7,819 research outputs found
Supernova Simulations with Boltzmann Neutrino Transport: A Comparison of Methods
Accurate neutrino transport has been built into spherically symmetric
simulations of stellar core collapse and postbounce evolution. The results of
such simulations agree that spherically symmetric models with standard
microphysical input fail to explode by the delayed, neutrino-driven mechanism.
Independent groups implemented fundamentally different numerical methods to
tackle the Boltzmann neutrino transport equation. Here we present a direct and
detailed comparison of such neutrino radiation-hydrodynamical simulations for
two codes, Agile-Boltztran of the Oak Ridge-Basel group and Vertex of the
Garching group. The former solves the Boltzmann equation directly by an
implicit, general relativistic discrete angle method on the adaptive grid of a
conservative implicit hydrodynamics code with second-order TVD advection. In
contrast, the latter couples a variable Eddington factor technique with an
explicit, moving-grid, conservative high-order Riemann solver with important
relativistic effects treated by an effective gravitational potential. The
presented study is meant to test both neutrino radiation-hydrodynamics
implementations and to provide a data basis for comparisons and verifications
of supernova codes to be developed in the future. Results are discussed for
simulations of the core collapse and post-bounce evolution of a 13 solar mass
star with Newtonian gravity and a 15 solar mass star with relativistic gravity.Comment: 23 pages, 13 figures, revised version, to appear in Ap
Safety and Effectiveness of Struvite from Black Water and Urine as a Phosphorus Fertilizer
To ensure food supply, phosphorus must be recycled, for which an appealing method is using struvite fertilizer from human excreta. One struvite from black water and another from urine were assessed for safety under Dutch regulations, and for effectiveness as P fertilizer in a maize field experiment and a literature review. Both struvites contained 12% P, 12% Mg, 6% N, and 0.5-1.5% of several micronutrients. Struvites did not exceed Dutch regulations for heavy metals or pathogens, and based on literature, organic toxins should be far below regulatory limits. In this study and 18 others, struvite appears to have similar effectiveness to soluble fertilizer. Early in the season, 200 kg P2O5 ha-1 of black water struvite and soluble phosphorus improved maize performance (
Anomalous Hall Effect due to the spin chirality in the Kagom\'{e} lattice
We consider a model for a two dimensional electron gas moving on a kagom\'{e}
lattice and locally coupled to a chiral magnetic texture. We show that the
transverse conductivity does not vanish even if spin-orbit
coupling is not present and it may exhibit unusual behavior. Model parameters
are the chirality, the number of conduction electrons and the amplitude of the
local coupling. Upon varying these parameters, a topological transition
characterized by change of the band Chern numbers occur. As a consequence,
can be quantized, proportional to the chirality or have a non
monotonic behavior upon varying these parameters.Comment: 8 pages, 7 figure
Comparison of Dissipative Particle Dynamics and Langevin thermostats for out-of-equilibrium simulations of polymeric systems
In this work we compare and characterize the behavior of Langevin and
Dissipative Particle Dynamics (DPD) thermostats in a broad range of
non-equilibrium simulations of polymeric systems. Polymer brushes in relative
sliding motion, polymeric liquids in Poiseuille and Couette flows, and
brush-melt interfaces are used as model systems to analyze the efficiency and
limitations of different Langevin and DPD thermostat implementations. Widely
used coarse-grained bead-spring models under good and poor solvent conditions
are employed to assess the effects of the thermostats. We considered
equilibrium, transient, and steady state examples for testing the ability of
the thermostats to maintain constant temperature and to reproduce the
underlying physical phenomena in non-equilibrium situations. The common
practice of switching-off the Langevin thermostat in the flow direction is also
critically revisited. The efficiency of different weight functions for the DPD
thermostat is quantitatively analyzed as a function of the solvent quality and
the non-equilibrium situation.Comment: 12 pages, introduction improved, references added, to appear in Phys.
Rev.
Topological Hall effect and Berry phase in magnetic nanostructures
We discuss the anomalous Hall effect in a two-dimensional electron gas
subject to a spatially varying magnetization. This topological Hall effect
(THE) does not require any spin-orbit coupling, and arises solely from Berry
phase acquired by an electron moving in a smoothly varying magnetization. We
propose an experiment with a structure containing 2D electrons or holes of
diluted magnetic semiconductor subject to the stray field of a lattice of
magnetic nanocylinders. The striking behavior predicted for such a system (of
which all relevant parameters are well known) allows to observe unambiguously
the THE and to distinguish it from other mechanisms.Comment: 5 pages with 4 figure
Static and dynamic properties of the interface between a polymer brush and a melt of identical chains
Molecular dynamics simulations of a short-chain polymer melt between two
brush-covered surfaces under shear have been performed. The end-grafted
polymers which constitute the brush have the same chemical properties as the
free chains in the melt and provide a soft deformable substrate. Polymer chains
are described by a coarse-grained bead-spring model with Lennard-Jones
interactions between the beads and a FENE potential between nearest neighbors
along the backbone of the chains. The grafting density of the brush layer
offers a way of controlling the behavior of the surface without altering the
molecular interactions. We perform equilibrium and non-equilibrium Molecular
Dynamics simulations at constant temperature and volume using the Dissipative
Particle Dynamics thermostat. The equilibrium density profiles and the behavior
under shear are studied as well as the interdigitation of the melt into the
brush, the orientation on different length scales (bond vectors, radius of
gyration, and end-to-end vector) of free and grafted chains, and velocity
profiles. The viscosity and slippage at the interface are calculated as
functions of grafting density and shear velocity.Comment: 12 pages, submitted to J Chem Phy
Numerical Simulation of an Electroweak Oscillon
Numerical simulations of the bosonic sector of the
electroweak Standard Model in 3+1 dimensions have demonstrated the existence of
an oscillon -- an extremely long-lived, localized, oscillatory solution to the
equations of motion -- when the Higgs mass is equal to twice the boson
mass. It contains total energy roughly 30 TeV localized in a region of radius
0.05 fm. A detailed description of these numerical results is presented.Comment: 12 pages, 8 figures, uses RevTeX4; v2: expanded results section,
fixed typo
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