads to the sphere-nanowire-sphere morphology transition with increasing oxygen partial pressure. Understanding the impact of chemical reaction on Li dendrite growth mechanisms and morphology evolution provided insights on the formation of the solid electrolyte interface (SEI) layer in a Li-ion battery. Finally, a shortcoming of the current charge transfer scheme (qEq) used in the ReaxFF MD simulation is discussed. It is demonstrated that qEq method will lead to overductile ionic materials in the MD simulation. A new Force field method and new parameters are proposed to mitigate this problem."--Pages ii-iii.
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- Title
- The quest for active media models : a self-consistent framework for simulating wave propagation in nonlinear systems
- Creator
- Glosser, Connor Adrian
- Date
- 2018
- Collection
- Electronic Theses & Dissertations
- Description
-
This work presents new approaches to simulations of active media at the level of individual particles. Active systems contain internal, nonlinear, processes beyond those of simple scattering systems; thus these new models afford high degrees of fidelity in exploring the underlying physics without recourse to continuum or spatially-averaged approximations.First, I examine the dynamics of microspheres set into motion by ambient acoustic radiation in a fluid described by potential flow in the...
Show moreThis work presents new approaches to simulations of active media at the level of individual particles. Active systems contain internal, nonlinear, processes beyond those of simple scattering systems; thus these new models afford high degrees of fidelity in exploring the underlying physics without recourse to continuum or spatially-averaged approximations.First, I examine the dynamics of microspheres set into motion by ambient acoustic radiation in a fluid described by potential flow in the long-wavelength limit. Variations in the local surface pressure caused by scattering from each microsphere set each microsphere into motion following Newton’s second law. By expanding this pressure in terms of spherical harmonics—natural eigenfunctions of the unretarded radiation kernel—I recover an analytic description of the force on individual microspheres due to an incident waveform. High-order numerical integrations then relate the surface potential on one microsphere to the surface pressure on the others, thereby coupling the microspheres’ trajectories. These simulations predict a dominant translational effect along the direction of propagation of the incident waveform, though they also reveal significant dipolar interactions between microspheres that produce secondary expansions and contractions of the collective microsphere system.Extending my approach from acoustic to electromagnetic systems, I apply it to a collection of quantum dots: “artificial” two-level atoms with a size-dependent energy structure. The optical Maxwell-Bloch equations give the evolution of quantum dots under the influence of electromagnetic fields; this evolution then produces secondary radiation that couples a collection of quantum dots together. In my computational model, I castmy secondary electromagnetic fields in terms of a point-to-point integral operator that accurately recovers both near- and far-field effects. These fields, then, drive a set of implicitly coupled Bloch equations (solved with an exponentially-fitted predictor/corrector scheme) to give the dynamics of the system as a whole. In ensembles of up to 10 000 quantum dots, my model predicts synchronized multiplets of particles that exchange energy, quantum dots that dynamically couple to screen the effect of incident external radiation, localization of the polarization due to randomness and interactions, as well as wavelength-scale regionsof enhanced and suppressed polarization.The remainder of the work uses the same physical quantum dot system while moving towards efficient computer-aided device design. I detail an improved propagation algorithm to reduce the time and space complexity of the simulation dramatically, thereby facilitating rapid analysis of promising device structures. The algorithm makes use of physical and numerical approximations to effect large-scale calculations in reasonable CPU time. A rotating-frame approximation removes high-frequency components in the evolution of the system while simultaneously preserving accurate interference phenomena in space,thereby affording far larger simulation timesteps. Additionally, projecting the source current distribution onto a regular spatial grid makes use of a low-rank approximation to the field propagator to communicate radiation information between distant groups of particles via fast Fourier transforms in a manner reminiscent of fast multipole methods.
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- Title
- Balancing exploration and exploitation in bottom-up organizational learning contexts
- Creator
- Walker, Ross Ian
- Date
- 2018
- Collection
- Electronic Theses & Dissertations
- Description
-
In order to keep pace with a rapidly changing environment, organizations must navigate a fundamental tension between exploration and exploitation. Over time, organizations often drift toward exploitation of known strengths and established resources, but this tendency can be harmful in a dynamic and competitive landscape. A classic simulation by James March (1991) demonstrated the importance of maintaining some degree of belief heterogeneity in an organization for the sake of long-term...
Show moreIn order to keep pace with a rapidly changing environment, organizations must navigate a fundamental tension between exploration and exploitation. Over time, organizations often drift toward exploitation of known strengths and established resources, but this tendency can be harmful in a dynamic and competitive landscape. A classic simulation by James March (1991) demonstrated the importance of maintaining some degree of belief heterogeneity in an organization for the sake of long-term learning. In March’s lineage, this thesis examines the effects of various exploratory strategies (i.e., individual experimentation, codification frequency, structural modularity, and employee turnover) on organizational learning in a bottom-up, networked, interpersonal learning context. Results demonstrate the complex interdependency of these variables in the exploration/exploitation tradeoff. Exploratory analyses suggest that a small degree of random individual experimentation has a favorable reward-to-risk ratio and that it is preferable to turnover as an exploratory strategy.
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- Title
- Modeling and control of pre-chamber initiated turbulent jet ignition combustion systems
- Creator
- Song, Ruitao
- Date
- 2018
- Collection
- Electronic Theses & Dissertations
- Description
-
"Turbulent jet ignition (TJI) combustion is a promising concept for achieving high thermal efficiency and low NOx (nitrogen oxides) emissions. A control-oriented TJI combustion model with satisfactory accuracy and low computational effort is usually a necessity for optimizing the TJI combustion system and developing the associated model-based TJI control strategies. A control-oriented TJI combustion model was first developed for a rapid compression machine (RCM) configured for TJI combustion....
Show more"Turbulent jet ignition (TJI) combustion is a promising concept for achieving high thermal efficiency and low NOx (nitrogen oxides) emissions. A control-oriented TJI combustion model with satisfactory accuracy and low computational effort is usually a necessity for optimizing the TJI combustion system and developing the associated model-based TJI control strategies. A control-oriented TJI combustion model was first developed for a rapid compression machine (RCM) configured for TJI combustion. A one-zone gas exchange model is developed to simulate the gas exchange process in both pre- and main-combustion chambers. The combustion process is modeled by a two-zone combustion model, where the ratio of the burned and unburned gases flowing between the two combustion chambers is variable. To simulate the influence of the turbulent jets to the rate of combustion in the main-combustion chamber, a new parameter-varying Wiebe function is proposed and used for mass fraction burned (MFB) calculation in the main-combustion chamber. The developed model is calibrated using the Least-Squares fitting and optimization procedure. The RCM model was then extended to a TJI engine model. The combustion process is modeled by a similar two-zone combustion model based on the newly proposed parameter-varying Wiebe function. The gas exchange process is simulated by one-zone model considering piston movement and intake and exhaust processes. Since the engine uses liquid fuel, a pre-chamber air-fuel mixing and vaporization model is developed. And correspondingly, the pre-chamber uses a chemical kinetics based model for combustion rate calculation. The model was validated using the experimental data from a single cylinder TJI engine under different operational conditions, and the simulation results show a good agreement with the experimental data. For control design, a nonlinear state-space engine model with cycle-to-cycle dynamics is developed based on the previous crank-angle-resolved (CAR) TJI engine model. The state-space model successfully linked the combustion processes in the two chambers using the parameter-varying Wiebe function. The validated CAR model is used to calibrate and validate the state-space engine model. The simulation results of the two engine models show a good agreement with each other. Thereafter, a linear-quadratic tracking controller is developed for combustion phasing control. Simulation results are presented and a baseline controller has been implemented on the research engine. Combustion phasing control is very important for internal combustion engines to achieve high thermal efficiency with low engine-out emissions. Traditional open-loop map-based control becomes less favorable in terms of calibration effort, robustness to engine aging, and especially control accuracy for TJI engines due to the increased number of control variables over conventional spark-ignition engines. In this research, a model-based feedforward controller is developed for the TJI engine, and a feedback controller is also designed based on the linear quadratic tracking control with output covariance constraint. Since the TJI main-chamber combustion is influenced by the pre-chamber one, the proposed controller optimizes the control variables in both combustion chambers. The proposed feedforward and feedback controllers show significant performance improvement over a group of baseline controllers through a series of dynamometer engine tests."--Pages ii-iii.
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- Title
- A global modeling framework for plasma kinetics : development and applications
- Creator
- Parsey, Guy Morland
- Date
- 2017
- Collection
- Electronic Theses & Dissertations
- Description
-
The modern study of plasmas, and applications thereof, has developed synchronously with com-puter capabilities since the mid-1950s. Complexities inherent to these charged-particle, many-body, systems have resulted in the development of multiple simulation methods (particle-in-cell,fluid, global modeling, etc.) in order to both explain observed phenomena and predict outcomesof plasma applications. Recognizing that different algorithms are chosen to best address specifictopics of interest, this...
Show moreThe modern study of plasmas, and applications thereof, has developed synchronously with com-puter capabilities since the mid-1950s. Complexities inherent to these charged-particle, many-body, systems have resulted in the development of multiple simulation methods (particle-in-cell,fluid, global modeling, etc.) in order to both explain observed phenomena and predict outcomesof plasma applications. Recognizing that different algorithms are chosen to best address specifictopics of interest, this thesis centers around the development of an open-source global model frame-work for the focused study of non-equilibrium plasma kinetics. After verification and validationof the framework, it was used to study two physical phenomena: plasma-assisted combustion andthe recently proposed optically-pumped rare gas metastable laser.Global models permeate chemistry and plasma science, relying on spatial averaging to focusattention on the dynamics of reaction networks. Defined by a set of species continuity and energyconservation equations, the required data and constructed systems are conceptually similar acrossmost applications, providing a light platform for exploratory and result-search parameter scan-ning. Unfortunately, it is common practice for custom code to be developed for each application-an enormous duplication of effort which negatively affects the quality of the software produced.Presented herein, the Python-based Kinetic Global Modeling framework (KGMf) was designed tosupport all modeling phases: collection and analysis of reaction data, construction of an exportablesystem of model ODEs, and a platform for interactive evaluation and post-processing analysis. Asymbolic ODE system is constructed for interactive manipulation and generation of a Jacobian,both of which are compiled as operation-optimized C-code.Plasma-assisted combustion and ignition (PAC/PAI) embody the modernization of burning fuelby opening up new avenues of control and optimization. With applications ranging from engineefficiency and pollution control to stabilized operation of scramjet technology in hypersonic flows,developing an understanding of the underlying plasma chemistry is of the utmost importance.While the use of equilibrium (thermal) plasmas in the combustion process extends back to the ad-vent of the spark-ignition engine, works from the last few decades have demonstrated fundamentaldifferences between PAC and classical combustion theory. The KGMf is applied to nanosecond-discharge systems in order to analyze the effects of electron energy distribution assumptions onreaction kinetics and highlight the usefulness of 0D modeling in systems defined by coupled andcomplex physics.With fundamentally different principles involved, the concept of optically-pumped rare gasmetastable lasing (RGL) presents a novel opportunity for scalable high-powered lasers by takingadvantage of similarities in the electronic structure of elements while traversing the periodic ta-ble. Building from the proven concept of diode-pumped alkali vapor lasers (DPAL), RGL systemsdemonstrate remarkably similar spectral characteristics without problems associated with heatedcaustic vapors. First introduced in 2012, numerical studies on the latent kinetics remain immature.This work couples an analytic model developed for DPAL with KGMf plasma chemistry to bet-ter understand the interaction of a non-equilibrium plasma with the induced laser processes anddetermine if optical pumping could be avoided through careful discharge selection.
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- Title
- Suicide, signals, and symbionts : evolving cooperation in agent-based systems
- Creator
- Vostinar, Anya E.
- Date
- 2017
- Collection
- Electronic Theses & Dissertations
- Description
-
Cooperation is ubiquitous in nature despite the constant pressure for organisms to cheat by receiving a benefit from cooperators, while not cooperating themselves. The continued evolution and persistence of countless forms of cooperation is a central topic in evolutionary theory. Extensive research has been done on the theoretical dynamics of cooperation through game theory and the natural examples of cooperation. However, it remains difficult to understand thoroughly the evolution of many...
Show moreCooperation is ubiquitous in nature despite the constant pressure for organisms to cheat by receiving a benefit from cooperators, while not cooperating themselves. The continued evolution and persistence of countless forms of cooperation is a central topic in evolutionary theory. Extensive research has been done on the theoretical dynamics of cooperation through game theory and the natural examples of cooperation. However, it remains difficult to understand thoroughly the evolution of many cooperative systems, due in part to the ancient origins of these systems and the long time scales required to see cooperation evolve in any natural populations. I have systematically analyzed the evolution of three broad types of cooperation: programmed cell death, quorum sensing, and mutualisms (cooperation across species). I have provided evidence that programmed cell death can originate due to kin selection. I have also created two new systems to enable the extensive exploration of factors that affect the evolution of public goods cooperation and mutualism. Using these systems, I determine the effects of environmental factors on the evolution of public goods cooperation and mutualism. By uniting the expansive theoretical work on these forms of cooperation with a fully-controlled experimental system, I contributed to our understanding of how these forms of cooperation can emerge and be maintained in industrial and medical applications that rely on bacterial cooperation.
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