Browsing by Subject "Vibrational"
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Item First-principles calculations of vibrational lifetimes in silicon(Texas Tech University, 2006-08) West, Damien; Estreicher, Stefan K.; Hase, Bill; Holtz, Mark; Lichti, Roger L.Time-resolved infrared absorption spectroscopy of the local vibrational modes (LVMs) of light impurities in crystalline silicon reveal that the vibrational lifetimes of almost identical LVMs sometimes differ by up to two orders of magnitude. Indeed, at low temperatures, the lifetimes of the 2062cm-1 mode of H2*, the 1998cm-1 mode of H+BC, and the 2072cm-1 mode of V2H2 are 4,8, and 295ps, respectively. Since the optical phonon of Si is about 530cm-1, these decays should all involve at least four phonons and have long and comparable lifetimes. More surprising still, the measured lifetime of the asymmetric stretch of interstitial O in Si changes by almost an order of magnitude with the isotope of O or one of its Si neighbors. In this work, ab-initio molecular dynamic simulations in periodic supercells are used to calculate the temperature dependence of vibrational lifetimes. The theoretical approach developed for these calculations can be applied to the calculation of any vibrational lifetime in any crystal. The calculations predict accurate lifetimes of the various defects in the range 50Item First-principles calculations of vibrational lifetimes in silicon(2006-08) West, Damien; Estreicher, Stefan K.; Hase, Bill; Holtz, Mark; Lichti, Roger L.Time-resolved infrared absorption spectroscopy of the local vibrational modes (LVMs) of light impurities in crystalline silicon reveal that the vibrational lifetimes of almost identical LVMs sometimes differ by up to two orders of magnitude. Indeed, at low temperatures, the lifetimes of the 2062cm-1 mode of H2*, the 1998cm-1 mode of H+BC, and the 2072cm-1 mode of V2H2 are 4,8, and 295ps, respectively. Since the optical phonon of Si is about 530cm-1, these decays should all involve at least four phonons and have long and comparable lifetimes. More surprising still, the measured lifetime of the asymmetric stretch of interstitial O in Si changes by almost an order of magnitude with the isotope of O or one of its Si neighbors. In this work, ab-initio molecular dynamic simulations in periodic supercells are used to calculate the temperature dependence of vibrational lifetimes. The theoretical approach developed for these calculations can be applied to the calculation of any vibrational lifetime in any crystal. The calculations predict accurate lifetimes of the various defects in the range 50Item Hypersonic nonequilibrium flow simulations over a blunt body using bgk simulations(2009-05-15) Jain, SunnyThere has been a continuous effort to unveil the physics of hypersonic flows both experimentally and numerically, in order to achieve an efficient hypersonic vehicle design. With the advent of the high speed computers, a lot of focus has been given on research pertaining to numerical approach to understand this physics. The features of such flows are quite different from those of subsonic, transonic and supersonic ones and thus normal CFD methodologies fail to capture the high speed flows efficiently. Such calculations are made even more challenging by the presence of nonequilibrium thermodynamic and chemical effects. Thus further research in the field of nonequilibrium thermodynamics is required for the accurate prediction of such high enthalpy flows. The objective of this thesis is to develop improved computational tools for hypersonic aerodynamics accounting for non-equilibrium effects. A survey of the fundamental theory and mathematical modeling pertaining to modeling high temperature flow physics is presented. The computational approaches and numerical methods pertaining to high speed flows are discussed. In the first part of this work, the fundamental theory and mathematical modeling pertaining to modeling high temperature flow physics is presented. Continuum based approach (Navier Stokes) and Boltzmann equation based approach (Gas Kinetic) are discussed. It is shown mathematically that unlike the most popular continuum based methods, Gas Kinetic method presented in this work satisfies the entropy condition. In the second part of this work, the computational approaches and numerical methods pertaining to high speed flows is discussed. In the continuum methods, the Steger Warming schemes and Roe?s scheme are discussed. The kinetic approach discussed is the Boltzmann equation with Bhatnagar Gross Krook (BGK) collision operator. In the third part, the results from new computational fluid dynamics code developed are presented. A range of validation and verification test cases are presented. A comparison of the two common reconstruction techniques: Green Gauss gradient method and MUSCL scheme are discussed. Two of the most common failings of continuum based methods: excessive numerical dissipation and carbuncle phenomenon techniques, are investigated. It is found that for the blunt body problem, Boltzmann BGK method is free of these failings.Item Multiscalar line measurements in nonisobaric high-pressure underexpanded supersonic jets using rotational-vibrational raman spectroscopy(2009-05-15) Cohen, Benjamin NathanThis work describes the development of a Raman spectroscopy system for measuring aerothermochemistry in high-speed jets and flames. A transmissive grating spectrometer was newly developed for capturing pure rotational Raman and rotationalvibrational Raman with a single CCD camera. Previous state-of-the-art experiments applied line imagining in known flowfields of constant pressure. The system described herein is designed to provide local measurement of pressure, with full thermochemistry, along a line. In every point, temperature will be measured by examining the Boltzmann decay of the rotational spectrum, while molar fraction will be measured from the vibrational Raman spectrum. The temperature and concentrations will then be combined to obtain partial pressure measurements via the equation of state. This work examines the phenomenology of rotational and vibrational Raman scattering and proposes algorithms that can be used for data extraction.