Browsing by Subject "scattering"
Now showing 1 - 8 of 8
Results Per Page
Sort Options
Item Analysis of a PML method applied to computation to resonances in open systems and acoustic scattering problems(2010-01-14) Kim, SeungilWe consider computation of resonances in open systems and acoustic scattering problems. These problems are posed on an unbounded domain and domain truncation is required for the numerical computation. In this paper, a perfectly matched layer (PML) technique is proposed for computation of solutions to the unbounded domain problems. For resonance problems, resonance functions are characterized as improper eigenfunction (non-zero solutions of the eigenvalue problem which are not square integrable) of the Helmholtz equation on an unbounded domain. We shall see that the application of the spherical PML converts the resonance problem to a standard eigenvalue problem on the infinite domain. Then, the goal will be to approximate the eigenvalues first by replacing the infinite domain by a finite computational domain with a convenient boundary condition and second by applying finite elements to the truncated problem. As approximation of eigenvalues of problems on a bounded domain is classical [12], we will focus on the convergence of eigenvalues of the (continuous) PML truncated problem to those of the infinite PML problem. Also, it will be shown that the domain truncation does not produce spurious eigenvalues provided that the size of computational domain is sufficiently large. The spherical PML technique has been successfully applied for approximation of scattered waves [13]. We develop an analysis for the case of a Cartesian PML application to the acoustic scattering problem, i.e., solvability of infinite and truncated Cartesian PML scattering problems and convergence of the truncated Cartesian PML problem to the solution of the original solution in the physical region as the size of computational domain increases.Item Asymptotic scattering wave function for three charged particles and astrophysical capture processes(Texas A&M University, 2006-08-16) Pirlepesov, FakhriddinThe asymptotic behavior of the wave functions of three charged particles has been investigated. There are two different types of three-body scattering wave functions. The first type of scattering wave function evolves from the incident three-body wave of three charged particles in the continuum. The second type of scattering wave function evolves from the initial two-body incident wave. In this work the asymptotic three-body incident wave has been derived in the asymptotic regions where two particles are close to each other and far away from the third particle. This wave function satisfies the Schrodinger equation up to terms O(1/3pa), where pa is the distance between the center of mass of two particles and the third particle. The derived asymptotic three-body incident wave transforms smoothly into Redmond??s asymptotic incident wave in the asymptotic region where all three particles are well separated. For the scattering wave function of the second type the asymptotic threebody scattered wave has been derived in all the asymptotic regions. In the asymptotic region where all three particles well separated, the derived asymptotic scattered wave coincides with the Peterkop asymptotic wave. In the asymptotic regions where two particles are close to each other and far away from the third one, this is a new expression which is free of the logarithmically diverging phase factors that appeared in the Peterkop approach. The derived asymptotic scattered wave resolves a long-standing phase-amplitude ambiguity. Based on these results the expressions for the exact prior and post breakup amplitudes have been obtained. The post breakup amplitude for charged particles has not been known and has been derived for the first time directly from the prior form. It turns out that the post form of the breakup amplitude is given by a surface integral in the six dimensional hyperspace, rather than a volume integral, with the transition operator expressed in terms of the interaction potentials. We also show how to derive a generalized distorted-wave-Born approximation amplitude (DWBA) from the exact prior form of the breakup amplitude. It is impossible to derive the DWBA amplitude from the post form. The three-body Coulomb incident wave is used to calculate the reaction rates of 7Be(ep, e)8B and 7Be(pp, p)8B nonradiative triple collisions in stellar environments.Item Developing models of aerosol representation to investigate composition, evolution, optical properties, and CCN spectra using measurements of size-resolved hygroscopicity(Texas A&M University, 2006-08-16) Gasparini, RobertoA Differential Mobility Analyzer/Tandem Differential Mobility Analyzer (DMA/TDMA) was used to measure size distributions, hygroscopicity, and volatility during the May 2003 Aerosol Intensive Operational Period at the Central Facility of the Atmospheric Radiation Measurement Southern Great Plains site. Hygroscopic growth factor distributions for particles at eight dry diameters ranging from 0.012 ??m to 0.600 ??m were measured. These measurements, along with backtrajectory clustering, were used to infer aerosol composition and evolution. The hygroscopic growth of the smallest and largest particles analyzed was typically less than that of particles with dry diameters of about 0.100 ??m. Condensation of secondary organic aerosol on nucleation mode particles may be responsible for the minimal growth observed at the smallest sizes. Growth factor distributions of the largest particles typically contained a non-hygroscopic mode believed to be composed of dust. A model was developed to characterize the hygroscopic properties of particles within a size distribution mode through analysis of the fixed-size hygroscopic growth measurements. This model was used to examine three cases in which the sampled aerosol evolved over a period of hours or days. Additionally, size and hygroscopicity information were combined to model the aerosol as a population of multi-component particles. With this model, the aerosol hygroscopic growth factor f(RH), relating the submicron scattering at high RH to that at low RH, is predicted. The f(RH) values predicted when the hygroscopic fraction of the aerosol is assumed to be metastable agree better with measurements than do those predicted under the assumption of crystalline aerosol. Agreement decreases at RH greater than 65%. This multi-component aerosol model is used to derive cloud condensation nuclei (CCN) spectra for comparison with spectra measured directly with two Desert Research Institute (DRI) CCN spectrometers. Among the 1490 pairs of DMA/TDMA-predicted and DRI-measured CCN concentrations at various critical supersaturations from 0.02-1.05%, the sample number-weighted mean R2 value is 0.74. CCN concentrations are slightly overpredicted at both the lowest (0.02-0.04%) and highest (0.80-1.05%) supersaturations measured. Overall, this multi-component aerosol model based on size distributions and size-resolved hygroscopicity yields reasonable predictions of the humidity-dependent optical properties and CCN spectra of the aerosol.Item Investigation of the optical and cloud forming properties of pollution, biomass burning, and mineral dust aerosols(Texas A&M University, 2006-08-16) Lee, Yong SeobThis dissertation describes the use of measured aerosol size distributions and size-resolved hygroscopic growth to examine the physical and chemical properties of several particle classes. The primary objective of this work was to investigate the optical and cloud forming properties of a range of ambient aerosol types measured in a number of different locations. The tool used for most of these analyses is a differential mobility analyzer / tandem differential mobility analyzer (DMA / TDMA) system developed in our research group. To collect the data described in two of the chapters of this dissertation, an aircraft-based version of the DMA / TDMA was deployed to Japan and California. The data described in two other chapters were conveniently collected during a period when the aerosol of interest came to us. The unique aspect of this analysis is the use of these data to isolate the size distributions of distinct aerosol types in order to quantify their optical and cloud forming properties. I used collected data during the Asian Aerosol Characterization Experiment (ACE-Asia) to examine the composition and homogeneity of a complex aerosol generated in the deserts and urban regions of China and other Asian countries. An aircraft-based TDMA was used for the first time during this campaign to examine the size-resolved hygroscopic properties of the aerosol. The Asian Dust Above Monterey (ADAM-2003) study was designed both to evaluate the degree to which models can predict the long-range transport of Asian dust, and to examine the physical and optical properties of that aged dust upon reaching the California coast. Aerosol size distributions and hygroscopic growth were measured in College Station, Texas to investigate the cloud nucleating and optical properties of a biomass burning aerosol generated from fires on the Yucatan Peninsula. Measured aerosol size distributions and size-resolved hygroscopicity and volatility were used to infer critical supersaturation distributions of the distinct particle types that were observed during this period. The predicted cloud condensation nuclei concentrations were used in a cloud model to determine the impact of the different aerosol types on the expected cloud droplet concentration. RH-dependent aerosol extinction coefficients were also calculated.Item Measurement and model assessment of fluorescence lifetime sensing in multiply scattering media(Texas A&M University, 2005-08-29) Kuwana, EddyThe generation and propagation of fluorescence light within biological tissue offers the potential for biomedical diagnostics and analyte sensing. Arising from an exogenous fluorescent dye injected as a contrast agent or immobilized in a polymer implant, the fluorescent decay kinetics can be sensitive to the tissue??s biochemical environment, providing quantitative in vivo information of the confined tissue site. The impact of light propagation and decay kinetics upon the measured signals is important for consideration, simply because tissue scatters light, giving rise to nanosecond photon time-of-flights that are comparable to fluorescence relaxation kinetics. The goal of this study is to develop a time-dependent model describing (i) the generation of fluorescence from dyes exhibiting multi-exponential or more complex kinetics and (ii) its propagation in scattering media. In the preliminary study, fluorescence lifetime spectroscopy is investigated in tissue-like scattering solution. Two fluorescent dyes, 3,3-diethylthiatricarbocyanine iodide (DTTCI) and Indocynanine Green (ICG), which exhibit distinctly different lifetimes and each exhibits single-exponential decay kinetics, were employed. Measurements of phase-modulation as a function of modulation frequency were made at varying concentration ratios of the two dyes to experimentally simulate fluorescence multi-exponential decay kinetics in non-scattering and scattering solutions. The results suggest that frequency-domain measurements of fluorescent decay kinetics along with models of light propagation may be enhanced by scatter in order to probe kinetics more sensitively than in non-scattering solutions. The next study involved fluorescence lifetime sensing in scattering and non-scattering solutions with a pH sensitive dye, Carboxy Seminaphthofluorescein-1 (C-SNAFL-1), which is known to exhibit multi-exponential decay kinetics. The results demonstrate accurate pH sensing in scattering solution via fluorescence kinetics using a simplified propagation model incorporating an average lifetime. Finally, fluorescence lifetime sensing in immobilized systems were investigated. C-SNAFL-1 was immobilized in poly(ethylene glycol) (PEG) microparticles that were immersed in buffered polystyrene solutions. The results demonstrate the ability to perform pH sensing with fluorescence lifetime without the confounding effect of fluorophore loading or the use of 'reference' measurement within multiply scattering systems. In addition, the stability of the immobilized fluorescence sensor and the reliability of fluorescence lifetime measurement verify the prospect of this technology for implantable purposes.Item Novel instrumentation for a scattering independent measurement of the absorption coefficient of natural waters, and a new diffuse reflector for spectroscopic instrumentation and close cavity coupling(Texas A&M University, 2007-04-25) Musser, Joseph AlanWe report results for the development of a flow-through integrated cavity absorption meter (ICAM.) Absorption measurements have been made with 2% or less change in the signal in the presence of up to 10 m-1 of scattering in the medium. The operating range of the ICAM ranges from 0.004 m-1 to over 40 m-1 of absorption. This range allows one to use a single instrument to measure the absorption from sediment laden rivers out to the cleanest of ocean waters. Further, the ICAM signal has been shown to be independent of the flow rate and turbulence in the medium. In addition we report the development of a diffuse reflector which, to our best knowledge, has the highest measured diffuse reflectivity of 0.998 at 532 nm and 0.996 at 266 nm. We also show that the average distance a photon travels between successive reflections in an integrating cavity of arbitrary shape is four times the volume divided by the surface area, 4 V/S. Further, for a cavity which is formed by planes tangent to an inscribed sphere and which maintains a homogeneous and isotropic field, the average distance traveled by a photon between successive reflections is equal to 4 V/S of the inscribed sphere. Thus, each cavity has the same ratio of V/S as the inscribed sphere. These advances lead to an increase in the sensitivity of absorption spectroscopy. The sensitivity approaches that of cavity ring down spectroscopy (CARS), without the adverse scattering effects traditionally associated with CARS.Item Physics of the Structural Color on the Skin of Cephalopods(2012-10-30) Gao, Meng 1981-The structural colors, produced by leucophore and iridophore cells, are important for cephalopod camouflage; however, their scattering properties have not been very well studied. These colors are mainly due to the scattering of the specific small scatterers inside of the cell. In this work we will summarize the theories and the numerical methods used to solve both the scattering problems for one scatterer and a collection of such scatterers. The reflection spectrum of iridophores is shown to depend on both particle orientation and incident angle of the light. The leucophores are shown to be a white Lambertian surface. Therefore, starting from the structure of the cells, we can predict their color appearance in the skin. This work provides a general framework for the study of the structural color of cephalpods, and can be applied to many species with different cell structures.Item Wave Energy Extraction from an Oscillating Water Column in a Truncated Circular Cylinder(2013-07-19) Wang, HaoOscillating Water Column (OWC) device is a relatively practical and convenient way that converts wave energy to a utilizable form, which is usually electricity. The OWC is kept inside a fixed truncated vertical cylinder, which is a hollow structure with one submerged open end in the water and with an air turbine at the top. The research adopts potential theory and Galerkin methods to solve the motion of the OWC. Based on the air-water interaction model, optimal OWC design for energy extraction from regular wave is explored. The hydrodynamic coefficients in scattering and radiation potential are solved using Galerkin approximation. The numerical results for the free surface elevation have been verified by a series of experiments conducted in the University of New Orleans Towing Tank. The effect of geometric parameters on the response amplitude operator (RAO) of OWC is studied and amendment of the equation for evaluating the natural frequency of the OWC is made. Using the model of air-water interaction under certain wave parameters and OWC geometric parameters, a computer program OWC Solution is developed to optimize the energy output from the system. Optimization results by the program OWC Solution lead to an effective method to design the OWC system.