362,618 research outputs found
Formation of plasma around a small meteoroid: simulation and theory
High‐power large‐aperture radars detect meteors by reflecting radio waves off dense plasma that surrounds a hypersonic meteoroid as it ablates in the Earth's atmosphere. If the plasma density profile around the meteoroid is known, the plasma's radar cross section can be used to estimate meteoroid properties such as mass, density, and composition. This paper presents head echo plasma density distributions obtained via two numerical simulations of a small ablating meteoroid and compares the results to an analytical solution found in Dimant and Oppenheim (2017a, https://doi.org/10.1002/2017JA023960, 2017b, https://doi.org/10.1002/2017JA023963). The first simulation allows ablated meteoroid particles to experience only a single collision to match an assumption in the analytical solution, while the second is a more realistic simulation by allowing multiple collisions. The simulation and analytical results exhibit similar plasma density distributions. At distances much less than λT, the average distance an ablated particle travels from the meteoroid before a collision with an atmospheric particle, the plasma density falls off as 1/R, where R is the distance from the meteoroid center. At distances substantially greater than λT, the plasma density profile has an angular dependence, falling off as 1/R^2 directly behind the meteoroid, 1/R^3 in a plane perpendicular to the meteoroid's path that contains the meteoroid center, and exp - 1.5(/λ)2/3/ in front of the meteoroid. When used for calculating meteoroid masses, this new plasma density model can give masses that are orders of magnitude different than masses calculated from a spherically symmetric Gaussian distribution, which has been used to calculate masses in the past.This work was supported by NSF grants AGS-1244842 and AGS-1056042. This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant ACI-1548562. The authors acknowledge the Texas Advanced Computing Center (TACC) at The University of Texas at Austin for providing HPC resources that have contributed to the research results reported within this paper; URL: http://www.tacc.utexas.edu. Simulation-produced data are archived at TACC and available upon request. (AGS-1244842 - NSF; AGS-1056042 - NSF; ACI-1548562 - National Science Foundation)First author draf
Induced Rotation in 3D Simulations of Core Collapse Supernovae: Implications for Pulsar Spins
It has been suggested that the observed rotation periods of radio pulsars
might be induced by a non-axisymmetric spiral-mode instability in the turbulent
region behind the stalled supernova bounce shock, even if the progenitor core
was not initially rotating. In this paper, using the three-dimensional AMR code
CASTRO with a realistic progenitor and equation of state and a simple neutrino
heating and cooling scheme, we present a numerical study of the evolution in 3D
of the rotational profile of a supernova core from collapse, through bounce and
shock stagnation, to delayed explosion. By the end of our simulation (420
ms after core bounce), we do not witness significant spin up of the
proto-neutron star core left behind. However, we do see the development before
explosion of strong differential rotation in the turbulent gain region between
the core and stalled shock. Shells in this region acquire high spin rates that
reach Hz, but this region contains too little mass and angular
momentum to translate, even if left behind, into rapid rotation for the full
neutron star. We find also that much of the induced angular momentum is likely
to be ejected in the explosion, and moreover that even if the optimal amount of
induced angular momentum is retained in the core, the resulting spin period is
likely to be quite modest. Nevertheless, induced periods of seconds are
possible.Comment: Accepted to the Astrophysical Journa
Fast Radiative Shocks in Dense Media. III. Properties of the Emission
Evolution of fast, radiative shocks in high density medium is presented.
Ionizing spectra and approximate broad band light curves of the shocked gas are
calculated. Emergent shock spectra, as seen by a distant observer, are obtained
from photoionization models. The emergent spectra have a power-law shape
with mean spectral index in
the energy range keV, and have a high-energy cutoff corresponding to
the original shock velocity. It is shown that the models exhibit promising
features that may account for some photometric and spectral properties of
Active Galactic Nuclei.Comment: 9 pages, 8 Postscript figures (not included), uses mn.sty, submitted
to MNRAS, revised version. A complete version with figures (self-unpacking
uuencoded archive) is available at
http://www.astrouw.edu.pl/~plewa/papers/pap3/ps/pap3.u
Effect of a rotor wake on the local heat transfer on the forward half of a circular cylinder
Turbine rotor-stator wake dynamics was simulated by a spoked wheel rotating in annular flow, generating rotor wakes. Spanwise averaged circumferentially local heat transfer in the circular cylindrical leading edge region of a turbine airfoil was obtained. Reynolds numbers ranged from 35,000 to 175,000. Strouhal numbers ranged from 0.63 to 2.50. Wakes were generated by 2 sets of circular cylindrical bars, 1.59 and 3.18 mm in diameter. The rotor could be rotated either clockwise or counterclockwise. Grid turbulence was introduced upstream yielding freestream turbulence of 1.0 to 2.5% at the stator. Data represented an extensive body of local heat transfer coefficients, which can be used to model the leading edge region of a turbine airfoil. In the presence of rotor wakes, an asymmetry from the leeward to windward side was noted. Windward side levels were 30 to 40% higher than the corresponding leeward side
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