378 research outputs found
Resolution Study for Three-dimensional Supernova Simulations with the Prometheus-Vertex Code
We present a carefully designed, systematic study of the angular resolution
dependence of simulations with the Prometheus-Vertex neutrino-hydrodynamics
code. Employing a simplified neutrino heating-cooling scheme in the Prometheus
hydrodynamics module allows us to sample the angular resolution between 4
degrees and 0.5 degrees. With a newly-implemented static mesh refinement (SMR)
technique on the Yin-Yang grid, the angular coordinates can be refined in
concentric shells, compensating for the diverging structure of the spherical
grid. In contrast to previous studies with Prometheus and other codes, we find
that higher angular resolution and therefore lower numerical viscosity provides
more favorable explosion conditions and faster shock expansion. We discuss the
possible reasons for the discrepant results. The overall dynamics seem to
converge at a resolution of about 1 degree. Applying the SMR setup to
marginally exploding progenitors is disadvantageous for the shock expansion,
however, because kinetic energy of downflows is dissipated to internal energy
at resolution interfaces, leading to a loss of turbulent pressure support and a
steeper temperature gradient. We also present a way to estimate the numerical
viscosity on grounds of the measured turbulent kinetic-energy spectrum, leading
to smaller values that are better compatible with the flow behavior witnessed
in our simulations than results following calculations in previous literature.
Interestingly, the numerical Reynolds numbers in the turbulent, neutrino-heated
postshock layer (some 10 to several 100) are in the ballpark of expected
neutrino-drag effects on the relevant length scales in the turbulent postshock
layer. We provide a formal derivation and quantitative assessment of the
neutrino drag terms in an appendix.Comment: 37 pages, 14 figures, 4 tables; revised version with neutrino drag
discussion extended for numerical evaluation; accepted by Ap
Fractography of the high temperature hydrogen attack of a medium carbon steel
Microscopic fracture processes were studied which are associated with hydrogen attack of a medium carbon steel in a well-controlled, high-temperature, high-purity hydrogen environment. Exposure to a hydrogen pressure and temperature of 3.5 MN/m2 and 575 C was found to degrade room temperature tensile properties with increasing exposure time. After 408 hr, yield and ultimate strengths were reduced by more than 40 percent and elongation was reduced to less than 2 percent. Initial fissure formation was found to be associated with manganese rich particles, most probably manganese oxide, aligned in the microstructure during the rolling operation. Fissure growth was found to be associated with a reduction in carbide content of the microstructure and was inhibited by the depletion of carbon. The interior surfaces of sectioned fissures or bubbles exhibit both primary and secondary cracking by intergranular separation. The grain surfaces were rough and rounded, suggesting a diffusion-associated separation process. Specimens that failed at room temperature after exposure to hydrogen were found to exhibit mixed mode fracture having varying amounts of intergranular separation, dimple formation, and cleavage, depending on exposure time
Parallelized Solution Method of the Three-dimensional Gravitational Potential on the Yin-Yang Grid
We present a new method for solving the three-dimensional gravitational
potential of a density field on the Yin-Yang grid. Our algorithm is based on a
multipole decomposition and completely symmetric with respect to the two
Yin-Yang grid patches. It is particularly efficient on distributed-memory
machines with a large number of compute tasks, because the amount of data being
explicitly communicated is minimized. All operations are performed on the
original grid without the need for interpolating data onto an auxiliary
spherical mesh.Comment: 8 pages, 4 figures; two minor additions after refereeing; accepted by
Ap
Supernova Simulations from a 3D Progenitor Model -- Impact of Perturbations and Evolution of Explosion Properties
We study the impact of large-scale perturbations from convective shell
burning on the core-collapse supernova explosion mechanism using
three-dimensional (3D) multi-group neutrino hydrodynamics simulations of an 18
solar mass progenitor. Seed asphericities in the O shell, obtained from a
recent 3D model of O shell burning, help trigger a neutrino-driven explosion
330ms after bounce whereas the shock is not revived in a model based on a
spherically symmetric progenitor for at least another 300ms. We tentatively
infer a reduction of the critical luminosity for shock revival by ~20% due to
pre-collapse perturbations. This indicates that convective seed perturbations
play an important role in the explosion mechanism in some progenitors. We
follow the evolution of the 18 solar mass model into the explosion phase for
more than 2s and find that the cycle of accretion and mass ejection is still
ongoing at this stage. With a preliminary value of 0.77 Bethe for the
diagnostic explosion energy, a baryonic neutron star mass of 1.85 solar masses,
a neutron star kick of ~600km/s and a neutron star spin period of ~20ms at the
end of the simulation, the explosion and remnant properties are slightly
atypical, but still lie comfortably within the observed distribution. Although
more refined simulations and a larger survey of progenitors are still called
for, this suggests that a solution to the problem of shock revival and
explosion energies in the ballpark of observations are within reach for
neutrino-driven explosions in 3D.Comment: 23 pages, 22 figures, accepted for publication in MNRA
Rotation-supported Neutrino-driven Supernova Explosions in Three Dimensions and the Critical Luminosity Condition
We present the first self-consistent, three-dimensional (3D) core-collapse
supernova simulations performed with the Prometheus-Vertex code for a rotating
progenitor star. Besides using the angular momentum of the 15 solar-mass model
as obtained in the stellar evolution calculation with an angular frequency of
about 0.001 rad/s (spin period of more than 6000 s) at the Si/Si-O interface,
we also computed 2D and 3D cases with no rotation and with a ~300 times shorter
rotation period and different angular resolutions. In 2D, only the nonrotating
and slowly rotating models explode, while rapid rotation prevents an explosion
within 500 ms after bounce because of lower radiated neutrino luminosities and
mean energies and thus reduced neutrino heating. In contrast, only the fast
rotating model develops an explosion in 3D when the Si/Si-O interface collapses
through the shock. The explosion becomes possible by the support of a powerful
SASI spiral mode, which compensates for the reduced neutrino heating and pushes
strong shock expansion in the equatorial plane. Fast rotation in 3D leads to a
"two-dimensionalization" of the turbulent energy spectrum (yielding roughly a
-3 instead of a -5/3 power-law slope at intermediate wavelengths) with enhanced
kinetic energy on the largest spatial scales. We also introduce a
generalization of the "universal critical luminosity condition" of Summa et al.
(2016) to account for the effects of rotation, and demonstrate its viability
for a set of more than 40 core-collapse simulations including 9 and 20
solar-mass progenitors as well as black-hole forming cases of 40 and 75
solar-mass stars to be discussed in forthcoming papers.Comment: 24 pages, 19 figures; refereed version with additional section on
resolution dependence; accepted by Ap
Effects of LESA in Three-Dimensional Supernova Simulations with Multi-Dimensional and Ray-by-Ray-plus Neutrino Transport
A set of eight self-consistent, time-dependent supernova (SN) simulations in
three spatial dimensions (3D) for 9 solar-mass and 20 solar-mass progenitors is
evaluated for the presence of dipolar asymmetries of the electron lepton-number
emission as discovered by Tamborra et al. and termed lepton-number emission
self-sustained asymmetry (LESA). The simulations were performed with the
Aenus-Alcar neutrino/hydrodynamics code, which treats the energy- and
velocity-dependent transport of neutrinos of all flavors by a two-moment scheme
with algebraic M1 closure. For each of the progenitors, results with fully
multi-dimensional (FMD) neutrino transport and with ray-by-ray-plus (RbR+)
approximation are considered for two different grid resolutions. While the 9
solar-mass models develop explosions, the 20 solar-mass progenitor does not
explode with the employed version of simplified neutrino opacities. In all 3D
models we observe the growth of substantial dipole amplitudes of the
lepton-number (electron neutrino minus antineutrino) flux with stable or slowly
time-evolving direction and overall properties fully consistent with the LESA
phenomenon. Models with RbR+ transport develop LESA dipoles somewhat faster and
with temporarily higher amplitudes, but the FMD calculations exhibit cleaner
hemispheric asymmetries with a far more dominant dipole. In contrast, the RbR+
results display much wider multipole spectra of the neutrino-emission
anisotropies with significant power also in the quadrupole and higher-order
modes. Our results disprove speculations that LESA is a numerical artifact of
RbR+ transport. We also discuss LESA as consequence of a dipolar convection
flow inside of the nascent neutron star and establish, tentatively, a
connection to Chandrasekhar's linear theory of thermal instability in spherical
shells.Comment: 20 pages, 9 figures; revised version accepted by ApJ; new Figs. 6,7,
and new panels in Fig.8 added; Sects. 4,5,6 considerably extended in reply to
referee question
Making in-class skills training more effective: the scope for interactive videos to complement the delivery of practical pedestrian training
Skills and awareness of young pedestrians can be improved with on-street practical pedestrian training, often delivered in schools in the United Kingdom by local authorities with the intention of improving road safety. This training is often supplemented by in-class paper based worksheet activities which are seen to be less effective than practical training in that they focus on knowledge acquisition rather than directly improving the correct application of safe pedestrian skills at the roadside. Previous research indicates that interactive video tools have the potential to develop procedural skills whilst offering an engaging road safety educational experience, which could positively impact on road crossing behaviour.In this paper, the design and development of a hazard-identification interactive road safety training video targeting child road crossing skills is presented. The interactive video was shown to be an engaging training resource for 6-7 year old children. The tool’s scope for improving pedestrians’ roadside skills is considered along with the wider implications for interactive video to aid safety training in other areas
Paleogene sediment from a fracture zone of the Mid-Atlantic Ridge
A dredge haul from a transverse fracture zone in the equatorial Atlantic Ocean yielded sediment of the Paleocene and the early Eocene ages. This sediment is the oldest thus far recorded from the Atlantic outside of the continental margins. Its occurrence is consistent with the view that transverse fracture zones are faults along which sea-floor spreading has taken place
The Ursinus Weekly, November 30, 1942
Y launches drive at Ursinus today in behalf of WSSF • Louis Adamic, Messiah, and senior ball to make the week of December seventh one of the biggest of the college year • Community club\u27s program questions way toward peace • Grad\u27s sister dies in Boston tragedy • Faculty sends four to profs\u27 conclave • Y heads to address frosh • Analysis of women re-written to include artistic viewpoint • Personality and broad musical experience distinguish Dr. Philip • Debaters to meet tonight • Snell\u27s belles give bear sports first unblemished season in five years • Giants finish perfect season with 6-0 win over Packers • English Club to meet at McClure\u27s home tonight • Beautiful and historical trees cover campus; Dean Kline is nature expert of the collegehttps://digitalcommons.ursinus.edu/weekly/1747/thumbnail.jp
The Ursinus Weekly, December 8, 1941
Guest vocalists and musical organizations will blend talents in Messiah presentation • Wenhold tells vespers that giving is one of most important virtues • First forum hears discussion of the post-war world • Gordon airs and novel tavern party to highlight gala senior weekend • President McClure calls for balance in time of crisis • Dr. N. W. Winkelman, Penn neurology prof, to address pre-meds • Michael named member of survey committee for secondary schools • Ursinus ranks high in grad school placements; stands 47th in survey of 215 leading colleges • Rec hall to have host, hostess; Appleget plans novelty events • Bomberger memorial maintains unceasing vigil as it celebrates completion of fiftieth year • Federal government to levy 10% tax on admissions to college activities • John and David Garvey present music recital in Bomberger Hall • Bear quintet faces Lehigh in season\u27s opener this Saturday • Twenty-one wrestlers prepare for opener against Kutztown Wednesday • Co-eds have last year\u27s team intact for season\u27s play • Jing Johnson announces athletic award winners • Captains elected! • 20 frosh candidates drill for season\u27s play under coach Pancoast • G-burg\u27s Bullets dominate bears opponent team; Petro unanimous • Bears make all teams of opponents and scribes • Giving: the spirit of Christmas • Vickland and Jamison to handle soprano, tenor parts in Messiahhttps://digitalcommons.ursinus.edu/weekly/1772/thumbnail.jp
- …