# Browsing by Subject "Instability"

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Item Determining Bounds for a Pressure Hazard Rating to Augment the NFPA 704 Standard(2012-02-14) Hodge, PhillipShow more Hazard communication is an essential part of a comprehensive safety plan, especially for those facilities that contain reactive chemicals. There are a variety of means of communicating a chemical hazard, but one of the most prevalent in the United States is the Instability Rating found in the NFPA 704 standard. While the NFPA 704 identifies hazards associated with exothermically decomposing compounds, it neglects compounds that decompose endothermicly to form large quantities of gas. Such compounds have been known to cause accidents due to pressure buildup, such as in the BP Amoco Polymers explosion in 2001. In this work, twenty-five compounds were examined via an APTAC to determine their pressure and temperature profiles. These profiles were then used to determine the amount of gas generated, the gas generation rate, the gas generation product, the onset temperature, and the instantaneous power density. These properties were analyzed to determine those that best represented the instability hazard of the chemical. Ultimately, the molar gas generation rate and onset temperature were chosen to rate the selected chemicals, and new cut-offs were established to divide the chemicals into revised instability groupings. Compounds that did not decompose in the temperature range examined were given the rating of zero. Compounds with low onset temperatures and high gas generation rates were assigned the rating of 4, while chemicals with high T_onset and low dn/dt_maxn were assigned a value of 1. Chemicals with high onset temperatures and high gas generation rates were grouped into rating 3. Group 2 included low onset temperature compounds with low gas generation rates. The cut-offs used to define these regions were 130 degrees C for the onset temperature and 0.01 (1/min) for the gas generation rate. The ratings were found to be comparable to the current NFPA system, but improved upon it by providing a valid rating (group 1) for the chemicals that endothermically generated gas. Detailed plots of the data are provided as well as suggestions for future work.Show more Item Diagnostics of subsynchronous vibrations in rotating machinery - methodologies to identify potential instability(Texas A&M University, 2005-11-01) Kar, RahulShow more Rotordynamic instability can be disastrous for the operation of high speed turbomachinery in the industry. Most ??instabilities?? are due to de-stabilizing cross coupled forces from variable fluid dynamic pressure around a rotor component, acting in the direction of the forward whirl and causing subsynchronous orbiting of the rotor. However, all subsynchronous whirling is not unstable and methods to diagnose the potentially unstable kind are critical to the health of the rotor-bearing system. The objective of this thesis is to explore means of diagnosing whether subsynchronous vibrations are benign or have the potential to become unstable. Several methods will be detailed to draw lines of demarcation between the two. Considerable focus of the research has been on subharmonic vibrations induced from non-linear bearing stiffness and the study of vibration signals typical to such cases. An analytical model of a short-rigid rotor with stiffness non-linearity is used for numerical simulations and the results are verified with actual experiments. Orbits filtered at the subsynchronous frequency are shown as a diagnostic tool to indicate benign vibrations as well as ??frequency tracking?? and agreement of the frequency with known eigenvalues. Several test rigs are utilized to practically demonstrate the above conclusions. A remarkable finding has been the possibility of diagnosing instability using the synchronous phase angle. The synchronous phase angle ?? is the angle by which the unbalance vector leads the vibration vector. Experiments have proved that ?? changes appreciably when there is a de-stabilizing cross coupled force acting on the rotor as compared to when there is none. A special technique to calculate the change in ?? with cross-coupling is outlined along with empirical results to exemplify the case. Subsequently, a correlation between the synchronous phase angle and the phase angle measured with most industrial balancing instruments is derived so that the actual measurement of the true phase angle is not a necessity for diagnosis. Requirements of advanced signal analysis techniques have led to the development of an extremely powerful rotordynamic measurement teststand ?? ??LVTRC??. The software was developed in tandem with this thesis project. It is a stand-alone application that can be used for field measurements and analysis by turbomachinery companies.Show more Item From growth-based to people-centered : how Chinese leaders have modified their governing strategies to sustain legitimacy in the reform era(2013-12) Zhang, Wenjie, active 2013; Galbraith, James K.Show more This dissertation analyzes changes in the ruling strategies of the Chinese Communist Party (CCP) in the context of economic reforms, beginning in 1978. By employing both quantitative and qualitative methods, this dissertation investigates how Chinese leaders have utilized legitimating strategies, while modifying their governing strategies, in order to a) solidify the population, b) consolidate ruling authority and c) maintain political and social stability. Specifically, this dissertation looks at how Chinese policymakers have developed effective public policies in response to rapidly rising wage inequality, one of the most pressing problems undermining the CCP’s ruling authority. By providing an original estimate of China’s wage inequality and analyzing the government’s response to it, this dissertation provides a unique look at how the CCP has transformed government functions from growth-based to people-centered to meet various social, political and economic challenges. A comparative statistical analysis helps illustrate the philosophical roots and sources of the CCP’s political legitimacy. The technique of Theil Statistics is applied to measure China’s wage inequality during the reform period. A multivariate hierarchical regression analysis is employed to measure the impact of rising inequality on Chinese society. Two models on social welfare system reform are studied in order to understand Hu Jintao and Wen Jiabao’s people-centered governing philosophy and the rationale for constructing a service-oriented government.Show more Item Instability of High Mach Number Poiseuille (Channel) Flow: Linear Analysis and Direct Numerical Simulations(2014-08-06) Xie, ZhiminShow more Poiseuille flow is a prototypical wall-bounded flow in which many fundamental aspects of fluid physics can be analyzed in isolation. The objective of this research is to establish the stability characteristics of high-speed laminar Poiseuille flow by examining the growth of small perturbations and their subsequent breakdown toward turbulence. The changing nature of pressure is considered critical to the transformation from incompressible to compressible behavior. The pressure-velocity interactions are central to the present investigation. The study employs both linear analysis and temporal direct numerical simulations (DNS) and consists of three distinct parts. The first study addresses the development and validation of the gas kinetic method (GKM) for wall-bounded high Mach number flows. It is shown that sustaining the Poiseuille flow using a body force rather than pressure-gradient is better suited for accurate numerical simulations. Effect of uniform and non-uniform grids on the simulation outcomes is examined. Grid resolution and time-step convergence studies are performed over the range of Mach numbers of interest. The next study establishes the stability characteristics at very high and very low Mach number limits. While stability at low Mach number limit is governed by the well-established Orr-Sommerfeld analysis, the pressure-released Navier-Stokes equation is shown to accurately characterize stability at the infinite Mach number limit. A semi-analytical stability evolution expression is derived. It is shown that the GKM numerical approach accurately captures the low and high Mach number solutions very precisely. The third study examines the critical effects of perturbation orientation and Mach number on linear stability, and investigates the various stages of perturbation evolution toward turbulent flow. This study can break into two parts. In the first part, an initial value linear analysis is performed to establish the self-similar scaling of pressure and velocity perturbations. The scaling then is confirmed with DNS. Based on analytical and numerical results, regions of stability and instability in the orientation space are established. Compressibility is shown to strongly stabilize streamwise perturbations. However, span-wise modes are relatively unaffected by Mach number. The multiple stages of temporal perturbation evolution are explained. The manner of Tollmien-Schlichting (TS) instability suppression due to compressibility is also described. In the second part, the progression from linear to nonlinear to preliminary stages of breakdown is examined. It is shown that nonlinear interactions between appropriate oblique perturbation mode pairs lead to span-wise and streamwise modes. The streamwise modes rapidly decay and span- wise perturbations are ultimately responsible for instability and breakdown toward turbulence. Overall, the studies performed in this research lead to fundamental advances toward understanding transition to turbulence in wall-bounded high-speed shear flows. Such an understanding is important for developing transition prediction tools and flow control strategies.Show more Item The macro- and micro-instabilities in the pedestal region of the Tokamak(2015-05) Ma, Jingfei; Morrison, Philip J.; Horton, C. W. (Claude Wendell), 1942-; Berk, Herbert; Fitzpatrick, Richard; Hallock, GaryShow more In this paper, we present the theoretical and numerical studies of the linear characteristics and nonlinear transport features of the instabilities driven by the steep profile gradient and edge current in the pedestal region of the tokamak. Two important instabilities, the peeling-ballooning (P-B) modes (macro-instability) and the drift-Alfven modes (micro-instability), are studied using the fluid analysis and the BOUT++ codes. In particular, the edge-localized modes (ELMs), which appear to be the energy burst in the nonlinear stage of the peeling-ballooning mode, are numerically studied and the results are compared with the experimental measurement. In addition, the features of the impurity transport in the edge region of the tokamak are theoretically analyzed. Firstly, we explore the fundamental characteristics of the P-B modes and the ELM bursts numerically using the three-field reduced MHD model under the BOUT++ framework, in the shifted-circular geometry, i.e. the limiter tokamak geometry. In the linear simulations, the growth rate and real frequency and the mode structure versus the toroidal mode number (n) are shown. The features of the ELM bursts are shown in the nonlinear simulations, including the time evolution of the relative energy loss (ELM size) and the pedestal profile. Secondly, two original research projects related to the P-B modes and the ELM burst are described. One is the study of the scaling law between the relative energy loss of ELMs and the edge collisionality. We generate a sequence of shifted-circular equilibria with different edge collisionality varying over four orders of magnitude using EFIT. The simulation results are in good agreement with the multi-tokamak experimental data. Another is the study of the differences of the linear behaviors of the P-B modes between the standard and snowflake divertor configurations. Using DIII-D H-mode ElMing equilibria, we found that the differences are due to the local magnetic shear change at the outboard midplane, which is the result of the realization of the snowflake configuration. Finally, the micro-instability, the drift-Alfven instability in the pedestal region of the DIII-D tokamak is studied. A modified six-field Landau fluid model under BOUT++ framework is used to study the linear characteristics and transport features of the drift-Alfven modes. Based on the DIII-D H-mode discharge, a sequence of divertor tokamak equilibria with different pedestal height is generated by the ’VARYPED’ tool for our studies. Qualitative agreement is obtained between theoretical analysis and the simulation results in the linear regime. Moreover, the heat transport induced by the drift-Alfven turbulence is explored and the convection level is estimated for both ions and electrons.Show more Item Stability and turbulence characteristics of a spiraling vortex filament using proper orthogonal decomposition(2015-05) Mula, Swathi Mahalaxmi; Tinney, Charles Edmund, 1975-Show more The stability and turbulence characteristics of a vortex filament emanating from a single-bladed rotor in hover are investigated using proper orthogonal decomposition. The rotor is operated at a tip chord Reynolds number and a tip Mach number of 218,000 and 0.22, respectively, and with a blade loading of CT /σ = 0.066. In-plane components of the velocity field (normal to the axis of the vortex filament) are captured by way of 2D particle image velocimetry with corrections for vortex wander being performed using the Γ1 method. Using the classical form of POD, the first POD mode alone is found to encompass nearly 75% of the energy for all vortex ages studied and is determined using a grid of sufficient resolution as to avoid numerical integration errors in the decomposition. The findings reveal an equal balance between the axisymmetric and helical modes during vortex roll-up which immediately transitions to helical mode dominance at all other vortex ages. This helical mode is one of the modes of the elliptic instability. While the snapshot POD is shown to reveal similar features of the first few energetic modes, the classical POD is employed here owing to the easier interpretation of the Fourier-azimuthal modes. The spatial eigenfunctions of the first few Fourier-azimuthal modes associated with the most energetic POD mode are shown to be sensitive to the choice of the wander correction technique used. Higher Fourier-azimuthal modes are observed in the outer portions of the vortex and appeared not to be affected by the choice of the wander correction technique used.Show more