Browsing by Subject "Transport theory"
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Item A thermodynamic model for membrane transport.(Texas Tech University, 1975-12) Narayan, Raghu SubramaniNot availableItem Abstract Riccati equations in a finite Lp1s space and applications to transport theory(Texas Tech University, 1987-08) Juang, JonqNot availableItem Adaptive finite elements for nonlinear transport equations(2003-12) Carnes, Brian Ross; Carey, Graham F.The a posteriori error analysis and estimation for conforming finite element approximation of stationary boundary value problems exhibiting certain classes of nonlinear reaction and nonlinear diffusion was investigated. Principal contributions were: (C1) Derivation of new rational local error indicators for both spatial and parameter error in parameterized nonlinear reaction–diffusion problems, (C2) New continuation algorithms for turning point prediction and calculation using adaptive mesh refinement (AMR), (C3) Improved linearization theory for nonlinear diffusion systems, (C4) A posteriori error analysis and new local error indicators for global error and error in output functionals for nonlinear diffusion systems, and (C5) A study of nonlinear diffusive mass transport in a PEM fuel cell cathode using AMR. For parameterized nonlinear reaction–diffusion problems, the solutions to a pair of local linear boundary value problems on each element were postprocessed to create local and global error indicators for both the spatial and parameter error, which were tested on representative problems, including the catalyst pellet problem from chemical engineering. The estimation of critical parameter values at simple turning points was demonstrated using AMR and the new local error indicator for the parameter error. The linearization theory for nondifferentiable, nonlinear diffusion operators with nonlinear solution–dependent diffusion coefficients was extended to systems, including the Stefan–Maxwell multicomponent diffusion operator. In addition, the application of the linearization arguments to the a posteriori error analysis of these operators was justified. Local error indicators for global error and error in output functionals were derived, based on solving local linear boundary value problems that approximate the primal and dual error. Numerical studies demonstrated the performance of the new indicators and confirmed the advantages of the linearization approach over simple estimates of the residuals. Finally, a study of nonlinear diffusive mass transport in the cathode of a PEM fuel cell was conducted, illustrating the use of AMR and the new local error indicators in an application problem of general interest. Calculation of an effectiveness factor that measures mass transport limitations in the cathode was also explored.Item An experimental and theoretical investigation of nonequilibrium behavior of electrons in gases(Texas Tech University, 1984-05) Young, Chris MorrowNot availableItem Analyses of device characteristics in low voltage p-, new material n-, and dual-channel organic field-effect transistors(2008-05) Jeong, Yeon Taek, 1971-; Dodabalapur, Ananth, 1963-This dissertation consists of three main chapters: Pentacene-based low voltage pchannel organic filed-effect transistors (OFETs) with anodized gate dielectrics; Charge transport in N,N’-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) [PDI-8CN2] based n-channel OFETs; and Dual-channel OFETs. Pentacene-based low voltage pchannel OFETs were realized using three different anodized gate dielectrics: a 470 Å SiO2, a 1,700 Å Ta2O5, and an 800 Å Ta2O5 formed by anodizing an n-Si wafer, a sputtered Ta thin film, and an e-beam evaporated Ta layer, respectively. Devices with the anodized SiO2 gate dielectric exhibited decent characteristics at VDS ≤ -10 V and VG ≤ -4 V, and the device performance was further improved by an octyltrichlorosilane (OTS) treatment. The two anodized Ta2O5 gate dielectrics were successfully employed to fabricate devices with high mobility at VDS ≤ -5 V and VG ≤ -2.5 V for the 1,700 Å Ta2O5 devices, and at VDS ≤ -10 V and VG ≤ -5 V for the 800 Å Ta2O5 devices. A hexamethyldisilazane (HMDS) treatment and a mono-docecyl phosphate (MDP) treatment proved to remarkably enhance the characteristics of the two Ta2O5 devices. However, the two treatments had the opposite influence on the threshold voltages of the devices from each other because of the capacitance difference resulting from their molecular length difference. In order to establish the suitable charge transport mechanisms in PDI-8CN2 and related n-channel organic semiconductors, the gate voltage and temperature dependence of electrical behavior and the contact resistance effects were studied in PDI-8CN2 based OFETs. The dependence of electrical behavior such as mobility, field-dependent mobility, trap density, and off current on gate voltage and temperature was derived using the multiple trapping and release (MTR) model. The contact resistance effects were determined by calculating the contact-corrected linear regime mobility and contact resistance by means of a four-probe measurement technique. Organic dual-channel OFETs were realized using poly-3-hexylthiophene (P3HT), PDI- 8CN2, and a polymeric dielectric (Merck® DS121) as the p-channel, n-channel, and gate dielectric materials, respectively. Coupled with each other, the p-FET and the n-FET showed acceptable characteristics at │VDS│ ≤ 50 V and │VG│ ≤ 50 V. Both the p-FET mode and the n-FET mode responded to delivered IPA and ethanol vapors with reasonably high sensitivity, which suggests that these organic dual-channel devices are effectively applicable to organic chemical sensing.Item Asymptotic analysis of the spatial weights of the arbitrarily high order transport method(2001-08) Elsawi, Mohamed Abdel Halim; Abdurrahman, Naeem M.; Koen, B. V.We perform an asymptotic analysis to the spatial weights of the Arbitrarily High Order Transport (AHOT) method that employs the method of characteristic as a means to relate the surface and the average fluxes of a computational cell in twodimensional Cartesian geometry. Previously, the spatial weights of the AHOT’s final discrete-variable equations have shown some numerical instabilities as the cell optical thickness approaches zero (or when computed on sufficiently thin cells). Our analysis is based on identifying the components of the spatial weights that are responsible for these instabilities, then expanding them in a truncated power series of the cell optical thickness that causes the instabilities when approaches zero. We then derive the conditions necessary to eliminate the singularity as the cell optical thickness approaches zero. We show that the method we adopted for computing the asymptotic spatial weights is very effective in eliminating these numerical instabilities by comparing the weights using the full analytical expressions and the new asymptotic ones, both being computed on fine grids. We implement the new asymptotic formulas for the spatial weights in the AHOT-C test code and generate benchmark quality solutions to some of the Burre’s Suite of Test Problems (BSTeP), which can be used as reference solutions to verify the accuracy of other solution methods and algorithms.Item Beam emission spectroscopy on the Alcator C-Mod Tokamak(2004) Sampsell, Matthew Brian; Bravenec, Ronald V.; Gentle, Kenneth W.Item Closures of the Vlasov-Poisson system(2003) Jones, Christopher Scott; Morrison, Philip J.Item Complex spectral representation of the Liouville operator : application to anomalous transport in anharmonic lattic(2001-12) Pereverzev, Andrey; Prigogine, I. (Ilya); Stanton, John (John F.)Item A conservative deterministic spectral method for rarefied gas flows(2008-08) Tharkabhushanam, Sri Harsha, 1979-; Martínez Gamba, Irene, 1957-The mathematical analysis of the Boltzmann equation for a wide range of important models is well developed. It describes physical phenomena which are often of great engineering importance (in aerospace industry, semiconductor design, etc.). For that reason, analytical and computational methods of solving the Boltzmann equation are studied extensively. The idea of describing processes on a scale of the order of the relaxation scales of time and space has been realized. The nonlinear Boltzmann equation possesses the important essence of a physically realistic equation, so it is possible not only to consider the flows of simple media but to formulate new problems due to the ability of this equation to describe nonequilibrium states. In this dissertation, a new spectral Lagrangian based deterministic solver for the non-linear Boltzmann transport equation for variable hard potential (VHP) collision kernels with conservative or non-conservative binary interactions is proposed. The method is based on symmetries of the Fourier transform of the collision integral, where the complexity in the collision integral computation is reduced to a separate integral over the unit sphere S2. In addition, the conservation of moments is enforced by Lagrangian constraints. The resulting scheme, implemented in free space, is very versatile and adjusts in a very simple manner to several cases that involve energy dissipation due to local micro-reversibility (inelastic interactions) or to elastic model of slowing down processes. We prove the accuracy, consistency and conservation properties of the proposed conservative spectral method. Existing spectral methods have consistency proofs which are only for elastic collisions, and also such methods do not conserve all the necessary moments of the collision integral. In this dissertation, error estimates for the conservation routine are provided. Such conservation correction is implemented as an extended isoperimetric problem with the moment conservation properties as the constraints. We use and extend an existing bound estimate of Gamba, Panferov and Villani for the inelastic/elastic space homogeneous Boltzmann collision operator. The result is an original extension to the work of Gustaffson. Using these estimates along with projection error estimates and conservation correction estimates, we prove that the conservation correction is bounded by the spectral accuracy. Simulations are benchmarked with available exact self-similar solutions, exact moment equations and analytical estimates for the homogeneous Boltzmann equation for both elastic and inelastic VHP interactions. Benchmarking of the self-similar simulations involves the selection of a time rescaling of the numerical distribution function which is performed using the continuous spectrum of the equation for Maxwell molecules. The method also produces accurate results in the case of inelastic diffusive Boltzmann equations for hard-spheres (inelastic collisions under thermal bath), where overpopulated non-Gaussian exponential tails have been conjectured in computations by stochastic methods. Recognizing the importance of the Boltzmann equation in the analysis of shock structures and nonequilibrium states, such a study is done for 1D(x) × 3D(v). The classic Riemann problem is numerically analyzed for Knudsen numbers close to continuum. The shock tube problem of Sone and Aoki, where the wall temperature is suddenly changed, is also studied. We consider the problem of heat transfer between two parallel plates with diffusive boundary conditions for a range of Knudsen numbers from close to continuum to a highly rarefied state. Finally, the classical infinite shock tube problem that generates a non-moving shock wave is studied. The point worth noting is that the flow in the final case turns from a supersonic flow to a subsonic flow across the shock.Item Heat-mass-momentum transfer in hollow fiber spinning(2004) Balasubramanian, Holly Ann; Lloyd, Douglas R., 1948-The goal of this research is to develop the requisite fundamental knowledge necessary to tailor the production of hollow fiber membranes. Specifically, the focus of this work is a model describing the simultaneous heat, mass, and momentum transfer for hollow fiber membrane spinning. The model predicts radial concentration gradients and spinline dimensions as functions of axial position, as well as axial profiles of temperature, velocity, and core pressure that evolve during hollow fiber spinning. The modeling procedure requires little computational time and is applicable to hollow fiber membranes spun under various conditions. Sensitivity of model-predicted spinline variables to surface tension effects was explored in the thin filament limit. While viscous effects dominate surface tension effects for typical pure-polymer melt spinning, membrane spinning results show that surface tension effects alter the evolution of spinline variables during the process, which can affect spinning stability. Experimental diameter and axial velocity profiles obtained during spinning of hollow fiber membranes using a twin-screw extruder indicate that system viscosity can vary significantly due to diluent evaporation at the clad–air quench interface, which creates the concentration gradients modeled in this work. Experimental results show more rapid attenuation of the spinline than predicted by the model without accounting for the concentration dependence of viscosity. Incorporating the viscosity dependence on both concentration and temperature helps to resolve the discrepancy between modelpredicted and measured spinline diameter and velocity profiles. The sensitivity of hollow fiber membrane extent of anisotropy, the fraction of the fiber cross-section possessing a pore size gradient, to processing conditions and spinning system physical properties was examined. Results indicate that extent of anisotropy is sensitive to spinning temperature, core gas flow rate, air gap length, and diffusion coefficient, showing an increase in extent of anisotropy for an increase in these parameters. These results have important implications for membrane research, where development and optimization are largely trial-and-error approaches. This work is an important precursor to development of a complete model to predict membrane macrostructure (inner and outer diameters) and microstructure (pore size, pore size distribution, and anisotropy) as functions of spinning conditions and material properties.Item Mathematical simulation of horizontal well performance in naturally fractured rock formations(Texas Tech University, 1998-12) Khan, Farid UddinThe purpose of horizontal drilling is to increase the well contact area with the formation, thereby enhancing production and injection. Application of horizontal wells in naturally fractured reservoirs has proven to be highly successful. In this study a dual-porosity, oil-water black oil simulator is developed and used to predict the performance of a horizontal well in fractured reservoirs. To avoid problems associated with discontinuous properties within the matrix and fracture system a fully implicit formulation is used to develop the flow equations. Point Successive Over Relaxation iterative method is successfully applied to solve the fully implicit system of equations for a dual porosity fractured reservoir. The model results were verified with volumetric calculations, analytic solution, and a dual porosity example. The matrix fracture transfer term is derived and found to be different from conventional expressions. It is seen that when a horizontal producer intersects high permeability vertical fractures more oil can be recovered which cannot be achieved by vertical wells. Hence application of horizontal wells can be very successful in formations having higher density of vertical fractures. Recovery has been found to be significantly sensitive to matrix water-oil relative permeability and capillary pressure but similar properties of the fracture system has much less effect.Item Novel intrinsic effects in charge and spin transport in semiconductors(2005) Culcer, Dimitrie Max; Niu, QianItem Production and subsurface vertical transport of radioxenon resulting from underground nuclear explosions(2010-12) Lowrey, Justin David; Biegalski, Steven R.; Deinert, MarkAtmospheric monitoring of radionuclides as part of the International Monitoring System requires the capability to differentiate between a radionuclide signature emanating from peaceful nuclear activity and one emanating from a well-contained underground nuclear explosion. While the radionuclide signatures of nuclear weapons are generally well known, radionuclides must first pass through hundreds of meters of earth to reach the surface where they can be detected and analyzed. Less well known is the affect that subsurface vertical transport has on the isotopic signatures of nuclear explosions. In this work, a model is developed, and tested, simulating the detonation of a simple underground nuclear explosion and the subsequent vertical transport of resulting radioxenon to the surface. First, the fast-fission burn of a fissile spherical core surrounded by a layer of geologic media is modeled, normalized to 1 kton total energy. The resulting source term is then used in the testing and evaluation of the constructed vertical transport model, which is based on the double-porosity model of underground fluid transport driven by barometric pumping. First, the ability of the vertical transport code to effectively model the underground pressure response from a varying surface pressure is demonstrated. Next, a 100-day simulation of the vertical migration of a static source is examined, and the resulting cumulative outflow of roughly 1% initial inventory outflow per cycle is found to closely follow the analytical predictions. Finally, calculated radioxenon source terms are utilized to model the resulting vertical transport and subsequent surface outflow. These results are found to be consistent with the physical expectations of the system, and lastly a cursory sensitivity analysis is conducted on several of the physical parameters of the model. The result is that the vertical transport model predicts isotopic fractionation of radioxenon that can potentially lie outside of currently accepted standard bounds.Item Quantum corrected full-band semiclassical Monte Carlo simulation research of charge transport in Si, stressed-Si, and SiGe MOSFETs(2006) Fan, Xiaofeng, 1978-; Banerjee, Sanjay; Register, Leonard F.This Ph.D. research is centered around a full-band Monte Carlo device simulator (“Monte Carlo at the University of Texas”, MCUT) with quantum corrections (based on one-dimensional Schrödinger equation solver). The code itself was based on a solid infrastructure of a Monte Carlo simulator, “MoCa” from the University of Illinois at Urbana-Champaign. To that there were added new methods and features during my Ph.D. program, including strained band structures, alternative (to conventional 100 ) surface orientations, full-band scattering mechanisms, and valley-dependent quantum correction. These features enable “MCUT” to be used to model various strained and/or alloyed silicon MOSFETs, as well as the MOSFETs composed of alternative materials such as Ge, in sub-100 nm regime. Monte Carlo simulation, itself, handles short channel effects and hot carriers in ultra small device well; full-band structure replaces the inaccurate and unknown (for new/strained materials) analytical formulae; and the quantum corrections approximate quantum-confinement effects on device performance. The goal is to understand and predict the device behavior of the so called “non-classical” CMOS ― beyond bulk Si based CMOS ― in the sub-100 nm regime.Item Quantum transport and control of atomic motion with light(2004) Gutiérrez-Medina, Braulio; Raizen, Mark GeorgeThis dissertation describes our experimental investigations of quantum transport and atomic motion control using optical potentials. The system we study consists of ultracold sodium atoms under the influence of light forces. First, we introduce the dynamics of neutral atoms in a periodic optical potential. The system resembles the textbook problem of an electron inside the crystalline lattice, and we review the main characteristics of the interaction for the atom optics case. In particular, atoms trapped in a lattice subject to large accelerations undergo Landau-Zener tunneling, process which makes the system unstable. The number of atoms trapped in the potential shows the characteristic exponential decay over time. However, deviations from this law are predicted by quantum mechanics. We use the experimental access to the non-exponential time to demonstrate the Quantum Zeno and Anti-Zeno effects. These effects show the influence of frequent observations on the decay rate of a quantum unstable system. The second part of the dissertation introduces a new system we plan to study, namely, the quantum interaction between sodium atoms in the ground state and a conductive surface. We are interested in the measurement of the Casimir-Polder potential with a precision of better than 1%. In order to do this, we have chosen to launch the atoms towards the surfaces at very small incident velocities (a few mm/s), and measure the influence of the interaction on the reflection probability. Atoms reflect from the purely attractive potential due to quantum reflection, an effect with no classical analogy. The experimental observation of quantum reflection requires atomic distributions with temperatures below 1 μK. For this purpose, we have pro- duced and studied a Bose-Einstein condensate (BEC) with sodium atoms. The region where the BEC is created is separated spatially form the surfaces by a distance of 10 cm, vertically. In order to bring the atoms close to the surfaces prior to their launching, we have developed an optical elevator. The eleva- tor uses a moving optical lattice in the regime where tunneling is negligible. Results of the macroscopic optical transport technique, and current progress towards a measurement of the Casimir-Polder interaction, are reported.Item Spin and charge transport through carbon based systems(2007) Jung, Suyong, 1976-; Yao, Zhen, Ph. D.In this thesis, we investigate spin-dependent transport through ferromagnet-contacted single-walled carbon nanotubes (SWCNTs), in which charge transport shows the Fabry-Perot (FP) interference effect, the Kondo effect and the Coulomb blockade effect at low temperatures. Hysteric magnetoresistance (MR) is observed in all three transport regimes, which can be controlled by both the external magnetic field and the gate voltage. The MR in the FP interference regime can be well understood by a model considering the intrinsic electronic structure of SWCNTs and the quantum interference effect. In the strongly interacting Kondo regime, the Kondo effect is not suppressed by the presence of nearby ferromagnetism. Several observed MR features including the non-splitted zero-bias Kondo peak and positive MR switching can be explained by the strong Kondo effect and weak ferromagnetism in the leads. In the Coulomb blockade regime, several effects that can be associated with the magneto-Coulomb effect have been observed, and isolated spin accumulation and transport through the SWCNT quantum dot have been realized by a four-probe non-local measurements. We also studied charge transport behavior through organic semiconductor pentacene thin film transistors (OTFTs) in the limit of single- or a few molecular layers of pentacene films. The charge transport in these devices can be well explained by the multiple trapping and release model. The structural disorders induced by the physical and chemical causes, such as grain boundaries, interactions with gate insulator, metal contacts and ambient conditions can be responsible for the localized trap states in the ultrathin layer OTFTs, which are further confirmed by the electric force microscopy (EFM) measurements.Item Stability of the diamond difference approximation in energy to the Spencer-Lewis equation of electron transport(Texas Tech University, 1987-12) Seth, Daniel L.Consider the Spencer-Lewis equation (S-L) of electron transport in an azimuthally symmetric slab geometry setting with energy restricted to a finite interval. Further, S-L is subject to boundary conditions in the form of known incident particle fluxes at the slab faces. The one-dimensional diamond difference approximation is applied in the energy variable to the continuous slowing down term (i.e., the energy derivative) of S-L, This results in a semi-discrete system of integro-differential equations in the spatial and angular variables (D.E.S-L). The numerical stability of D,E.S-L as an approximation to S-L is demonstrated for solutions of D.E,S-L that belong to an L2 function space. The system of integro-differential equations may be rewritten as a system of integral equations. Under certain reasonable conditions on the data, the existence-uniqueness of solutions of the integral equations in a Banach space of square integrable functions with weighted norms is established. This implies the existence of L2 solutions of the integral equations. Under further assumptions on the data, the solutions of the integral equations are shown to be the required solutions of the integro-differential equations as well. Moreover, if the data are all bounded, the solutions are bounded.Item The Nonlinear Integrodifferential Initial-Value Problem for the Reflection Kernel of an Isotropically Scattering Slab: Direct Existence Proofs(Texas Tech University, 1985-05) Juang, JonqNot Available.