125,841 research outputs found
Radiation Spectral Synthesis of Relativistic Filamentation
Radiation from many astrophysical sources, e.g. gamma-ray bursts and active
galactic nuclei, is believed to arise from relativistically shocked
collisionless plasmas. Such sources often exhibit highly transient spectra
evolving rapidly, compared with source lifetimes. Radiation emitted from these
sources is typically associated with non-linear plasma physics, complex field
topologies and non-thermal particle distributions. In such circumstances a
standard synchrotron paradigm may fail to produce accurate conclusions
regarding the underlying physics. Simulating spectral emission and spectral
evolution numerically in various relativistic shock scenarios is then the only
viable method to determine the detailed physical origin of the emitted spectra.
In this Letter we present synthetic radiation spectra representing the early
stage development of the filamentation (streaming) instability of an initially
unmagnetized plasma, which is relevant for both collisionless shock formation
and reconnection dynamics in relativistic astrophysical outflows, as well as
for laboratory astrophysics experiments. Results were obtained using a highly
efficient "in situ" diagnostics method, based on detailed particle-in-cell
modeling of collisionless plasmas. The synthetic spectra obtained here are
compared with those predicted by a semi-analytical model for jitter radiation
from the filamentation instability, the latter including self-consistent
generated field topologies and particle distributions obtained from the
simulations reported upon here. Spectra exhibit dependence on the presence - or
absence - of an inert plasma constituent, when comparing baryonic plasmas (i.e.
containing protons) with pair plasmas. The results also illustrate that
considerable care should be taken when using lower-dimensional models to obtain
information about the astrophysical phenomena generating observed spectra.Comment: 5 pages, 5 figures, accepted in Astrophysical Journal Letter
Reduced dimension modeling of leading edge turbulent interaction noise
A computational aeroacoustics approach is used to model the effects of real airfoil geometry on leading edge turbulent interaction noise for symmetric airfoils at zero angle of attack. For the first time, one-component (transverse), two-component (transverse and streamwise), and three-component (transverse, streamwise, and spanwise) synthesized turbulent disturbances are modeled instead of single frequency transverse gusts, which previous computational studies of leading edge noise have been confined to. The effects of the inclusion of streamwise and spanwise disturbances on the noise are assessed, and it is shown that accurate noise predictions for symmetric airfoils can be made by modeling only the transverse disturbances, which reduces the computational expense of simulations. Additionally, the two-component turbulent synthesis method is used to model the effects of airfoil thickness on the noise for thicknesses ranging from 2% to 12%. By using sufficient airfoil thicknesses to show trends, it is found that airfoil thickness will reduce the noise at high frequency, and that the sound power P will reduce linearly with increasing airfoil thickness
Towards a Realistic, Data-Driven Thermodynamic MHD Model of the Global Solar Corona
In this work we describe our implementation of a thermodynamic energy
equation into the global corona model of the Space Weather Modeling Framework
(SWMF), and its development into the new Lower Corona (LC) model. This work
includes the integration of the additional energy transport terms of coronal
heating, electron heat conduction, and optically thin radiative cooling into
the governing magnetohydrodynamic (MHD) energy equation. We examine two
different boundary conditions using this model; one set in the upper transition
region (the Radiative Energy Balance model), as well as a uniform chromospheric
condition where the transition region can be modeled in its entirety. Via
observation synthesis from model results and the subsequent comparison to full
sun extreme ultraviolet (EUV) and soft X-Ray observations of Carrington
Rotation (CR) 1913 centered on Aug 27, 1996, we demonstrate the need for these
additional considerations when using global MHD models to describe the unique
conditions in the low corona. Through multiple simulations we examine ability
of the LC model to asses and discriminate between coronal heating models, and
find that a relative simple empirical heating model is adequate in reproducing
structures observed in the low corona. We show that the interplay between
coronal heating and electron heat conduction provides significant feedback onto
the 3D magnetic topology in the low corona as compared to a potential field
extrapolation, and that this feedback is largely dependent on the amount of
mechanical energy introduced into the corona.Comment: 17 pages, 11 figures, Submitted to ApJ on 12/08/200
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Freeform Bioprinting of Liver Encapsulated in Alginate Hydrogels Tissue Constructs for Pharmacokinetic Study
An in vitro model that can be realistically and inexpensively used to predict human response to
various drug administration and toxic chemical exposure is needed. By fabricating a microscale
3D physiological tissue construct consisting of an array of channels and tissue-embedded
chambers, one can selectively develop various biomimicking mammalian tissues for a number of
pharmaceutical applications, for example, experimental pharmaceutical screening for drug
efficacy and toxicity along with apprehending the disposition and metabolic profile of a
candidate drug. This paper addresses issues relating to the development and implementation of a
bioprinting process for freeform fabrication of a 3D cell-encapsulated hydrogel-based tissue
construct, the direct integration onto a microfluidic device for pharmacokinetic study, and the
underlying engineering science for the fabrication of a 3D microscale tissue chamber as well as
its application in pharmacokinetic study. To this end, a prototype 3D microfluidic tissue chamber
embedded with liver cells encapsulated within a hydrogel matrix construct is bioprinted as a
physiological in vitro model for pharmacokinetic study. The developed fabrication processes are
further validated and parameters optimized by assessing cell viability and liver cell phenotype, in
which metabolic and synthetic liver functions are quantitated.Mechanical Engineerin
Computational Simulation and 3D Virtual Reality Engineering Tools for Dynamical Modeling and Imaging of Composite Nanomaterials
An adventure at engineering design and modeling is possible with a Virtual
Reality Environment (VRE) that uses multiple computer-generated media to let a
user experience situations that are temporally and spatially prohibiting. In
this paper, an approach to developing some advanced architecture and modeling
tools is presented to allow multiple frameworks work together while being
shielded from the application program. This architecture is being developed in
a framework of workbench interactive tools for next generation
nanoparticle-reinforced damping/dynamic systems. Through the use of system, an
engineer/programmer can respectively concentrate on tailoring an engineering
design concept of novel system and the application software design while using
existing databases/software outputs.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
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