1,772 research outputs found
Numerical Analysis of First and Second Order Unconditional Energy Stable Schemes for Nonlocal Cahn-Hilliard and Allen-Cahn Equations
This PhD dissertation concentrates on the numerical analysis of a family of fully discrete, energy stable schemes for nonlocal Cahn-Hilliard and Allen-Cahn type equations, which are integro-partial differential equations (IPDEs). These two IPDEs -- along with the evolution equation from dynamical density functional theory (DDFT), which is a generalization of the nonlocal Cahn-Hilliard equation -- are used to model a variety of physical and biological processes such as crystallization, phase transformations, and tumor growth. This dissertation advances the computational state-of-the-art related to this field in the following main contributions: (I) We propose and analyze a family of two-dimensional unconditionally energy stable schemes for these IPDEs. Specifically, we prove that the schemes are (a) uniquely solvable, independent of time and space step sizes; (b) energy stable, independent of time and space step sizes; and (c) convergent, provided the time step sizes are sufficiently small. (II) We develop a highly efficient solver for schemes we propose. These schemes are semi-implicit and contain nonlinear implicit terms, which makes numerical solutions challenging. To overcome this difficulty, a nearly-optimally efficient nonlinear multigrid method is employed. (III) Via our numerical methods, we are able to simulate crystal nucleation and growth phenomena, with arbitrary crystalline anisotropy, with properly chosen parameters for nonlocal Cahn-Hilliard equation, in a very efficient and straightforward way. To our knowledge these contributions do not exist in any form in any of the previous works in the literature
Designing isoelectronic counterparts to layered group V semiconductors
In analogy to III-V compounds, which have significantly broadened the scope
of group IV semiconductors, we propose IV-VI compounds as isoelectronic
counterparts to layered group V semiconductors. Using {\em ab initio} density
functional theory, we study yet unrealized structural phases of silicon
mono-sulfide (SiS). We find the black-phosphorus-like -SiS to be almost
equally stable as the blue-phosphorus-like -SiS. Both -SiS and
-SiS monolayers display a significant, indirect band gap that depends
sensitively on the in-layer strain. Unlike 2D semiconductors of group V
elements with the corresponding nonplanar structure, different SiS allotropes
show a strong polarization either within or normal to the layers. We find that
SiS may form both lateral and vertical heterostructures with phosphorene at a
very small energy penalty, offering an unprecedented tunability in structural
and electronic properties of SiS-P compounds.Comment: 7 pages, 5 figure
Qualitative and Quantitative Comparisons between Spar and Semi-Submersible Platforms
Spars and semisubmersibles are two floating platforms commonly used in offshore deepwater exploration and production. However, currently lack of studies are available to comparatively evaluate the two platforms under similar cost range. This study compares the hydrodynamic motions of a truss spar and a semisubmersible under similar cost and wave environment. The platforms were numerically modelled using the radiation/diffraction software HydroSTAR. The response amplitude operators (RAO) were obtained for surge, heave and pitch respectively. The findings indicate the responses for the truss spar are generally lower than the semisubmersible, with the exception for the peak heave and pitch RAO. At 1000 m, the percentage difference of the peak surge RAO for spar and semisubmersible is 0.005%; heave RAO for semisubmersible is 34% of the spar’s value; and pitch RAO for semisubmersible is 79% of the spar’s value. Additionally, the semisubmersible achieved its peak RAO at a higher frequency for the heave and pitch. The findings proved that overall the spar’s dynamic responses are better for the majority of wave frequencies
The Nature of the Interlayer Interaction in Bulk and Few-Layer Phosphorus
An outstanding challenge of theoretical electronic structure is the
description of van der Waals (vdW) interactions in molecules and solids.
Renewed interest in resolving this is in part motivated by the technological
promise of layered systems including graphite, transition metal
dichalcogenides, and more recently, black phosphorus, in which the interlayer
interaction is widely believed to be dominated by these types of forces. We
report a series of quantum Monte Carlo (QMC) calculations for bulk black
phosphorus and related few-layer phosphorene, which elucidate the nature of the
forces that bind these systems and provide benchmark data for the energetics of
these systems. We find a significant charge redistribution due to the
interaction between electrons on adjacent layers. Comparison to density
functional theory (DFT) calculations indicate not only wide variability even
among different vdW corrected functionals, but the failure of these functionals
to capture the trend of reorganization predicted by QMC. The delicate interplay
of steric and dispersive forces between layers indicate that few-layer
phosphorene presents an unexpected challenge for the development of vdW
corrected DFT.Comment: 8 pages, 6 figure
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