Advanced numerical methods for the simulation of alloy solidification with high Lewis number

A fully-implicit numerical method based upon adaptively refined meshes for the thermal-solutal simulation of alloy solidification in 2D is presented. In addition we combine an unconditional stable second-order fully-implicit time discretisation scheme with variable step size control to obtain an adaptive time and space discretisation method, where a robust and fast multigrid solver for systems of non-linear algebraic equations is used to solve the intermediate approximations per time step. For the isothermal case, the superiority of this method, compared to widely used fully-explicit methods, with respect to CPU time and accuracy, has been demonstrated and published previously. Here, the new proposed method has been applied to the thermalsolutal case with high Lewis number, where stability issues and time step restrictions have been major constraints in previous research

A fully implicit, fully adaptive time and space discretisation method for phase-field simulation of binary alloy solidification

A fully-implicit numerical method based upon adaptively refined meshes for the simulation of binary alloy solidification in 2D is presented. In addition we combine a second-order fully-implicit time discretisation scheme with variable steps size control to obtain an adaptive time and space discretisation method. The superiority of this method, compared to widely used fully-explicit methods, with respect to CPU time and accuracy, is shown. Due to the high non-linearity of the governing equations a robust and fast solver for systems of nonlinear algebraic equations is needed to solve the intermediate approximations per time step. We use a nonlinear multigrid solver which shows almost h-independent convergence behaviour

An adaptive, fully implicit multigrid phase-field model for the quantitative simulation of non-isothermal binary alloy solidification

Using state-of-the-art numerical techniques, such as mesh adaptivity, implicit time-stepping and a non-linear multi-grid solver, the phase-field equations for the non-isothermal solidification of a dilute binary alloy have been solved. Using the quantitative, thin-interface formulation of the problem we have found that at high Lewis number a minimum in the dendrite tip radius is predicted with increasing undercooling, as predicted by marginal stability theory. Over the dimensionless undercooling range 0.2–0.8 the radius selection parameter, σ*, was observed to vary by over a factor of 2 and in a non-monotonic fashion, despite the anisotropy strength being constant

Quantitative phase-field modeling of solidification at high Lewis number

A phase-field model of nonisothermal solidification in dilute binary alloys is used to study the variation of growth velocity, dendrite tip radius, and radius selection parameter as a function of Lewis number at fixed undercooling. By the application of advanced numerical techniques, we have been able to extend the analysis to Lewis numbers of order 10 000, which are realistic for metals. A large variation in the radius selection parameter is found as the Lewis number is increased from 1 to 10 000

Efeito da temperatura e da restrição hídrica na germinação de sementes de neoglaziovia variegata (Arruda) Mez.

Neoglaziovia variegata (Arruda) Mez conhecida como caroá, é uma Bromeliaceae nativa da Caatinga brasileira, cujas fibras retiradas das folhas geram trabalho e renda para diversas famílias nordestinas. Além disso, suas flores têm valor ornamental. Contudo, essa bromeliácea é coletada de forma extrativista e ainda é pouca estudada, havendo necessidade de desenvolver um método de propagação eficiente para estabelecer um sistema de cultivo e produção.PDF. 204

Digestibilidade aparente de nutrientes de dietas contendo farelo da castanha de caju (Anacardium occidentale L.) em caprinos

Foi avaliada a digestibilidade aparente dos nutrientes em dietas contendo farelo da castanha de caju (FCC) e feno de Tifton 85 em diferentes níveis de inclusão. Utilizaram-se doze caprinos com peso vivo inicial de 17,2 kg, distribuídos num delineamento inteiramente casualizado com quatro tratamentos e seis repetições no tempo. As inclusões de FCC nas dietas foram de 10; 15; 20 e 25% com base na matéria seca (MS). A estimativa da produção fecal foi avaliada por coleta total. A inclusão nas dietas de até 25% de FCC na MS promoveu o aumento (P<0,05) das digestibilidades aparente da proteína bruta e do extrato etéreo. As inclusões de FCC ocasionaram decréscimos lineares tanto na digestibilidade aparente da fibra em detergente neutro quanto da fibra em detergente ácido, nessas porções fibrosas foram encontrados decréscimos de 0,24 e 0,50 unidades percentuais das digestibilidades à cada 1% da inclusão do FCC, respectivamente

Intraband Dynamics in the Optically Excited Wannier-Stark Ladder Spectrum of Semiconductor Superlattices

In semiconductor superlattices, the carrier band structure can be tailored by the proper choice of their geometry. Therefore, superlattices are a model system for the study of coherent high-field transport phenomena in a periodic potential with applied static electric field. This thesis is structured in two parts. I. Zener Tunneling in Semiconductor Superlattices. In this work,semiconductor superlattices with shallow barriers and narrow band gaps were employed to investigate the Zener breakdown. In these samples, tunneling in the electron Wannier-Stark ladder spectrum is addressed as coupling of the electron states of a single bound below-barrier band to the states of the above-barrier spectrum. The field-dependent evolution of the Wannier-Stark ladder states was traced in the optical interband spectrum. Superlattices with different geometries were employed, to clarify the influence of the particular miniband structure on the Zener tunneling behavior. It was shown that in the presence of Zener tunneling, the Wannier-Stark ladder picture becomes invalid. Tunneling is demonstrated to lead to a field-induced delocalization of Wannier-Stark ladder states. In addition, the coherent polarization lifetime was analyzed as a measure of the tunneling probability. II. Terahertz Emission of Exciton Wave Packets in Semiconductor Superlattices. By means of Terahertz spectroscopy, the coherent intraband dynamics of exciton wave packets in biased superlattices after the selective ultrafast excitation of the Wannier-Stark ladder spectrum was investigated. The dynamics of Bloch oscillations was investigated under broadband excitation. It is demonstrated, that the Bloch oscillation amplitude can be controlled by altering the pump pulse energy. The xperimental results can only be explained in a full exciton picture, incorporating bound 1s exciton states and the associated exciton in-plane continuum. The intraband dipole of single Wannier-Stark ladder excitons was measured by detecting the Terhartz response after excitation of the Wannier-Stark ladder with a spectrally narrow rectangular pump pulse. In addition, experiments revealed a previously unknown mechanism for the generation of Bloch oscillating exciton wave packets. This was demonstrated for an incident pump spectrum which was too narrow to excite a superposition of Wannier-Stark ladder states. The effect is based on the sudden, non-adiabatic, change in the net dc internal field due to creation of electron-hole pairs with permanent dipole moments. The non-adiabatic generation of Bloch oscillations is a highly nonlinear effect mediated by strong exciton-exciton interactions.The central role that play exciton-exciton interactions in the intraband dynamics became especially evident when the Wannier-Stark ladder was selectively excited by two spectrally narrow laser lines. The experiments demonstrated a resonant enhancement of the intraband transition matrix element when 1s exciton wavepackets are excited.In Halbleiter-Übergittern kann die Bandstruktur von Ladungsträgern durch die geeignete Wahl der Geometrie eingestellt werden. Deshalb sind Halbleiter-Übergitter ein Modellsystem für Untersuchungen des kohärenten Ladungstransportes im periodischen Potential bei hohen, statischen, elektrischen Feldern. Diese Doktorarbeit ist in zwei Teile untergliedert. I. Zener-Tunneln in Halbleiter-Übergittern In dieser Arbeit werden Halbleiter-Übergitter mit flachen Barrieren und schmalen Bandlücken eingesetzt, um den Effekt des Zener-Durchbruchs zu untersuchen. In diesen Strukturen wird das Zener-Tunneln im Elektronen-Spektrum der Wannier-Stark-Leiter adressiert. Dabei handelt es sich um die Kopplung von Elektronen-Zuständen eines einzelnen Minibandes unterhalb der Potentialbarriere des Quantentopfes mit Zuständen oberhalb der Barriere. Die Feldabhängigkeit der Wannier-Stark-Leiter-Zustände wurde im optischen Interband-Spektrum detektiert. Übergitter mit unterschiedlichen Geometrien wurden untersucht, um den Einfluss der spezifischen Miniband-Struktur auf die Charakteristiken des Zener-Tunnelns aufzuklären. Es wurde gezeigt, dass im Regime des Zener-Tunnelns das Wannier-Stark-Leiter-Bild nicht mehr gültig ist. Dabei wird demonstriert, dass Tunneln zu einer feldabhängigen Delokalisierung der Wannier-Stark-Leiter-Zustände führt. Außerdem wird die Kohärenz-Lebensdauer der Polarisation analysiert. Sie bildet die Tunneln-Wahrscheinlichkeit ab. II. Terahertz Emission von Exzitonen-Wellen-Paketen in Halbleiter-Übergittern Mit Hilfe von Terahertz-Spektroskopie wurde die kohärente Intraband-Dynamik von Exzitonen-Wellen-Paketen in vorgespannten Halbleiter-Übergittern nach der selektiven, ultrakurzen Anregung des Wannier-Stark-Leiter-Spektrums untersucht. Die Dynamik von Bloch-Oszillatonen wurde durch spektral breitbandiger Anregung detektiert. Es wird gezeigt, dass die Amplitude von Bloch-Oszillationen durch die Änderung der Energie des Anrege-Pulses beeinflusst werden kann. Die experimentellen Resultate können nur in einem ganzheitlichen Exzitonenbild erklärt werden. Es umfaßt die gebundenen 1s-Exziton-Zustände und das zugehörige Exzitonen-Kontinuum in der Quantentopfschicht. Der Intraband-Dipol einzelner Wannier-Stark-Leiter-Exzitonen wurde durch die Detektion der Terahertz-Antwort auf die Anregung der Wannier-Stark-Leiter mit einem spektral schmalen Anrege-Puls vermessen. Außerdem wird in den Experimenten ein zuvor ungekannten Mechanismus der Anregung von bloch-oszillierenden Wellen-Paketen beobachtet. Dieser Effekt wird für ein eingestrahltes Anrege-Spektrum, welches spektral zu schmal für die Anregung einer Überlagerung von Wannier-Stark-Leiter-Zuständen ist, demonstriert. Der Mechanismus basiert auf die unmittelbare, nicht-adiabatische Änderung des effektiven, internen, statischen Feldes auf Grund der Anregung von Elektron-Loch-Paaren mit permanentem Dipolmoment. Die nicht-adiabatische Anregung von Bloch-Oszillationen ist ein hoch nicht-linearer Effekt, der durch starke Exziton-Exziton Wechselwirkung vermittelt wird. Die zentrale Rolle, die die Exziton-Exziton Wechselwirkung in der Intraband-Dynamik spielt, wurde besonders deutlich bei der selektiven Anregung der Wannier-Stark-Leiter durch zwei spekral schmale Laserlinien. Die Experimente demonstrieren eine resonante Überhöhung des Intraband-Übergangs-Matrix-Elements, wenn 1s-Exziton-Wellen-Pakete angeregt werden