1,571 research outputs found
Neutrino-driven supernova of a low-mass iron-core progenitor boosted by three-dimensional turbulent convection
We present the first successful simulation of a neutrino-driven supernova
explosion in three dimensions (3D), using the Prometheus-Vertex code with an
axis-free Yin-Yang grid and a sophisticated treatment of three-flavor,
energy-dependent neutrino transport. The progenitor is a nonrotating,
zero-metallicity 9.6 Msun star with an iron core. While in spherical symmetry
outward shock acceleration sets in later than 300 ms after bounce, a successful
explosion starts at ~130 ms postbounce in two dimensions (2D). The 3D model
explodes at about the same time but with faster shock expansion than in 2D and
a more quickly increasing and roughly 10 percent higher explosion energy of
>10^50 erg. The more favorable explosion conditions in 3D are explained by
lower temperatures and thus reduced neutrino emission in the cooling layer
below the gain radius. This moves the gain radius inward and leads to a bigger
mass in the gain layer, whose larger recombination energy boosts the explosion
energy in 3D. These differences are caused by less coherent, less massive, and
less rapid convective downdrafts associated with postshock convection in 3D.
The less violent impact of these accretion downflows in the cooling layer
produces less shock heating and therefore diminishes energy losses by neutrino
emission. We thus have, for the first time, identified a reduced mass accretion
rate, lower infall velocities, and a smaller surface filling factor of
convective downdrafts as consequences of 3D postshock turbulence that
facilitate neutrino-driven explosions and strengthen them compared to the 2D
case.Comment: 7 pages, 5 figures; revised version with more discussion of
resolution dependence and differences to other 3D results; accepted by ApJ
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
TAWFIVE: A user's guide
The Transonic Analysis of a Wing and Fuselage with Interacted Viscous Effects (TAWFIVE) was developed. A finite volume full potential method is used to model the outer inviscid flow field. First-order viscous effects are modeled by a three dimensional integral boundary layer method. Both turbulent and laminar boundary layers are treated. Wake thickness and curvature effects are modeled using a two dimensional strip method. A very brief discussion of the engineering aspects of the program is given. The input and use of the program are covered in great detail
Updated users' guide for TAWFIVE with multigrid
A program for the Transonic Analysis of a Wing and Fuselage with Interacted Viscous Effects (TAWFIVE) was improved by the incorporation of multigrid and a method to specify lift coefficient rather than angle-of-attack. A finite volume full potential multigrid method is used to model the outer inviscid flow field. First order viscous effects are modeled by a 3-D integral boundary layer method. Both turbulent and laminar boundary layers are treated. Wake thickness effects are modeled using a 2-D strip method. A brief discussion of the engineering aspects of the program is given. The input, output, and use of the program are covered in detail. Sample results are given showing the effects of boundary layer corrections and the capability of the lift specification method
Use of CYBER 203 and CYBER 205 computers for three-dimensional transonic flow calculations
Experiences are discussed for modifying two three-dimensional transonic flow computer programs (FLO 22 and FLO 27) for use on the CDC CYBER 203 computer system. Both programs were originally written for use on serial machines. Several methods were attempted to optimize the execution of the two programs on the vector machine: leaving the program in a scalar form (i.e., serial computation) with compiler software used to optimize and vectorize the program, vectorizing parts of the existing algorithm in the program, and incorporating a vectorizable algorithm (ZEBRA I or ZEBRA II) in the program. Comparison runs of the programs were made on CDC CYBER 175. CYBER 203, and two pipe CDC CYBER 205 computer systems
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
Studying Mathematical Relationships Using TI Calculator
Students use mathematical modeling/ multiple representation to provide a means of presenting, interpreting, communicating, and connecting mathematical information and relationships. They will be able to use the graphic calculator to model the data and choose proper mathematical model to simulate the data
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
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