824 research outputs found
Theoretical and Numerical Study of Free Molecular-Flow Problems
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77130/1/AIAA-25893-547.pd
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
Coupled Flow Field Simulations of Charring Ablators with Nonequilibrium Surface Chemistry
This paper describes the coupling of a Navier-Stokes solver to a material response code to simulate nonequilibrium gas-surface interactions. The Navier-Stokes solver used in this study is LeMANS, which is a three-dimensional computational fluid dynamics code that can simulate hypersonic reacting flows including thermo-chemical nonequilibrium effects. The material response code employed in this study is MOPAR, which uses the one-dimensional control volume nite-element method to model heat conduction and pyrolysis gas behavior. This coupling is demonstrated using a test case based on the Stardust sample return capsule. Coupled simulations are performed at three different trajectory conditions. The effects of the pyrolysis gas chemistry are evaluated by assuming that the gas is either in chemical equilibrium or composed entirely of non-reacting phenol. The results show that the non-reacting pyrolysis gas assumption produces higher convective heat fluxes, surface temperatures, and mass blowing rates. These effects are mainly due to the composition of the pyrolysis gas. The additional species produced by the pyrolysis gas in the chemical equilibrium case react with oxygen and nitrogen atoms in the gas-phase. This results in fewer atoms participating in the exothermic surface reactions, which reduces the heat transfer to the vehicle
Sharp interface limits of phase-field models
The use of continuum phase-field models to describe the motion of
well-defined interfaces is discussed for a class of phenomena, that includes
order/disorder transitions, spinodal decomposition and Ostwald ripening,
dendritic growth, and the solidification of eutectic alloys. The projection
operator method is used to extract the ``sharp interface limit'' from phase
field models which have interfaces that are diffuse on a length scale . In
particular,phase-field equations are mapped onto sharp interface equations in
the limits and , where and are
respectively the interface curvature and velocity and is the diffusion
constant in the bulk. The calculations provide one general set of sharp
interface equations that incorporate the Gibbs-Thomson condition, the
Allen-Cahn equation and the Kardar-Parisi-Zhang equation.Comment: 17 pages, 9 figure
Finite difference lattice Boltzmann model with flux limiters for liquid-vapor systems
In this paper we apply a finite difference lattice Boltzmann model to study
the phase separation in a two-dimensional liquid-vapor system. Spurious
numerical effects in macroscopic equations are discussed and an appropriate
numerical scheme involving flux limiter techniques is proposed to minimize them
and guarantee a better numerical stability at very low viscosity. The phase
separation kinetics is investigated and we find evidence of two different
growth regimes depending on the value of the fluid viscosity as well as on the
liquid-vapor ratio.Comment: 10 pages, 10 figures, to be published in Phys. Rev.
Conceptual Analysis of Electron Transpiration Cooling for the Leading Edges of Hypersonic Vehicles
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140441/1/6.2014-2674.pd
Tailoring Metallodielectric Structures for Super Resolution and Superguiding Applications in the Visible and Near IR Ranges
We discuss propagation effects in realistic, transparent, metallo-dielectric
photonic band gap structures in the context of negative refraction and
super-resolution in the visible and near infrared ranges. In the resonance
tunneling regime, we find that for transverse-magnetic incident polarization,
field localization effects contribute to a waveguiding phenomenon that makes it
possible for the light to remain confined within a small fraction of a
wavelength, without any transverse boundaries, due to the suppression of
diffraction. This effect is related to negative refraction of the Poynting
vector inside each metal layer, balanced by normal refraction inside the
adjacent dielectric layer: The degree of field localization and material
dispersion together determine the total momentum that resides within any given
layer, and thus the direction of energy flow. We find that the transport of
evanescent wave vectors is mediated by the excitation of quasi-stationary, low
group velocity surface waves responsible for relatively large losses. As
representative examples we consider transparent metallo-dielectric stacks such
as Ag/TiO2 and Ag/GaP and show in detail how to obtain the optimum conditions
for high transmittance of both propagating and evanescent modes for
super-guiding and super resolution applications across the visible and near IR
ranges. Finally, we study the influence of gain on super-resolution. We find
that the introduction of gain can compensate the losses caused by the
excitation of surface plasmons, improves the resolving characteristics of the
lens, and leads to gain-tunable super-resolution
Interactions of Single-Nozzle Sonic Propulsive Deceleration Jets on Mars Entry Aeroshells
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83612/1/AIAA-2010-4888-277.pd
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