# Browsing by Subject "Civil engineering"

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Item Embargo A Combined Experimental and Modeling Approach Unraveling the Mechanics Behind Drying-induced Fractures in Soils(2023) Chen, RuoyuThis dissertation aims to understand the physics of geomaterial under different loading rates, employing a combination of experimental analysis and theoretical modeling. After obtaining a comprehensive understanding of geophysics, the objective shifts to exploring methods for enhancing the mechanical properties of materials to mitigate or prevent structural failures.

One particular focus in experimental studies is placed on understanding the phenomenon of desiccation, where the volumetric shrinkage rate of the geomaterial can be manipulated by adjusting atmospheric conditions. The experimental results from desiccation tests, supported by triaxial tests, reveal that the behavior of geomaterials is dependent on the loading rate, indicating a rate-dependent response. This observation highlights the need to consider viscoplasticity in the mechanical analysis of these geomaterials. Subsequently, a theoretical mathematical model incorporating viscoplasticity is utilized to describe the stress distribution within the geomaterial. By comparing the predicted locations and the number of stress singularities obtained from the model with the observed locations and the number of cracks in desiccation tests conducted under controlled atmospheric conditions, the effectiveness of the model in capturing the mechanical behavior of the geomaterial is assessed.

Once the mechanical behavior is understood and the corresponding theoretical model is validated, modifications in soil properties can be achieved through adjustments to viscosity. Initially, increasing the viscosity results in the formation of more cracks with narrower spacing. However, as viscosity continues to increase, it eventually leads to the complete prevention of failure. Desiccation experiments containing fluids with varying viscosities were conducted to validate the predicted failure pattern. The experimental results align with the theoretical predictions, providing confirmation of the anticipated behavior.

During desiccation tests conducted on amended soil samples, it was observed that crack development was mitigated, indicating that cracks initially appeared but remained suspended during the dehydration process. Due to the complexity of solving time-dependent partial differential equations with shifting boundary conditions, capillary experiments were introduced to provide insights into the force development within soil particles with the loss of water. The morphology and force development from capillary tests revealed distinct outcomes during dehydration: in the capillary system with distilled water and low viscosity fluid, a rapid force reduction (drop to zero) occurred as the capillary bridges broke, while the presence of high viscosity fluid resulted in a rebound followed by a high attraction force due to bonding formation in the capillary system.

In all, this dissertation offers a novel perspective on describing soil behavior and provides a micro-scale explanation of force development in soil dehydration.

Item Open Access A Mean Field Approach to Watershed Hydrology(2016) Bartlett Jr., Mark StephanSociety-induced changes to the environment are altering the effectiveness of existing management strategies for sustaining natural and agricultural ecosystem productivity. At the watershed scale, natural and agro-ecosystems represent complex spatiotemporal stochastic processes. In time, they respond to random rainfall events, evapotranspiration and other losses that are spatially variable because of heterogeneities in soil properties, root distributions, topography, and other factors. To quantify the environmental impact of anthropogenic activities, it is essential that we characterize the evolution of space and time patterns of ecosystem fluxes (e.g., energy, water, and nutrients). Such a characterization then provides a basis for assessing and managing future anthropogenic risks to the sustainability of ecosystem productivity.

To characterize the space and time evolution of watershed scale processes, this dissertation introduces a mean field approach to watershed hydrology. Mean field theory (also known as self-consistent field theory) is commonly used in statistical physics when modeling the space-time behavior of complex systems. The mean field theory approximates a complex multi-component system by considering a lumped (or average) effect of all individual components acting on a single component. Thus, the many body problem is reduced to a one body problem. For watershed hydrology, a mean field theory reduces the numerous point component effects to more tractable watershed averages resulting in a consistent method for linking the average watershed fluxes (evapotranspiration, runoff, etc.) to the local fluxes at each point.

The starting point for this work is a general point description of the soil moisture, rainfall, and runoff system. For this system, we find the joint PDF that describes the temporal variability of the soil water, rainfall, and runoff processes. Since this approach does not account for the spatial variability of runoff, we introduce a probabilistic storage (ProStor) framework for constructing a lumped (unit area) rainfall-runoff response from the spatial distribution of watershed storage. This framework provides a basis for unifying and extending common event-based hydrology models (e.g. Soil Conservation Service curve number (SCS-CN) method) with more modern semi-distributed models (e.g. Variable Infiltration Capacity (VIC) model, the Probability Distributed (PDM) model, and TOPMODEL). In each case, we obtain simple equations for the fractions of the different source areas of runoff, the spatial variability of runoff and soil moisture, and the average runoff value (i.e., the so-called runoff curve). Finally, we link the temporal and spatial descriptions with a mean field approach for watershed hydrology. By applying this mean field approach, we upscale the point description with the spatial distribution of soil moisture and parameterize the numerous local interactions related to lateral fluxes of soil water in terms of its average. With this approach, we then derive PDFs that represent the space and time distribution of soil water and associated watershed fluxes such as evapotranspiration and runoff.

Item Open Access A Study in Laterally Restrained Buckled Beams for the use in a Vertical Isolation System(2021) McManus, Michael AllenLinear vibration isolation systems, used to reduce the transmissibility of vertical vibration, requires a vertical static displacement that increases with the square of the natural period of the isolation system. The static displacement of a vertical isolation system with a one second natural period is 0.25 m. The nonlinear stiffness of buckled beams loaded in the transverse direction can be designed to reduce the vertical static displacement requirement of vertical systems. This study presents an analysis of large displacement mechanics of slender beams that buckle against a constraint, and extracts the transverse constraint force via the Lagrange multiplier enforcing the constraint. The constraint prescribes a maximum allowable lateral displacement along the length of the beam and a specified longitudinal displacement at the mid span of the beam. No small curvature assumption is involved. Lateral and longitudinal displacements are parameterized in terms of Fourier coefficients. Coefficient values for constrained equilibria are found by minimizing the bending strain energy such that lateral and longitudinal constraints are satisfied. Because the full expression for curvature is used, this is a nonlinear constrained optimization problem.

Edge and mid-point horizontal constraint positions are varied to gain a better understanding of the constraint forces at each position. This modeling approach is then used to design a system of post-buckled leaf springs in order to meet vibration isolation requirements without over-stressing the springs. This process is discussed in detail along with the process and challenges associated with the physical model. Theoretical predictions are compared to laboratory scale measurements. Experimental results from the physical model are compared to the theoretical and numerical simulation results. The potential for rocking responses of the vertical isolation system are quantified via the modeling of the nonlinear dynamics of a platform supported by a system of springs and carrying a mass concentrated above the platform.

Item Open Access Adaptive Spline-based Finite Element Method with Application to Phase-field Models of Biomembranes(2015) Jiang, WenInterfaces play a dominant role in governing the response of many biological systems and they pose many challenges to traditional finite element. For sharp-interface model, traditional finite element methods necessitate the finite element mesh to align with surfaces of discontinuities. Diffuse-interface model replaces the sharp interface with continuous variations of an order parameter resulting in significant computational effort. To overcome these difficulties, we focus on developing a computationally efficient spline-based finite element method for interface problems.

A key challenge while employing B-spline basis functions in finite-element methods is the robust imposition of Dirichlet boundary conditions. We begin by examining weak enforcement of such conditions for B-spline basis functions, with application to both second- and fourth-order problems based on Nitsche's approach. The use of spline-based finite elements is further examined along with a Nitsche technique for enforcing constraints on an embedded interface. We show that how the choice of weights and stabilization parameters in the Nitsche consistency terms has a great influence on the accuracy and robustness of the method. In the presence of curved interface, to obtain optimal rates of convergence we employ a hierarchical local refinement approach to improve the geometrical representation of interface.

In multiple dimensions, a spline basis is obtained as a tensor product of the one-dimensional basis. This necessitates a rectangular grid that cannot be refined locally in regions of embedded interfaces. To address this issue, we develop an adaptive spline-based finite element method that employs hierarchical refinement and coarsening techniques. The process of refinement and coarsening guarantees linear independence and remains the regularity of the basis functions. We further propose an efficient data transfer algorithm during both refinement and coarsening which yields to accurate results.

The adaptive approach is applied to vesicle modeling which allows three-dimensional simulation to proceed efficiently. In this work, we employ a continuum approach to model the evolution of microdomains on the surface of Giant Unilamellar Vesicles. The chemical energy is described by a Cahn-Hilliard type density functional that characterizes the line energy between domains of different species. The generalized Canham-Helfrich-Evans model provides a description of the mechanical energy of the vesicle membrane. This coupled model is cast in a diffuse-interface form using the phase-field framework. The effect of coupling is seen through several numerical examples of domain formation coupled to vesicle shape changes.

Item Open Access Analysis of the Stability and Response of Deep-Seated Landslides by Monitoring their Basal Temperature(2020) Seguí, CarolinaDeep-seated landslides are known as large slides involving millions of cubic meters that move as a rigid block on top of a deep (below the roots of the trees and the groundwater level) basal layer of heavily deformed minerals. This kind of landslides geometrically shears as translational/rotational (depending on the stratigraphy of the area), with very low velocities (cm/year) during long periods (years to tens of years). However, their collapse is usually very sudden, happening within minutes and without previous warning, reaching high velocities up to 120m/s (as the 1963 Vaiont landslide in Italy, \cite{Muller1964}). The catastrophic and fast collapse of this kind of landslides makes the evacuation of the area that is going to be affected almost impossible, thereby possibly causing fatalities and infrastructure damages. Moreover, the lack of understanding of the physical processes behind the mechanisms of failure of this kind of landslides makes the development of reliable early warning systems (or tools/protocols to stop the acceleration of the landslide) challenging, therefore potentially causing significant damages to civil infrastructures. The landslide-prone areas are widespread around the world, having a detrimental fatality rate of tens of thousands. Hence, landslides are a globally threatening natural hazard with disproportional consequences.

This thesis focuses on the understanding of the mechanism of the fast collapse of large deep-seated landslides and provides the first-stage tool for the early warning system. First, is described the Vardoulakis Forecasting Model (VFM), which is a heat energy-based mathematical model that considers that the temperature of the shear band material is critical in the behavior and stability of the landslide. The model contemplates the external and internal factors of this kind of landslide. The external factors of a landslide are considered as the loading conditions, such as groundwater level. And the internal factors of the landslide are focused on the thin shear band, such as the reduction of the friction coefficient of the material, hence, the loss of resistance of the basal material due to continuous friction and cycles of loading-unloading of external forces such as the groundwater. Moreover, the constitutive law used in the VFM theoretically implies that the material of the shear band (usually clay or clay-like material) is rate (velocity) hardening and thermal softening \cite{Vardoulakis2002}, but this assumption has never been tested experimentally. This model has been applied previously by \cite{Veveakis2007} for the case of the famous Vaiont landslide, which collapsed catastrophically in 1963 causing over 2000 fatalities. However, the study did not consider a time-dependence of the loading conditions, and the parameters of the basal material were taken from the literature. This thesis thus presents an extension of the work that the late Professor Vardoulakis and Professor Veveakis started from 2002 until 2007, by implementing the VFM to other case studies with time-dependent loading conditions. Moreover, the present thesis proves the theory that the temperature plays a critical role in the behavior of deep-seated landslides by instrumenting an active deep-seated landslide for the first time, called El Forn landslide (Andorra), with a thermometer in the shear band. The log-samples of this landslide have been studied in the laboratory in different ways, firstly in the triaxial machine to test the theoretical constitutive law of Vardoulakis that the clay material inside the shear band is rate hardening and thermal softening. The tests performed in the triaxial machine have validated for the first time that, indeed, the basal material (as a clay-like material) behaves as Vardoulakis postulated. Furthermore, micro-scale tests, such as X-Ray diffraction, SEM-EDS, MicroCT, and Plasticity Index have been performed to understand the effect of this behavior. Hence, mineralogical, textural, porosity, and plasticity results have been obtained for the samples, and, indeed exists a correlation of why the basal material is velocity and thermal sensitive.

Field data of the El Forn landslide has been obtained, such as the shear band's temperature, groundwater pressure, and displacement of the landslide. The data has demonstrated that, indeed, the temperature of the material of the shear band varies when the pressure changes, and then the landslide accelerates. The field data has shown that for this case study, the material is thermal sensitive when the water pressure varies, not when the landslide accelerates and, due to friction, the material heats.

The VFM model has been applied to four different cases, Vaiont (Italy), Shuping (Three Gorges Dam, China), Mud Creek (California, USA), and the El Forn (Andorra) landslides. The first three landslides have been implemented in the model by using literature data, and the model has reproduced with accuracy the behavior of the three landslides. Finally, the El Forn landslide has been applied to the VFM by implementing field and experimental data, thus reducing the uncertainty of the mathematical model, which accurately reproduces its behavior as well.

The VFM allows to forecast and control deep-seated landslides by using the heat-energy based mathematical model, and the constitutive law. This model works in a dimensionless form of the parameters, to avoid complications in the model by working with so many parameters. Furthermore, this unique model allows accounting in it the external loading and several parameters of the material of the shear band. By taking the heat-diffusion equation in dimensionless form, allows working with only a single dimensionless parameter, that includes the material parameters and the external loading. The single dimensionless parameter is then plotted against the temperature of the shear band (calculated by the model) and is, thus, mapped in the phase space. The phase-space is a curve calculated by the heat equation in the dimensionless form at a steady-state. It is a generic curve for all materials and allows to map the behavior of the landslide with the single dimensionless parameter against the temperature. This mapping allows to locate the creeping stage of the landslide and see if the landslide is close to collapse. Hence, the VFM can become a very useful tool to control and forecast the behavior of a deep-seated landslide and take remediation measures in time.

Item Open Access Application of Numerical Methods to Study Arrangement and Fracture of Lithium-Ion Microstructure(2016) Stershic, Andrew JosephThe focus of this work is to develop and employ numerical methods that provide characterization of granular microstructures, dynamic fragmentation of brittle materials, and dynamic fracture of three-dimensional bodies.

We first propose the fabric tensor formalism to describe the structure and evolution of lithium-ion electrode microstructure during the calendaring process. Fabric tensors are directional measures of particulate assemblies based on inter-particle connectivity, relating to the structural and transport properties of the electrode. Applying this technique to X-ray computed tomography of cathode microstructure, we show that fabric tensors capture the evolution of the inter-particle contact distribution and are therefore good measures for the internal state of and electronic transport within the electrode.

We then shift focus to the development and analysis of fracture models within finite element simulations. A difficult problem to characterize in the realm of fracture modeling is that of fragmentation, wherein brittle materials subjected to a uniform tensile loading break apart into a large number of smaller pieces. We explore the effect of numerical precision in the results of dynamic fragmentation simulations using the cohesive element approach on a one-dimensional domain. By introducing random and non-random field variations, we discern that round-off error plays a significant role in establishing a mesh-convergent solution for uniform fragmentation problems. Further, by using differing magnitudes of randomized material properties and mesh discretizations, we find that employing randomness can improve convergence behavior and provide a computational savings.

The Thick Level-Set model is implemented to describe brittle media undergoing dynamic fragmentation as an alternative to the cohesive element approach. This non-local damage model features a level-set function that defines the extent and severity of degradation and uses a length scale to limit the damage gradient. In terms of energy dissipated by fracture and mean fragment size, we find that the proposed model reproduces the rate-dependent observations of analytical approaches, cohesive element simulations, and experimental studies.

Lastly, the Thick Level-Set model is implemented in three dimensions to describe the dynamic failure of brittle media, such as the active material particles in the battery cathode during manufacturing. The proposed model matches expected behavior from physical experiments, analytical approaches, and numerical models, and mesh convergence is established. We find that the use of an asymmetrical damage model to represent tensile damage is important to producing the expected results for brittle fracture problems.

The impact of this work is that designers of lithium-ion battery components can employ the numerical methods presented herein to analyze the evolving electrode microstructure during manufacturing, operational, and extraordinary loadings. This allows for enhanced designs and manufacturing methods that advance the state of battery technology. Further, these numerical tools have applicability in a broad range of fields, from geotechnical analysis to ice-sheet modeling to armor design to hydraulic fracturing.

Item Open Access Applications of Deep Representation Learning to Natural Language Processing and Satellite Imagery(2020) Wang, GuoyinDeep representation learning has shown its effectiveness in many tasks such as text classification and image processing. Many researches have been done to directly improve the representation quality. However, how to improve the representation quality by cooperating ancillary data source or by interacting with other representations is still not fully explored. Also, using representation learning to help other tasks is worth further exploration.

In this work, we explore these directions by solving various problems in natural language processing and image processing. In the natural language processing part, we first discuss how to introduce alternative representations to improve the original representation quality and hence boost the model performance. We then discuss a text representation matching algorithm. By introducing such matching algorithm, we can better align different text representations in text generation models and hence improve the generation qualities.

For the image processing part, we consider a real-world air condition prediction problem: ground-level $PM_{2.5}$ estimation. To solve this problem, we introduce a joint model to improve image representation learning by incorporating image encoder with ancillary data source and random forest model. We the further extend this model with ranking information for semi-supervised learning setup. The semi-supervised model can then utilize low-cost sensors for $PM_{2.5}$ estimation.

Finally, we introduce a recurrent kernel machine concept to explain the representation interaction mechanism within time-dependent neural network models and hence unified a variety of algorithms into a generalized framework.

Item Open Access Aquifer Parametrization and Evaluation of Dipole Flow in Recirculation Wells(2015) Embon, Michelle NataliThe dipole-flow test is a novel aquifer characterization technique that utilizes a single-borehole measurement system to yield the vertical hydraulic conductivity, horizontal hydraulic conductivity, and storativity within confined aquifers. The test implements a packer and a pump system that creates a hydraulic dipole flow pattern by pumping water at a constant rate thought a suction screen, transferring it within the well to a second chamber, and injecting it back into the aquifer. Various mathematical models have been developed to derive the drawdown in each chamber and estimating water flow parameters. This thesis derives and proposes a new mathematical model that deals with packers containing asymmetrical chamber lengths. It further tests this formula by implementing in on a particular aquifer of interest and contrasting the numerical findings with those obtained in field testing and simulations as described in a Johnson and Simmon 2007 publication.

In order to derive this equation we utilize the principles of superposition, the Taylor series, the Newton Raphson model, and the implementation of an error function. We also draw elements of the Hantush leaky well function and the infinity aquifer simplifications suggested by Zlotnik. The results obtain from this computation demonstrated that this developed hydrologic model yields accurate and rational measurements for drawdown and conductivity. We conclude that our modeled formulas surpass those proposed in the Johnson article, and provides experimenters with a valuable and efficient mathematical tool for aquifer characterization.

Item Open Access Characterization and Mechanism of Rigidity in Columns of Star-shaped Granular Particles(2020) Zhao, YuchenAn important challenge in the science of granular materials is to understand the connection between the shapes of individual grains and the macroscopic response of the aggregate. Granular packings of concave or elongated particles can form free-standing structures like walls or arches, in sharp contrast to the behaviors of spherical grains. For some particle shapes, such as staples, the rigidity arises from interlocking of pairs of particles, but the origins of rigidity for non-interlocking particles remains unclear. In addition to their intrinsic interest, these packings are relevant to lightweight and reconfigurable structures in civil, geotechnical and material engineering applications.

In this thesis, we report on experiments and numerical simulations of packings of star-shaped particles consisting of three mutually orthogonal sphero-cylinders whose centers coincide. The first set of experiments studies the chance of obtaining a free-standing column when the confining tube of the column is removed, which we will call it as ``intrinsic stability''. We prepare monodisperse packings of star-shaped particles with different length-to-arm diameter aspect ratio α, interparticle friction and particle-base friction. We also vary packing density by vibrating the packings when they are in the tube. We find that the intrinsic stability depends on packing dimension: columns of greater diameter or shorter height are more stable. Both arm length and interparticle friction can greatly increase the intrinsic stability, while the packing density and basal friction have limited effects on the intrinsic stability.

The second set of experiments involves stability of free-standing columns (prepared from the first set of experiments) under three different external perturbations: (1) base tilting; (2) static axial loading; and (3) vertical vibration. For the base tilting test, we gradually tilt the base of the column and observe column collapse as a function of tilt angle. We find that columns of low friction particles are more fragile than those of high friction particles. For the axial loading test, we gradually increase the loading on a column until it collapses. We find that tall columns are more fragile. For the vibration test, we apply vertical sinusoidal vibration from the base to destabilize the column. Both interparticle and basal friction improve packing stability in terms of increasing relaxation time under vibration. We also find that tall columns are more sensitive to the vibration in the sense that they collapse faster than short ones under the same vibration.

In the third set of experiments, we vary α and subject the packings to quasistatic direct shear. For small α, we observe a finite yield stress. For large α, however, the packings become rigid when sheared, supporting stresses that increase sharply with increasing strain. Analysis of x-ray micro-computed tomography data collected during the shear reveals that the stiffening is associated with a tilted, oblate cluster of particles near the nominal shear plane in which particle deformation and average contact number both increase.

Molecular dynamics simulations that closely match the third experiments are used to investigate the finite yield stress and the stiffening. In simulation, interparticle contact forces are known to us. For yield packings (small α), simulations suggest no apparent cohesion. For stiffening packings (large α), simulation results show that the particles are collectively under tension along one direction even though they do not interlock pairwise. These tensions come from contact forces with large associated torques, and they are perpendicular to the compressive stresses in the packing. They counteract the tendency to dilate, thus stabilizing the particle cluster.

Item Open Access Chemo-Hygro-Geomechanics of Enhanced Crack Propagation(2015) Hu, ManmanThis dissertation studies the chemo-hygro-mechanical coupling involved in the process of crack propagation encountered both in natural and engineered context. Chemical processes are likely to affect the mechanical properties of geo-materials, resulting in possible weakening effect. The deformation and micro-cracking induced by material weakening in turn enhances the overall mass removal. In this study, several models within both elasticity and plasticity domain are developed for a better understanding of the enhanced crack propagation. A deformational plasticity model based on experimental observations is addressed. Rigid-plasticity models are applied to various boundary conditions. In the chemo-elasticity model, chemical dissolution is assumed to be a function of a comprehensive strain invariant. One-way coupling and two-way coupling models are discussed. In the two-way coupling model, volumetric strain coupling and deviatoric strain coupling are compared. A variety of loading modes are adopted to investigate the chemical enhancement of propagation of a single crack. The behavior of the material is either rigid-plastic, or elastic with the variable of mass removal enters the constitutive equation as a chemical strain. Comparison between the results from two models is presented and discussed.

Item Open Access Combined Deterministic-Stochastic Identification with Application to Control of Wave Energy Harvesting Systems(2012) Li, QuanThis thesis proposes an integrated procedure for identifying the nominal models of the deterministic part and the stochastic part of a system, as well as their model error bounds in different uncertainty structures (e.g. $\mathcal{H}_2$-norm and $\mathcal{H}_{\infty}$-norm) based on the measurement data. In particular, the deterministic part of a system is firstly identified by closed-loop instrumental variable method in which a known external signal sequence uncorrelated with the system noises is injected in the control input for the identifiability of the system in closed loop. By exploiting the second-order statistics of the noise-driven output components, the stochastic part of a system is identified by the improved subspace approach in which a new and straightforward linear-matrix-inequality-based optimization is proposed to obtain a valid model even under insufficient measurement data.

To derive an explicit model error bound on the identification model, we investigate a complete asymptotic analysis for identification of the stochastic part of the system. We first derive the asymptotically normal distributions of the empirical sample covariance and block-Hankel matrix of the outputs. Thanks to these asymptotic distributions and the perturbation analysis of singular value decomposition and discrete algebraic Riccati equation, several central limit theorems for the identified controllability matrix, observability matrix, and the state-space matrices in the associated covariance model are derived, as well as the norm bounds of Kalman gain and the innovations covariance matrix in the innovations model. By combining these asymptotic results, the explicit $\mathcal{H}_2$-norm and $\mathcal{H}_{\infty}$-norm bounds of the model error are identified with a given confidence level.

Practical applicability of the proposed combined deterministic-stochastic identification procedure is illustrated by the application to indirect adaptive control of a multi-generator wave energy harvesting system.

Item Open Access Control of Vibratory Energy Harvesters in the Presence of Nonlinearities and Power-Flow Constraints(2012) Cassidy, Ian LernerOver the past decade, a significant amount of research activity has been devoted to developing electromechanical systems that can convert ambient mechanical vibrations into usable electric power. Such systems, referred to as vibratory energy harvesters, have a number of useful of applications, ranging in scale from self-powered wireless sensors for structural health monitoring in bridges and buildings to energy harvesting from ocean waves. One of the most challenging aspects of this technology concerns the efficient extraction and transmission of power from transducer to storage. Maximizing the rate of power extraction from vibratory energy harvesters is further complicated by the stochastic nature of the disturbance. The primary purpose of this dissertation is to develop feedback control algorithms which optimize the average power generated from stochastically-excited vibratory energy harvesters.

This dissertation will illustrate the performance of various controllers using two vibratory energy harvesting systems: an electromagnetic transducer embedded within a flexible structure, and a piezoelectric bimorph cantilever beam. Compared with piezoelectric systems, large-scale electromagnetic systems have received much less attention in the literature despite their ability to generate power at the watt--kilowatt scale. Motivated by this observation, the first part of this dissertation focuses on developing an experimentally validated predictive model of an actively controlled electromagnetic transducer. Following this experimental analysis, linear-quadratic-Gaussian control theory is used to compute unconstrained state feedback controllers for two ideal vibratory energy harvesting systems. This theory is then augmented to account for competing objectives, nonlinearities in the harvester dynamics, and non-quadratic transmission loss models in the electronics.

In many vibratory energy harvesting applications, employing a bi-directional power electronic drive to actively control the harvester is infeasible due to the high levels of parasitic power required to operate the drive. For the case where a single-directional drive is used, a constraint on the directionality of power-flow is imposed on the system, which necessitates the use of nonlinear feedback. As such, a sub-optimal controller for power-flow-constrained vibratory energy harvesters is presented, which is analytically guaranteed to outperform the optimal static admittance controller. Finally, the last section of this dissertation explores a numerical approach to compute optimal discretized control manifolds for systems with power-flow constraints. Unlike the sub-optimal nonlinear controller, the numerical controller satisfies the necessary conditions for optimality by solving the stochastic Hamilton-Jacobi equation.

Item Open Access Correlation of Finite Element Analysis to Impacted Composite Plates(2011) Berry, Jessica LynnThe purpose of this thesis was to examine progressive composite damage models available within LS-DYNA and to correlate the results of these models with drop weight impact testing and with the non-destructive evaluation techniques of shearography, thermography, and ultrasonic testing. The secondary purpose of this study was to assess whether shearography and thermography provide an adequate less expensive replacement to ultrasonic testing. For this investigation, three models were chosen: Chang-Chang, Chang-Chang + Tsai-Wu, and a Faceted Failure Surface

Model. For the experimental impact testing, two sets of specimens were chosen: a 16-ply lay-up and a 32-ply lay-up of carbon fiber pre-preg material. The panel specimens were tested at various impact energies and the displacement and force history of the impactor were recorded. The models showed good correlation for the force history with the experiments. Furthermore, the 16-ply models correlated well with the displacement history. However, due the penalty method implementation, the 32-ply models did not show similar peak displacement output. The damage shown by the models was compared to non-destructive evaluation techniques. The shearography and thermography showed significantly less damage than the ultrasonic scans, and therefore do not provide an adequate replacement to ultrasonic scanning. In looking at correlation between the models and the non-destructive evaluation techniques, the faceted failure surface showed significantly more damage due to its elastic-plastic type formulation.

Item Open Access Crack Nucleation and Branching in the eXtended Finite Element Method(2013) Merewether, Mark ThomasThe eXtended Finite Element Method (X-FEM) has proven to be a robust method for simulating crack propagation, but relatively little work has focused on the important problem of crack initiation or nucleation. In this work, we examine various options for nucleating cracks within a cohesive framework and the X-FEM. Attention is confined to shell problems. We discuss the details of the methods and their strengths and weaknesses. With the introduction of such nucleation algorithms, the need to model more complex crack growth topologies also arises. In particular, we examine algorithms for enabling crack branching, focusing on both the mechanics and element kinematic considerations. The results of various benchmark problems for the nucleation and branching algorithms are also presented and discussed.

Item Open Access Data Transfer between Meshes for Large Deformation Frictional Contact Problems(2013) Kindo, Temesgen MarkosIn the finite element simulation of problems with contact there arises

the need to change the mesh and continue the simulation on a new mesh.

This is encountered when the mesh has to be changed because the original mesh experiences severe distortion or the mesh is adapted to minimize errors in the solution. In such instances a crucial component is the transfer of data from the old mesh to the new one.

This work proposes a strategy by which such remeshing can be accomplished in the presence of mortar-discretized contact,

focusing in particular on the remapping of contact variables which must occur to make the method robust and efficient.

By splitting the contact stress into normal and tangential components and transferring the normal component as a scalar and the tangential component by parallel transporting on the contact surface an accurate and consistent transfer scheme is obtained. Penalty and augmented Lagrangian formulations are considered. The approach is demonstrated by a number of two and three dimensional numerical examples.

Item Open Access Development of Water and Wastewater Biofiltration Technologies for the Developing World using Locally Available Packing Media: Case Studies in Vietnam and Haiti(2014) Thomson, Ashley AnneWater and sanitation are two of the world's most urgent current challenges (Elimelech, 2006). With a population racing towards seven billion people, over one sixth of the human population does not have access to adequate water and sanitation. Drinking water is inaccessible for approximately 783 million people living in the developing world (WHO, 2014). This is especially critical for people at risk of exposure to deadly pathogens such as Vibrio cholerae, Shigella, and Salmonella, such as those living in Haiti as Vibrio cholerae is now ubiquitous (Enserink, 2010). On the sanitation side, more than 2.5 billion people in the world still lack access to adequate resources (WHO, 2014). Almost half of these people have access to no sanitation facilities at all and practice open defecation (WHO, 2014). Thousands of small children still die every day from preventable diseases caused by inadequate sanitation (WHO, 2014). As global climate change is expected to exacerbate these issues, there is an urgent need for the development of sustainable treatment technologies to ensure a better tomorrow for our world (Ford, 1999). Safe water and sanitation technologies, while often disjointed, should be considered together as pathogens transmitted via drinking water are predominantly of fecal origin (Ashbolt, 2004; Montgomery, 2007).

In this dissertation project, I explore the use of both drinking water and wastewater treatment technologies which are cost effective and rely on locally available materials in low-income countries. For the drinking water treatment side, I focus on the use of biosand filters in Haiti with a specific interest in understanding their ability to remove the pathogen Vibrio cholerae, the causative agent for cholera. The wastewater treatment technology consists of biofilters packed with cocopeat, a waste product generated during coconut husk processing, and I investigate their use for the treatment of septic tank effluent in Vietnam. Both of these projects combine lab and field work. The specific objectives of this dissertation project are to 1) compare the removal efficiency of V. cholerae to indicator bacteria in field biosand filters and determine the parameters controlling removal; 2) investigate the correlation between removal efficiency of pathogens in field biosand filters having operated for varying lengths of time to schmutzdecke bacterial composition and influent water characteristics; 3) determine the effect of number of charges, total organic carbon loading, and schmutzdecke composition on V. cholerae removal efficacy; 4) isolate the effect of biological removal mechanisms and physical/chemical removal mechanisms on V. cholerae removal efficiency and determine the correlation to TOC concentration in water; 5) evaluate cocopeat as a packing medium for biofilters in terms of nitrogen, phosphorus and biological oxygen demand removal from simulated wastewater as compared to other traditional packing media; and 6) conduct an assessment of cocopeat-packed, vertical flow constructed wetlands treating septic tank effluent in the Mekong Delta of Vietnam.

In the first part of this dissertation, biosand filters in the Artibonite Valley of Haiti, the epicenter of the cholera epidemic, were tested for total coliform and V. cholerae removal efficiencies. In addition, schmutzdecke samples were collected in order to measure the amount of EPS in the biofilm, as well as characterize the microbial community. Total coliform and V. cholerae concentration were measured using novel membrane filtration technique methods. It was found that total coliform concentration does not indicate V. cholerae concentration in water, and total coliform removal efficiency does not indicate V. cholerae removal efficiency within biosand filters. Additionally, parameters controlling biosand filter performance include: schmutzdecke composition, time in operation, and idle time.

In the second part of this dissertation, V. cholerae challenge tests were performed on laboratory-operated biosand filters receiving high, medium or low TOC influents in order to determine the effect of number of charges, total organic carbon loading, and schmutzdecke composition on V. cholerae removal efficacy, as well as to isolate the effect of biological removal mechanisms and physical/chemical removal mechanisms on V. cholerae removal efficiency and determine the correlation to TOC concentration in water. To this end, three biosand filters were operated in the lab. Each received lake water or diluted lake water with high, medium or low concentrations of TOC. After being charged once per day for 6 days, the filters were charged with four consecutive charges of pure cultures of V. cholerae suspended in PBS buffer, at concentrations of 102, 103, 105, and 107 cfu/mL. This challenge was repeated each time the filters received an additional 6 charges, up to 66 total charges. This was done to determine how number of charges, TOC loading, and schmutzdecke composition affects removal efficiency. Schmutzdecke was analyzed for amount of EPS and microbial community. It was found that parameters controlling biosand filter performance include: TOC loading, schmutzdecke composition, time in operation, and physical/chemical attachment. Additionally, it was shown that physical/chemical attachment is critical during startup, especially at low TOC concentrations. At steady state, physical/chemical attachment is more important than schmutzdecke effects in filters receiving low TOC, and schmutzdecke effect is more important than physical/chemical attachment in filters receiving high TOC.

For the third section of this dissertation, columns packed with cocopeat, celite, or sphagnum peat were charged with simulated wastewater and removal efficiencies of nitrogen, phosphorus, and biological oxygen demand were measured. Additionally, different redox zones were tested to determine if cocopeat could successfully accomplish nitrification and denitrification. It was found that cocopeat is comparable to traditional packing media and can successfully accomplish nitrification and denitrification in the treatment of synthetic wastewater.

In the final section of this dissertation, constructed wetlands were built and packed with cocopeat to determine if cocopeat is a suitable packing media in constructed wetlands treating wastewater in Vietnam. Removal efficiencies of nitrogen, phosphorus, and biological demand were measured. Microbial community samples were collected periodically in order to analyze community shifts between wetlands and over time. This work concluded that cocopeat can be used successfully as a packing media in constructed wetlands treating wastewater for the removal of nitrogen, phosphorus, and total coliform.

Overall, this dissertation work contributes to the body of knowledge on point-of-use water and wastewater technologies. The biosand filter was studied in both lab and field conditions and it was found that total coliform is not a reliable indicator for V. cholerae, and that there are several factors controlling biosand filter performance, including idle time, TOC, filter time in operation, physical/chemical attachment, and schmutzdecke composition. Cocopeat was studied for its ability to promote nitrification and denitrification in lab-scale vertical flow columns treating synthetic wastewater. It was shown that cocopeat achieved similar levels of nitrification and denitrification as traditional packing media. Finally, cocopeat packed vertical flow constructed wetlands were operated in Vietnam for the treatment of septic tank effluent. This setup proved effective for the removal of nitrogen, phosphorus, and total coliform in the treatment of wastewater.

Item Open Access Effects of Earthquake Source Recurrence on the Conditional Seismic Hazard Analysis(2012) Kuzucu, Ismail BahadirIn earthquake engineering, the selection of input ground motions for nonlinear structural dynamic analysis is an important element in performance based structural design. Probabilistic seismic hazard analysis (PSHA) provides a basis for determination of ground motion characteristics by incorporating regressions on ground motion metrics from past recorded earthquakes for known seismic sources, propagation paths and local site conditions. Due to aleatoric variability in the processes of fault rupture, seismic wave propagation, and local site response; and, the epistemic uncertainty in models of these phenomena due in part to limited data, there exist uncertainties in seismic hazard assesments. This uncertainty should be interpreted carefully before the resulting ground motion predictions are adopted for dynamic analysis and design of structures. This thesis examines the sensitivity of algorithmic parameters and the choice of earthquake magnitude recurrence relations on the conditional ground motion characteristics. For this purpose, new correlation coefficients between pseudo-spectral acceleration (PSA), peak ground velocity (PGV), and cumulative absolute velocity (CAV) are derived using data from the PEER-NGA earthquake database. Finally, the sensitivity of the conditional mean spectrum to earthquake recurrence models is investigated. It is concluded that the choice of the Gutenberg-Richter or Characteristic Magnitude model can significantly affect the conditional mean spectra.

Item Open Access Elucidating Atmospheric Turbulence Across Scales in Numerical Models: Land-Atmosphere Interaction Controls of Moist Processes(2020) Eghdami, MasihThis research aims to understand the development of the atmospheric energy spectrum and energy transfer mechanisms across scales. A clear understanding of energy spectrum development and transfer mechanisms is necessary for improving the representation of multiscale processes in the atmosphere, modeling turbulence in the boundary layer, and understanding the limits of atmospheric predictability. This work consists of three parts.

The first part investigates the Navier Stokes Equations (NSE) behavior under idealized conditions relevant to large-scale atmospheric turbulence. This study uses a series of direct numerical simulations (DNS) of two-dimensional (2D) with forcing applied at different scales to investigate energy transfer between the synoptic scale and the mesoscale. The DNS results, forced by a single kinetic energy source at large scales, show that the energy spectra slopes of the direct enstrophy cascade are steeper than the theoretically predicted -3 slope. Next, the presence of two inertial ranges in 2D turbulence at intermediate scales is investigated by introducing a second energy source in the meso-a-scale range. The energy spectra for the simulations with two kinetic energy sources exhibit flatter slopes closer to -5/3, consistent with the observed kinetic energy spectra of horizontal winds in the atmosphere at synoptic scales. Further, the results are independent of model resolution and scale separation between the two energy sources, with a robust transition region between the lower synoptic and the upper meso-a scales in agreement with classical observations in the upper troposphere. These results suggest the existence of mesoscale feedback on synoptic-scale predictability that emerges from the concurrence of the direct (downscale) enstrophy transfer in the synoptic scales and the inverse (upscale) kinetic energy transfer from the mesoscale to the synoptic-scale in the troposphere.

The second part of this research is devoted to the characterization of atmospheric energy spectra over mountainous terrain under various environmental conditions using the Weather and Research Forecasting (WRF) model. First, a comprehensive analysis of climatology and mesoscale structure of cold air intrusions (CAIs) over the Andes shows that the events are responsible for localized heavy rainfall events (200 mm, less than 6 hours) in the mid-elevations (~1,500) Peruvian Andes. The climatology analysis uses European Center Medium-Range Weather Forecasts (ECMWF) reanalysis, Tropical Rainfall Measurement Mission (TRMM) data products, and rain-gauge observations. This analysis emphasized characterizing year-round CAI frequency, CAI interactions with Andes topography, and their impact on orographic precipitation climatology. The results show a robust enhancement of the diurnal cycle of precipitation during CAI events in all seasons, particularly in the increase in surface rainfall rate during early morning at intermediate elevations (~ 1,500m), the eastern Andes orographic maximum. This link between CAI frequency and rainfall suggests that they play an essential role in maintaining the Andes to Amazon year-round terrestrial connectivity through runoff production and transport by the river networks. Second, the next step examines the transient behavior of horizontal scaling in the vertical during the evolution of extreme orographic precipitation storms. Furthermore, a mechanistic framework to examine the implications of ongoing deforestation in the western Amazon on orographic precipitation in the tropical Andes through land-atmosphere interactions is provided. Understanding the interplay between large-scale dynamics and land-atmosphere interactions is critical to developing an effective boundary layer processes parameterizations for future numerical weather prediction models. The study includes a case over the Appalachians as middle mountains in comparison to high mountains (Andes) highlighting terrain and weather contrasts. Previous work showed that atmospheric model simulations exhibit different scaling behavior of vertically averaged horizontal wind (u, v) and moisture (q) in the mesoscales for convective (spectral slopes β~−5/3) and non-convective (β~−11/5) conditions. Here, the results show that β exhibits a strong diurnal cycle switching between convective and non-convective behavior following the space‐time evolution of atmospheric stability in the lower troposphere (below 700 hPa) depending on latitude, topography, landform, and the synoptic environment. Anomalous flattening of the wind and moisture spectra (i.e., spectral saturation, ∣β ∣ < 5/3) at high wavenumbers and up to 200 hPa is tied to convective activity, where and when strong vertical motions develop, corresponding to an abrupt directional switch from horizontal energy transfer to vertical energy transfer including latent heating release and parameterized microphysical processes. In the small mesoscales (<50 km), β~ − 5/3 at all times up to 200 hPa with nighttime steepening (β~−11/5) below the orographic envelope where cold air pools form at low elevations and vertical motion weakens in the Appalachians. In the Andes, at a high elevation, the scaling behavior exhibits a stronger diurnal cycle at low levels (below 700 hPa) with significant shoaling between tropical and high latitudes. Furthermore, blocking and strong modification of regional circulations result in nighttime anisotropy at midlevels on the altitudinal profile along the North‐South topographic divide.

The third part focuses on modeling turbulent fluxes near the surface, which is essential for an accurate representation of the heterogeneous surface boundary layer. Second-moment turbulent models have been widely used in numerical weather prediction models for parameterizing the planetary boundary layer (PBL). The most commonly used schemes are based on the Mellor and Yamada (1982) model, which are currently implemented to only account for the contribution of the vertical divergences of the vertical turbulent fluxes in the WRF model. Horizontal tendencies are parameterized separately based on a Smagorinsky scheme. Although this approach may be successful in coarse grid models (e.g., dx~12-2 km), the influence of horizontal gradients becomes more important when the grid spacing is smaller (less than 1 km). Recently, a full 3D PBL scheme (3DPBL) has been implemented in WRF to reconcile the representation of the vertical and horizontal turbulent mixing. The model integrates 3DPBL parameterization with a diagnostic model of the three-dimensional second-order turbulent properties of the flow in the surface layer. A set of modifications introduced to the surface parameters provides a better diagnosis of the surface layer covering different flow regimes based on anisotropy and stability conditions. The near-surface diagnostic variables are analyzed and compared with the data from the Weather Forecast Improvement Project II (WFIPII).

Finally, the dissertation discusses and recommends potential directions for future research focusing on boundary layer processes.

Item Open Access Experimental Study on Geomaterial’s Moisture Content Distribution and Deformation During Drying Process(2022) Wu, FeiKnowledge of the drying process of geomaterials is meaningful and helpful in the field of geotechnical and geo-environmental engineering. This experimental study focuses on drying tests on geomaterial samples with monitored surface moisture contents and controlled environmental conditions. Using the digital image correlation (DIC) method to analyze the sample’s displacement, the 3D displacement plots and volumetric strain maps are obtained after calculation. By combining the monitored moisture content with analyzed displacement and volumetric strain plots, the phenomenon and characteristics of a geomaterial’s drying process are discussed and concluded. This study offers a better understanding of the deformation of geomaterials during the drying process in 3D.

Item Open Access Exploring Chemical Enhancement of Subcritical Fractures in Geomaterials(2013) Hu, ManmanPropagation of subcritical cracks is studied in a geomaterial subject to weakening by the presence of water, which dissolves a mineral component of it. Such weakening is common when tensile micro-cracks develop, constituting sites of an enhanced mineral dissolution. Meanwhile, the dissolution process at each active site of the inter-surface is affected by the chemical properties of the environment, e.g. the PH value. In this research, a previous concept of reactive chemo-plasticity is adopted with the yield limit depending on the mineral mass dissolved and causing a chemical softening. The dissolution is described by a rate equation and is a function of a variable internal specific surface area, which in turn is assumed to be a function of the dilative plastic deformation. Two loading modes are adopted to investigate the chemical enhancement of propagation of a single crack. The behavior of the material is rigid-plastic with a chemical softening. The extended Johnson approximation is adopted, meaning that all the fields involved are axisymmetric around the crack tip with a small, unstressed cavity around it. An initial dissolution proportional to the initial porosity activates the plastic yielding. The total dissolved mass diffuses out from the process zone, and the exiting mineral mass flux can be correlated with the displacement of the crack tip. A calibration against available data will be performed in the future, followed by a series of experiments to simulate the real case.

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