Traditional studies of the modernization-instability thesis have neglected the simultaneous influence of time and place on the relationship between modernization (social mobilization and political participation) and political instability, and the possible causal linkage between the two concepts. Empirical support for modernization-instability hypothesis will be obtained if and only if there is a strong positive correlation between modernization and political instability and the former causes the latter unidirectionally. Only then can one assert that modernization is exogenous, and that a policy geared toward restricting modernization is a proper anti-instability policy. This work attempts to address the question of correlation and causality through a pooled time-series cross-sectional data design and the use of Granger-causality tests. Particular attention is paid to the error structure of the models. Using pooled regression, a model of political instability is estimated for a total of 35 countries for the period 1960-1982. Granger tests are performed on twelve separate countries randomly selected from the 35. The results indicate that there is the expected positive relationship between modernization and political instability. Further, political institutionalization and economic well-being have strong negative influence on political instability. With regard to causality, the results vary by country. Some countries experience no causality between modernization and political instability, while some witness bidirectional causality. Further, some nations experience unidirectional causality running from modernization to political instability, while some depict a reverse causation. The main results suggest that modernization and political instability are positively related, and that political instability can have causal influence on modernization, just as modernization can exert causal influence on political instability.

Arch-shaped microelectromechanical systems (MEMS) have been used as mechanical memories, micro-sensors, micro-actuators, and micro-valves. A bi-stable structure, such as an arch, is characterized by a multivalued load deflection curve. Here we study the symmetry breaking, the snap-through instability, and the pull-in instability of bi-stable arch shaped MEMS under steady and transient electric loads. We analyze transient finite electroelastodynamic deformations of perfect electrically conducting clamped-clamped beams and arches suspended over a flat rigid semi-infinite perfect conductor. The coupled nonlinear partial differential equations (PDEs) for mechanical deformations are solved numerically by the finite element method (FEM) and those for the electrical problem by the boundary element method. The coupled nonlinear PDE governing transient deformations of the arch based on the Euler-Bernoulli beam theory is solved numerically using the Galerkin method, mode shapes for a beam as basis functions, and integrated numerically with respect to time. For the static problem, the displacement control and the pseudo-arc length continuation (PALC) methods are used to obtain the bifurcation curve of arch's deflection versus the electric potential. The displacement control method fails to compute arch's asymmetric deformations that are found by the PALC method. For the dynamic problem, two distinct mechanisms of the snap-through instability are found. It is shown that critical loads and geometric parameters for instabilities of an arch with and without the consideration of mechanical inertia effects are quite different. A phase diagram between a critical load parameter and the arch height is constructed to delineate different regions of instabilities. The local water slamming refers to the impact of a part of a ship hull on stationary water for a short duration during which high local pressures occur. We simulate slamming impact of rigid and deformable hull bottom panels by using the coupled Lagrangian and Eulerian formulation in the commercial FE software LS-DYNA. The Lagrangian formulation is used to describe planestrain deformations of the wedge and the Eulerian description of motion for deformations of the water. A penalty contact algorithm couples the wedge with the water surface. Damage and delamination induced, respectively, in a fiber reinforced composite panel and a sandwich composite panel and due to hydroelastic pressure are studied. / Ph. D.

The interaction of vortices passing near free and solid surfaces has been examined using direct numerical simulation (DNS). A computer code was developed which solves the unsteady, three-dimensional Navier-Stokes equations for incompressible flow. A critical element of the numerical scheme is the efficient solution of Poisson's equation. A state of the art solver based on multigrid techniques was developed which gives excellent convergence rates. The result is a tool capable of modeling complex three-dimensional flows in a variety of configurations. Three different flow fields have been examined in order to determine some of the complex interactions involved between a vortex and a surface. The first concerns the two-dimensional interaction between a boundary layer and a convecting vortex. The size and height above the wall of the vortex are the same order of magnitude as the boundary layer thickness. A strong primary vortex creates a secondary vortex which causes the rebound of the primary, a response observed in many previous studies. However, weaker vortices as well do not follow the inviscid trajectory despite the absence of a secondary vortex. Rather than creating vorticity at the wall, a weaker vortex mainly redistributes the vorticity of the boundary layer. The redistributed vorticity alters the path of the vortex in ways not seen for vortex/wall interactions. / Ph. D.

We use molecular mechanics (MM) simulations with the tight-binding (TB) potential to study local and global instabilities in initially defect-free finite specimens of gold crystals deformed in shear, simple shear, tension/compression, simple tension/compression, and triaxial tension/compression. The criteria used to delineate local instabilities in a system include the following: (i) a second order spatial derivative of the displacement field having large values relative to its average value in the body, (ii) the minimum eigenvalue of the Hessian of the potential energy of an atom becoming nonpositive, (iii) and structural changes represented by a high value of the common neighborhood parameter. A specimen becomes globally unstable when its potential energy decreases significantly with a small increase in its deformations. It is found that the three criteria for local instability are satisfied essentially simultaneously at the same atomic position. Deformations of a specimen are quite different when it is deformed with some bounding surfaces free from external forces as opposed to essential boundary conditions prescribed on all bounding surfaces. It is found that the initial unloaded configuration (or the reference configuration) of the minimum potential energy has significant in-plane stresses on the bounding surfaces and nonzero normal stresses at interior points. In tensile/compressive deformations of a rectangular prismatic nanobar the yield stress defined as the average axial stress when the average axial stress vs. the average axial strain curve exhibits a sharp discontinuity depends upon the specimen size; a similar result holds for simulations of shear deformations. Specimens deformed with essential boundary conditions on all bounding surfaces experience instabilities at a higher value of the average strain than identical specimens deformed similarly but with one or more pairs of opposite bounding surfaces traction free. For the former set of deformations, the response of a specimen prior to the onset of instability is the same as that of a hyperelastic body with the strain energy derived from the TB potential and deformations obeying the Cauchy-Born rule. Specimens with some traction free bounding surfaces experience local instabilities prior to the onset of a global instability but the two instabilities occur simultaneously in specimens with essential boundary conditions prescribed on all bounding surfaces. It is believed that because of residual stresses in the reference configuration, the average axial stress at yield in compression is nearly one-half of that in tension. / Ph. D.

Stimulated Electromagnetic Emissions SEEs may provide important diagnostic information about space plasma composition, energetics, and dynamics during active experiments in which ground-based high powered radio waves are transmitted into the ionosphere. The nonlinear plasma processes producing this secondary radiation are not well understood particularly for some recent observations where the transmitter (pump) frequency is near the second harmonic of the electron gyro-frequency. New, more comprehensive, experimental observations of spectral features within 1kHz of the pump wave frequency are reported here in order to begin more careful comparisons of the experimental observations and a possible theoretical underpinning which is also provided. The experimental observations typically show two distinct types of secondary radiation spectra which are a) discrete narrowband harmonic spectral structures ordered by the ion gyro-frequency, so-called stimulated ion Bernstein Scatter (SIBS) and, b) broadband spectral structure with center frequency near 500 Hz and similar spectral bandwidth named ion acoustic parametric decay (IAPD). A theoretical model is provided that interprets these spectral features as resulting from parametric decay instabilities in which the pump field ultimately decays into high frequency upper hybrid/electron Bernstein and low frequency neutralized ion Bernstein and/or obliquely propagating ion acoustic waves at the upper hybrid interaction altitude. Detailed calculations of the threshold level, growth rate, unstable wavenumber and frequency bandwidth of the instabilities are provided for comparisons with experimental observations. An assessment of the effect of the critical instability parameters are provided including pump electric field strength, proximity of the pump frequency to the electron gyro-frequency and pump electric field geometry. The model shows quite reasonable agreement with the experimental observations. Next, a two dimensional Particle-In-Cell Monte-Carlo Collision computational model (PIC-MCC) is employed in order to consider nonlinear aspects such as 1) electron acceleration through wave-particle heating, 2) more complex nonlinear wave-wave processes and 3) temporal evolution of electron irregularities through nonlinear saturation. The simulation results show that the IB associated parametric decay is primarily associated with electron acceleration perpendicular to the geomagnetic field. More gyro-harmonic lines are typically associated with more intense heating. Heating is reduced when the pump frequency is sufficiently close to 2fce. The IA associated parametric decay instability is primarily associated with electron tail heating along the magnetic field and heating is reduced when the pump frequency is sufficiently close to 2fce. Characteristics of caviton collapse behavior become prevalent in this case. Results are discussed within the context of some recent experimental observations. Further discussions are provided of connections with past observed SEE spectral features and potential new diagnostic information provided by these newly categorized spectra. / Ph. D.

<p dir="ltr">Thermo-acoustic interactions, characterized by the coupling between heat release and acoustic waves, are a phenomenon that can lead to combustion instability in high-speed propulsion devices. These interactions are highly undesirable as they can damage engine components and, in severe cases, cause catastrophic failure of the entire propulsion system. Mitigating these instabilities is crucial for ensuring reliable combustor operation. This work presents a computational investigation of combustion instability in Purdue's Continuously Variable Resonance Combustor (CVRC), focusing on the prediction of instability trend over the entire oxidizer-post length range. Computational fluid dynamics (CFD) studies in the past mainly focused on individual CVRC cases with specific oxidizer-post lengths. Those studies help understand the instability mechanism for individual CVRC cases but are limited in examining the applicability of model predictions over a wide range of instability conditions. No studies have been reported to assess the model predictivity over the entire oxidizer-post range in CVRC. </p><p dir="ltr">In this work, we first conduct a series of CFD simulations that cover the entire oxidizer-post length in CVRC to assess the models for a wide range of instability conditions. It is found that the CFD models generally fail to capture the instability trend over the entire oxidizer-post length although they can capture some individual cases. To understand the model failure, parametric studies are often deemed the first step of investigation. Such parameter studies, however, are expensive for CVRC since more than ten simulation cases to cover the entire oxidizer-post range are needed for each parametric study. Multiple parametric studies are typically needed to cover various uncertainties from numerics and physical models and those involved in the experimental conditions, making parametric studies for CVRC a computationally expensive task. Therefore, our focus next is on developing faster approaches.</p><p dir="ltr">The second part of this work is to develop a reduced-order model to quickly conduct the needed parametric studies. The developed reduced-order model leverages the instability mechanisms observed from the CFD simulations conducted in the first part. Monte Carlo approaches are employed to replace expensive CFD simulations by replicating the randomness in the combustor through statistical sampling. The developed reduced-order model is first validated by comparing its predictions with the CFD simulation results in a number of cases. The reduced-order model, despite its simplicity, reasonably reproduced the overall trend of instability from CFD simulations, making it an attractive alternative to the detailed model simulations for parametric studies. </p><p dir="ltr">The validated reduced-order model is then applied to parametric studies of CVRC to help identify the uncertainties of CFD predictions of CVRC. Four sets of parametric studies are conducted to provide a rapid examination of the effect of heat loss, the effect of oxidizer temperature, the effect of equivalence ratio, and the effect of turbulence on the instability predictions in CVRC. From the rapid reduced-order parametric studies, we found that the heat losses in upstream of the oxidizer inlet and the combustor wall are the two most contributing factors to the uncertainties of CFD model predictions. The turbulence level and the error involved in the equivalence ratio due to experimental uncertainties play an insignificant role in contributing to the CFD prediction uncertainties. </p><p dir="ltr">This work is a significant contribution to the combustion instability community by enabling an alternative rapid assessment of CFD model predictions. This capability facilitates the identification of major contributing factors of CFD modeling uncertainties with much less computational cost, thereby allowing for a more focused approach to CFD analysis and ultimately accelerating the improvement of CFD models for combustion instability studies. </p>

Recent concepts for controlled magneto-inertial fusion (MIF), such as magnetized liner inertial fusion (MagLIF), have suffered from magnetohydrodynamic (MHD) instabilities that lead to degradations in fusion yield. High levels of azimuthally-correlated MHD instability structures have been observed on cylindrical liner experiments without a pre-imposed axial magnetic field (Bz=0) elsewhere in the literature and are believed to be seeded from surface machining roughness. This dissertation uses highly resolved (0.5 μm and less resolution) 1D and 2D resistive magnetohydrodynamics (MHD) arbitrary-Lagrangian-Eulerian (ALE) simulations of electrical wire explosions (EWEs) and liner implosions to show that micrometer-scale surface roughness seeds the electrothermal instability (ETI), which induces early melting in pockets across the conductor and leads to millimeter-scale instability growth. The relationship between the ETI and the MRTI in liner implosions is also described in this dissertation, which shows that the traditional growth rates associated with these modes are coupled together and are not linearly independent. This dissertation also describes the preliminary implementation of a Koopman neural network architecture for learning the nonlinear dynamics of a high energy density (HED) exploding or imploding electrical conductor. / Doctor of Philosophy / Researchers have been working on controlling nuclear fusion and harnessing it as a power source since the discovery that nuclear fusion powers stars. In many of these controlled nuclear fusion concepts the aim is to heat the fuel until it forms a high-temperature plasma state of matter and then compress it to the point that the atoms are close enough and at high enough speeds that they collide fuse together. In the magnetized liner inertial fusion (MagLIF) concept these temperatures, densities, and pressures are achieved by surrounding the fusion fuel with a cylindrical piece of metal called a liner and using magnetic fields to implode the liner inward. Experiments have shown, however, that these liner implosions do not occur smoothly and that the system becomes unstable and can mix liner material into the fuel, which disrupts the fusion process. This dissertation investigates the stability of liner implosions and electrical wire explosions. In particular, this dissertation shows that surface roughness imparted on the surface of a solid fusion target by a machining process can grow into a millimeter-scale perturbation. It also describes the relationship between two common types of instabilities found in current-driven nuclear fusion: the magneto-Rayleigh-Taylor instability and the electrothermal instability. Finally, it looks at using neural networks to better understand the dynamics of electrical wire explosions.

A computational study aimed at improving the understanding of rotating instability in the LP steam turbine last stage working under low flow rate conditions is described in this thesis. A numerical simulation framework has been developed to investigate into the instability flow field. Two LP model turbine stages are studied under various flow rate conditions. By using the 2D simulations as reference and comparing the results to those of the 3D simulations, the basic physical mechanism of rotating instability is analysed. The pressure ratio characteristics across the rotor row tip are found to be crucial to the inception of rotating instability. The captured instability demonstrates a 2D mechanism based on the circumferential variation of unsteady separation flow in the rotor row. The 3D tip clearance flow is found not a necessary cause of the instability onset. Several influential parameters on the instability flow are also investigated by a set of detailed studies on different turbine configurations. The results show that the instability flow pattern and characteristics can be altered by the gap distance between the stator and rotor row, the rotor blading and the stator row stagger angle. Some flow control approaches are proposed based on the observations, which may also serve as design reference. The tip region 3D vortex flow upstream to the rotor row is also captured by the simulations under low flow rate conditions. Its appearance is found to be able to suppress the inception of rotating instability by disrupting the interaction between the rotor separation flow and the incoming flow. Finally, some recommendations for further work are proposed.

This project explores the link between exposure to family instability through maternal repartnering and the development of romantic interest and involvement during childhood and preadolescence. Family instability has long reaching consequences, especially for children's well being. Exposure to family instability spills over into children's own lives, implying parents and children's lives are inextricably linked. The actions, experiences, and choices of parents have a direct and measurable impact on children's lives, as evidenced by links between parents' marital lives and their offspring's romantic involvement during adolescence and adulthood. The goal of this project is twofold. First, I build on this literature to explore how maternal repartnering impacts children's romantic interest and involvement. Through various mechanisms such as increased awareness of romance or searching for a substitute in response to maternal repartnering, I expect children's romantic interest and involvement will be associated with maternal repartnering behaviors and attitudes. The second goal of this project explores this association among a sample of children and preadolescents, examining this link during an earlier period in the life course than existing literature. This includes an in depth examination of the characteristics and developmental trajectories of romantic interest and involvement beginning at age five and extending to age 13. Results suggest that romantic interest and involvement emerges during childhood and is associated with both individual and family level characteristics. As children mature, they report substantially greater levels of romantic involvement with each passing year. Changes in romantic involvement correspond to greater social and pubertal development. Children's reports of romantic interest did not show developmental change but were relatively stable over time. There were no consistent effects of maternal repartnering on children's romantic involvement. Children's romantic interest was linked with maternal repartnering attitudes however. Higher levels of maternal focus on repartnering were associated with greater romantic interest among children, suggesting that maternal repartnering does impact children's romantic trajectories. / text

Repetitive stress injuries are common in the workplace where workers perform repetitive tasks continuously throughout the day. Muscle fatigue may lead to injury either directly through muscle damage or indirectly through changes in coordination, development of muscle imbalances, kinematic and muscle activation variability, and/or movement instability. To better understand the role of muscle fatigue in changes in movement parameters, we studied how muscle fatigue and muscle imbalances affected the control of movement timing, variability, and stability during a repetitive upper extremity sawing task. Since muscle fatigue leads to delayed muscle and cognitive response times, we might expect the ability to maintain movement timing would decline with muscle fatigue. We compared timing errors pre- and post-fatigue as subjects performed this repetitive sawing task synchronized with a metronome using standard techniques and a goalequivalent manifold (GEM) approach. No differences in basic performance parameters were found. Significant decreases in the temporal correlations of the timing errors and velocities indicated that subjects made more frequent corrections to their movements post-fatigue. Muscle fatigue may lead to movement instability through a variety of mechanisms including delayed muscle response times and muscle imbalances. To measure movement stability, we must first define a state space that describes the movement. We compared a variety of different state space definitions and found that state spaces composed of angles and velocities with little redundant information provide the most consistent results. We then studied the affect of fatigue on the shoulder flexor muscles and general fatigue of the arm on movement stability. Subjects were able to maintain stability in spite of muscle fatigue, shoulder strength imbalance and decreased muscle cocontraction. Little is known about the time course for adaptations in response to fatigue. We studied the effect of muscle fatigue on movement coordination, kinematic variability and movement stability while subjects performed the same sawing task at two work heights. Increasing the height of the task caused subjects to make more adjustments to their movement patterns in response to muscle fatigue. Subjects also exhibited some increases in kinematic variability at the shoulder but no changes in movement stability. These findings suggest that people alter their kinematic patterns in response to fatigue possibly to maintain stability at the expense of increased variability. / text