2,547 research outputs found
Fully-Coupled Simulation of Cosmic Reionization. I: Numerical Methods and Tests
We describe an extension of the Enzo code to enable fully-coupled radiation
hydrodynamical simulation of inhomogeneous reionization in large cosmological volumes with thousands to millions of point sources. We
solve all dynamical, radiative transfer, thermal, and ionization processes
self-consistently on the same mesh, as opposed to a postprocessing approach
which coarse-grains the radiative transfer. We do, however, employ a simple
subgrid model for star formation which we calibrate to observations. Radiation
transport is done in the grey flux-limited diffusion (FLD) approximation, which
is solved by implicit time integration split off from the gas energy and
ionization equations, which are solved separately. This results in a faster and
more robust scheme for cosmological applications compared to the earlier
method. The FLD equation is solved using the hypre optimally scalable geometric
multigrid solver from LLNL. By treating the ionizing radiation as a grid field
as opposed to rays, our method is scalable with respect to the number of
ionizing sources, limited only by the parallel scaling properties of the
radiation solver. We test the speed and accuracy of our approach on a number of
standard verification and validation tests. We show by direct comparison with
Enzo's adaptive ray tracing method Moray that the well-known inability of FLD
to cast a shadow behind opaque clouds has a minor effect on the evolution of
ionized volume and mass fractions in a reionization simulation validation test.
We illustrate an application of our method to the problem of inhomogeneous
reionization in a 80 Mpc comoving box resolved with Eulerian grid
cells and dark matter particles.Comment: 32 pages, 23 figures. ApJ Supp accepted. New title and substantial
revisions re. v
Experimental Synthetic Aperture Radar with Dynamic Metasurfaces
We investigate the use of a dynamic metasurface as the transmitting antenna
for a synthetic aperture radar (SAR) imaging system. The dynamic metasurface
consists of a one-dimensional microstrip waveguide with complementary electric
resonator (cELC) elements patterned into the upper conductor. Integrated into
each of the cELCs are two diodes that can be used to shift each cELC resonance
out of band with an applied voltage. The aperture is designed to operate at K
band frequencies (17.5 to 20.3 GHz), with a bandwidth of 2.8 GHz. We
experimentally demonstrate imaging with a fabricated metasurface aperture using
existing SAR modalities, showing image quality comparable to traditional
antennas. The agility of this aperture allows it to operate in spotlight and
stripmap SAR modes, as well as in a third modality inspired by computational
imaging strategies. We describe its operation in detail, demonstrate
high-quality imaging in both 2D and 3D, and examine various trade-offs
governing the integration of dynamic metasurfaces in future SAR imaging
platforms
Cosmological Radiation Hydrodynamics with ENZO
We describe an extension of the cosmological hydrodynamics code ENZO to
include the self-consistent transport of ionizing radiation modeled in the
flux-limited diffusion approximation. A novel feature of our algorithm is a
coupled implicit solution of radiation transport, ionization kinetics, and gas
photoheating, making the timestepping for this portion of the calculation
resolution independent. The implicit system is coupled to the explicit
cosmological hydrodynamics through operator splitting and solved with scalable
multigrid methods. We summarize the numerical method, present a verification
test on cosmological Stromgren spheres, and then apply it to the problem of
cosmological hydrogen reionization.Comment: 14 pages, 3 figures, to appear in Recent Directions in Astrophysical
Quantitative Spectroscopy and Radiation Hydrodynamics, Ed. I. Hubeny,
American Institute of Physics (2009
Steepest-Entropy-Ascent Quantum Thermodynamics Models in Materials Science
Steepest-entropy-ascent quantum thermodynamics, or SEAQT, is a unified
approach of quantum mechanics and thermodynamics that avoids many of the
inconsistencies that can arise between the two theories. Given a set of energy
levels, i.e., energy eigenstructure, accessible to a given physical system,
SEAQT predicts the unique kinetic path from any initial non-equilibrium state
to stable equilibrium by solving a master equation that directs the system
along the path of steepest entropy ascent. There are no intrinsic limitations
on the length and time scales the method can treat so it is well-suited for
calculations where the dynamics over multiple spacial scales need to be taken
into account within a single framework. In this paper, the theoretical
framework and its advantages are described, and several applications are
presented to illustrate the use of the SEAQT equation of motion and the
construction of a simplified, reduced-order, energy eigenstructure.Comment: 18 page
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